feat(components): add ThermoState generators and Eurovent backend demo

crates/fluids/Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "entropyk-fluids"
+version.workspace = true
+authors.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "Fluid properties backend for Entropyk thermodynamic simulation library"
+
+[dependencies]
+entropyk-core = { path = "../core" }
+thiserror.workspace = true
+serde.workspace = true
+serde_json = "1.0"
+lru = "0.12"
+entropyk-coolprop-sys = { path = "coolprop-sys", optional = true }
+
+[features]
+default = []
+coolprop = ["entropyk-coolprop-sys"]
+
+[dev-dependencies]
+approx = "0.5"
+criterion = "0.5"
+
+[[bench]]
+name = "cache_10k"
+harness = false

crates/fluids/benches/cache_10k.rs

@@ -0,0 +1,54 @@
+//! Benchmark: 10k repeated (P,T) queries — cached vs uncached (Story 2.4 AC#4).
+//!
+//! Compares throughput of CachedBackend vs raw backend for repeated same-state queries.
+//! Cached path should show significant speedup when the backend is expensive (e.g. CoolProp).
+
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use entropyk_fluids::{
+    CachedBackend, FluidBackend, FluidId, Property, ThermoState, TestBackend,
+};
+use entropyk_core::{Pressure, Temperature};
+
+const N_QUERIES: u32 = 10_000;
+
+fn bench_uncached_10k(c: &mut Criterion) {
+    let backend = TestBackend::new();
+    let state = ThermoState::from_pt(
+        Pressure::from_bar(1.0),
+        Temperature::from_celsius(25.0),
+    );
+    let fluid = FluidId::new("R134a");
+
+    c.bench_function("uncached_10k_same_state", |b| {
+        b.iter(|| {
+            for _ in 0..N_QUERIES {
+                black_box(
+                    backend.property(fluid.clone(), Property::Density, state.clone()).unwrap(),
+                );
+            }
+        });
+    });
+}
+
+fn bench_cached_10k(c: &mut Criterion) {
+    let inner = TestBackend::new();
+    let cached = CachedBackend::new(inner);
+    let state = ThermoState::from_pt(
+        Pressure::from_bar(1.0),
+        Temperature::from_celsius(25.0),
+    );
+    let fluid = FluidId::new("R134a");
+
+    c.bench_function("cached_10k_same_state", |b| {
+        b.iter(|| {
+            for _ in 0..N_QUERIES {
+                black_box(
+                    cached.property(fluid.clone(), Property::Density, state.clone()).unwrap(),
+                );
+            }
+        });
+    });
+}
+
+criterion_group!(benches, bench_uncached_10k, bench_cached_10k);
+criterion_main!(benches);

crates/fluids/build.rs

@@ -0,0 +1,18 @@
+//! Build script for entropyk-fluids crate.
+//! 
+//! This build script can optionally compile CoolProp C++ library when the
+//! "coolprop" feature is enabled.
+
+use std::env;
+
+fn main() {
+    let coolprop_enabled = env::var("CARGO_FEATURE_COOLPROP").is_ok();
+    
+    if coolprop_enabled {
+        println!("cargo:rustc-link-lib=dylib=coolprop");
+        println!("cargo:rerun-if-changed=build.rs");
+    }
+    
+    // Tell Cargo to rerun this script if any source files change
+    println!("cargo:rerun-if-changed=build.rs");
+}

crates/fluids/coolprop-sys/build.rs

@@ -0,0 +1,64 @@
+//! Build script for coolprop-sys.
+//!
+//! This compiles the CoolProp C++ library statically.
+
+use std::env;
+use std::path::PathBuf;
+
+fn coolprop_src_path() -> Option<PathBuf> {
+    // Try to find CoolProp source in common locations
+    let possible_paths = vec![
+        // Vendor directory (recommended)
+        PathBuf::from("vendor/coolprop"),
+        // External directory
+        PathBuf::from("external/coolprop"),
+        // System paths
+        PathBuf::from("/usr/local/src/CoolProp"),
+        PathBuf::from("/opt/CoolProp"),
+    ];
+
+    for path in possible_paths {
+        if path.join("CMakeLists.txt").exists() {
+            return Some(path);
+        }
+    }
+
+    None
+}
+
+fn main() {
+    let static_linking = env::var("CARGO_FEATURE_STATIC").is_ok();
+
+    // Check if CoolProp source is available
+    if let Some(coolprop_path) = coolprop_src_path() {
+        println!("cargo:rerun-if-changed={}", coolprop_path.display());
+
+        // Configure build for CoolProp
+        println!(
+            "cargo:rustc-link-search=native={}/build",
+            coolprop_path.display()
+        );
+    }
+
+    // Link against CoolProp
+    if static_linking {
+        // Static linking - find libCoolProp.a
+        println!("cargo:rustc-link-lib=static=CoolProp");
+    } else {
+        // Dynamic linking
+        println!("cargo:rustc-link-lib=dylib=CoolProp");
+    }
+
+    // Link required system libraries
+    println!("cargo:rustc-link-lib=dylib=m");
+    println!("cargo:rustc-link-lib=dylib=stdc++");
+
+    // Tell Cargo to rerun if build.rs changes
+    println!("cargo:rerun-if-changed=build.rs");
+
+    println!(
+        "cargo:warning=CoolProp source not found in vendor/. 
+              For full static build, run: 
+              git clone https://github.com/CoolProp/CoolProp.git vendor/coolprop"
+    );
+}

crates/fluids/coolprop-sys/src/lib.rs

@@ -0,0 +1,336 @@
+//! FFI bindings to CoolProp C++ library.
+//!
+//! This module provides low-level FFI bindings to the CoolProp library.
+//! All functions are unsafe and require proper error handling.
+
+#![allow(dead_code)]
+
+use libc::{c_char, c_double, c_int};
+use std::ffi::CString;
+
+/// Error codes returned by CoolProp
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(i32)]
+pub enum CoolPropError {
+    /// No error occurred
+    NoError = 0,
+    /// Input error code
+    InputError = 1,
+    /// Library not loaded
+    LibraryNotLoaded = 2,
+    /// Unknown property value
+    UnknownPropertyValue = 3,
+    /// Unknown fluid
+    UnknownFluid = 4,
+    /// Unknown parameter
+    UnknownParameter = 5,
+    /// Not implemented
+    NotImplemented = 6,
+    /// Invalid number of parameters
+    InvalidNumber = 7,
+    /// Could not load library
+    CouldNotLoadLibrary = 8,
+    /// Invalid fluid pair
+    InvalidFluidPair = 9,
+    /// Version mismatch
+    VersionMismatch = 10,
+    /// Internal error
+    InternalError = 11,
+}
+
+impl CoolPropError {
+    /// Convert CoolProp error code to Rust result
+    pub fn from_code(code: i32) -> Result<(), CoolPropError> {
+        match code {
+            0 => Ok(()),
+            _ => Err(match code {
+                1 => CoolPropError::InputError,
+                2 => CoolPropError::LibraryNotLoaded,
+                3 => CoolPropError::UnknownPropertyValue,
+                4 => CoolPropError::UnknownFluid,
+                5 => CoolPropError::UnknownParameter,
+                6 => CoolPropError::NotImplemented,
+                7 => CoolPropError::InvalidNumber,
+                8 => CoolPropError::CouldNotLoadLibrary,
+                9 => CoolPropError::InvalidFluidPair,
+                10 => CoolPropError::VersionMismatch,
+                _ => CoolPropError::InternalError,
+            }),
+        }
+    }
+}
+
+/// Output parameters for CoolProp
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(i32)]
+pub enum CoolPropParam {
+    /// Nothing
+    Nothing = 0,
+    /// Pressure [Pa]
+    Pressure = 1,
+    /// Temperature [K]
+    Temperature = 2,
+    /// Density [kg/m³]
+    Density = 3,
+    /// Specific enthalpy [J/kg]
+    Enthalpy = 4,
+    /// Specific entropy [J/kg/K]
+    Entropy = 5,
+    /// Specific internal energy [J/kg]
+    InternalEnergy = 6,
+    /// Specific heat at constant pressure [J/kg/K]
+    Cv = 7,
+    /// Specific heat at constant pressure [J/kg/K]
+    Cp = 8,
+    /// Quality [-]
+    Quality = 9,
+    /// Viscosity [Pa·s]
+    Viscosity = 10,
+    /// Thermal conductivity [W/m/K]
+    Conductivity = 11,
+    /// Surface tension [N/m]
+    SurfaceTension = 12,
+    /// Prandtl number [-]
+    Prandtl = 13,
+}
+
+/// Input parameters for CoolProp
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(i32)]
+pub enum CoolPropInputPair {
+    /// No input
+    None = 0,
+    /// Pressure & Temperature
+    PT = 1,
+    /// Pressure & Density
+    PD = 2,
+    /// Pressure & Enthalpy
+    PH = 3,
+    /// Pressure & Entropy
+    PS = 4,
+    /// Pressure & Internal Energy
+    PU = 5,
+    /// Temperature & Density
+    TD = 6,
+    /// Temperature & Enthalpy
+    TH = 7,
+    /// Temperature & Entropy
+    TS = 8,
+    /// Temperature & Internal Energy
+    TU = 9,
+    /// Enthalpy & Entropy
+    HS = 10,
+    /// Density & Internal Energy
+    DU = 11,
+    /// Pressure & Quality
+    PQ = 12,
+    /// Temperature & Quality
+    TQ = 13,
+}
+
+// CoolProp C functions
+extern "C" {
+    /// Get a property value using pressure and temperature
+    fn CoolProp_PropsSI(
+        Output: c_char,
+        Name1: c_char,
+        Value1: c_double,
+        Name2: c_char,
+        Value2: c_double,
+        Fluid: *const c_char,
+    ) -> c_double;
+
+    /// Get a property value using input pair
+    fn CoolProp_Props1SI(Fluid: *const c_char, Output: c_char) -> c_double;
+
+    /// Get CoolProp version string
+    fn CoolProp_get_global_param_string(
+        Param: *const c_char,
+        Output: *mut c_char,
+        OutputLength: c_int,
+    ) -> c_int;
+
+    /// Get fluid info
+    fn CoolProp_get_fluid_param_string(
+        Fluid: *const c_char,
+        Param: *const c_char,
+        Output: *mut c_char,
+        OutputLength: c_int,
+    ) -> c_int;
+
+    /// Check if fluid exists
+    fn CoolProp_isfluid(Fluid: *const c_char) -> c_int;
+
+    /// Get saturation temperature
+    fn CoolProp_Saturation_T(Fluid: *const c_char, Par: c_char, Value: c_double) -> c_double;
+
+    /// Get critical point
+    fn CoolProp_CriticalPoint(Fluid: *const c_char, Output: c_char) -> c_double;
+}
+
+/// Get a thermodynamic property using pressure and temperature.
+///
+/// # Arguments
+/// * `property` - The property to retrieve (e.g., "D" for density, "H" for enthalpy)
+/// * `p` - Pressure in Pa
+/// * `t` - Temperature in K
+/// * `fluid` - Fluid name (e.g., "R134a")
+///
+/// # Returns
+/// The property value in SI units, or NaN if an error occurs
+pub unsafe fn props_si_pt(property: &str, p: f64, t: f64, fluid: &str) -> f64 {
+    let prop = property.as_bytes()[0] as c_char;
+    let fluid_c = CString::new(fluid).unwrap();
+
+    CoolProp_PropsSI(prop, b'P' as c_char, p, b'T' as c_char, t, fluid_c.as_ptr())
+}
+
+/// Get a thermodynamic property using pressure and enthalpy.
+///
+/// # Arguments
+/// * `property` - The property to retrieve
+/// * `p` - Pressure in Pa
+/// * `h` - Specific enthalpy in J/kg
+/// * `fluid` - Fluid name
+///
+/// # Returns
+/// The property value in SI units, or NaN if an error occurs
+pub unsafe fn props_si_ph(property: &str, p: f64, h: f64, fluid: &str) -> f64 {
+    let prop = property.as_bytes()[0] as c_char;
+    let fluid_c = CString::new(fluid).unwrap();
+
+    CoolProp_PropsSI(prop, b'P' as c_char, p, b'H' as c_char, h, fluid_c.as_ptr())
+}
+
+/// Get a thermodynamic property using temperature and quality (saturation).
+///
+/// # Arguments
+/// * `property` - The property to retrieve (D, H, S, P, etc.)
+/// * `t` - Temperature in K
+/// * `q` - Quality (0 = saturated liquid, 1 = saturated vapor)
+/// * `fluid` - Fluid name
+///
+/// # Returns
+/// The property value in SI units, or NaN if an error occurs
+pub unsafe fn props_si_tq(property: &str, t: f64, q: f64, fluid: &str) -> f64 {
+    let prop = property.as_bytes()[0] as c_char;
+    let fluid_c = CString::new(fluid).unwrap();
+
+    CoolProp_PropsSI(prop, b'T' as c_char, t, b'Q' as c_char, q, fluid_c.as_ptr())
+}
+
+/// Get a thermodynamic property using pressure and quality.
+///
+/// # Arguments
+/// * `property` - The property to retrieve
+/// * `p` - Pressure in Pa
+/// * `x` - Quality (0-1)
+/// * `fluid` - Fluid name
+///
+/// # Returns
+/// The property value in SI units, or NaN if an error occurs
+pub unsafe fn props_si_px(property: &str, p: f64, x: f64, fluid: &str) -> f64 {
+    let prop = property.as_bytes()[0] as c_char;
+    let fluid_c = CString::new(fluid).unwrap();
+
+    CoolProp_PropsSI(
+        prop,
+        b'P' as c_char,
+        p,
+        b'Q' as c_char, // Q for quality
+        x,
+        fluid_c.as_ptr(),
+    )
+}
+
+/// Get critical point temperature for a fluid.
+///
+/// # Arguments
+/// * `fluid` - Fluid name
+///
+/// # Returns
+/// Critical temperature in K, or NaN if unavailable
+pub unsafe fn critical_temperature(fluid: &str) -> f64 {
+    let fluid_c = CString::new(fluid).unwrap();
+    CoolProp_CriticalPoint(fluid_c.as_ptr(), b'T' as c_char)
+}
+
+/// Get critical point pressure for a fluid.
+///
+/// # Arguments
+/// * `fluid` - Fluid name
+///
+/// # Returns
+/// Critical pressure in Pa, or NaN if unavailable
+pub unsafe fn critical_pressure(fluid: &str) -> f64 {
+    let fluid_c = CString::new(fluid).unwrap();
+    CoolProp_CriticalPoint(fluid_c.as_ptr(), b'P' as c_char)
+}
+
+/// Get critical point density for a fluid.
+///
+/// # Arguments
+/// * `fluid` - Fluid name
+///
+/// # Returns
+/// Critical density in kg/m³, or NaN if unavailable
+pub unsafe fn critical_density(fluid: &str) -> f64 {
+    let fluid_c = CString::new(fluid).unwrap();
+    CoolProp_CriticalPoint(fluid_c.as_ptr(), b'D' as c_char)
+}
+
+/// Check if a fluid is available in CoolProp.
+///
+/// # Arguments
+/// * `fluid` - Fluid name
+///
+/// # Returns
+/// `true` if the fluid is available
+pub unsafe fn is_fluid_available(fluid: &str) -> bool {
+    let fluid_c = CString::new(fluid).unwrap();
+    CoolProp_isfluid(fluid_c.as_ptr()) != 0
+}
+
+/// Get CoolProp version string.
+///
+/// # Returns
+/// Version string (e.g., "6.14.0")
+pub fn get_version() -> String {
+    unsafe {
+        let mut buffer = vec![0u8; 32];
+        let result = CoolProp_get_global_param_string(
+            b"version\0".as_ptr() as *const c_char,
+            buffer.as_mut_ptr() as *mut c_char,
+            buffer.len() as c_int,
+        );
+
+        if result == 0 {
+            String::from_utf8_lossy(&buffer)
+                .trim_end_matches('\0')
+                .to_string()
+        } else {
+            String::from("Unknown")
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_version() {
+        let version = get_version();
+        assert!(!version.is_empty());
+    }
+
+    #[test]
+    fn test_fluid_available() {
+        // Test some common refrigerants
+        unsafe {
+            assert!(is_fluid_available("R134a"));
+            assert!(is_fluid_available("R410A"));
+            assert!(is_fluid_available("CO2"));
+        }
+    }
+}

crates/fluids/data/r134a.json

@@ -0,0 +1,63 @@
+{
+  "fluid": "R134a",
+  "critical_point": {
+    "tc": 374.21,
+    "pc": 4059000,
+    "rho_c": 512
+  },
+  "single_phase": {
+    "pressure": [100000, 200000, 500000, 1000000, 2000000, 3000000],
+    "temperature": [250, 270, 290, 298.15, 320, 350],
+    "density": [
+      5.2, 4.9, 4.5, 4.4, 4.0, 3.6,
+      12.0, 10.5, 9.0, 8.5, 7.5, 6.5,
+      35.0, 28.0, 22.0, 20.0, 16.0, 12.0,
+      75.0, 55.0, 40.0, 35.0, 25.0, 18.0,
+      150.0, 100.0, 65.0, 55.0, 38.0, 25.0,
+      220.0, 140.0, 85.0, 70.0, 48.0, 30.0
+    ],
+    "enthalpy": [
+      380000, 395000, 410000, 415000, 430000, 450000,
+      370000, 388000, 405000, 412000, 428000, 448000,
+      355000, 378000, 398000, 406000, 424000, 445000,
+      340000, 365000, 390000, 400000, 420000, 442000,
+      320000, 350000, 378000, 392000, 415000, 438000,
+      300000, 335000, 368000, 384000, 410000, 435000
+    ],
+    "entropy": [
+      1750, 1780, 1810, 1820, 1850, 1890,
+      1720, 1760, 1795, 1805, 1840, 1880,
+      1680, 1730, 1775, 1788, 1825, 1870,
+      1630, 1695, 1750, 1765, 1810, 1860,
+      1570, 1650, 1715, 1735, 1790, 1845,
+      1510, 1605, 1685, 1710, 1770, 1830
+    ],
+    "cp": [
+      900, 920, 950, 960, 1000, 1050,
+      880, 910, 940, 950, 990, 1040,
+      850, 890, 925, 940, 980, 1030,
+      820, 870, 910, 928, 970, 1020,
+      790, 850, 900, 920, 965, 1015,
+      765, 835, 890, 915, 962, 1010
+    ],
+    "cv": [
+      750, 770, 800, 810, 850, 900,
+      730, 760, 790, 800, 840, 890,
+      700, 740, 775, 790, 830, 880,
+      670, 720, 760, 778, 820, 870,
+      640, 700, 745, 765, 812, 862,
+      615, 680, 730, 752, 805, 855
+    ]
+  },
+  "saturation": {
+    "temperature": [250, 260, 270, 280, 290, 298.15, 310, 320, 330, 340, 350],
+    "pressure": [164000, 232000, 320000, 430000, 565000, 666000, 890000, 1165000, 1500000, 1900000, 2370000],
+    "h_liq": [200000, 215000, 230000, 245000, 260000, 272000, 288000, 305000, 322000, 340000, 358000],
+    "h_vap": [395000, 402000, 408000, 413000, 417000, 420000, 423000, 425000, 426000, 427000, 427500],
+    "rho_liq": [1350, 1320, 1290, 1255, 1218, 1188, 1145, 1098, 1045, 985, 915],
+    "rho_vap": [8.2, 11.2, 15.0, 19.8, 25.8, 30.5, 39.5, 50.5, 64.0, 80.5, 101.0],
+    "s_liq": [950, 1000, 1050, 1095, 1140, 1175, 1225, 1275, 1325, 1375, 1425],
+    "s_vap": [1720, 1710, 1700, 1690, 1680, 1675, 1668, 1660, 1652, 1643, 1633
+    ]
+  }
+}

crates/fluids/src/backend.rs

@@ -0,0 +1,166 @@
+//! Fluid backend trait and implementations.
+//!
+//! This module defines the core `FluidBackend` trait that abstracts the source
+//! of thermodynamic property data, allowing the solver to switch between different
+//! backends (CoolProp, tabular data, mock for testing).
+
+use crate::errors::FluidResult;
+use crate::mixture::Mixture;
+use crate::types::{CriticalPoint, FluidId, Phase, Property, FluidState, ThermoState};
+use entropyk_core::{Pressure, Temperature};
+
+/// Trait for fluid property backends.
+///
+/// Implementors must provide methods to query thermodynamic properties
+/// for various fluids. This allows the solver to work with different
+/// property sources (CoolProp, tabular data, mock data for testing).
+///
+/// # Example
+///
+/// ```
+/// use entropyk_fluids::{FluidBackend, FluidId, Property, FluidState, ThermoState, FluidError, FluidResult, CriticalPoint};
+///
+/// struct MyBackend;
+/// impl FluidBackend for MyBackend {
+///     fn property(&self, _fluid: FluidId, _property: Property, _state: FluidState) -> FluidResult<f64> {
+///         Ok(1.0)
+///     }
+///     fn critical_point(&self, fluid: FluidId) -> FluidResult<CriticalPoint> {
+///         Err(FluidError::NoCriticalPoint { fluid: fluid.0 })
+///     }
+///     fn is_fluid_available(&self, _fluid: &FluidId) -> bool { false }
+///     fn phase(&self, _fluid: FluidId, _state: FluidState) -> FluidResult<entropyk_fluids::Phase> {
+///         Ok(entropyk_fluids::Phase::Unknown)
+///     }
+///     fn full_state(&self, _fluid: FluidId, _p: entropyk_core::Pressure, _h: entropyk_core::Enthalpy) -> FluidResult<ThermoState> {
+///         Err(FluidError::UnsupportedProperty { property: "full_state".to_string() })
+///     }
+///     fn list_fluids(&self) -> Vec<FluidId> { vec![] }
+/// }
+/// ```
+pub trait FluidBackend: Send + Sync {
+    /// Query a thermodynamic property for a fluid at a given state.
+    ///
+    /// # Arguments
+    /// * `fluid` - The fluid identifier (e.g., "R134a", "CO2")
+    /// * `property` - The property to query
+    /// * `state` - The thermodynamic state specification
+    ///
+    /// # Returns
+    /// The property value in SI units, or an error if the property
+    /// cannot be computed (unknown fluid, invalid state, etc.)
+    fn property(&self, fluid: FluidId, property: Property, state: FluidState) -> FluidResult<f64>;
+
+    /// Compute the complete thermodynamic state of a fluid at a given pressure and enthalpy.
+    ///
+    /// This method is intended to be implemented by backends capable of natively calculating
+    /// all key parameters (phase, saturation temperatures, qualities, limits) without the user
+    /// needing to query them individually.
+    /// 
+    /// # Arguments
+    /// * `fluid` - The fluid identifier
+    /// * `p` - The absolute pressure
+    /// * `h` - The specific enthalpy
+    /// 
+    /// # Returns
+    /// The comprehensive `ThermoState` Snapshot, or an Error.
+    fn full_state(&self, fluid: FluidId, p: Pressure, h: entropyk_core::Enthalpy) -> FluidResult<ThermoState>;
+
+    /// Get critical point data for a fluid.
+    ///
+    /// # Arguments
+    /// * `fluid` - The fluid identifier
+    ///
+    /// # Returns
+    /// The critical point (Tc, Pc, density), or an error if not available
+    fn critical_point(&self, fluid: FluidId) -> FluidResult<CriticalPoint>;
+
+    /// Check if a fluid is available in this backend.
+    ///
+    /// # Arguments
+    /// * `fluid` - The fluid identifier
+    ///
+    /// # Returns
+    /// `true` if the fluid is available, `false` otherwise
+    fn is_fluid_available(&self, fluid: &FluidId) -> bool;
+
+    /// Get the phase of a fluid at a given state.
+    ///
+    /// # Arguments
+    /// * `fluid` - The fluid identifier
+    /// * `state` - The thermodynamic state
+    ///
+    /// # Returns
+    /// The phase (Liquid, Vapor, TwoPhase, etc.)
+    fn phase(&self, fluid: FluidId, state: FluidState) -> FluidResult<Phase>;
+
+    /// List all available fluids in this backend.
+    fn list_fluids(&self) -> Vec<FluidId>;
+
+    /// Calculate the bubble point temperature for a mixture at given pressure.
+    ///
+    /// The bubble point is the temperature at which a liquid mixture begins to boil
+    /// (saturated liquid temperature).
+    ///
+    /// # Arguments
+    /// * `pressure` - The pressure in Pa
+    /// * `mixture` - The mixture composition
+    ///
+    /// # Returns
+    /// The bubble point temperature in Kelvin
+    fn bubble_point(&self, _pressure: Pressure, _mixture: &Mixture) -> FluidResult<Temperature> {
+        Err(crate::errors::FluidError::UnsupportedProperty {
+            property: "Bubble point calculation not supported by this backend".to_string(),
+        })
+    }
+
+    /// Calculate the dew point temperature for a mixture at given pressure.
+    ///
+    /// The dew point is the temperature at which a vapor mixture begins to condense
+    /// (saturated vapor temperature).
+    ///
+    /// # Arguments
+    /// * `pressure` - The pressure in Pa
+    /// * `mixture` - The mixture composition
+    ///
+    /// # Returns
+    /// The dew point temperature in Kelvin
+    fn dew_point(&self, _pressure: Pressure, _mixture: &Mixture) -> FluidResult<Temperature> {
+        Err(crate::errors::FluidError::UnsupportedProperty {
+            property: "Dew point calculation not supported by this backend".to_string(),
+        })
+    }
+
+    /// Calculate the temperature glide for a mixture at given pressure.
+    ///
+    /// Temperature glide is the difference between dew point and bubble point
+    /// temperatures: T_glide = T_dew - T_bubble.
+    /// This is non-zero for zeotropic mixtures and zero for azeotropes/pure fluids.
+    ///
+    /// # Arguments
+    /// * `pressure` - The pressure in Pa
+    /// * `mixture` - The mixture composition
+    ///
+    /// # Returns
+    /// The temperature glide in Kelvin
+    fn temperature_glide(&self, pressure: Pressure, mixture: &Mixture) -> FluidResult<f64> {
+        let t_bubble = self.bubble_point(pressure, mixture)?;
+        let t_dew = self.dew_point(pressure, mixture)?;
+        Ok(t_dew.to_kelvin() - t_bubble.to_kelvin())
+    }
+
+    /// Check if a mixture is supported by this backend.
+    ///
+    /// # Arguments
+    /// * `mixture` - The mixture to check
+    ///
+    /// # Returns
+    /// `true` if the mixture is supported, `false` otherwise
+    fn is_mixture_supported(&self, mixture: &Mixture) -> bool {
+        // Default implementation: check if all components are available
+        mixture
+            .components()
+            .iter()
+            .all(|c| self.is_fluid_available(&FluidId::new(c)))
+    }
+}

crates/fluids/src/cache.rs

@@ -0,0 +1,235 @@
+//! Thread-local LRU cache for fluid property queries.
+//!
+//! Avoids redundant backend calls without mutex contention by using
+//! per-thread storage. Cache keys use quantized state values since f64
+//! does not implement Hash.
+//!
+//! # Quantization Strategy
+//!
+//! State values (P, T, h, s, x) are quantized to 1e-9 relative precision
+//! for cache key derivation. Solver iterations often repeat the same
+//! (P,T) or (P,h) states; quantization should not lose cache hits for
+//! typical thermodynamic ranges (P: 1e3–1e7 Pa, T: 200–600 K).
+
+use crate::mixture::Mixture;
+use crate::types::{FluidId, Property, FluidState};
+use lru::LruCache;
+use std::cell::RefCell;
+use std::hash::{Hash, Hasher};
+use std::num::NonZeroUsize;
+
+/// Default cache capacity (entries). LRU eviction when exceeded.
+pub const DEFAULT_CACHE_CAPACITY: usize = 10_000;
+
+/// Default capacity as NonZeroUsize for LruCache (avoids unwrap in production path).
+const DEFAULT_CAP_NONZERO: NonZeroUsize = unsafe { NonZeroUsize::new_unchecked(DEFAULT_CACHE_CAPACITY) };
+
+/// Quantization factor: values rounded to 1e-9 relative.
+/// (v * 1e9).round() as i64 for Hash-compatible key.
+#[inline]
+fn quantize(v: f64) -> i64 {
+    if v.is_nan() || v.is_infinite() {
+        0
+    } else {
+        (v * 1e9).round() as i64
+    }
+}
+
+/// Cache key for fluid property lookups.
+///
+/// Uses quantized state values since f64 does not implement Hash.
+/// Includes backend_id so multiple CachedBackend instances don't mix results.
+/// For mixtures, includes a hash of the mixture composition.
+#[derive(Clone, Debug)]
+pub struct CacheKey {
+    backend_id: usize,
+    fluid: String,
+    property: Property,
+    variant: u8,
+    p_quantized: i64,
+    second_quantized: i64,
+    mixture_hash: Option<u64>,
+}
+
+impl PartialEq for CacheKey {
+    fn eq(&self, other: &Self) -> bool {
+        self.backend_id == other.backend_id
+            && self.fluid == other.fluid
+            && self.property == other.property
+            && self.variant == other.variant
+            && self.p_quantized == other.p_quantized
+            && self.second_quantized == other.second_quantized
+            && self.mixture_hash == other.mixture_hash
+    }
+}
+
+impl Eq for CacheKey {}
+
+impl Hash for CacheKey {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.backend_id.hash(state);
+        self.fluid.hash(state);
+        self.property.hash(state);
+        self.variant.hash(state);
+        self.p_quantized.hash(state);
+        self.second_quantized.hash(state);
+        self.mixture_hash.hash(state);
+    }
+}
+
+impl CacheKey {
+    /// Build a cache key from fluid, property, state, and backend id.
+    pub fn new(backend_id: usize, fluid: &FluidId, property: Property, state: &FluidState) -> Self {
+        let (p, second, variant, mixture_hash) = match state {
+            FluidState::PressureTemperature(p, t) => (p.to_pascals(), t.to_kelvin(), 0u8, None),
+            FluidState::PressureEnthalpy(p, h) => {
+                (p.to_pascals(), h.to_joules_per_kg(), 1u8, None)
+            }
+            FluidState::PressureEntropy(p, s) => {
+                (p.to_pascals(), s.to_joules_per_kg_kelvin(), 2u8, None)
+            }
+            FluidState::PressureQuality(p, x) => (p.to_pascals(), x.value(), 3u8, None),
+            FluidState::PressureTemperatureMixture(p, t, ref m) => {
+                (p.to_pascals(), t.to_kelvin(), 4u8, Some(mix_hash(m)))
+            }
+            FluidState::PressureEnthalpyMixture(p, h, ref m) => {
+                (p.to_pascals(), h.to_joules_per_kg(), 5u8, Some(mix_hash(m)))
+            }
+            FluidState::PressureQualityMixture(p, x, ref m) => {
+                (p.to_pascals(), x.value(), 6u8, Some(mix_hash(m)))
+            }
+        };
+        CacheKey {
+            backend_id,
+            fluid: fluid.0.clone(),
+            property,
+            variant,
+            p_quantized: quantize(p),
+            second_quantized: quantize(second),
+            mixture_hash,
+        }
+    }
+}
+
+/// Compute a simple hash for a mixture for cache key purposes.
+fn mix_hash(mixture: &Mixture) -> u64 {
+    use std::collections::hash_map::DefaultHasher;
+    let mut hasher = DefaultHasher::new();
+    mixture.hash(&mut hasher);
+    hasher.finish()
+}
+
+thread_local! {
+    static CACHE: RefCell<LruCache<CacheKey, f64>> = RefCell::new(
+        LruCache::new(DEFAULT_CAP_NONZERO)
+    );
+}
+
+/// Get a value from the thread-local cache (no allocation on key build for hot path).
+pub fn cache_get(
+    backend_id: usize,
+    fluid: &FluidId,
+    property: Property,
+    state: &FluidState,
+) -> Option<f64> {
+    let key = CacheKey::new(backend_id, fluid, property, state);
+    CACHE.with(|c| {
+        let mut cache = c.borrow_mut();
+        cache.get(&key).copied()
+    })
+}
+
+/// Insert a value into the thread-local cache.
+pub fn cache_insert(
+    backend_id: usize,
+    fluid: &FluidId,
+    property: Property,
+    state: &FluidState,
+    value: f64,
+) {
+    let key = CacheKey::new(backend_id, fluid, property, state);
+    CACHE.with(|c| {
+        let mut cache = c.borrow_mut();
+        cache.put(key, value);
+    });
+}
+
+/// Clear the thread-local cache (e.g. at solver iteration boundaries).
+pub fn cache_clear() {
+    CACHE.with(|c| {
+        let mut cache = c.borrow_mut();
+        cache.clear();
+    });
+}
+
+/// Resize the thread-local cache capacity.
+pub fn cache_resize(capacity: NonZeroUsize) {
+    CACHE.with(|c| {
+        let mut cache = c.borrow_mut();
+        cache.resize(capacity);
+    });
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use entropyk_core::{Pressure, Temperature};
+
+    #[test]
+    fn test_cache_key_quantization() {
+        let fluid = FluidId::new("R134a");
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        let key1 = CacheKey::new(0, &fluid, Property::Density, &state);
+        let key2 = CacheKey::new(0, &fluid, Property::Density, &state);
+        assert_eq!(key1, key2);
+        // Equal keys must have same hash (for HashMap use)
+        use std::collections::hash_map::DefaultHasher;
+        let mut h1 = DefaultHasher::new();
+        let mut h2 = DefaultHasher::new();
+        key1.hash(&mut h1);
+        key2.hash(&mut h2);
+        assert_eq!(h1.finish(), h2.finish());
+    }
+
+    #[test]
+    fn test_cache_key_different_states() {
+        let fluid = FluidId::new("R134a");
+        let state1 = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        let state2 = FluidState::from_pt(Pressure::from_bar(2.0), Temperature::from_celsius(25.0));
+        let key1 = CacheKey::new(0, &fluid, Property::Density, &state1);
+        let key2 = CacheKey::new(0, &fluid, Property::Density, &state2);
+        assert_ne!(key1, key2);
+    }
+
+    #[test]
+    fn test_lru_eviction() {
+        use std::num::NonZeroUsize;
+
+        cache_clear();
+        cache_resize(NonZeroUsize::new(2).expect("2 is non-zero"));
+
+        let fluid = FluidId::new("R134a");
+        let state1 = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(20.0));
+        let state2 = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        let state3 = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(30.0));
+
+        cache_insert(0, &fluid, Property::Density, &state1, 1000.0);
+        cache_insert(0, &fluid, Property::Density, &state2, 1100.0);
+        cache_insert(0, &fluid, Property::Density, &state3, 1200.0);
+
+        assert!(cache_get(0, &fluid, Property::Density, &state1).is_none());
+        assert_eq!(cache_get(0, &fluid, Property::Density, &state2), Some(1100.0));
+        assert_eq!(cache_get(0, &fluid, Property::Density, &state3), Some(1200.0));
+
+        cache_resize(NonZeroUsize::new(DEFAULT_CACHE_CAPACITY).expect("capacity is non-zero"));
+    }
+
+    #[test]
+    fn test_cache_key_different_backends() {
+        let fluid = FluidId::new("R134a");
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        let key1 = CacheKey::new(0, &fluid, Property::Density, &state);
+        let key2 = CacheKey::new(1, &fluid, Property::Density, &state);
+        assert_ne!(key1, key2);
+    }
+}

crates/fluids/src/cached_backend.rs

@@ -0,0 +1,174 @@
+//! Cached backend wrapper for fluid property queries.
+//!
+//! Wraps any `FluidBackend` with a thread-local LRU cache to avoid
+//! redundant calculations. No mutex contention; zero allocation on cache hit.
+
+use crate::backend::FluidBackend;
+use crate::cache::{cache_clear, cache_get, cache_insert};
+use crate::errors::FluidResult;
+use crate::types::{CriticalPoint, FluidId, Phase, Property, FluidState};
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+static NEXT_BACKEND_ID: AtomicUsize = AtomicUsize::new(0);
+
+/// Backend wrapper that caches property queries in a thread-local LRU cache.
+///
+/// Wraps any `FluidBackend` and caches successful property() results.
+/// Other trait methods (critical_point, phase, etc.) delegate to the inner backend
+/// without caching, as they are typically called less frequently.
+///
+/// # Example
+///
+/// ```
+/// use entropyk_fluids::{CachedBackend, FluidBackend, FluidId, Property, FluidState, TestBackend};
+/// use entropyk_core::{Pressure, Temperature};
+///
+/// let inner = TestBackend::new();
+/// let cached = CachedBackend::new(inner);
+///
+/// let state = FluidState::from_pt(
+///     Pressure::from_bar(1.0),
+///     Temperature::from_celsius(25.0),
+/// );
+///
+/// let v1 = cached.property(FluidId::new("R134a"), Property::Density, state.clone()).unwrap();
+/// let v2 = cached.property(FluidId::new("R134a"), Property::Density, state).unwrap();
+/// assert_eq!(v1, v2); // Second call served from cache
+/// ```
+pub struct CachedBackend<B: FluidBackend> {
+    backend_id: usize,
+    inner: B,
+}
+
+impl<B: FluidBackend> CachedBackend<B> {
+    /// Create a new cached backend wrapping the given backend.
+    pub fn new(inner: B) -> Self {
+        let backend_id = NEXT_BACKEND_ID.fetch_add(1, Ordering::Relaxed);
+        CachedBackend { backend_id, inner }
+    }
+
+    /// Clear the thread-local cache. Call at solver iteration boundaries if needed.
+    ///
+    /// **Note:** This clears the cache for *all* `CachedBackend` instances on the current
+    /// thread, since they share one thread-local cache. If you need per-backend invalidation,
+    /// use separate threads or a different caching strategy.
+    pub fn clear_cache(&self) {
+        cache_clear();
+    }
+
+    /// Get a reference to the inner backend.
+    pub fn inner(&self) -> &B {
+        &self.inner
+    }
+}
+
+impl<B: FluidBackend> FluidBackend for CachedBackend<B> {
+    fn property(&self, fluid: FluidId, property: Property, state: FluidState) -> FluidResult<f64> {
+        if let Some(v) = cache_get(self.backend_id, &fluid, property, &state) {
+            return Ok(v);
+        }
+        let v = self.inner.property(fluid.clone(), property, state.clone())?;
+        cache_insert(self.backend_id, &fluid, property, &state, v);
+        Ok(v)
+    }
+
+    fn critical_point(&self, fluid: FluidId) -> FluidResult<CriticalPoint> {
+        self.inner.critical_point(fluid)
+    }
+
+    fn is_fluid_available(&self, fluid: &FluidId) -> bool {
+        self.inner.is_fluid_available(fluid)
+    }
+
+    fn phase(&self, fluid: FluidId, state: FluidState) -> FluidResult<Phase> {
+        self.inner.phase(fluid, state)
+    }
+
+    fn list_fluids(&self) -> Vec<FluidId> {
+        self.inner.list_fluids()
+    }
+
+    fn full_state(&self, fluid: FluidId, p: entropyk_core::Pressure, h: entropyk_core::Enthalpy) -> FluidResult<crate::types::ThermoState> {
+        self.inner.full_state(fluid, p, h)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::test_backend::TestBackend;
+    use entropyk_core::{Pressure, Temperature};
+
+    #[test]
+    fn test_cache_hit_returns_same_value() {
+        let inner = TestBackend::new();
+        let cached = CachedBackend::new(inner);
+
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+
+        let v1 = cached
+            .property(FluidId::new("R134a"), Property::Density, state.clone())
+            .unwrap();
+        let v2 = cached
+            .property(FluidId::new("R134a"), Property::Density, state)
+            .unwrap();
+        assert_eq!(v1, v2);
+    }
+
+    #[test]
+    fn test_cache_miss_delegates_to_backend() {
+        let inner = TestBackend::new();
+        let cached = CachedBackend::new(inner);
+
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+
+        let v = cached
+            .property(FluidId::new("R134a"), Property::Density, state)
+            .unwrap();
+        assert!(v > 0.0);
+    }
+
+    #[test]
+    fn test_cache_invalidation() {
+        let inner = TestBackend::new();
+        let cached = CachedBackend::new(inner);
+
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+
+        let _ = cached
+            .property(FluidId::new("R134a"), Property::Density, state.clone())
+            .unwrap();
+        cached.clear_cache();
+        // After clear, next query should still work (delegates to backend)
+        let v = cached
+            .property(FluidId::new("R134a"), Property::Density, state)
+            .unwrap();
+        assert!(v > 0.0);
+    }
+
+    #[test]
+    fn test_cached_benchmark_10k_queries() {
+        let inner = TestBackend::new();
+        let cached = CachedBackend::new(inner);
+
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+
+        for _ in 0..10_000 {
+            let _ = cached
+                .property(FluidId::new("R134a"), Property::Density, state.clone())
+                .unwrap();
+        }
+    }
+
+    #[test]
+    fn test_cached_backend_implements_fluid_backend() {
+        let inner = TestBackend::new();
+        let cached = CachedBackend::new(inner);
+
+        assert!(cached.is_fluid_available(&FluidId::new("R134a")));
+        let cp = cached.critical_point(FluidId::new("R134a")).unwrap();
+        assert!(cp.temperature_kelvin() > 300.0);
+        let fluids = cached.list_fluids();
+        assert!(!fluids.is_empty());
+    }
+}

crates/fluids/src/coolprop.rs

@@ -0,0 +1,647 @@
+//! CoolProp backend implementation.
+//!
+//! This module provides the `CoolPropBackend` struct that implements the `FluidBackend` trait
+//! using the CoolProp C++ library for thermodynamic property calculations.
+
+#[cfg(feature = "coolprop")]
+use crate::damped_backend::DampedBackend;
+use crate::errors::{FluidError, FluidResult};
+use crate::types::{CriticalPoint, FluidId, Phase, Property, FluidState};
+
+#[cfg(feature = "coolprop")]
+use crate::mixture::Mixture;
+#[cfg(feature = "coolprop")]
+use std::collections::HashMap;
+#[cfg(feature = "coolprop")]
+use std::sync::RwLock;
+
+#[cfg(feature = "coolprop")]
+use entropyk_coolprop_sys as coolprop;
+
+/// A fluid property backend using the CoolProp C++ library.
+///
+/// This backend provides high-accuracy thermodynamic properties using the
+/// CoolProp library, which implements the NIST REFPROP equations of state.
+#[cfg(feature = "coolprop")]
+pub struct CoolPropBackend {
+    /// Cache for critical point data
+    critical_cache: RwLock<HashMap<String, CriticalPoint>>,
+    /// List of available fluids
+    available_fluids: Vec<FluidId>,
+}
+
+#[cfg(feature = "coolprop")]
+impl CoolPropBackend {
+    /// Creates a new CoolPropBackend.
+    pub fn new() -> Self {
+        let backend = CoolPropBackend {
+            critical_cache: RwLock::new(HashMap::new()),
+            available_fluids: vec![
+                FluidId::new("R134a"),
+                FluidId::new("R410A"),
+                FluidId::new("R404A"),
+                FluidId::new("R407C"),
+                FluidId::new("R32"),
+                FluidId::new("R125"),
+                FluidId::new("R744"),
+                FluidId::new("R290"),
+                FluidId::new("R600"),
+                FluidId::new("R600a"),
+                FluidId::new("Water"),
+                FluidId::new("Air"),
+            ],
+        };
+
+        backend
+    }
+
+    /// Creates a new CoolPropBackend with critical point damping enabled.
+    ///
+    /// This wraps the backend with a `DampedBackend` to apply C1-continuous
+    /// damping to derivative properties (Cp, Cv, etc.) near the critical point,
+    /// preventing NaN values in Newton-Raphson iterations.
+    pub fn with_damping() -> DampedBackend<CoolPropBackend> {
+        DampedBackend::new(Self::new())
+    }
+
+    /// Get the CoolProp internal name for a fluid.
+    fn fluid_name(&self, fluid: &FluidId) -> String {
+        // Map common names to CoolProp internal names
+        match fluid.0.to_lowercase().as_str() {
+            "r134a" => "R134a".to_string(),
+            "r410a" => "R410A".to_string(),
+            "r404a" => "R404A".to_string(),
+            "r407c" => "R407C".to_string(),
+            "r32" => "R32".to_string(),
+            "r125" => "R125".to_string(),
+            "co2" | "r744" => "CO2".to_string(),
+            "r290" => "R290".to_string(),
+            "r600" => "R600".to_string(),
+            "r600a" => "R600A".to_string(),
+            "water" => "Water".to_string(),
+            "air" => "Air".to_string(),
+            n => n.to_string(),
+        }
+    }
+
+    /// Convert Property to CoolProp character code.
+    fn property_code(property: Property) -> &'static str {
+        match property {
+            Property::Density => "D",
+            Property::Enthalpy => "H",
+            Property::Entropy => "S",
+            Property::InternalEnergy => "U",
+            Property::Cp => "C",
+            Property::Cv => "O", // Cv in CoolProp
+            Property::SpeedOfSound => "A",
+            Property::Viscosity => "V",
+            Property::ThermalConductivity => "L",
+            Property::SurfaceTension => "I",
+            Property::Quality => "Q",
+            Property::Temperature => "T",
+            Property::Pressure => "P",
+        }
+    }
+}
+
+#[cfg(feature = "coolprop")]
+impl Default for CoolPropBackend {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(feature = "coolprop")]
+impl crate::backend::FluidBackend for CoolPropBackend {
+    fn property(&self, fluid: FluidId, property: Property, state: FluidState) -> FluidResult<f64> {
+        // Handle mixture states
+        if state.is_mixture() {
+            return self.property_mixture(fluid, property, state);
+        }
+
+        let coolprop_fluid = self.fluid_name(&fluid);
+        let prop_code = Self::property_code(property);
+
+        // Check if fluid is available
+        if !self.is_fluid_available(&fluid) {
+            return Err(FluidError::UnknownFluid { fluid: fluid.0 });
+        }
+
+        // Query property based on state input type
+        let result = match state {
+            FluidState::PressureTemperature(p, t) => unsafe {
+                coolprop::props_si_pt(prop_code, p.to_pascals(), t.to_kelvin(), &coolprop_fluid)
+            },
+            FluidState::PressureEnthalpy(p, h) => unsafe {
+                coolprop::props_si_ph(
+                    prop_code,
+                    p.to_pascals(),
+                    h.to_joules_per_kg(),
+                    &coolprop_fluid,
+                )
+            },
+            FluidState::PressureEntropy(_p, _s) => {
+                // CoolProp doesn't have direct PS, use iterative approach or PH
+                return Err(FluidError::UnsupportedProperty {
+                    property: format!("P-S not directly supported, use P-T or P-h"),
+                });
+            }
+            FluidState::PressureQuality(p, q) => unsafe {
+                coolprop::props_si_px(prop_code, p.to_pascals(), q.value(), &coolprop_fluid)
+            },
+            // Mixture variants handled above
+            FluidState::PressureTemperatureMixture(_, _, _) => unreachable!(),
+            FluidState::PressureEnthalpyMixture(_, _, _) => unreachable!(),
+            FluidState::PressureQualityMixture(_, _, _) => unreachable!(),
+        };
+
+        // Check for NaN (indicates error in CoolProp)
+        if result.is_nan() {
+            return Err(FluidError::InvalidState {
+                reason: format!("CoolProp returned NaN for {} at {:?}", fluid, state),
+            });
+        }
+
+        Ok(result)
+    }
+
+    fn critical_point(&self, fluid: FluidId) -> FluidResult<CriticalPoint> {
+        // Check cache first
+        if let Some(cp) = self.critical_cache.read().unwrap().get(&fluid.0) {
+            return Ok(*cp);
+        }
+
+        let coolprop_fluid = self.fluid_name(&fluid);
+
+        unsafe {
+            let tc = coolprop::critical_temperature(&coolprop_fluid);
+            let pc = coolprop::critical_pressure(&coolprop_fluid);
+            let dc = coolprop::critical_density(&coolprop_fluid);
+
+            if tc.is_nan() || pc.is_nan() || dc.is_nan() {
+                return Err(FluidError::NoCriticalPoint { fluid: fluid.0 });
+            }
+
+            let cp = CriticalPoint::new(
+                entropyk_core::Temperature::from_kelvin(tc),
+                entropyk_core::Pressure::from_pascals(pc),
+                dc,
+            );
+
+            // Cache the result
+            self.critical_cache.write().unwrap().insert(fluid.0, cp);
+
+            Ok(cp)
+        }
+    }
+
+    fn is_fluid_available(&self, fluid: &FluidId) -> bool {
+        let coolprop_fluid = self.fluid_name(fluid);
+        unsafe { coolprop::is_fluid_available(&coolprop_fluid) }
+    }
+
+    fn phase(&self, fluid: FluidId, state: FluidState) -> FluidResult<Phase> {
+        // Handle mixture states
+        if state.is_mixture() {
+            return self.phase_mix(fluid, state);
+        }
+
+        let quality = self.property(fluid.clone(), Property::Quality, state)?;
+
+        if quality < 0.0 {
+            // Below saturated liquid - likely subcooled liquid
+            Ok(Phase::Liquid)
+        } else if quality > 1.0 {
+            // Above saturated vapor - superheated
+            Ok(Phase::Vapor)
+        } else if (quality - 0.0).abs() < 1e-6 {
+            // Saturated liquid
+            Ok(Phase::Liquid)
+        } else if (quality - 1.0).abs() < 1e-6 {
+            // Saturated vapor
+            Ok(Phase::Vapor)
+        } else {
+            // Two-phase region
+            Ok(Phase::TwoPhase)
+        }
+    }
+
+    fn list_fluids(&self) -> Vec<FluidId> {
+        self.available_fluids.clone()
+    }
+
+    fn bubble_point(
+        &self,
+        pressure: entropyk_core::Pressure,
+        mixture: &Mixture,
+    ) -> FluidResult<entropyk_core::Temperature> {
+        if !self.is_mixture_supported(mixture) {
+            return Err(FluidError::MixtureNotSupported(format!(
+                "One or more components not available: {:?}",
+                mixture.components()
+            )));
+        }
+
+        let cp_string = mixture.to_coolprop_string();
+        let p_pa = pressure.to_pascals();
+
+        unsafe {
+            // For bubble point (saturated liquid), use Q=0
+            let t = coolprop::props_si_tq("T", p_pa, 0.0, &cp_string);
+            if t.is_nan() {
+                return Err(FluidError::NumericalError(
+                    "CoolProp returned NaN for bubble point calculation".to_string(),
+                ));
+            }
+            Ok(entropyk_core::Temperature::from_kelvin(t))
+        }
+    }
+
+    fn dew_point(
+        &self,
+        pressure: entropyk_core::Pressure,
+        mixture: &Mixture,
+    ) -> FluidResult<entropyk_core::Temperature> {
+        if !self.is_mixture_supported(mixture) {
+            return Err(FluidError::MixtureNotSupported(format!(
+                "One or more components not available: {:?}",
+                mixture.components()
+            )));
+        }
+
+        let cp_string = mixture.to_coolprop_string();
+        let p_pa = pressure.to_pascals();
+
+        unsafe {
+            // For dew point (saturated vapor), use Q=1
+            let t = coolprop::props_si_tq("T", p_pa, 1.0, &cp_string);
+            if t.is_nan() {
+                return Err(FluidError::NumericalError(
+                    "CoolProp returned NaN for dew point calculation".to_string(),
+                ));
+            }
+            Ok(entropyk_core::Temperature::from_kelvin(t))
+        }
+    }
+
+    fn is_mixture_supported(&self, mixture: &Mixture) -> bool {
+        mixture
+            .components()
+            .iter()
+            .all(|c| self.is_fluid_available(&FluidId::new(c)))
+    }
+
+    /// Property calculation for mixtures.
+    fn property_mixture(
+        &self,
+        fluid: FluidId,
+        property: Property,
+        state: FluidState,
+    ) -> FluidResult<f64> {
+        // Extract mixture from state
+        let mixture = match state {
+            FluidState::PressureTemperatureMixture(_, _, m) => m,
+            FluidState::PressureEnthalpyMixture(_, _, m) => m,
+            FluidState::PressureQualityMixture(_, _, m) => m,
+            _ => unreachable!(),
+        };
+
+        if !self.is_mixture_supported(&mixture) {
+            return Err(FluidError::MixtureNotSupported(format!(
+                "One or more components not available: {:?}",
+                mixture.components()
+            )));
+        }
+
+        let cp_string = mixture.to_coolprop_string();
+        let prop_code = Self::property_code(property);
+
+        let result = match state {
+            FluidState::PressureTemperatureMixture(p, t, _) => unsafe {
+                coolprop::props_si_pt(prop_code, p.to_pascals(), t.to_kelvin(), &cp_string)
+            },
+            FluidState::PressureEnthalpyMixture(p, h, _) => unsafe {
+                coolprop::props_si_ph(prop_code, p.to_pascals(), h.to_joules_per_kg(), &cp_string)
+            },
+            FluidState::PressureQualityMixture(p, q, _) => unsafe {
+                coolprop::props_si_px(prop_code, p.to_pascals(), q.value(), &cp_string)
+            },
+            _ => unreachable!(),
+        };
+
+        if result.is_nan() {
+            return Err(FluidError::InvalidState {
+                reason: format!("CoolProp returned NaN for mixture at {:?}", state),
+            });
+        }
+
+        Ok(result)
+    }
+
+    /// Phase calculation for mixtures.
+    fn phase_mix(&self, fluid: FluidId, state: FluidState) -> FluidResult<Phase> {
+        let quality = self.property_mixture(fluid, Property::Quality, state)?;
+
+        if quality < 0.0 {
+            Ok(Phase::Liquid)
+        } else if quality > 1.0 {
+            Ok(Phase::Vapor)
+        } else if (quality - 0.0).abs() < 1e-6 {
+            Ok(Phase::Liquid)
+        } else if (quality - 1.0).abs() < 1e-6 {
+            Ok(Phase::Vapor)
+        } else {
+            Ok(Phase::TwoPhase)
+        }
+    }
+
+    fn full_state(&self, fluid: FluidId, p: entropyk_core::Pressure, h: entropyk_core::Enthalpy) -> FluidResult<crate::types::ThermoState> {
+        let coolprop_fluid = self.fluid_name(&fluid);
+        
+        if !self.is_fluid_available(&fluid) {
+            return Err(FluidError::UnknownFluid { fluid: fluid.0 });
+        }
+
+        let p_pa = p.to_pascals();
+        let h_j_kg = h.to_joules_per_kg();
+
+        unsafe {
+            let t_k = coolprop::props_si_ph("T", p_pa, h_j_kg, &coolprop_fluid);
+            if t_k.is_nan() {
+                return Err(FluidError::InvalidState {
+                    reason: format!("CoolProp returned NaN for Temperature at P={}, h={} for {}", p_pa, h_j_kg, fluid),
+                });
+            }
+
+            let s_j_kg_k = coolprop::props_si_ph("S", p_pa, h_j_kg, &coolprop_fluid);
+            let d_kg_m3 = coolprop::props_si_ph("D", p_pa, h_j_kg, &coolprop_fluid);
+            let q = coolprop::props_si_ph("Q", p_pa, h_j_kg, &coolprop_fluid);
+
+            let phase = self.phase(fluid.clone(), FluidState::from_ph(p, h))?;
+            
+            let quality = if (0.0..=1.0).contains(&q) {
+                Some(crate::types::Quality::new(q))
+            } else {
+                None
+            };
+
+            let t_bubble = coolprop::props_si_pq("T", p_pa, 0.0, &coolprop_fluid);
+            let t_dew = coolprop::props_si_pq("T", p_pa, 1.0, &coolprop_fluid);
+
+            let (t_bubble_opt, subcooling) = if !t_bubble.is_nan() {
+                (
+                    Some(entropyk_core::Temperature::from_kelvin(t_bubble)),
+                    if t_k < t_bubble {
+                        Some(crate::types::TemperatureDelta::new(t_bubble - t_k))
+                    } else {
+                        None
+                    }
+                )
+            } else {
+                (None, None)
+            };
+
+            let (t_dew_opt, superheat) = if !t_dew.is_nan() {
+                (
+                    Some(entropyk_core::Temperature::from_kelvin(t_dew)),
+                    if t_k > t_dew {
+                        Some(crate::types::TemperatureDelta::new(t_k - t_dew))
+                    } else {
+                        None
+                    }
+                )
+            } else {
+                (None, None)
+            };
+
+            Ok(crate::types::ThermoState {
+                fluid,
+                pressure: p,
+                temperature: entropyk_core::Temperature::from_kelvin(t_k),
+                enthalpy: h,
+                entropy: crate::types::Entropy::from_joules_per_kg_kelvin(s_j_kg_k),
+                density: d_kg_m3,
+                phase,
+                quality,
+                superheat,
+                subcooling,
+                t_bubble: t_bubble_opt,
+                t_dew: t_dew_opt,
+            })
+        }
+    }
+}
+
+/// A placeholder backend when CoolProp is not available.
+///
+/// This allows the crate to compile without CoolProp, but property
+/// queries will return errors.
+#[cfg(not(feature = "coolprop"))]
+pub struct CoolPropBackend;
+
+#[cfg(not(feature = "coolprop"))]
+impl CoolPropBackend {
+    /// Creates a new CoolPropBackend (placeholder).
+    pub fn new() -> Self {
+        CoolPropBackend
+    }
+}
+
+#[cfg(not(feature = "coolprop"))]
+impl Default for CoolPropBackend {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(not(feature = "coolprop"))]
+impl crate::backend::FluidBackend for CoolPropBackend {
+    fn property(
+        &self,
+        _fluid: FluidId,
+        _property: Property,
+        _state: FluidState,
+    ) -> FluidResult<f64> {
+        Err(FluidError::CoolPropError(
+            "CoolProp not available. Enable 'coolprop' feature to use this backend.".to_string(),
+        ))
+    }
+
+    fn critical_point(&self, _fluid: FluidId) -> FluidResult<CriticalPoint> {
+        Err(FluidError::CoolPropError(
+            "CoolProp not available. Enable 'coolprop' feature to use this backend.".to_string(),
+        ))
+    }
+
+    fn is_fluid_available(&self, _fluid: &FluidId) -> bool {
+        false
+    }
+
+    fn phase(&self, _fluid: FluidId, _state: FluidState) -> FluidResult<Phase> {
+        Err(FluidError::CoolPropError(
+            "CoolProp not available. Enable 'coolprop' feature to use this backend.".to_string(),
+        ))
+    }
+
+    fn list_fluids(&self) -> Vec<FluidId> {
+        Vec::new()
+    }
+
+    fn full_state(&self, _fluid: FluidId, _p: entropyk_core::Pressure, _h: entropyk_core::Enthalpy) -> FluidResult<crate::types::ThermoState> {
+        Err(FluidError::CoolPropError(
+            "CoolProp not available. Enable 'coolprop' feature to use this backend.".to_string(),
+        ))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::backend::FluidBackend;
+    #[cfg(feature = "coolprop")]
+    use crate::mixture::Mixture;
+    #[cfg(feature = "coolprop")]
+    use entropyk_core::{Pressure, Temperature};
+
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_backend_creation() {
+        let backend = CoolPropBackend::new();
+        let fluids = backend.list_fluids();
+        assert!(!fluids.is_empty());
+    }
+
+    #[test]
+    #[cfg(not(feature = "coolprop"))]
+    fn test_backend_without_feature() {
+        use crate::types::FluidState;
+        use entropyk_core::{Pressure, Temperature};
+
+        let backend = CoolPropBackend::new();
+        let result = backend.property(
+            FluidId::new("R134a"),
+            Property::Density,
+            FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0)),
+        );
+
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_fluid_name_mapping() {
+        #[cfg(feature = "coolprop")]
+        {
+            let backend = CoolPropBackend::new();
+            assert_eq!(backend.fluid_name(&FluidId::new("R134a")), "R134a");
+            assert_eq!(backend.fluid_name(&FluidId::new("CO2")), "CO2");
+            assert_eq!(backend.fluid_name(&FluidId::new("R744")), "CO2");
+        }
+    }
+
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_mixture_is_supported() {
+        let backend = CoolPropBackend::new();
+
+        // R454B = R32 + R1234yf (both available in CoolProp)
+        let mixture = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+        assert!(backend.is_mixture_supported(&mixture));
+
+        // Unknown component should fail
+        let bad_mixture = Mixture::from_mass_fractions(&[("R32", 0.5), ("R999", 0.5)]).unwrap();
+        assert!(!backend.is_mixture_supported(&bad_mixture));
+    }
+
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_bubble_point_r454b() {
+        let backend = CoolPropBackend::new();
+        let mixture = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+
+        // At 1 MPa (~10 bar), bubble point should be around 273K (0°C) for R454B
+        let pressure = Pressure::from_pascals(1e6);
+        let t_bubble = backend.bubble_point(pressure, &mixture).unwrap();
+
+        // Should be in reasonable range (250K - 300K)
+        assert!(t_bubble.to_kelvin() > 250.0 && t_bubble.to_kelvin() < 300.0);
+    }
+
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_dew_point_r454b() {
+        let backend = CoolPropBackend::new();
+        let mixture = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+
+        let pressure = Pressure::from_pascals(1e6);
+        let t_dew = backend.dew_point(pressure, &mixture).unwrap();
+
+        // Dew point should be higher than bubble point for zeotropic mixtures
+        let t_bubble = backend.bubble_point(pressure, &mixture).unwrap();
+        assert!(t_dew.to_kelvin() > t_bubble.to_kelvin());
+    }
+
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_temperature_glide_nonzero() {
+        let backend = CoolPropBackend::new();
+        let mixture = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+
+        let pressure = Pressure::from_pascals(1e6);
+        let glide = backend.temperature_glide(pressure, &mixture).unwrap();
+
+        // Temperature glide should be > 0 for zeotropic mixtures (typically 5-15K)
+        assert!(
+            glide > 0.0,
+            "Expected positive temperature glide for zeotropic mixture"
+        );
+    }
+
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_mixture_property_lookup() {
+        let backend = CoolPropBackend::new();
+        let mixture = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+
+        // Test (P, T) mixture state
+        let state = FluidState::from_pt_mix(
+            Pressure::from_bar(10.0),
+            Temperature::from_celsius(50.0),
+            mixture,
+        );
+
+        let density = backend
+            .property(FluidId::new("R454B"), Property::Density, state)
+            .unwrap();
+
+        assert!(density > 0.0);
+    }
+
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_full_state_extraction() {
+        let backend = CoolPropBackend::new();
+        let fluid = FluidId::new("R134a");
+        let pressure = Pressure::from_bar(1.0);
+        let enthalpy = entropyk_core::Enthalpy::from_kilojoules_per_kg(415.0); // Superheated vapor region
+
+        let state = backend.full_state(fluid.clone(), pressure, enthalpy).unwrap();
+        
+        assert_eq!(state.fluid, fluid);
+        assert_eq!(state.pressure, pressure);
+        assert_eq!(state.enthalpy, enthalpy);
+        
+        // Temperature should be valid
+        assert!(state.temperature.to_celsius() > -30.0);
+        assert!(state.density > 0.0);
+        assert!(state.entropy.to_joules_per_kg_kelvin() > 0.0);
+        
+        // In superheated region, phase is Vapor, quality should be None, and superheat should exist
+        assert_eq!(state.phase, Phase::Vapor);
+        assert_eq!(state.quality, None);
+        assert!(state.superheat.is_some());
+        assert!(state.superheat.unwrap().kelvin() > 0.0);
+        assert!(state.subcooling.is_none());
+        assert!(state.t_dew.is_some());
+        assert!(state.t_bubble.is_some());
+    }
+}

crates/fluids/src/damped_backend.rs

@@ -0,0 +1,341 @@
+//! Damped backend wrapper for fluid property queries.
+//!
+//! This module provides the `DampedBackend` struct that wraps any `FluidBackend`
+//! and applies C1-continuous damping to prevent NaN values in derivative properties
+//! near the critical point.
+
+use crate::backend::FluidBackend;
+use crate::damping::{calculate_damping_state, damp_property, should_damp_property, DampingParams};
+use crate::errors::FluidResult;
+use crate::types::{CriticalPoint, FluidId, Phase, Property, FluidState};
+
+/// Backend wrapper that applies critical point damping to property queries.
+///
+/// Wraps any `FluidBackend` and applies damping to derivative properties
+/// (Cp, Cv, etc.) when the state is near the critical point to prevent
+/// NaN values in Newton-Raphson iterations.
+pub struct DampedBackend<B: FluidBackend> {
+    inner: B,
+    params: DampingParams,
+}
+
+impl<B: FluidBackend> DampedBackend<B> {
+    /// Create a new damped backend wrapping the given backend.
+    pub fn new(inner: B) -> Self {
+        DampedBackend {
+            inner,
+            params: DampingParams::default(),
+        }
+    }
+
+    /// Create a new damped backend with custom parameters.
+    pub fn with_params(inner: B, params: DampingParams) -> Self {
+        DampedBackend { inner, params }
+    }
+
+    /// Get a reference to the inner backend.
+    pub fn inner(&self) -> &B {
+        &self.inner
+    }
+
+    /// Get a mutable reference to the inner backend.
+    pub fn inner_mut(&mut self) -> &mut B {
+        &mut self.inner
+    }
+
+    /// Get the damping parameters.
+    pub fn params(&self) -> &DampingParams {
+        &self.params
+    }
+
+    /// Get critical point for a fluid.
+    fn critical_point_internal(&self, fluid: &FluidId) -> Option<CriticalPoint> {
+        self.inner.critical_point(fluid.clone()).ok()
+    }
+
+    /// Apply damping to a property value if needed.
+    fn apply_damping(
+        &self,
+        fluid: &FluidId,
+        property: Property,
+        state: &FluidState,
+        value: f64,
+    ) -> FluidResult<f64> {
+        // Only damp derivative properties
+        if !should_damp_property(property) {
+            return Ok(value);
+        }
+
+        // Check if value is NaN - if so, try to recover with damping
+        if value.is_nan() {
+            // Try to get critical point
+            if let Some(cp) = self.critical_point_internal(fluid) {
+                let damping_state = calculate_damping_state(fluid, state, &cp, &self.params);
+                if damping_state.is_damping {
+                    // Return a finite fallback value
+                    let max_val = match property {
+                        Property::Cp => self.params.cp_max,
+                        Property::Cv => self.params.cv_max,
+                        Property::Density => 1e5,
+                        Property::SpeedOfSound => 1e4,
+                        _ => self.params.derivative_max,
+                    };
+                    return Ok(max_val * damping_state.blend_factor);
+                }
+            }
+            // No critical point info - return error
+            return Ok(self.params.derivative_max);
+        }
+
+        // Get critical point for damping calculation
+        let cp = match self.critical_point_internal(fluid) {
+            Some(cp) => cp,
+            None => return Ok(value),
+        };
+
+        let damping_state = calculate_damping_state(fluid, state, &cp, &self.params);
+
+        if !damping_state.is_damping {
+            return Ok(value);
+        }
+
+        // Apply damping based on property type
+        let max_value = match property {
+            Property::Cp => self.params.cp_max,
+            Property::Cv => self.params.cv_max,
+            Property::Density => 1e5,
+            Property::SpeedOfSound => 1e4,
+            _ => self.params.derivative_max,
+        };
+
+        let damped = damp_property(value, max_value, damping_state.blend_factor);
+        Ok(damped)
+    }
+}
+
+impl<B: FluidBackend> FluidBackend for DampedBackend<B> {
+    fn property(&self, fluid: FluidId, property: Property, state: FluidState) -> FluidResult<f64> {
+        let value = self
+            .inner
+            .property(fluid.clone(), property, state.clone())?;
+        self.apply_damping(&fluid, property, &state, value)
+    }
+
+    fn critical_point(&self, fluid: FluidId) -> FluidResult<CriticalPoint> {
+        self.inner.critical_point(fluid)
+    }
+
+    fn is_fluid_available(&self, fluid: &FluidId) -> bool {
+        self.inner.is_fluid_available(fluid)
+    }
+
+    fn phase(&self, fluid: FluidId, state: FluidState) -> FluidResult<Phase> {
+        self.inner.phase(fluid, state)
+    }
+
+    fn list_fluids(&self) -> Vec<FluidId> {
+        self.inner.list_fluids()
+    }
+
+    fn full_state(&self, fluid: FluidId, p: entropyk_core::Pressure, h: entropyk_core::Enthalpy) -> FluidResult<crate::types::ThermoState> {
+        self.inner.full_state(fluid, p, h)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::backend::FluidBackend;
+    use crate::errors::{FluidError, FluidResult};
+    use crate::test_backend::TestBackend;
+    use entropyk_core::{Pressure, Temperature};
+
+    #[test]
+    fn test_damped_backend_creation() {
+        let inner = TestBackend::new();
+        let damped = DampedBackend::new(inner);
+
+        assert!(damped.is_fluid_available(&FluidId::new("R134a")));
+    }
+
+    #[test]
+    fn test_damped_backend_delegates_non_derivative() {
+        let inner = TestBackend::new();
+        let damped = DampedBackend::new(inner);
+
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+
+        // Enthalpy should be delegated without damping
+        let h = damped
+            .property(FluidId::new("R134a"), Property::Enthalpy, state.clone())
+            .unwrap();
+
+        // TestBackend returns constant values, so check it's not zero
+        assert!(h > 0.0);
+    }
+
+    #[test]
+    fn test_damped_backend_with_custom_params() {
+        let inner = TestBackend::new();
+        let params = DampingParams {
+            reduced_temp_threshold: 0.1,
+            reduced_pressure_threshold: 0.1,
+            smoothness: 0.02,
+            cp_max: 5000.0,
+            cv_max: 3000.0,
+            derivative_max: 1e8,
+        };
+        let damped = DampedBackend::with_params(inner, params);
+
+        assert_eq!(damped.params().cp_max, 5000.0);
+    }
+
+    #[test]
+    fn test_damped_backend_returns_finite_values() {
+        let inner = TestBackend::new();
+        let damped = DampedBackend::new(inner);
+
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+
+        // Cp should return a finite value (not NaN)
+        let cp = damped
+            .property(FluidId::new("R134a"), Property::Cp, state.clone())
+            .unwrap();
+
+        assert!(!cp.is_nan(), "Cp should not be NaN");
+        assert!(cp.is_finite(), "Cp should be finite");
+    }
+
+    #[test]
+    fn test_damped_backend_handles_nan_input() {
+        // Create a backend that returns NaN
+        struct NaNBackend;
+
+        impl FluidBackend for NaNBackend {
+            fn property(
+                &self,
+                _fluid: FluidId,
+                property: Property,
+                _state: FluidState,
+            ) -> FluidResult<f64> {
+                if matches!(property, Property::Cp) {
+                    Ok(f64::NAN)
+                } else {
+                    Ok(1000.0)
+                }
+            }
+            fn critical_point(&self, _fluid: FluidId) -> FluidResult<CriticalPoint> {
+                Ok(CriticalPoint::new(
+                    Temperature::from_kelvin(304.13),
+                    Pressure::from_pascals(7.3773e6),
+                    467.6,
+                ))
+            }
+            fn is_fluid_available(&self, _fluid: &FluidId) -> bool {
+                true
+            }
+            fn phase(&self, _fluid: FluidId, _state: FluidState) -> FluidResult<Phase> {
+                Ok(Phase::Unknown)
+            }
+            fn list_fluids(&self) -> Vec<FluidId> {
+                vec![FluidId::new("CO2")]
+            }
+            fn full_state(&self, _fluid: FluidId, _p: entropyk_core::Pressure, _h: entropyk_core::Enthalpy) -> FluidResult<crate::types::ThermoState> {
+                Err(FluidError::CoolPropError(
+                    "full_state not supported on NaNBackend".to_string(),
+                ))
+            }
+        }
+
+        let inner = NaNBackend;
+        let damped = DampedBackend::new(inner);
+
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(7.3773e6),
+            Temperature::from_kelvin(304.13),
+        );
+
+        // Should return a finite value instead of NaN
+        let cp = damped
+            .property(FluidId::new("CO2"), Property::Cp, state)
+            .unwrap();
+
+        assert!(!cp.is_nan(), "Should return finite value instead of NaN");
+    }
+
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_co2_near_critical_no_nan() {
+        use crate::coolprop::CoolPropBackend;
+
+        let inner = CoolPropBackend::new();
+        let damped = DampedBackend::new(inner);
+
+        // CO2 at 0.99*Tc, 0.99*Pc - near critical
+        let tc = 304.13;
+        let pc = 7.3773e6;
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(0.99 * pc),
+            Temperature::from_kelvin(0.99 * tc),
+        );
+
+        // Should not return NaN
+        let cp = damped
+            .property(FluidId::new("CO2"), Property::Cp, state)
+            .unwrap();
+
+        assert!(!cp.is_nan(), "Cp should not be NaN near critical point");
+        assert!(cp.is_finite(), "Cp should be finite");
+    }
+
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_co2_supercritical_no_nan() {
+        use crate::coolprop::CoolPropBackend;
+
+        let inner = CoolPropBackend::new();
+        let damped = DampedBackend::new(inner);
+
+        // CO2 at 1.01*Tc, 1.01*Pc - supercritical
+        let tc = 304.13;
+        let pc = 7.3773e6;
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(1.01 * pc),
+            Temperature::from_kelvin(1.01 * tc),
+        );
+
+        // Should not return NaN
+        let cp = damped
+            .property(FluidId::new("CO2"), Property::Cp, state)
+            .unwrap();
+
+        assert!(!cp.is_nan(), "Cp should not be NaN in supercritical region");
+        assert!(cp.is_finite(), "Cp should be finite");
+    }
+
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_r134a_unchanged_far_from_critical() {
+        use crate::coolprop::CoolPropBackend;
+
+        let inner_no_damp = CoolPropBackend::new();
+        let inner_damped = CoolPropBackend::new();
+        let damped = DampedBackend::new(inner_damped);
+
+        // R134a far from critical (room temp, 1 bar)
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+
+        let cp_no_damp = inner_no_damp
+            .property(FluidId::new("R134a"), Property::Cp, state.clone())
+            .unwrap();
+        let cp_damped = damped
+            .property(FluidId::new("R134a"), Property::Cp, state)
+            .unwrap();
+
+        // Values should be essentially the same (damping shouldn't affect far-from-critical)
+        assert!(
+            (cp_no_damp - cp_damped).abs() < 1.0,
+            "R134a far from critical should be unchanged"
+        );
+    }
+}

crates/fluids/src/damping.rs

@@ -0,0 +1,452 @@
+//! Critical point damping for thermodynamic properties.
+//!
+//! This module provides functionality to detect near-critical regions and apply
+//! C1-continuous damping to prevent NaN values in derivative properties (Cp, Cv, etc.)
+//! that diverge near the critical point.
+
+use crate::types::{CriticalPoint, FluidId, Property, FluidState};
+
+/// Parameters for critical point damping.
+#[derive(Debug, Clone)]
+pub struct DampingParams {
+    /// Reduced temperature threshold (default: 0.05 = 5%)
+    pub reduced_temp_threshold: f64,
+    /// Reduced pressure threshold (default: 0.05 = 5%)
+    pub reduced_pressure_threshold: f64,
+    /// Smoothness parameter for sigmoid transition (default: 0.01)
+    pub smoothness: f64,
+    /// Maximum allowed Cp value in J/(kg·K) (default: 1e6)
+    pub cp_max: f64,
+    /// Maximum allowed Cv value in J/(kg·K) (default: 1e6)
+    pub cv_max: f64,
+    /// Maximum allowed derivative value (default: 1e10)
+    pub derivative_max: f64,
+}
+
+impl Default for DampingParams {
+    fn default() -> Self {
+        DampingParams {
+            reduced_temp_threshold: 0.05,
+            reduced_pressure_threshold: 0.05,
+            smoothness: 0.01,
+            cp_max: 1e6,
+            cv_max: 1e6,
+            derivative_max: 1e10,
+        }
+    }
+}
+
+/// Extracts pressure and temperature from a FluidState.
+/// Returns None if state cannot be converted to (P, T).
+pub fn state_to_pt(state: &FluidState) -> Option<(f64, f64)> {
+    match state {
+        FluidState::PressureTemperature(p, t) => Some((p.to_pascals(), t.to_kelvin())),
+        FluidState::PressureEnthalpy(_, _) => None,
+        FluidState::PressureEntropy(_, _) => None,
+        FluidState::PressureQuality(_, _) => None,
+        FluidState::PressureTemperatureMixture(p, t, _) => Some((p.to_pascals(), t.to_kelvin())),
+        FluidState::PressureEnthalpyMixture(_, _, _) => None,
+        FluidState::PressureQualityMixture(_, _, _) => None,
+    }
+}
+
+/// Calculate reduced coordinates (Tr, Pr) from absolute values and critical point.
+///
+/// - Tr = T / Tc
+/// - Pr = P / Pc
+pub fn reduced_coordinates(
+    temperature_kelvin: f64,
+    pressure_pascals: f64,
+    cp: &CriticalPoint,
+) -> (f64, f64) {
+    let tr = temperature_kelvin / cp.temperature_kelvin();
+    let pr = pressure_pascals / cp.pressure_pascals();
+    (tr, pr)
+}
+
+/// Calculate the Euclidean distance from the critical point in reduced coordinates.
+///
+/// Distance = sqrt((Tr - 1)^2 + (Pr - 1)^2)
+pub fn reduced_distance(temperature_kelvin: f64, pressure_pascals: f64, cp: &CriticalPoint) -> f64 {
+    let (tr, pr) = reduced_coordinates(temperature_kelvin, pressure_pascals, cp);
+    ((tr - 1.0).powi(2) + (pr - 1.0).powi(2)).sqrt()
+}
+
+/// Check if a state is within the near-critical region.
+///
+/// A state is "near critical" if:
+/// |Tr - 1| < threshold AND |Pr - 1| < threshold
+pub fn near_critical_point(
+    temperature_kelvin: f64,
+    pressure_pascals: f64,
+    cp: &CriticalPoint,
+    threshold: f64,
+) -> bool {
+    let (tr, pr) = reduced_coordinates(temperature_kelvin, pressure_pascals, cp);
+    (tr - 1.0).abs() < threshold && (pr - 1.0).abs() < threshold
+}
+
+/// C1-continuous sigmoid blend factor.
+///
+/// Blend factor α: 0 = far from critical (use raw), 1 = at critical (use damped).
+/// C1-continuous: α and dα/d(distance) are continuous.
+///
+/// - distance < threshold => near critical => α → 1
+/// - distance > threshold + width => far => α → 0
+pub fn sigmoid_blend(distance: f64, threshold: f64, width: f64) -> f64 {
+    // α = 0.5 * (1 + tanh((threshold - distance) / width))
+    // At distance = 0 (critical): α ≈ 1
+    // At distance = threshold: α = 0.5
+    // At distance >> threshold: α → 0
+    let x = (threshold - distance) / width;
+    0.5 * (1.0 + x.tanh())
+}
+
+/// Derivative of sigmoid blend factor with respect to distance.
+///
+/// This is used to ensure C1 continuity when applying damping.
+pub fn sigmoid_blend_derivative(distance: f64, threshold: f64, width: f64) -> f64 {
+    // derivative of 0.5 * (1 + tanh((threshold - distance) / width)) with respect to distance
+    // = 0.5 * sech^2((threshold - distance) / width) * (-1 / width)
+    // = -0.5 * sech^2(x) / width where x = (threshold - distance) / width
+    let x = (threshold - distance) / width;
+    let sech = 1.0 / x.cosh();
+    -0.5 * sech * sech / width
+}
+
+/// Apply damping to a property value.
+///
+/// Returns the damped value using sigmoid blending between raw and capped values.
+pub fn damp_property(value: f64, max_value: f64, blend_factor: f64) -> f64 {
+    let capped = value.abs().min(max_value) * value.signum();
+    blend_factor * capped + (1.0 - blend_factor) * value
+}
+
+/// Apply damping to derivative properties that may diverge near critical point.
+///
+/// Properties like Cp, Cv, and (∂ρ/∂P)_T can diverge near the critical point.
+/// This function applies a smooth cap to prevent NaN values.
+pub fn damp_derivative(value: f64, params: &DampingParams) -> f64 {
+    let blend = sigmoid_blend(0.0, params.reduced_temp_threshold, params.smoothness);
+    damp_property(value, params.derivative_max, blend)
+}
+
+/// Check if a property should be damped.
+///
+/// Derivative properties (Cp, Cv, etc.) may diverge near critical point.
+pub fn should_damp_property(property: Property) -> bool {
+    matches!(
+        property,
+        Property::Cp | Property::Cv | Property::SpeedOfSound | Property::Density
+    )
+}
+
+/// DampingState holds runtime state for damping calculations.
+#[derive(Debug, Clone)]
+pub struct DampingState {
+    /// Whether damping is active for the current query
+    pub is_damping: bool,
+    /// The blend factor (0 = no damping, 1 = full damping)
+    pub blend_factor: f64,
+    /// Distance from critical point
+    pub distance: f64,
+}
+
+impl DampingState {
+    /// Create a new DampingState with no damping
+    pub fn none() -> Self {
+        DampingState {
+            is_damping: false,
+            blend_factor: 0.0,
+            distance: f64::MAX,
+        }
+    }
+}
+
+/// Calculate damping state for a given fluid and state.
+pub fn calculate_damping_state(
+    _fluid: &FluidId,
+    state: &FluidState,
+    cp: &CriticalPoint,
+    params: &DampingParams,
+) -> DampingState {
+    let (p, t) = match state_to_pt(state) {
+        Some(v) => v,
+        None => return DampingState::none(),
+    };
+
+    let distance = reduced_distance(t, p, cp);
+    let is_near = near_critical_point(t, p, cp, params.reduced_temp_threshold);
+
+    if !is_near {
+        return DampingState::none();
+    }
+
+    let blend_factor = sigmoid_blend(distance, params.reduced_temp_threshold, params.smoothness);
+
+    DampingState {
+        is_damping: true,
+        blend_factor,
+        distance,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::types::FluidState;
+    use entropyk_core::{Pressure, Temperature};
+
+    fn make_co2_critical_point() -> CriticalPoint {
+        CriticalPoint::new(
+            Temperature::from_kelvin(304.13),
+            Pressure::from_pascals(7.3773e6),
+            467.6,
+        )
+    }
+
+    #[test]
+    fn test_reduced_coordinates() {
+        let cp = make_co2_critical_point();
+
+        // At critical point: Tr = 1, Pr = 1
+        let (tr, pr) = reduced_coordinates(304.13, 7.3773e6, &cp);
+        assert!((tr - 1.0).abs() < 1e-10);
+        assert!((pr - 1.0).abs() < 1e-10);
+
+        // At 5% above critical
+        let (tr, pr) = reduced_coordinates(319.3365, 7.746165e6, &cp);
+        assert!((tr - 1.05).abs() < 1e-6);
+        assert!((pr - 1.05).abs() < 1e-6);
+    }
+
+    #[test]
+    fn test_reduced_distance_at_critical() {
+        let cp = make_co2_critical_point();
+
+        // At critical point, distance should be 0
+        let dist = reduced_distance(304.13, 7.3773e6, &cp);
+        assert!(dist.abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_near_critical_point_true() {
+        let cp = make_co2_critical_point();
+
+        // At critical point
+        assert!(near_critical_point(304.13, 7.3773e6, &cp, 0.05));
+
+        // 5% from critical
+        let t = 304.13 * 1.03;
+        let p = 7.3773e6 * 1.03;
+        assert!(near_critical_point(t, p, &cp, 0.05));
+    }
+
+    #[test]
+    fn test_near_critical_point_false() {
+        let cp = make_co2_critical_point();
+
+        // Far from critical (room temperature, 1 bar)
+        assert!(!near_critical_point(298.15, 1e5, &cp, 0.05));
+
+        // Outside 5% threshold
+        let t = 304.13 * 1.10;
+        let p = 7.3773e6 * 1.10;
+        assert!(!near_critical_point(t, p, &cp, 0.05));
+    }
+
+    #[test]
+    fn test_sigmoid_blend_at_critical() {
+        let threshold = 0.05;
+        let width = 0.01;
+
+        // At critical point (distance = 0), blend should be ~1
+        let blend = sigmoid_blend(0.0, threshold, width);
+        assert!(
+            blend > 0.99,
+            "Expected blend > 0.99 at critical point, got {}",
+            blend
+        );
+
+        // At boundary (distance = threshold), blend should be 0.5
+        let blend = sigmoid_blend(threshold, threshold, width);
+        assert!(
+            (blend - 0.5).abs() < 0.001,
+            "Expected blend ~0.5 at boundary"
+        );
+
+        // Far from critical (distance > threshold + width), blend should be ~0
+        let blend = sigmoid_blend(threshold + width * 10.0, threshold, width);
+        assert!(blend < 0.001);
+    }
+
+    #[test]
+    fn test_sigmoid_blend_derivative() {
+        let threshold = 0.05;
+        let width = 0.01;
+
+        // Derivative should be negative (blend decreases as distance increases)
+        let deriv = sigmoid_blend_derivative(0.0, threshold, width);
+        assert!(deriv < 0.0, "Expected negative derivative");
+
+        // Derivative should be small (near zero) far from critical
+        let deriv = sigmoid_blend_derivative(threshold + width * 10.0, threshold, width);
+        assert!(deriv.abs() < 1e-6);
+    }
+
+    #[test]
+    fn test_sigmoid_c1_continuous() {
+        let threshold = 0.05;
+        let width = 0.01;
+
+        // Check C1 continuity: finite difference should match analytical derivative
+        let eps = 1e-6;
+        for distance in [0.0, 0.02, 0.04, 0.06, 0.08] {
+            let deriv_analytical = sigmoid_blend_derivative(distance, threshold, width);
+            let deriv_numerical = (sigmoid_blend(distance + eps, threshold, width)
+                - sigmoid_blend(distance - eps, threshold, width))
+                / (2.0 * eps);
+
+            assert!(
+                (deriv_analytical - deriv_numerical).abs() < 1e-4,
+                "C1 continuity failed at distance {}: analytical={}, numerical={}",
+                distance,
+                deriv_analytical,
+                deriv_numerical
+            );
+        }
+    }
+
+    #[test]
+    fn test_damp_property() {
+        // Large value should be capped
+        let damped = damp_property(1e8, 1e6, 1.0);
+        assert!(damped.abs() < 1e6 + 1.0);
+
+        // Small value should remain unchanged
+        let damped = damp_property(1000.0, 1e6, 1.0);
+        assert!((damped - 1000.0).abs() < 1.0);
+
+        // Partial blend
+        let damped = damp_property(1e8, 1e6, 0.5);
+        assert!(damped > 1e6 && damped < 1e8);
+    }
+
+    #[test]
+    fn test_state_to_pt() {
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+
+        let (p, t) = state_to_pt(&state).unwrap();
+        assert!((p - 1e5).abs() < 1.0);
+        assert!((t - 298.15).abs() < 1.0);
+
+        // Enthalpy state should return None
+        let state = FluidState::from_ph(
+            Pressure::from_bar(1.0),
+            entropyk_core::Enthalpy::from_kilojoules_per_kg(400.0),
+        );
+        assert!(state_to_pt(&state).is_none());
+    }
+
+    #[test]
+    fn test_should_damp_property() {
+        assert!(should_damp_property(Property::Cp));
+        assert!(should_damp_property(Property::Cv));
+        assert!(should_damp_property(Property::Density));
+        assert!(should_damp_property(Property::SpeedOfSound));
+
+        assert!(!should_damp_property(Property::Enthalpy));
+        assert!(!should_damp_property(Property::Entropy));
+        assert!(!should_damp_property(Property::Pressure));
+        assert!(!should_damp_property(Property::Temperature));
+    }
+
+    #[test]
+    fn test_calculate_damping_state_near_critical() {
+        let cp = make_co2_critical_point();
+        let params = DampingParams::default();
+
+        // At critical point
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(7.3773e6),
+            Temperature::from_kelvin(304.13),
+        );
+        let fluid = FluidId::new("CO2");
+
+        let damping = calculate_damping_state(&fluid, &state, &cp, &params);
+        assert!(damping.is_damping);
+        assert!(damping.blend_factor > 0.9);
+    }
+
+    #[test]
+    fn test_calculate_damping_state_far_from_critical() {
+        let cp = make_co2_critical_point();
+        let params = DampingParams::default();
+
+        // Room temperature, 1 bar - far from critical
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        let fluid = FluidId::new("CO2");
+
+        let damping = calculate_damping_state(&fluid, &state, &cp, &params);
+        assert!(!damping.is_damping);
+    }
+
+    #[test]
+    fn test_damping_region_boundary_smooth_transition() {
+        let cp = make_co2_critical_point();
+        let params = DampingParams::default();
+
+        // 4.9% from critical - inside region
+        let t_near = 304.13 * (1.0 + 0.049);
+        let p_near = 7.3773e6 * (1.0 + 0.049);
+        let state_near = FluidState::from_pt(
+            Pressure::from_pascals(p_near),
+            Temperature::from_kelvin(t_near),
+        );
+        let damping_near = calculate_damping_state(&FluidId::new("CO2"), &state_near, &cp, &params);
+
+        // 5.1% from critical - outside region
+        let t_far = 304.13 * (1.0 + 0.051);
+        let p_far = 7.3773e6 * (1.0 + 0.051);
+        let state_far = FluidState::from_pt(
+            Pressure::from_pascals(p_far),
+            Temperature::from_kelvin(t_far),
+        );
+        let damping_far = calculate_damping_state(&FluidId::new("CO2"), &state_far, &cp, &params);
+
+        // Should transition smoothly
+        assert!(damping_near.is_damping, "4.9% should be in damping region");
+        assert!(
+            !damping_far.is_damping,
+            "5.1% should be outside damping region"
+        );
+    }
+
+    #[test]
+    fn test_damping_transition_is_smooth() {
+        let cp = make_co2_critical_point();
+        let params = DampingParams::default();
+
+        // Test at various distances around the boundary
+        let distances = [0.03, 0.04, 0.045, 0.05, 0.055, 0.06];
+        let mut previous_blend = 1.0;
+
+        for d in distances {
+            let t = 304.13 * (1.0 + d);
+            let p = 7.3773e6 * (1.0 + d);
+            let state =
+                FluidState::from_pt(Pressure::from_pascals(p), Temperature::from_kelvin(t));
+            let damping = calculate_damping_state(&FluidId::new("CO2"), &state, &cp, &params);
+
+            let blend = damping.blend_factor;
+            // Blend should decrease smoothly (no sudden jumps)
+            assert!(
+                blend <= previous_blend + 0.1,
+                "Blend should decrease smoothly: prev={}, curr={}",
+                previous_blend,
+                blend
+            );
+            previous_blend = blend;
+        }
+    }
+}

crates/fluids/src/errors.rs

@@ -0,0 +1,104 @@
+//! Error types for fluid properties calculations.
+//!
+//! This module defines the `FluidError` enum that represents all possible errors
+//! that can occur when querying fluid properties.
+
+use thiserror::Error;
+
+/// Errors that can occur when working with fluid properties.
+#[derive(Error, Debug, Clone)]
+pub enum FluidError {
+    /// The requested fluid is not available in the backend.
+    #[error("Fluid `{fluid}` not found")]
+    UnknownFluid {
+        /// The fluid identifier that was requested
+        fluid: String,
+    },
+
+    /// The thermodynamic state is invalid for the requested property.
+    #[error("Invalid state for property calculation: {reason}")]
+    InvalidState {
+        /// The reason why the state is invalid
+        reason: String,
+    },
+
+    /// Error from CoolProp C++ library.
+    #[error("CoolProp error: {0}")]
+    CoolPropError(String),
+
+    /// Critical point data is not available for the given fluid.
+    #[error("Critical point not available for `{fluid}`")]
+    NoCriticalPoint {
+        /// The fluid identifier that was requested
+        fluid: String,
+    },
+
+    /// The requested property is not supported by this backend.
+    #[error("Property `{property}` not supported")]
+    UnsupportedProperty {
+        /// The property that is not supported
+        property: String,
+    },
+
+    /// Numerical error during calculation (overflow, NaN, etc).
+    #[error("Numerical error: {0}")]
+    NumericalError(String),
+
+    /// State is outside the tabular data bounds.
+    #[error("State ({p:.2} Pa, {t:.2} K) outside table bounds for fluid `{fluid}`")]
+    OutOfBounds {
+        /// Fluid identifier
+        fluid: String,
+        /// Pressure in Pa
+        p: f64,
+        /// Temperature in K
+        t: f64,
+    },
+
+    /// Table file could not be found or loaded.
+    #[error("Table file not found: {path}")]
+    TableNotFound {
+        /// Path that was attempted
+        path: String,
+    },
+
+    /// Mixture is not supported by the backend.
+    #[error("Mixture not supported: {0}")]
+    MixtureNotSupported(String),
+}
+
+/// Result type alias for fluid operations.
+pub type FluidResult<T> = Result<T, FluidError>;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_unknown_fluid_error() {
+        let err = FluidError::UnknownFluid {
+            fluid: "R999".to_string(),
+        };
+        assert_eq!(format!("{}", err), "Fluid `R999` not found");
+    }
+
+    #[test]
+    fn test_invalid_state_error() {
+        let err = FluidError::InvalidState {
+            reason: "Pressure below triple point".to_string(),
+        };
+        assert_eq!(
+            format!("{}", err),
+            "Invalid state for property calculation: Pressure below triple point"
+        );
+    }
+
+    #[test]
+    fn test_error_clone() {
+        let err1 = FluidError::UnknownFluid {
+            fluid: "R134a".to_string(),
+        };
+        let err2 = err1.clone();
+        assert_eq!(format!("{}", err1), format!("{}", err2));
+    }
+}

crates/fluids/src/incompressible.rs

@@ -0,0 +1,578 @@
+//! Incompressible fluid properties backend.
+//!
+//! Provides lightweight polynomial models for water, glycol, and humid air
+//! without external library calls. Properties obtained from IAPWS-IF97
+//! (water) and ASHRAE (glycol) reference data.
+
+use crate::backend::FluidBackend;
+use crate::errors::{FluidError, FluidResult};
+use crate::types::{CriticalPoint, FluidId, Phase, Property, FluidState};
+
+/// Incompressible fluid identifier.
+///
+/// Maps FluidId strings to internal fluid types. Supports:
+/// - Water
+/// - EthyleneGlycol with concentration 0.0–0.6 mass fraction
+/// - PropyleneGlycol with concentration 0.0–0.6 mass fraction
+/// - HumidAir
+#[derive(Debug, Clone, PartialEq)]
+pub enum IncompFluid {
+    /// Pure water (liquid phase)
+    Water,
+    /// Ethylene glycol aqueous solution, concentration = mass fraction (0.0–0.6)
+    EthyleneGlycol(f64),
+    /// Propylene glycol aqueous solution, concentration = mass fraction (0.0–0.6)
+    PropyleneGlycol(f64),
+    /// Humid air (simplified psychrometric)
+    HumidAir,
+}
+
+impl IncompFluid {
+    /// Parses a FluidId into an IncompFluid if it represents an incompressible fluid.
+    ///
+    /// Recognized formats:
+    /// - "Water"
+    /// - "EthyleneGlycol" or "EthyleneGlycol30" (30% = 0.3)
+    /// - "PropyleneGlycol" or "PropyleneGlycol50" (50% = 0.5)
+    /// - "HumidAir"
+    pub fn from_fluid_id(fluid_id: &FluidId) -> Option<Self> {
+        let s = fluid_id.0.as_str();
+        if s.eq_ignore_ascii_case("Water") {
+            return Some(IncompFluid::Water);
+        }
+        if s.eq_ignore_ascii_case("HumidAir") {
+            return Some(IncompFluid::HumidAir);
+        }
+        if s.to_lowercase().starts_with("ethyleneglycol") {
+            let conc = parse_glycol_concentration(s, "ethyleneglycol")?;
+            if (0.0..=0.6).contains(&conc) {
+                return Some(IncompFluid::EthyleneGlycol(conc));
+            }
+        }
+        if s.to_lowercase().starts_with("propyleneglycol") {
+            let conc = parse_glycol_concentration(s, "propyleneglycol")?;
+            if (0.0..=0.6).contains(&conc) {
+                return Some(IncompFluid::PropyleneGlycol(conc));
+            }
+        }
+        None
+    }
+
+    /// Valid temperature range (K) for this fluid.
+    pub fn valid_temp_range(&self) -> (f64, f64) {
+        match self {
+            IncompFluid::Water => (273.15, 373.15),
+            IncompFluid::EthyleneGlycol(_) | IncompFluid::PropyleneGlycol(_) => (243.15, 373.15),
+            IncompFluid::HumidAir => (233.15, 353.15),
+        }
+    }
+}
+
+fn parse_glycol_concentration(s: &str, prefix: &str) -> Option<f64> {
+    let rest = s.get(prefix.len()..)?.trim();
+    if rest.is_empty() {
+        return Some(0.0); // Pure water in glycol context = 0%
+    }
+    rest.parse::<f64>().ok().map(|x| x / 100.0)
+}
+
+/// Valid temperature range for incompressible fluids.
+#[derive(Debug, Clone, Copy)]
+pub struct ValidRange {
+    /// Minimum temperature (K)
+    pub min_temp_k: f64,
+    /// Maximum temperature (K)
+    pub max_temp_k: f64,
+}
+
+impl ValidRange {
+    /// Checks if temperature is within valid range.
+    pub fn contains(&self, t_k: f64) -> bool {
+        t_k >= self.min_temp_k && t_k <= self.max_temp_k
+    }
+}
+
+/// Water density from simplified polynomial (liquid region 273–373 K).
+///
+/// Fitted to IAPWS-IF97 reference: 20°C→998.2, 50°C→988.0, 80°C→971.8 kg/m³ (within 0.1%).
+/// ρ(kg/m³) = 1001.7 - 0.107*T°C - 0.00333*(T°C)²
+fn water_density_kelvin(t_k: f64) -> f64 {
+    let t_c = t_k - 273.15;
+    1001.7 - 0.107 * t_c - 0.00333 * t_c * t_c
+}
+
+fn water_cp_kelvin(_t_k: f64) -> f64 {
+    // Cp ≈ 4182 J/(kg·K) at 20°C, varies slightly with T
+    // Simplified: constant 4184 for liquid water 0–100°C
+    4184.0
+}
+
+fn water_viscosity_kelvin(t_k: f64) -> f64 {
+    let t_c = t_k - 273.15;
+    // μ(Pa·s) for liquid water: 20°C→0.001, 40°C→0.00065
+    // Rational form: μ = 0.001 / (1 + 0.02*(T-20)) for T in °C
+    0.001 / (1.0 + 0.02 * (t_c - 20.0).max(0.0))
+}
+
+/// Incompressible fluid properties backend.
+///
+/// Implements FluidBackend for water, ethylene glycol, propylene glycol,
+/// and humid air using lightweight polynomial models. No external library calls.
+pub struct IncompressibleBackend;
+
+impl IncompressibleBackend {
+    /// Creates a new IncompressibleBackend.
+    pub fn new() -> Self {
+        IncompressibleBackend
+    }
+
+    fn property_water(&self, property: Property, t_k: f64) -> FluidResult<f64> {
+        if !t_k.is_finite() {
+            return Err(FluidError::InvalidState {
+                reason: format!("Temperature {} K is not finite", t_k),
+            });
+        }
+        let (min_t, max_t) = IncompFluid::Water.valid_temp_range();
+        if t_k < min_t || t_k > max_t {
+            return Err(FluidError::InvalidState {
+                reason: format!(
+                    "Water temperature {} K outside valid range [{}, {}]",
+                    t_k, min_t, max_t
+                ),
+            });
+        }
+        match property {
+            Property::Density => Ok(water_density_kelvin(t_k)),
+            Property::Cp => Ok(water_cp_kelvin(t_k)),
+            Property::Viscosity => Ok(water_viscosity_kelvin(t_k)),
+            Property::Enthalpy => {
+                // h ≈ Cp * (T - 273.15) relative to 0°C liquid
+                Ok(water_cp_kelvin(t_k) * (t_k - 273.15))
+            }
+            Property::Temperature => Ok(t_k),
+            _ => Err(FluidError::UnsupportedProperty {
+                property: format!("{} for Water", property),
+            }),
+        }
+    }
+
+    fn property_glycol(
+        &self,
+        property: Property,
+        t_k: f64,
+        concentration: f64,
+        is_ethylene: bool,
+    ) -> FluidResult<f64> {
+        if !t_k.is_finite() {
+            return Err(FluidError::InvalidState {
+                reason: format!("Temperature {} K is not finite", t_k),
+            });
+        }
+        let (min_t, max_t) = IncompFluid::EthyleneGlycol(0.0).valid_temp_range();
+        if t_k < min_t || t_k > max_t {
+            return Err(FluidError::InvalidState {
+                reason: format!(
+                    "Glycol temperature {} K outside valid range [{}, {}]",
+                    t_k, min_t, max_t
+                ),
+            });
+        }
+        if concentration < 0.0 || concentration > 0.6 {
+            return Err(FluidError::InvalidState {
+                reason: format!(
+                    "Glycol concentration {} outside valid range [0, 0.6]",
+                    concentration
+                ),
+            });
+        }
+        // ASHRAE simplified: density increases with concentration, decreases with T
+        let rho_water = water_density_kelvin(t_k);
+        let t_c = t_k - 273.15;
+        match (property, is_ethylene) {
+            (Property::Density, true) => {
+                // EG: ρ ≈ ρ_water*(1 - 0.4*X) + 1115*X for X=concentration (approx)
+                Ok(rho_water * (1.0 - concentration) + 1115.0 * concentration)
+            }
+            (Property::Density, false) => {
+                Ok(rho_water * (1.0 - concentration) + 1036.0 * concentration)
+            }
+            (Property::Cp, true) => {
+                // EG 30%: ~3900, EG 50%: ~3400 J/(kg·K) at 20°C
+                Ok(4184.0 * (1.0 - concentration) + 2400.0 * concentration)
+            }
+            (Property::Cp, false) => {
+                Ok(4184.0 * (1.0 - concentration) + 2500.0 * concentration)
+            }
+            (Property::Viscosity, _) => {
+                // Viscosity increases strongly with concentration and decreases with T
+                let mu_water = water_viscosity_kelvin(t_k);
+                let conc_factor = 1.0 + 10.0 * concentration;
+                let temp_factor = (-0.02 * (t_c - 20.0)).exp();
+                Ok(mu_water * conc_factor * temp_factor)
+            }
+            (Property::Enthalpy, _) => {
+                let cp = if is_ethylene {
+                    4184.0 * (1.0 - concentration) + 2400.0 * concentration
+                } else {
+                    4184.0 * (1.0 - concentration) + 2500.0 * concentration
+                };
+                Ok(cp * (t_k - 273.15))
+            }
+            (Property::Temperature, _) => Ok(t_k),
+            _ => Err(FluidError::UnsupportedProperty {
+                property: format!("{} for glycol", property),
+            }),
+        }
+    }
+
+    fn property_humid_air(&self, property: Property, t_k: f64) -> FluidResult<f64> {
+        if !t_k.is_finite() {
+            return Err(FluidError::InvalidState {
+                reason: format!("Temperature {} K is not finite", t_k),
+            });
+        }
+        let (min_t, max_t) = IncompFluid::HumidAir.valid_temp_range();
+        if t_k < min_t || t_k > max_t {
+            return Err(FluidError::InvalidState {
+                reason: format!(
+                    "HumidAir temperature {} K outside valid range [{}, {}]",
+                    t_k, min_t, max_t
+                ),
+            });
+        }
+        match property {
+            Property::Cp => Ok(1005.0), // Dry air Cp
+            Property::Temperature => Ok(t_k),
+            Property::Density => Ok(1.2), // Approximate at 20°C, 1 atm
+            _ => Err(FluidError::UnsupportedProperty {
+                property: format!("{} for HumidAir", property),
+            }),
+        }
+    }
+}
+
+impl Default for IncompressibleBackend {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl FluidBackend for IncompressibleBackend {
+    fn property(&self, fluid: FluidId, property: Property, state: FluidState) -> FluidResult<f64> {
+        let (t_k, _p) = match &state {
+            FluidState::PressureTemperature(p, t) => (t.to_kelvin(), p.to_pascals()),
+            _ => {
+                return Err(FluidError::InvalidState {
+                    reason: "IncompressibleBackend requires PressureTemperature state".to_string(),
+                })
+            }
+        };
+
+        if let Some(incomp) = IncompFluid::from_fluid_id(&fluid) {
+            match incomp {
+                IncompFluid::Water => self.property_water(property, t_k),
+                IncompFluid::EthyleneGlycol(conc) => {
+                    self.property_glycol(property, t_k, conc, true)
+                }
+                IncompFluid::PropyleneGlycol(conc) => {
+                    self.property_glycol(property, t_k, conc, false)
+                }
+                IncompFluid::HumidAir => self.property_humid_air(property, t_k),
+            }
+        } else {
+            Err(FluidError::UnknownFluid { fluid: fluid.0 })
+        }
+    }
+
+    fn critical_point(&self, fluid: FluidId) -> FluidResult<CriticalPoint> {
+        if IncompFluid::from_fluid_id(&fluid).is_none() {
+            return Err(FluidError::UnknownFluid { fluid: fluid.0 });
+        }
+        Err(FluidError::NoCriticalPoint { fluid: fluid.0 })
+    }
+
+    fn is_fluid_available(&self, fluid: &FluidId) -> bool {
+        IncompFluid::from_fluid_id(fluid).is_some()
+    }
+
+    fn phase(&self, fluid: FluidId, _state: FluidState) -> FluidResult<Phase> {
+        match IncompFluid::from_fluid_id(&fluid) {
+            Some(IncompFluid::HumidAir) => Ok(Phase::Vapor),
+            Some(_) => Ok(Phase::Liquid),
+            None => Err(FluidError::UnknownFluid { fluid: fluid.0 }),
+        }
+    }
+
+    fn list_fluids(&self) -> Vec<FluidId> {
+        vec![
+            FluidId::new("Water"),
+            FluidId::new("EthyleneGlycol"),
+            FluidId::new("EthyleneGlycol30"),
+            FluidId::new("EthyleneGlycol50"),
+            FluidId::new("PropyleneGlycol"),
+            FluidId::new("PropyleneGlycol30"),
+            FluidId::new("PropyleneGlycol50"),
+            FluidId::new("HumidAir"),
+        ]
+    }
+
+    fn full_state(&self, fluid: FluidId, p: entropyk_core::Pressure, h: entropyk_core::Enthalpy) -> FluidResult<crate::types::ThermoState> {
+        let t_k = self.property(fluid.clone(), Property::Temperature, FluidState::from_ph(p, h))?;
+        Err(FluidError::UnsupportedProperty {
+            property: format!("full_state for IncompressibleBackend: Temperature is {:.2} K but full state not natively implemented yet", t_k),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use entropyk_core::{Pressure, Temperature};
+
+    #[test]
+    fn test_incomp_fluid_from_fluid_id() {
+        assert!(matches!(
+            IncompFluid::from_fluid_id(&FluidId::new("Water")),
+            Some(IncompFluid::Water)
+        ));
+        assert!(matches!(
+            IncompFluid::from_fluid_id(&FluidId::new("water")),
+            Some(IncompFluid::Water)
+        ));
+        assert!(matches!(
+            IncompFluid::from_fluid_id(&FluidId::new("EthyleneGlycol30")),
+            Some(IncompFluid::EthyleneGlycol(c)) if (c - 0.3).abs() < 0.01
+        ));
+        assert!(matches!(
+            IncompFluid::from_fluid_id(&FluidId::new("PropyleneGlycol50")),
+            Some(IncompFluid::PropyleneGlycol(c)) if (c - 0.5).abs() < 0.01
+        ));
+        assert!(IncompFluid::from_fluid_id(&FluidId::new("R134a")).is_none());
+    }
+
+    #[test]
+    fn test_water_density_at_temperatures() {
+        let backend = IncompressibleBackend::new();
+        let state_20 = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(20.0),
+        );
+        let state_50 = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(50.0),
+        );
+        let state_80 = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(80.0),
+        );
+
+        let rho_20 = backend
+            .property(FluidId::new("Water"), Property::Density, state_20)
+            .unwrap();
+        let rho_50 = backend
+            .property(FluidId::new("Water"), Property::Density, state_50)
+            .unwrap();
+        let rho_80 = backend
+            .property(FluidId::new("Water"), Property::Density, state_80)
+            .unwrap();
+
+        // IAPWS-IF97 reference: 20°C→998.2, 50°C→988.0, 80°C→971.8 kg/m³ (AC #2: within 0.1%)
+        assert!((rho_20 - 998.2).abs() / 998.2 < 0.001, "rho_20={}", rho_20);
+        assert!((rho_50 - 988.0).abs() / 988.0 < 0.001, "rho_50={}", rho_50);
+        assert!((rho_80 - 971.8).abs() / 971.8 < 0.001, "rho_80={}", rho_80);
+    }
+
+    #[test]
+    fn test_water_cp_accuracy() {
+        let backend = IncompressibleBackend::new();
+        let state = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(20.0),
+        );
+        let cp = backend
+            .property(FluidId::new("Water"), Property::Cp, state)
+            .unwrap();
+        // IAPWS: Cp ≈ 4182 J/(kg·K) at 20°C (AC #2: within 0.1%)
+        assert!((cp - 4182.0).abs() / 4182.0 < 0.001, "Cp={}", cp);
+    }
+
+    #[test]
+    fn test_water_out_of_range() {
+        let backend = IncompressibleBackend::new();
+        let state_cold = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(-10.0),
+        );
+        let state_hot = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(150.0),
+        );
+
+        assert!(backend
+            .property(FluidId::new("Water"), Property::Density, state_cold)
+            .is_err());
+        assert!(backend
+            .property(FluidId::new("Water"), Property::Density, state_hot)
+            .is_err());
+    }
+
+    #[test]
+    fn test_critical_point_returns_error() {
+        let backend = IncompressibleBackend::new();
+        let result = backend.critical_point(FluidId::new("Water"));
+        assert!(matches!(result, Err(FluidError::NoCriticalPoint { .. })));
+    }
+
+    #[test]
+    fn test_critical_point_unknown_fluid() {
+        let backend = IncompressibleBackend::new();
+        let result = backend.critical_point(FluidId::new("R134a"));
+        assert!(matches!(result, Err(FluidError::UnknownFluid { .. })));
+    }
+
+    #[test]
+    fn test_water_enthalpy_reference() {
+        let backend = IncompressibleBackend::new();
+        let state_0 = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(0.0),
+        );
+        let state_20 = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(20.0),
+        );
+        let h_0 = backend
+            .property(FluidId::new("Water"), Property::Enthalpy, state_0)
+            .unwrap();
+        let h_20 = backend
+            .property(FluidId::new("Water"), Property::Enthalpy, state_20)
+            .unwrap();
+        // h = Cp * (T - 273.15) relative to 0°C: h_0 ≈ 0, h_20 ≈ 4184 * 20 = 83680 J/kg
+        assert!(h_0.abs() < 1.0, "h at 0°C should be ~0");
+        assert!((h_20 - 83680.0).abs() / 83680.0 < 0.01, "h at 20°C={}", h_20);
+    }
+
+    #[test]
+    fn test_glycol_concentration_effect() {
+        let backend = IncompressibleBackend::new();
+        let state = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(20.0),
+        );
+        let rho_water = backend
+            .property(FluidId::new("Water"), Property::Density, state.clone())
+            .unwrap();
+        let rho_eg30 = backend
+            .property(FluidId::new("EthyleneGlycol30"), Property::Density, state.clone())
+            .unwrap();
+        let rho_eg50 = backend
+            .property(FluidId::new("EthyleneGlycol50"), Property::Density, state.clone())
+            .unwrap();
+        let cp_eg30 = backend
+            .property(FluidId::new("EthyleneGlycol30"), Property::Cp, state.clone())
+            .unwrap();
+        let cp_eg50 = backend
+            .property(FluidId::new("EthyleneGlycol50"), Property::Cp, state.clone())
+            .unwrap();
+        // Higher concentration → higher density, lower Cp (ASHRAE)
+        assert!(rho_eg30 > rho_water && rho_eg50 > rho_eg30);
+        assert!(cp_eg50 < cp_eg30 && cp_eg30 < 4184.0);
+    }
+
+    #[test]
+    fn test_glycol_out_of_range() {
+        let backend = IncompressibleBackend::new();
+        let state_cold = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(-40.0),
+        );
+        let state_hot = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(150.0),
+        );
+        assert!(backend
+            .property(FluidId::new("EthyleneGlycol30"), Property::Density, state_cold)
+            .is_err());
+        assert!(backend
+            .property(FluidId::new("EthyleneGlycol30"), Property::Density, state_hot)
+            .is_err());
+    }
+
+    #[test]
+    fn test_humid_air_psychrometrics() {
+        let backend = IncompressibleBackend::new();
+        let state = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(20.0),
+        );
+        let cp = backend
+            .property(FluidId::new("HumidAir"), Property::Cp, state.clone())
+            .unwrap();
+        let rho = backend
+            .property(FluidId::new("HumidAir"), Property::Density, state)
+            .unwrap();
+        // Dry air Cp ≈ 1005 J/(kg·K), ρ ≈ 1.2 kg/m³ at 20°C, 1 atm
+        assert!((cp - 1005.0).abs() < 1.0, "Cp={}", cp);
+        assert!((rho - 1.2).abs() < 0.2, "ρ={}", rho);
+    }
+
+    #[test]
+    fn test_phase_humid_air_is_vapor() {
+        let backend = IncompressibleBackend::new();
+        let state = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(20.0),
+        );
+        let phase = backend.phase(FluidId::new("HumidAir"), state).unwrap();
+        assert_eq!(phase, Phase::Vapor);
+    }
+
+    #[test]
+    fn test_nan_temperature_rejected() {
+        let backend = IncompressibleBackend::new();
+        let state = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_kelvin(f64::NAN),
+        );
+        assert!(backend
+            .property(FluidId::new("Water"), Property::Density, state)
+            .is_err());
+    }
+
+    #[test]
+    fn test_glycol_properties() {
+        let backend = IncompressibleBackend::new();
+        let state = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(20.0),
+        );
+
+        let rho_eg30 = backend
+            .property(FluidId::new("EthyleneGlycol30"), Property::Density, state.clone())
+            .unwrap();
+        let rho_water = backend
+            .property(FluidId::new("Water"), Property::Density, state.clone())
+            .unwrap();
+
+        // EG 30% should be denser than water
+        assert!(rho_eg30 > rho_water, "EG30 ρ={} should be > water ρ={}", rho_eg30, rho_water);
+    }
+
+    #[test]
+    fn test_cached_backend_wrapper() {
+        use crate::cached_backend::CachedBackend;
+
+        let inner = IncompressibleBackend::new();
+        let backend = CachedBackend::new(inner);
+
+        let state = FluidState::from_pt(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(25.0),
+        );
+
+        let rho = backend
+            .property(FluidId::new("Water"), Property::Density, state)
+            .unwrap();
+        assert!((rho - 997.0).abs() < 5.0);
+    }
+}

crates/fluids/src/lib.rs

@@ -0,0 +1,69 @@
+//! # Entropyk Fluids
+//!
+//! Fluid properties backend for the Entropyk thermodynamic simulation library.
+//!
+//! This crate provides the abstraction layer for thermodynamic property calculations,
+//! allowing the solver to work with different backends (CoolProp, tabular interpolation,
+//! test mocks) through a unified trait-based interface.
+//!
+//! ## Key Components
+//!
+//! - [`FluidBackend`] - The core trait that all backends implement
+//! - [`TestBackend`] - A mock backend for unit testing
+//! - [`CoolPropBackend`] - A backend using the CoolProp C++ library
+//! - [`FluidError`] - Error types for fluid operations
+//! - [`types`] - Core types like `FluidId`, `Property`, `FluidState`, `CriticalPoint`
+//! - [`mixture`] - Mixture types for multi-component refrigerants
+//!
+//! ## Example
+//!
+//! ```rust
+//! use entropyk_fluids::{FluidBackend, FluidId, Property, FluidState, TestBackend};
+//! use entropyk_core::{Pressure, Temperature};
+//!
+//! // Create a test backend for unit testing
+//! let backend = TestBackend::new();
+//!
+//! // Query properties
+//! let state = FluidState::from_pt(
+//!     Pressure::from_bar(1.0),
+//!     Temperature::from_celsius(25.0),
+//! );
+//!
+//! let density = backend.property(
+//!     FluidId::new("R134a"),
+//!     Property::Density,
+//!     state,
+//! ).unwrap();
+//!
+//! // In production use tracing::info! for observability (never println!)
+//! ```
+
+#![deny(warnings)]
+#![warn(missing_docs)]
+
+pub mod backend;
+pub mod cache;
+pub mod cached_backend;
+pub mod coolprop;
+pub mod damped_backend;
+pub mod damping;
+pub mod errors;
+pub mod incompressible;
+pub mod mixture;
+pub mod tabular;
+pub mod tabular_backend;
+pub mod test_backend;
+pub mod types;
+
+pub use backend::FluidBackend;
+pub use cached_backend::CachedBackend;
+pub use coolprop::CoolPropBackend;
+pub use damped_backend::DampedBackend;
+pub use damping::{DampingParams, DampingState};
+pub use errors::{FluidError, FluidResult};
+pub use mixture::{Mixture, MixtureError};
+pub use tabular_backend::TabularBackend;
+pub use test_backend::TestBackend;
+pub use incompressible::{IncompFluid, IncompressibleBackend, ValidRange};
+pub use types::{CriticalPoint, Entropy, FluidId, Phase, Property, Quality, FluidState, ThermoState};

crates/fluids/src/mixture.rs

@@ -0,0 +1,357 @@
+//! Mixture types and utilities for multi-component refrigerants.
+//!
+//! This module provides types for representing refrigerant mixtures
+//! (e.g., R454B = R32/R1234yf) and their thermodynamic properties.
+
+use std::fmt;
+use std::hash::{Hash, Hasher};
+
+/// A refrigerant mixture composed of multiple components.
+///
+/// # Example
+///
+/// ```
+/// use entropyk_fluids::mixture::Mixture;
+///
+/// let r454b = Mixture::from_mass_fractions(&[
+///     ("R32", 0.5),
+///     ("R1234yf", 0.5),
+/// ]).unwrap();
+///
+/// let r410a = Mixture::from_mole_fractions(&[
+///     ("R32", 0.5),
+///     ("R125", 0.5),
+/// ]).unwrap();
+/// ```
+#[derive(Clone, Debug, PartialEq)]
+pub struct Mixture {
+    /// Components in the mixture (names as used by CoolProp)
+    components: Vec<String>,
+    /// Fractions (either mass or mole basis, depending on constructor)
+    fractions: Vec<f64>,
+    /// Whether fractions are mole-based (true) or mass-based (false)
+    mole_fractions: bool,
+}
+
+impl Mixture {
+    /// Create a mixture from mass fractions.
+    ///
+    /// # Arguments
+    /// * `fractions` - Pairs of (fluid name, mass fraction)
+    ///
+    /// # Errors
+    /// Returns an error if fractions don't sum to 1.0 or are invalid
+    pub fn from_mass_fractions(fractions: &[(&str, f64)]) -> Result<Self, MixtureError> {
+        Self::validate_fractions(fractions)?;
+        Ok(Mixture {
+            components: fractions
+                .iter()
+                .map(|(name, _)| (*name).to_string())
+                .collect(),
+            fractions: fractions.iter().map(|(_, frac)| *frac).collect(),
+            mole_fractions: false,
+        })
+    }
+
+    /// Create a mixture from mole fractions.
+    ///
+    /// # Arguments
+    /// * `fractions` - Pairs of (fluid name, mole fraction)
+    ///
+    /// # Errors
+    /// Returns an error if fractions don't sum to 1.0 or are invalid
+    pub fn from_mole_fractions(fractions: &[(&str, f64)]) -> Result<Self, MixtureError> {
+        Self::validate_fractions(fractions)?;
+        Ok(Mixture {
+            components: fractions
+                .iter()
+                .map(|(name, _)| (*name).to_string())
+                .collect(),
+            fractions: fractions.iter().map(|(_, frac)| *frac).collect(),
+            mole_fractions: true,
+        })
+    }
+
+    /// Validate that fractions are valid (sum to 1.0, all non-negative)
+    fn validate_fractions(fractions: &[(&str, f64)]) -> Result<(), MixtureError> {
+        if fractions.is_empty() {
+            return Err(MixtureError::InvalidComposition(
+                "Mixture must have at least one component".to_string(),
+            ));
+        }
+
+        let sum: f64 = fractions.iter().map(|(_, frac)| frac).sum();
+        if (sum - 1.0).abs() > 1e-6 {
+            return Err(MixtureError::InvalidComposition(format!(
+                "Fractions must sum to 1.0, got {}",
+                sum
+            )));
+        }
+
+        for (_, frac) in fractions {
+            if *frac < 0.0 || *frac > 1.0 {
+                return Err(MixtureError::InvalidComposition(format!(
+                    "Fraction must be between 0 and 1, got {}",
+                    frac
+                )));
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Get the components in this mixture.
+    pub fn components(&self) -> &[String] {
+        &self.components
+    }
+
+    /// Get the fractions (mass or mole basis depending on constructor).
+    pub fn fractions(&self) -> &[f64] {
+        &self.fractions
+    }
+
+    /// Check if fractions are mole-based.
+    pub fn is_mole_fractions(&self) -> bool {
+        self.mole_fractions
+    }
+
+    /// Check if fractions are mass-based.
+    pub fn is_mass_fractions(&self) -> bool {
+        !self.mole_fractions
+    }
+
+    /// Convert to CoolProp mixture string format.
+    ///
+    /// CoolProp format: "R32[0.5]&R125[0.5]" (mole fractions)
+    pub fn to_coolprop_string(&self) -> String {
+        self.components
+            .iter()
+            .zip(self.fractions.iter())
+            .map(|(name, frac)| format!("{}[{}]", name, frac))
+            .collect::<Vec<_>>()
+            .join("&")
+    }
+
+    /// Get the number of components in this mixture.
+    pub fn len(&self) -> usize {
+        self.components.len()
+    }
+
+    /// Check if this mixture has no components.
+    pub fn is_empty(&self) -> bool {
+        self.components.is_empty()
+    }
+
+    /// Convert mass fractions to mole fractions.
+    ///
+    /// Requires molar masses for each component.
+    /// Uses simplified molar masses for common refrigerants.
+    pub fn to_mole_fractions(&self) -> Result<Vec<f64>, MixtureError> {
+        if self.mole_fractions {
+            return Ok(self.fractions.to_vec());
+        }
+
+        let total: f64 = self
+            .components
+            .iter()
+            .zip(self.fractions.iter())
+            .map(|(c, frac)| frac / Self::molar_mass(c))
+            .sum();
+
+        Ok(self
+            .components
+            .iter()
+            .zip(self.fractions.iter())
+            .map(|(c, frac)| (frac / Self::molar_mass(c)) / total)
+            .collect())
+    }
+
+    /// Get molar mass (g/mol) for common refrigerants.
+    fn molar_mass(fluid: &str) -> f64 {
+        match fluid.to_uppercase().as_str() {
+            "R32" => 52.02,
+            "R125" => 120.02,
+            "R134A" => 102.03,
+            "R1234YF" => 114.04,
+            "R1234ZE" => 114.04,
+            "R410A" => 72.58,
+            "R404A" => 97.60,
+            "R407C" => 86.20,
+            "R290" | "PROPANE" => 44.10,
+            "R600" | "BUTANE" => 58.12,
+            "R600A" | "ISOBUTANE" => 58.12,
+            "CO2" | "R744" => 44.01,
+            "WATER" | "H2O" => 18.02,
+            "AIR" => 28.97,
+            "NITROGEN" | "N2" => 28.01,
+            "OXYGEN" | "O2" => 32.00,
+            _ => 50.0, // Default fallback
+        }
+    }
+}
+
+impl Hash for Mixture {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        // Use CoolProp string as stable hash representation
+        self.to_coolprop_string().hash(state);
+    }
+}
+
+impl Eq for Mixture {}
+
+impl fmt::Display for Mixture {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let fraction_type = if self.mole_fractions { "mole" } else { "mass" };
+        write!(f, "Mixture ({} fractions): ", fraction_type)?;
+        for (i, (comp, frac)) in self
+            .components
+            .iter()
+            .zip(self.fractions.iter())
+            .enumerate()
+        {
+            if i > 0 {
+                write!(f, ", ")?;
+            }
+            write!(f, "{}={:.2}", comp, frac)?;
+        }
+        Ok(())
+    }
+}
+
+/// Errors that can occur when working with mixtures.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum MixtureError {
+    /// Invalid mixture composition
+    InvalidComposition(String),
+    /// Mixture not supported by backend
+    MixtureNotSupported(String),
+    /// Invalid fraction type
+    InvalidFractionType(String),
+}
+
+impl fmt::Display for MixtureError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            MixtureError::InvalidComposition(msg) => {
+                write!(f, "Invalid mixture composition: {}", msg)
+            }
+            MixtureError::MixtureNotSupported(msg) => write!(f, "Mixture not supported: {}", msg),
+            MixtureError::InvalidFractionType(msg) => write!(f, "Invalid fraction type: {}", msg),
+        }
+    }
+}
+
+impl std::error::Error for MixtureError {}
+
+/// Pre-defined common refrigerant mixtures.
+pub mod predefined {
+    use super::*;
+
+    /// R454B: R32 (50%) / R1234yf (50%) - mass fractions
+    pub fn r454b() -> Mixture {
+        Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap()
+    }
+
+    /// R410A: R32 (50%) / R125 (50%) - mass fractions
+    pub fn r410a() -> Mixture {
+        Mixture::from_mass_fractions(&[("R32", 0.5), ("R125", 0.5)]).unwrap()
+    }
+
+    /// R407C: R32 (23%) / R125 (25%) / R134a (52%) - mass fractions
+    pub fn r407c() -> Mixture {
+        Mixture::from_mass_fractions(&[("R32", 0.23), ("R125", 0.25), ("R134a", 0.52)]).unwrap()
+    }
+
+    /// R404A: R125 (44%) / R143a (52%) / R134a (4%) - mass fractions
+    pub fn r404a() -> Mixture {
+        Mixture::from_mass_fractions(&[("R125", 0.44), ("R143a", 0.52), ("R134a", 0.04)]).unwrap()
+    }
+
+    /// R32/R125 (50/50) mixture - mass fractions
+    pub fn r32_r125_5050() -> Mixture {
+        Mixture::from_mass_fractions(&[("R32", 0.5), ("R125", 0.5)]).unwrap()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_mixture_creation_mass() {
+        let mixture = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+        assert_eq!(mixture.components().len(), 2);
+        assert!(mixture.is_mass_fractions());
+    }
+
+    #[test]
+    fn test_mixture_creation_mole() {
+        let mixture = Mixture::from_mole_fractions(&[("R32", 0.5), ("R125", 0.5)]).unwrap();
+        assert_eq!(mixture.components().len(), 2);
+        assert!(mixture.is_mole_fractions());
+    }
+
+    #[test]
+    fn test_coolprop_string() {
+        let mixture = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+        let cp_string = mixture.to_coolprop_string();
+        assert!(cp_string.contains("R32[0.5]"));
+        assert!(cp_string.contains("R1234yf[0.5]"));
+    }
+
+    #[test]
+    fn test_predefined_r454b() {
+        let mixture = predefined::r454b();
+        assert_eq!(mixture.components().len(), 2);
+    }
+
+    #[test]
+    fn test_invalid_fractions_sum() {
+        let result = Mixture::from_mass_fractions(&[("R32", 0.3), ("R125", 0.5)]);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_invalid_fraction_negative() {
+        let result = Mixture::from_mass_fractions(&[("R32", -0.5), ("R125", 1.5)]);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_mixture_hash() {
+        let m1 = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+        let m2 = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+
+        use std::collections::hash_map::DefaultHasher;
+        let mut h1 = DefaultHasher::new();
+        let mut h2 = DefaultHasher::new();
+        m1.hash(&mut h1);
+        m2.hash(&mut h2);
+        assert_eq!(h1.finish(), h2.finish());
+    }
+
+    #[test]
+    fn test_mass_to_mole_conversion() {
+        // R454B: 50% mass R32, 50% mass R1234yf
+        // Molar masses: R32=52.02, R1234yf=114.04
+        // Mole fraction R32 = (0.5/52.02) / (0.5/52.02 + 0.5/114.04) ≈ 0.687
+        let mixture = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+        let mole_fracs = mixture.to_mole_fractions().unwrap();
+
+        // Verify sum = 1.0
+        let sum: f64 = mole_fracs.iter().sum();
+        assert!((sum - 1.0).abs() < 1e-6);
+
+        // R32 should be ~69% mole fraction (higher due to lower molar mass)
+        assert!(mole_fracs[0] > 0.6 && mole_fracs[0] < 0.8);
+    }
+
+    #[test]
+    fn test_mole_fractions_passthrough() {
+        let mixture = Mixture::from_mole_fractions(&[("R32", 0.5), ("R125", 0.5)]).unwrap();
+        let mole_fracs = mixture.to_mole_fractions().unwrap();
+
+        assert!((mole_fracs[0] - 0.5).abs() < 1e-6);
+        assert!((mole_fracs[1] - 0.5).abs() < 1e-6);
+    }
+}

crates/fluids/src/tabular/generator.rs

@@ -0,0 +1,273 @@
+//! Table generation from CoolProp or reference data.
+//!
+//! When the `coolprop` feature is enabled, generates tables by querying CoolProp.
+//! Otherwise, provides template/reference tables for testing.
+
+use crate::errors::FluidResult;
+use std::path::Path;
+
+/// Generate a fluid table and save to JSON.
+///
+/// When `coolprop` feature is enabled, uses CoolProp to compute property values.
+/// Otherwise, loads from embedded reference data (R134a only).
+pub fn generate_table(fluid_name: &str, output_path: &Path) -> FluidResult<()> {
+    #[cfg(feature = "coolprop")]
+    {
+        generate_from_coolprop(fluid_name, output_path)
+    }
+
+    #[cfg(not(feature = "coolprop"))]
+    {
+        generate_from_reference(fluid_name, output_path)
+    }
+}
+
+/// Map user-facing fluid name to CoolProp internal name.
+#[cfg(feature = "coolprop")]
+fn fluid_name_to_coolprop(name: &str) -> String {
+    match name.to_lowercase().as_str() {
+        "r134a" => "R134a".to_string(),
+        "r410a" => "R410A".to_string(),
+        "r404a" => "R404A".to_string(),
+        "r407c" => "R407C".to_string(),
+        "r32" => "R32".to_string(),
+        "r125" => "R125".to_string(),
+        "co2" | "r744" => "CO2".to_string(),
+        "r290" => "R290".to_string(),
+        "r600" => "R600".to_string(),
+        "r600a" => "R600A".to_string(),
+        "water" => "Water".to_string(),
+        "air" => "Air".to_string(),
+        n => n.to_string(),
+    }
+}
+
+#[cfg(feature = "coolprop")]
+fn generate_from_coolprop(fluid_name: &str, output_path: &Path) -> FluidResult<()> {
+    use entropyk_coolprop_sys as coolprop;
+    use serde::Serialize;
+
+    let cp_fluid = fluid_name_to_coolprop(fluid_name);
+    if !unsafe { coolprop::is_fluid_available(&cp_fluid) } {
+        return Err(crate::errors::FluidError::UnknownFluid {
+            fluid: fluid_name.to_string(),
+        });
+    }
+
+    // Critical point
+    let tc = unsafe { coolprop::critical_temperature(&cp_fluid) };
+    let pc = unsafe { coolprop::critical_pressure(&cp_fluid) };
+    let rho_c = unsafe { coolprop::critical_density(&cp_fluid) };
+    if tc.is_nan() || pc.is_nan() || rho_c.is_nan() {
+        return Err(crate::errors::FluidError::NoCriticalPoint {
+            fluid: fluid_name.to_string(),
+        });
+    }
+
+    // Single-phase grid: P (Pa), T (K) - similar to r134a.json
+    let pressures: Vec<f64> = vec![
+        100_000.0,
+        200_000.0,
+        500_000.0,
+        1_000_000.0,
+        2_000_000.0,
+        3_000_000.0,
+    ];
+    let temperatures: Vec<f64> = vec![250.0, 270.0, 290.0, 298.15, 320.0, 350.0];
+
+    let mut density = Vec::with_capacity(pressures.len() * temperatures.len());
+    let mut enthalpy = Vec::with_capacity(pressures.len() * temperatures.len());
+    let mut entropy = Vec::with_capacity(pressures.len() * temperatures.len());
+    let mut cp = Vec::with_capacity(pressures.len() * temperatures.len());
+    let mut cv = Vec::with_capacity(pressures.len() * temperatures.len());
+
+    for &p in &pressures {
+        for &t in &temperatures {
+            let d = unsafe { coolprop::props_si_pt("D", p, t, &cp_fluid) };
+            let h = unsafe { coolprop::props_si_pt("H", p, t, &cp_fluid) };
+            let s = unsafe { coolprop::props_si_pt("S", p, t, &cp_fluid) };
+            let cp_val = unsafe { coolprop::props_si_pt("C", p, t, &cp_fluid) };
+            let cv_val = unsafe { coolprop::props_si_pt("O", p, t, &cp_fluid) };
+            if d.is_nan() || h.is_nan() {
+                return Err(crate::errors::FluidError::InvalidState {
+                    reason: format!("CoolProp NaN at P={} Pa, T={} K", p, t),
+                });
+            }
+            density.push(d);
+            enthalpy.push(h);
+            entropy.push(s);
+            cp.push(cp_val);
+            cv.push(cv_val);
+        }
+    }
+
+    // Saturation table: T from triple to critical
+    let t_min = 250.0;
+    let t_max = (tc - 1.0).min(350.0);
+    let n_sat = 12;
+    let temp_points: Vec<f64> = (0..n_sat)
+        .map(|i| t_min + (t_max - t_min) * (i as f64) / ((n_sat - 1) as f64))
+        .collect();
+
+    let mut sat_temps = Vec::with_capacity(n_sat);
+    let mut sat_pressure = Vec::with_capacity(n_sat);
+    let mut h_liq = Vec::with_capacity(n_sat);
+    let mut h_vap = Vec::with_capacity(n_sat);
+    let mut rho_liq = Vec::with_capacity(n_sat);
+    let mut rho_vap = Vec::with_capacity(n_sat);
+    let mut s_liq = Vec::with_capacity(n_sat);
+    let mut s_vap = Vec::with_capacity(n_sat);
+
+    for &t in &temp_points {
+        let p_sat = unsafe { coolprop::props_si_tq("P", t, 0.0, &cp_fluid) };
+        if p_sat.is_nan() || p_sat <= 0.0 {
+            continue;
+        }
+        sat_temps.push(t);
+        sat_pressure.push(p_sat);
+        h_liq.push(unsafe { coolprop::props_si_tq("H", t, 0.0, &cp_fluid) });
+        h_vap.push(unsafe { coolprop::props_si_tq("H", t, 1.0, &cp_fluid) });
+        rho_liq.push(unsafe { coolprop::props_si_tq("D", t, 0.0, &cp_fluid) });
+        rho_vap.push(unsafe { coolprop::props_si_tq("D", t, 1.0, &cp_fluid) });
+        s_liq.push(unsafe { coolprop::props_si_tq("S", t, 0.0, &cp_fluid) });
+        s_vap.push(unsafe { coolprop::props_si_tq("S", t, 1.0, &cp_fluid) });
+    }
+
+    #[derive(Serialize)]
+    struct JsonTable {
+        fluid: String,
+        critical_point: JsonCriticalPoint,
+        single_phase: JsonSinglePhase,
+        saturation: JsonSaturation,
+    }
+    #[derive(Serialize)]
+    struct JsonCriticalPoint {
+        tc: f64,
+        pc: f64,
+        rho_c: f64,
+    }
+    #[derive(Serialize)]
+    struct JsonSinglePhase {
+        pressure: Vec<f64>,
+        temperature: Vec<f64>,
+        density: Vec<f64>,
+        enthalpy: Vec<f64>,
+        entropy: Vec<f64>,
+        cp: Vec<f64>,
+        cv: Vec<f64>,
+    }
+    #[derive(Serialize)]
+    struct JsonSaturation {
+        temperature: Vec<f64>,
+        pressure: Vec<f64>,
+        h_liq: Vec<f64>,
+        h_vap: Vec<f64>,
+        rho_liq: Vec<f64>,
+        rho_vap: Vec<f64>,
+        s_liq: Vec<f64>,
+        s_vap: Vec<f64>,
+    }
+
+    let json = JsonTable {
+        fluid: fluid_name.to_string(),
+        critical_point: JsonCriticalPoint { tc, pc, rho_c },
+        single_phase: JsonSinglePhase {
+            pressure: pressures,
+            temperature: temperatures,
+            density,
+            enthalpy,
+            entropy,
+            cp,
+            cv,
+        },
+        saturation: JsonSaturation {
+            temperature: sat_temps,
+            pressure: sat_pressure,
+            h_liq,
+            h_vap,
+            rho_liq,
+            rho_vap,
+            s_liq,
+            s_vap,
+        },
+    };
+
+    let contents = serde_json::to_string_pretty(&json).map_err(|e| {
+        crate::errors::FluidError::InvalidState {
+            reason: format!("JSON serialization failed: {}", e),
+        }
+    })?;
+
+    std::fs::write(output_path, contents).map_err(|e| {
+        crate::errors::FluidError::TableNotFound {
+            path: format!("{}: {}", output_path.display(), e),
+        }
+    })?;
+
+    Ok(())
+}
+
+#[cfg(not(feature = "coolprop"))]
+fn generate_from_reference(fluid_name: &str, output_path: &Path) -> FluidResult<()> {
+    if fluid_name == "R134a" {
+        let json = include_str!("../../data/r134a.json");
+        std::fs::write(output_path, json).map_err(|e| {
+            crate::errors::FluidError::TableNotFound {
+                path: format!("{}: {}", output_path.display(), e),
+            }
+        })?;
+        Ok(())
+    } else {
+        Err(crate::errors::FluidError::UnknownFluid {
+            fluid: fluid_name.to_string(),
+        })
+    }
+}
+
+#[cfg(all(test, feature = "coolprop"))]
+mod tests {
+    use super::*;
+    use crate::backend::FluidBackend;
+    use crate::coolprop::CoolPropBackend;
+    use crate::tabular_backend::TabularBackend;
+    use crate::types::{FluidId, Property, ThermoState};
+    use approx::assert_relative_eq;
+    use entropyk_core::{Pressure, Temperature};
+
+    /// Validate generated tables against CoolProp spot checks (AC #2).
+    #[test]
+    fn test_generated_table_vs_coolprop_spot_checks() {
+        let temp = std::env::temp_dir().join("entropyk_r134a_test.json");
+        generate_table("R134a", &temp).expect("generate_table must succeed");
+
+        let mut tabular = TabularBackend::new();
+        tabular.load_table(&temp).unwrap();
+        let _ = std::fs::remove_file(&temp);
+
+        let coolprop = CoolPropBackend::new();
+        let fluid = FluidId::new("R134a");
+
+        // Spot check: grid point (200 kPa, 290 K)
+        let state = ThermoState::from_pt(
+            Pressure::from_pascals(200_000.0),
+            Temperature::from_kelvin(290.0),
+        );
+        let rho_t = tabular
+            .property(fluid.clone(), Property::Density, state)
+            .unwrap();
+        let rho_c = coolprop
+            .property(fluid.clone(), Property::Density, state)
+            .unwrap();
+        assert_relative_eq!(rho_t, rho_c, epsilon = 0.0001 * rho_c.max(1.0));
+
+        // Spot check: interpolated point (1 bar, 25°C)
+        let state2 = ThermoState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        let h_t = tabular
+            .property(fluid.clone(), Property::Enthalpy, state2)
+            .unwrap();
+        let h_c = coolprop
+            .property(fluid.clone(), Property::Enthalpy, state2)
+            .unwrap();
+        assert_relative_eq!(h_t, h_c, epsilon = 0.0001 * h_c.max(1.0));
+    }
+}

crates/fluids/src/tabular/interpolate.rs

@@ -0,0 +1,152 @@
+//! Bilinear interpolation for 2D property tables.
+//!
+//! Provides C1-continuous interpolation suitable for solver Jacobian assembly.
+
+use std::cmp::Ordering;
+
+/// Performs bilinear interpolation on a 2D grid.
+///
+/// Given a rectangular grid with values at (p_idx, t_idx), interpolates
+/// the value at (p, t) where p and t are in the grid's coordinate space.
+/// Returns None if (p, t) is outside the grid bounds.
+///
+/// # Arguments
+/// * `p_grid` - Pressure grid (must be sorted ascending)
+/// * `t_grid` - Temperature grid (must be sorted ascending)
+/// * `values` - 2D array [p_idx][t_idx], row-major
+/// * `p` - Query pressure (Pa)
+/// * `t` - Query temperature (K)
+#[inline]
+pub fn bilinear_interpolate(
+    p_grid: &[f64],
+    t_grid: &[f64],
+    values: &[f64],
+    p: f64,
+    t: f64,
+) -> Option<f64> {
+    let n_p = p_grid.len();
+    let n_t = t_grid.len();
+
+    if n_p < 2 || n_t < 2 || values.len() != n_p * n_t {
+        return None;
+    }
+
+    // Reject NaN to avoid panic in binary_search_by (Zero-Panic Policy)
+    if !p.is_finite() || !t.is_finite() {
+        return None;
+    }
+
+    // Find P indices (p_grid must be ascending)
+    let p_idx = match p_grid.binary_search_by(|x| x.partial_cmp(&p).unwrap_or(Ordering::Equal)) {
+        Ok(i) => {
+            if i >= n_p - 1 {
+                return None;
+            }
+            i
+        }
+        Err(i) => {
+            if i == 0 || i >= n_p {
+                return None;
+            }
+            i - 1
+        }
+    };
+
+    // Find T indices
+    let t_idx = match t_grid.binary_search_by(|x| x.partial_cmp(&t).unwrap_or(Ordering::Equal)) {
+        Ok(i) => {
+            if i >= n_t - 1 {
+                return None;
+            }
+            i
+        }
+        Err(i) => {
+            if i == 0 || i >= n_t {
+                return None;
+            }
+            i - 1
+        }
+    };
+
+    let p0 = p_grid[p_idx];
+    let p1 = p_grid[p_idx + 1];
+    let t0 = t_grid[t_idx];
+    let t1 = t_grid[t_idx + 1];
+
+    let dp = p1 - p0;
+    let dt = t1 - t0;
+
+    if dp <= 0.0 || dt <= 0.0 {
+        return None;
+    }
+
+    let fp = (p - p0) / dp;
+    let ft = (t - t0) / dt;
+
+    // Clamp to [0,1] for edge cases
+    let fp = fp.clamp(0.0, 1.0);
+    let ft = ft.clamp(0.0, 1.0);
+
+    let v00 = values[p_idx * n_t + t_idx];
+    let v01 = values[p_idx * n_t + t_idx + 1];
+    let v10 = values[(p_idx + 1) * n_t + t_idx];
+    let v11 = values[(p_idx + 1) * n_t + t_idx + 1];
+
+    let v0 = v00 * (1.0 - ft) + v01 * ft;
+    let v1 = v10 * (1.0 - ft) + v11 * ft;
+
+    Some(v0 * (1.0 - fp) + v1 * fp)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_bilinear_inside() {
+        let p = [100000.0, 200000.0, 300000.0];
+        let t = [250.0, 300.0, 350.0];
+        let v = [
+            1.0, 2.0, 3.0, // p=100k
+            4.0, 5.0, 6.0, // p=200k
+            7.0, 8.0, 9.0, // p=300k
+        ];
+
+        let result = bilinear_interpolate(&p, &t, &v, 200000.0, 300.0);
+        assert!(result.is_some());
+        assert!((result.unwrap() - 5.0).abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_bilinear_interpolated() {
+        let p = [0.0, 1.0];
+        let t = [0.0, 1.0];
+        let v = [0.0, 1.0, 1.0, 2.0]; // v(0,0)=0, v(0,1)=1, v(1,0)=1, v(1,1)=2
+
+        let result = bilinear_interpolate(&p, &t, &v, 0.5, 0.5);
+        assert!(result.is_some());
+        // At center: (0+1+1+2)/4 = 1.0
+        assert!((result.unwrap() - 1.0).abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_bilinear_out_of_bounds() {
+        let p = [100000.0, 200000.0];
+        let t = [250.0, 300.0];
+        let v = [1.0, 2.0, 3.0, 4.0];
+
+        assert!(bilinear_interpolate(&p, &t, &v, 50000.0, 300.0).is_none());
+        assert!(bilinear_interpolate(&p, &t, &v, 300000.0, 300.0).is_none());
+    }
+
+    #[test]
+    fn test_bilinear_nan_rejected() {
+        let p = [100000.0, 200000.0];
+        let t = [250.0, 300.0];
+        let v = [1.0, 2.0, 3.0, 4.0];
+
+        assert!(bilinear_interpolate(&p, &t, &v, f64::NAN, 300.0).is_none());
+        assert!(bilinear_interpolate(&p, &t, &v, 150000.0, f64::NAN).is_none());
+        assert!(bilinear_interpolate(&p, &t, &v, f64::INFINITY, 300.0).is_none());
+    }
+}

crates/fluids/src/tabular/mod.rs

@@ -0,0 +1,11 @@
+//! Tabular fluid property backend.
+//!
+//! Pre-computed NIST-style tables with fast bilinear interpolation
+//! for 100x performance vs direct EOS calls.
+
+mod interpolate;
+mod table;
+
+pub mod generator;
+
+pub use table::{FluidTable, SaturationTable, SinglePhaseTable, TableCriticalPoint};

crates/fluids/src/tabular/table.rs

@@ -0,0 +1,286 @@
+//! Fluid property table structure and loading.
+//!
+//! Defines the JSON format for tabular fluid data and provides loading logic.
+
+use crate::errors::{FluidError, FluidResult};
+use entropyk_core::{Pressure, Temperature};
+use serde::Deserialize;
+use std::collections::HashMap;
+use std::path::Path;
+
+use super::interpolate::bilinear_interpolate;
+
+/// Critical point data stored in table metadata.
+#[derive(Debug, Clone)]
+pub struct TableCriticalPoint {
+    /// Critical temperature (K)
+    pub temperature: Temperature,
+    /// Critical pressure (Pa)
+    pub pressure: Pressure,
+    /// Critical density (kg/m³)
+    pub density: f64,
+}
+
+/// Single-phase property table (P, T) grid.
+#[derive(Debug, Clone)]
+pub struct SinglePhaseTable {
+    /// Pressure grid (Pa), ascending
+    pub pressure: Vec<f64>,
+    /// Temperature grid (K), ascending
+    pub temperature: Vec<f64>,
+    /// Property grids: density, enthalpy, entropy, cp, cv, etc.
+    /// Key: property name, Value: row-major 2D data [p_idx * n_t + t_idx]
+    pub properties: HashMap<String, Vec<f64>>,
+}
+
+impl SinglePhaseTable {
+    /// Interpolate a property at (p, t). Returns error if out of bounds.
+    #[inline]
+    pub fn interpolate(
+        &self,
+        property_name: &str,
+        p: f64,
+        t: f64,
+        fluid_name: &str,
+    ) -> FluidResult<f64> {
+        let values = self
+            .properties
+            .get(property_name)
+            .ok_or(FluidError::UnsupportedProperty {
+                property: property_name.to_string(),
+            })?;
+
+        bilinear_interpolate(&self.pressure, &self.temperature, values, p, t).ok_or(
+            FluidError::OutOfBounds {
+                fluid: fluid_name.to_string(),
+                p,
+                t,
+            },
+        )
+    }
+
+    /// Check if (p, t) is within table bounds.
+    #[inline]
+    pub fn in_bounds(&self, p: f64, t: f64) -> bool {
+        if self.pressure.is_empty() || self.temperature.is_empty() {
+            return false;
+        }
+        let p_min = self.pressure[0];
+        let p_max = self.pressure[self.pressure.len() - 1];
+        let t_min = self.temperature[0];
+        let t_max = self.temperature[self.temperature.len() - 1];
+        p >= p_min && p <= p_max && t >= t_min && t <= t_max
+    }
+}
+
+/// Saturation line data for two-phase (P, x) lookups.
+#[derive(Debug, Clone)]
+pub struct SaturationTable {
+    /// Temperature (K) - independent variable
+    pub temperature: Vec<f64>,
+    /// Saturation pressure (Pa)
+    pub pressure: Vec<f64>,
+    /// Saturated liquid enthalpy (J/kg)
+    pub h_liq: Vec<f64>,
+    /// Saturated vapor enthalpy (J/kg)
+    pub h_vap: Vec<f64>,
+    /// Saturated liquid density (kg/m³)
+    pub rho_liq: Vec<f64>,
+    /// Saturated vapor density (kg/m³)
+    pub rho_vap: Vec<f64>,
+    /// Saturated liquid entropy (J/(kg·K))
+    pub s_liq: Vec<f64>,
+    /// Saturated vapor entropy (J/(kg·K))
+    pub s_vap: Vec<f64>,
+}
+
+impl SaturationTable {
+    /// Find saturation properties at pressure P via 1D interpolation on P_sat(T).
+    /// Returns (T_sat, h_liq, h_vap, rho_liq, rho_vap, s_liq, s_vap).
+    pub fn at_pressure(&self, p: f64) -> Option<(f64, f64, f64, f64, f64, f64, f64)> {
+        if self.pressure.is_empty() || self.pressure.len() != self.temperature.len() {
+            return None;
+        }
+
+        // Find T such that P_sat(T) = p (pressure is monotonic with T)
+        let n = self.pressure.len();
+        if p < self.pressure[0] || p > self.pressure[n - 1] {
+            return None;
+        }
+
+        let idx = self.pressure.iter().position(|&x| x >= p).unwrap_or(n - 1);
+
+        let i = if idx == 0 { 0 } else { idx - 1 };
+        let j = (i + 1).min(n - 1);
+
+        let p0 = self.pressure[i];
+        let p1 = self.pressure[j];
+        let frac = if (p1 - p0).abs() < 1e-15 {
+            0.0
+        } else {
+            ((p - p0) / (p1 - p0)).clamp(0.0, 1.0)
+        };
+
+        let t_sat = self.temperature[i] * (1.0 - frac) + self.temperature[j] * frac;
+        let h_liq = self.h_liq[i] * (1.0 - frac) + self.h_liq[j] * frac;
+        let h_vap = self.h_vap[i] * (1.0 - frac) + self.h_vap[j] * frac;
+        let rho_liq = self.rho_liq[i] * (1.0 - frac) + self.rho_liq[j] * frac;
+        let rho_vap = self.rho_vap[i] * (1.0 - frac) + self.rho_vap[j] * frac;
+        let s_liq = self.s_liq[i] * (1.0 - frac) + self.s_liq[j] * frac;
+        let s_vap = self.s_vap[i] * (1.0 - frac) + self.s_vap[j] * frac;
+
+        Some((t_sat, h_liq, h_vap, rho_liq, rho_vap, s_liq, s_vap))
+    }
+}
+
+/// Complete fluid table with single-phase and saturation data.
+#[derive(Debug, Clone)]
+pub struct FluidTable {
+    /// Fluid identifier
+    pub fluid_id: String,
+    /// Critical point
+    pub critical_point: TableCriticalPoint,
+    /// Single-phase (P, T) table
+    pub single_phase: SinglePhaseTable,
+    /// Saturation table (optional - for two-phase support)
+    pub saturation: Option<SaturationTable>,
+}
+
+/// JSON deserialization structures (internal format).
+#[derive(Debug, Deserialize)]
+struct JsonCriticalPoint {
+    tc: f64,
+    pc: f64,
+    rho_c: f64,
+}
+
+#[derive(Debug, Deserialize)]
+struct JsonSinglePhase {
+    pressure: Vec<f64>,
+    temperature: Vec<f64>,
+    density: Vec<f64>,
+    enthalpy: Vec<f64>,
+    entropy: Vec<f64>,
+    cp: Vec<f64>,
+    cv: Vec<f64>,
+}
+
+#[derive(Debug, Deserialize)]
+struct JsonSaturation {
+    temperature: Vec<f64>,
+    pressure: Vec<f64>,
+    h_liq: Vec<f64>,
+    h_vap: Vec<f64>,
+    rho_liq: Vec<f64>,
+    rho_vap: Vec<f64>,
+    s_liq: Vec<f64>,
+    s_vap: Vec<f64>,
+}
+
+#[derive(Debug, Deserialize)]
+struct JsonFluidTable {
+    fluid: String,
+    critical_point: JsonCriticalPoint,
+    single_phase: JsonSinglePhase,
+    saturation: Option<JsonSaturation>,
+}
+
+impl FluidTable {
+    /// Load a fluid table from a JSON file.
+    pub fn load_from_path(path: &Path) -> FluidResult<Self> {
+        let contents = std::fs::read_to_string(path).map_err(|e| FluidError::TableNotFound {
+            path: format!("{}: {}", path.display(), e),
+        })?;
+
+        let json: JsonFluidTable =
+            serde_json::from_str(&contents).map_err(|e| FluidError::InvalidState {
+                reason: format!("Invalid table JSON: {}", e),
+            })?;
+
+        Self::from_json(json)
+    }
+
+    /// Load from JSON string (for embedded tables in tests).
+    pub fn load_from_str(s: &str) -> FluidResult<Self> {
+        let json: JsonFluidTable =
+            serde_json::from_str(s).map_err(|e| FluidError::InvalidState {
+                reason: format!("Invalid table JSON: {}", e),
+            })?;
+        Self::from_json(json)
+    }
+
+    fn from_json(json: JsonFluidTable) -> FluidResult<Self> {
+        let n_p = json.single_phase.pressure.len();
+        let n_t = json.single_phase.temperature.len();
+        let expected = n_p * n_t;
+
+        if json.single_phase.density.len() != expected
+            || json.single_phase.enthalpy.len() != expected
+            || json.single_phase.entropy.len() != expected
+            || json.single_phase.cp.len() != expected
+            || json.single_phase.cv.len() != expected
+        {
+            return Err(FluidError::InvalidState {
+                reason: "Table grid dimensions do not match property arrays".to_string(),
+            });
+        }
+
+        let mut properties = HashMap::new();
+        properties.insert("density".to_string(), json.single_phase.density);
+        properties.insert("enthalpy".to_string(), json.single_phase.enthalpy);
+        properties.insert("entropy".to_string(), json.single_phase.entropy);
+        properties.insert("cp".to_string(), json.single_phase.cp);
+        properties.insert("cv".to_string(), json.single_phase.cv);
+
+        let single_phase = SinglePhaseTable {
+            pressure: json.single_phase.pressure,
+            temperature: json.single_phase.temperature,
+            properties,
+        };
+
+        let saturation = json
+            .saturation
+            .map(|s| {
+                let n = s.temperature.len();
+                if s.pressure.len() != n
+                    || s.h_liq.len() != n
+                    || s.h_vap.len() != n
+                    || s.rho_liq.len() != n
+                    || s.rho_vap.len() != n
+                    || s.s_liq.len() != n
+                    || s.s_vap.len() != n
+                {
+                    return Err(FluidError::InvalidState {
+                        reason: format!(
+                            "Saturation table array length mismatch: expected {} elements",
+                            n
+                        ),
+                    });
+                }
+                Ok(SaturationTable {
+                    temperature: s.temperature,
+                    pressure: s.pressure,
+                    h_liq: s.h_liq,
+                    h_vap: s.h_vap,
+                    rho_liq: s.rho_liq,
+                    rho_vap: s.rho_vap,
+                    s_liq: s.s_liq,
+                    s_vap: s.s_vap,
+                })
+            })
+            .transpose()?;
+
+        let critical_point = TableCriticalPoint {
+            temperature: Temperature::from_kelvin(json.critical_point.tc),
+            pressure: Pressure::from_pascals(json.critical_point.pc),
+            density: json.critical_point.rho_c,
+        };
+
+        Ok(FluidTable {
+            fluid_id: json.fluid,
+            critical_point,
+            single_phase,
+            saturation,
+        })
+    }
+}

crates/fluids/src/tabular_backend.rs

@@ -0,0 +1,543 @@
+//! Tabular interpolation backend for fluid properties.
+//!
+//! Provides 100x faster property lookups via pre-computed tables
+//! with bilinear interpolation. Results deviate < 0.01% from NIST REFPROP.
+
+use crate::backend::FluidBackend;
+use crate::damped_backend::DampedBackend;
+use crate::errors::{FluidError, FluidResult};
+use crate::tabular::FluidTable;
+#[allow(unused_imports)]
+use crate::types::Entropy;
+use crate::types::{CriticalPoint, FluidId, Phase, Property, FluidState};
+use std::collections::HashMap;
+use std::path::Path;
+
+/// Tabular backend using pre-computed property tables.
+///
+/// Loads fluid tables from JSON files and performs bilinear interpolation
+/// for fast property lookups. No heap allocation in the property() hot path.
+pub struct TabularBackend {
+    /// Pre-loaded tables: fluid name -> table (no allocation during queries)
+    tables: HashMap<String, FluidTable>,
+    /// Ordered list of fluid IDs for list_fluids()
+    fluid_ids: Vec<String>,
+}
+
+impl TabularBackend {
+    /// Create an empty TabularBackend.
+    pub fn new() -> Self {
+        TabularBackend {
+            tables: HashMap::new(),
+            fluid_ids: Vec::new(),
+        }
+    }
+
+    /// Creates a new TabularBackend with critical point damping enabled.
+    ///
+    /// This wraps the backend with a `DampedBackend` to apply C1-continuous
+    /// damping to derivative properties (Cp, Cv, etc.) near the critical point.
+    pub fn with_damping() -> DampedBackend<TabularBackend> {
+        DampedBackend::new(Self::new())
+    }
+
+    /// Load a fluid table from a JSON file and register it.
+    pub fn load_table(&mut self, path: &Path) -> FluidResult<()> {
+        let table = FluidTable::load_from_path(path)?;
+        let id = table.fluid_id.clone();
+        if !self.fluid_ids.contains(&id) {
+            self.fluid_ids.push(id.clone());
+        }
+        self.tables.insert(id, table);
+        Ok(())
+    }
+
+    /// Load a fluid table from a JSON string (for embedded/test data).
+    pub fn load_table_from_str(&mut self, json: &str) -> FluidResult<()> {
+        let table = FluidTable::load_from_str(json)?;
+        let id = table.fluid_id.clone();
+        if !self.fluid_ids.contains(&id) {
+            self.fluid_ids.push(id.clone());
+        }
+        self.tables.insert(id, table);
+        Ok(())
+    }
+
+    /// Get a reference to a fluid table. Returns None if not loaded.
+    #[inline]
+    fn get_table(&self, fluid: &FluidId) -> Option<&FluidTable> {
+        self.tables.get(&fluid.0)
+    }
+
+    /// Resolve FluidState to (p, t) in Pascals and Kelvin.
+    /// For (P,x) uses saturation temperature at P.
+    fn resolve_state(&self, fluid: &FluidId, state: FluidState) -> FluidResult<(f64, f64)> {
+        match state {
+            FluidState::PressureTemperature(p, t) => Ok((p.to_pascals(), t.to_kelvin())),
+            FluidState::PressureEnthalpy(p, h) => {
+                let table = self.get_table(fluid).ok_or(FluidError::UnknownFluid {
+                    fluid: fluid.0.clone(),
+                })?;
+                self.find_t_from_ph(table, p.to_pascals(), h.to_joules_per_kg())
+            }
+            FluidState::PressureQuality(p, _x) => {
+                let table = self.get_table(fluid).ok_or(FluidError::UnknownFluid {
+                    fluid: fluid.0.clone(),
+                })?;
+                if let Some(ref sat) = table.saturation {
+                    let (t_sat, _, _, _, _, _, _) =
+                        sat.at_pressure(p.to_pascals())
+                            .ok_or(FluidError::OutOfBounds {
+                                fluid: fluid.0.clone(),
+                                p: p.to_pascals(),
+                                t: 0.0,
+                            })?;
+                    Ok((p.to_pascals(), t_sat))
+                } else {
+                    Err(FluidError::InvalidState {
+                        reason: "Two-phase (P,x) requires saturation table".to_string(),
+                    })
+                }
+            }
+            FluidState::PressureEntropy(_, _) => Err(FluidError::InvalidState {
+                reason: "TabularBackend does not yet support (P,s) state".to_string(),
+            }),
+            FluidState::PressureTemperatureMixture(_, _, _)
+            | FluidState::PressureEnthalpyMixture(_, _, _)
+            | FluidState::PressureQualityMixture(_, _, _) => {
+                // TabularBackend does not support mixtures - fallback to error
+                Err(FluidError::MixtureNotSupported(
+                    "TabularBackend does not support mixtures. Use CoolPropBackend.".to_string(),
+                ))
+            }
+        }
+    }
+
+    /// Find T given (P, h) using Newton iteration on the enthalpy table.
+    fn find_t_from_ph(&self, table: &FluidTable, p: f64, h_target: f64) -> FluidResult<(f64, f64)> {
+        // Initial guess: use midpoint of T range
+        let t_grid = &table.single_phase.temperature;
+        if t_grid.len() < 2 {
+            return Err(FluidError::InvalidState {
+                reason: "Table too small for (P,h) lookup".to_string(),
+            });
+        }
+
+        let mut t = (t_grid[0] + t_grid[t_grid.len() - 1]) / 2.0;
+        let dt_fd = 0.1; // K, for finite difference
+
+        for _ in 0..20 {
+            let h = table
+                .single_phase
+                .interpolate("enthalpy", p, t, &table.fluid_id)?;
+            let err = h - h_target;
+
+            if err.abs() < 1.0 {
+                return Ok((p, t));
+            }
+
+            let h_plus = table
+                .single_phase
+                .interpolate("enthalpy", p, t + dt_fd, &table.fluid_id)
+                .unwrap_or(h);
+            let dh_dt = (h_plus - h) / dt_fd;
+
+            if dh_dt.abs() < 1e-10 {
+                return Err(FluidError::NumericalError(
+                    "Zero dh/dT in (P,h) Newton iteration".to_string(),
+                ));
+            }
+
+            t -= err / dh_dt;
+
+            if t < t_grid[0] || t > t_grid[t_grid.len() - 1] {
+                return Err(FluidError::OutOfBounds {
+                    fluid: table.fluid_id.clone(),
+                    p,
+                    t,
+                });
+            }
+        }
+
+        Err(FluidError::NumericalError(
+            "Newton iteration did not converge for (P,h)".to_string(),
+        ))
+    }
+
+    /// Get property for two-phase (P, x) via linear blend.
+    fn property_two_phase(
+        &self,
+        table: &FluidTable,
+        p: f64,
+        x: f64,
+        property: Property,
+    ) -> FluidResult<f64> {
+        let sat = table.saturation.as_ref().ok_or(FluidError::InvalidState {
+            reason: "Two-phase requires saturation table".to_string(),
+        })?;
+
+        let (t_sat, h_liq, h_vap, rho_liq, rho_vap, s_liq, s_vap) =
+            sat.at_pressure(p).ok_or(FluidError::OutOfBounds {
+                fluid: table.fluid_id.clone(),
+                p,
+                t: 0.0,
+            })?;
+
+        let val = match property {
+            Property::Enthalpy => h_liq * (1.0 - x) + h_vap * x,
+            Property::Density => {
+                let v_liq = 1.0 / rho_liq;
+                let v_vap = 1.0 / rho_vap;
+                let v = v_liq * (1.0 - x) + v_vap * x;
+                1.0 / v
+            }
+            Property::Entropy => s_liq * (1.0 - x) + s_vap * x,
+            Property::Quality => x,
+            Property::Temperature => t_sat,
+            Property::Pressure => p,
+            _ => {
+                return Err(FluidError::UnsupportedProperty {
+                    property: property.to_string(),
+                })
+            }
+        };
+
+        Ok(val)
+    }
+
+    /// Map Property enum to table property name.
+    fn property_table_name(property: Property) -> Option<&'static str> {
+        match property {
+            Property::Density => Some("density"),
+            Property::Enthalpy => Some("enthalpy"),
+            Property::Entropy => Some("entropy"),
+            Property::Cp => Some("cp"),
+            Property::Cv => Some("cv"),
+            Property::Temperature => Some("temperature"),
+            Property::Pressure => Some("pressure"),
+            _ => None,
+        }
+    }
+}
+
+impl Default for TabularBackend {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::backend::FluidBackend;
+    use approx::assert_relative_eq;
+    use entropyk_core::{Pressure, Temperature};
+
+    fn make_test_backend() -> TabularBackend {
+        let mut backend = TabularBackend::new();
+        let path = std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("data/r134a.json");
+        backend.load_table(&path).unwrap();
+        backend
+    }
+
+    #[test]
+    fn test_tabular_load_r134a() {
+        let backend = make_test_backend();
+        assert!(backend.is_fluid_available(&FluidId::new("R134a")));
+        assert!(!backend.is_fluid_available(&FluidId::new("R999")));
+    }
+
+    #[test]
+    fn test_tabular_property_pt() {
+        let backend = make_test_backend();
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+
+        let density = backend
+            .property(FluidId::new("R134a"), Property::Density, state.clone())
+            .unwrap();
+        assert!(density > 1.0 && density < 100.0);
+
+        let enthalpy = backend
+            .property(FluidId::new("R134a"), Property::Enthalpy, state)
+            .unwrap();
+        assert!(enthalpy > 300_000.0 && enthalpy < 500_000.0);
+    }
+
+    /// Accuracy: at grid point (200 kPa, 290 K), density must match table exactly.
+    #[test]
+    fn test_tabular_accuracy_at_grid_point() {
+        let backend = make_test_backend();
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(200_000.0),
+            Temperature::from_kelvin(290.0),
+        );
+        let density = backend
+            .property(FluidId::new("R134a"), Property::Density, state)
+            .unwrap();
+        assert_relative_eq!(density, 9.0, epsilon = 1e-10);
+    }
+
+    /// Accuracy: interpolated value within 1% (table self-consistency check).
+    #[test]
+    fn test_tabular_accuracy_interpolated() {
+        let backend = make_test_backend();
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(200_000.0),
+            Temperature::from_kelvin(300.0),
+        );
+        let density = backend
+            .property(FluidId::new("R134a"), Property::Density, state)
+            .unwrap();
+        assert_relative_eq!(density, 8.415, epsilon = 0.01);
+    }
+
+    #[test]
+    fn test_tabular_critical_point() {
+        let backend = make_test_backend();
+        let cp = backend.critical_point(FluidId::new("R134a")).unwrap();
+        assert!((cp.temperature_kelvin() - 374.21).abs() < 1.0);
+        assert!((cp.pressure_pascals() - 4.059e6).abs() < 1e4);
+    }
+
+    #[test]
+    fn test_tabular_list_fluids() {
+        let backend = make_test_backend();
+        let fluids = backend.list_fluids();
+        assert_eq!(fluids.len(), 1);
+        assert_eq!(fluids[0].0, "R134a");
+    }
+
+    #[test]
+    fn test_tabular_unknown_fluid() {
+        let backend = make_test_backend();
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        let result = backend.property(FluidId::new("R999"), Property::Density, state);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_tabular_out_of_bounds() {
+        let backend = make_test_backend();
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(50_000.0),
+            Temperature::from_kelvin(200.0),
+        );
+        let result = backend.property(FluidId::new("R134a"), Property::Density, state);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_tabular_ph_state() {
+        let backend = make_test_backend();
+        let state = FluidState::from_ph(
+            Pressure::from_bar(1.0),
+            entropyk_core::Enthalpy::from_kilojoules_per_kg(415.0),
+        );
+        let density = backend
+            .property(FluidId::new("R134a"), Property::Density, state)
+            .unwrap();
+        assert!(density > 1.0);
+    }
+
+    #[test]
+    fn test_tabular_px_state() {
+        let backend = make_test_backend();
+        let state = FluidState::from_px(
+            Pressure::from_pascals(500_000.0),
+            crate::types::Quality::new(0.5),
+        );
+        let enthalpy = backend
+            .property(FluidId::new("R134a"), Property::Enthalpy, state)
+            .unwrap();
+        assert!(enthalpy > 300_000.0 && enthalpy < 450_000.0);
+    }
+
+    #[test]
+    fn test_tabular_benchmark_10k_queries() {
+        let backend = make_test_backend();
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        let start = std::time::Instant::now();
+        for _ in 0..10_000 {
+            let _ = backend.property(FluidId::new("R134a"), Property::Density, state.clone());
+        }
+        let elapsed = start.elapsed();
+        assert!(
+            elapsed.as_millis() < 100,
+            "10k queries took {}ms, expected < 100ms (debug mode)",
+            elapsed.as_millis()
+        );
+    }
+
+    /// Release build: 10k queries must complete in < 10ms (AC #3).
+    #[test]
+    #[cfg_attr(debug_assertions, ignore = "run with cargo test --release")]
+    fn test_tabular_benchmark_10k_queries_release() {
+        let backend = make_test_backend();
+        let state = FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        let start = std::time::Instant::now();
+        for _ in 0..10_000 {
+            let _ = backend.property(FluidId::new("R134a"), Property::Density, state.clone());
+        }
+        let elapsed = start.elapsed();
+        assert!(
+            elapsed.as_millis() < 10,
+            "10k queries took {}ms, expected < 10ms in release",
+            elapsed.as_millis()
+        );
+    }
+
+    /// Compare TabularBackend vs CoolPropBackend. Embedded r134a.json may be from
+    /// reference data; use epsilon 1% for compatibility. CoolProp-generated tables
+    /// achieve < 0.01% (validated in generator::test_generated_table_vs_coolprop_spot_checks).
+    #[test]
+    #[cfg(feature = "coolprop")]
+    fn test_tabular_vs_coolprop_accuracy() {
+        use crate::coolprop::CoolPropBackend;
+        use crate::types::Quality;
+
+        let mut tabular = TabularBackend::new();
+        let path = std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("data/r134a.json");
+        tabular.load_table(&path).unwrap();
+
+        let coolprop = CoolPropBackend::new();
+        let fluid = FluidId::new("R134a");
+
+        // (P, T) at 1 bar, 25°C
+        let state_pt =
+            FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        let rho_t = tabular
+            .property(fluid.clone(), Property::Density, state_pt)
+            .unwrap();
+        let rho_c = coolprop
+            .property(fluid.clone(), Property::Density, state_pt)
+            .unwrap();
+        assert_relative_eq!(rho_t, rho_c, epsilon = 0.01 * rho_c.max(1.0));
+
+        let h_t = tabular
+            .property(fluid.clone(), Property::Enthalpy, state_pt)
+            .unwrap();
+        let h_c = coolprop
+            .property(fluid.clone(), Property::Enthalpy, state_pt)
+            .unwrap();
+        assert_relative_eq!(h_t, h_c, epsilon = 0.01 * h_c.max(1.0));
+
+        // (P, h) at 1 bar, h ≈ 415 kJ/kg
+        let state_ph = FluidState::from_ph(
+            Pressure::from_bar(1.0),
+            entropyk_core::Enthalpy::from_kilojoules_per_kg(415.0),
+        );
+        let rho_t_ph = tabular
+            .property(fluid.clone(), Property::Density, state_ph)
+            .unwrap();
+        let rho_c_ph = coolprop
+            .property(fluid.clone(), Property::Density, state_ph)
+            .unwrap();
+        assert_relative_eq!(rho_t_ph, rho_c_ph, epsilon = 0.01 * rho_c_ph.max(1.0));
+
+        // (P, x) at 500 kPa, x = 0.5
+        let state_px = FluidState::from_px(Pressure::from_pascals(500_000.0), Quality::new(0.5));
+        let h_t_px = tabular
+            .property(fluid.clone(), Property::Enthalpy, state_px)
+            .unwrap();
+        let h_c_px = coolprop
+            .property(fluid.clone(), Property::Enthalpy, state_px)
+            .unwrap();
+        assert_relative_eq!(h_t_px, h_c_px, epsilon = 0.01 * h_c_px.max(1.0));
+    }
+}
+
+impl FluidBackend for TabularBackend {
+    fn property(&self, fluid: FluidId, property: Property, state: FluidState) -> FluidResult<f64> {
+        let table = self.get_table(&fluid).ok_or(FluidError::UnknownFluid {
+            fluid: fluid.0.clone(),
+        })?;
+
+        // Handle (P, x) two-phase explicitly
+        if let FluidState::PressureQuality(p, x) = state {
+            return self.property_two_phase(table, p.to_pascals(), x.value(), property);
+        }
+
+        let (p, t) = self.resolve_state(&fluid, state)?;
+
+        // Temperature and Pressure are direct
+        if property == Property::Temperature {
+            return Ok(t);
+        }
+        if property == Property::Pressure {
+            return Ok(p);
+        }
+
+        let name = Self::property_table_name(property).ok_or(FluidError::UnsupportedProperty {
+            property: property.to_string(),
+        })?;
+
+        table.single_phase.interpolate(name, p, t, &fluid.0)
+    }
+
+    fn critical_point(&self, fluid: FluidId) -> FluidResult<CriticalPoint> {
+        let table = self.get_table(&fluid).ok_or(FluidError::NoCriticalPoint {
+            fluid: fluid.0.clone(),
+        })?;
+
+        let cp = &table.critical_point;
+        Ok(CriticalPoint::new(cp.temperature, cp.pressure, cp.density))
+    }
+
+    fn is_fluid_available(&self, fluid: &FluidId) -> bool {
+        self.tables.contains_key(&fluid.0)
+    }
+
+    fn phase(&self, fluid: FluidId, state: FluidState) -> FluidResult<Phase> {
+        let table = self.get_table(&fluid).ok_or(FluidError::UnknownFluid {
+            fluid: fluid.0.clone(),
+        })?;
+
+        let (p, t) = self.resolve_state(&fluid, state.clone())?;
+        let pc = table.critical_point.pressure.to_pascals();
+        let tc = table.critical_point.temperature.to_kelvin();
+
+        if p > pc && t > tc {
+            return Ok(Phase::Supercritical);
+        }
+
+        if let Some(ref sat) = table.saturation {
+            if let Some((_, h_liq, h_vap, _, _, _, _)) = sat.at_pressure(p) {
+                if let FluidState::PressureEnthalpy(_, h) = state {
+                    let hv = h.to_joules_per_kg();
+                    if hv <= h_liq {
+                        return Ok(Phase::Liquid);
+                    }
+                    if hv >= h_vap {
+                        return Ok(Phase::Vapor);
+                    }
+                    return Ok(Phase::TwoPhase);
+                }
+                if let FluidState::PressureQuality(_, x) = state {
+                    if x.value() <= 0.0 {
+                        return Ok(Phase::Liquid);
+                    }
+                    if x.value() >= 1.0 {
+                        return Ok(Phase::Vapor);
+                    }
+                    return Ok(Phase::TwoPhase);
+                }
+            }
+        }
+
+        Ok(Phase::Unknown)
+    }
+
+    fn list_fluids(&self) -> Vec<FluidId> {
+        self.fluid_ids
+            .iter()
+            .map(|s| FluidId::new(s.clone()))
+            .collect()
+    }
+
+    fn full_state(&self, fluid: FluidId, p: entropyk_core::Pressure, h: entropyk_core::Enthalpy) -> FluidResult<crate::types::ThermoState> {
+        let t_k = self.property(fluid.clone(), Property::Temperature, FluidState::from_ph(p, h))?;
+        Err(FluidError::UnsupportedProperty {
+            property: format!("full_state for TabularBackend: Temperature is {:.2} K", t_k),
+        })
+    }
+}

crates/fluids/src/test_backend.rs

@@ -0,0 +1,430 @@
+//! Test backend implementation for unit testing.
+//!
+//! This module provides a mock backend that returns simplified/idealized
+//! property values for testing without requiring external dependencies
+//! like CoolProp.
+
+use crate::backend::FluidBackend;
+use crate::errors::{FluidError, FluidResult};
+#[cfg(test)]
+use crate::mixture::Mixture;
+use crate::types::{CriticalPoint, FluidId, Phase, Property, FluidState};
+use entropyk_core::{Pressure, Temperature};
+use std::collections::HashMap;
+
+/// Test backend for unit testing.
+///
+/// This backend provides simplified thermodynamic property calculations
+/// suitable for testing without external dependencies. Values are idealized
+/// approximations and should NOT be used for real simulations.
+pub struct TestBackend {
+    /// Map of fluid names to critical points
+    critical_points: HashMap<String, CriticalPoint>,
+    /// List of available test fluids
+    available_fluids: Vec<String>,
+}
+
+impl TestBackend {
+    /// Creates a new TestBackend with default test fluids.
+    pub fn new() -> Self {
+        let mut critical_points = HashMap::new();
+
+        // CO2 (R744)
+        critical_points.insert(
+            "CO2".to_string(),
+            CriticalPoint::new(
+                Temperature::from_kelvin(304.13),
+                Pressure::from_pascals(7.3773e6),
+                467.0,
+            ),
+        );
+
+        // R134a
+        critical_points.insert(
+            "R134a".to_string(),
+            CriticalPoint::new(
+                Temperature::from_kelvin(374.21),
+                Pressure::from_pascals(4.059e6),
+                512.0,
+            ),
+        );
+
+        // R410A
+        critical_points.insert(
+            "R410A".to_string(),
+            CriticalPoint::new(
+                Temperature::from_kelvin(344.49),
+                Pressure::from_pascals(4.926e6),
+                458.0,
+            ),
+        );
+
+        // R32
+        critical_points.insert(
+            "R32".to_string(),
+            CriticalPoint::new(
+                Temperature::from_kelvin(351.25),
+                Pressure::from_pascals(5.782e6),
+                360.0,
+            ),
+        );
+
+        // Water
+        critical_points.insert(
+            "Water".to_string(),
+            CriticalPoint::new(
+                Temperature::from_kelvin(647.096),
+                Pressure::from_pascals(22.064e6),
+                322.0,
+            ),
+        );
+
+        let available_fluids = vec![
+            "CO2".to_string(),
+            "R134a".to_string(),
+            "R410A".to_string(),
+            "R32".to_string(),
+            "Water".to_string(),
+            "Nitrogen".to_string(),
+            "Oxygen".to_string(),
+            "Air".to_string(),
+        ];
+
+        TestBackend {
+            critical_points,
+            available_fluids,
+        }
+    }
+
+    /// Simplified ideal gas property calculation.
+    fn ideal_property(
+        &self,
+        fluid: &str,
+        property: Property,
+        state: FluidState,
+    ) -> FluidResult<f64> {
+        // Simple ideal gas approximations for testing
+        // Real implementation would use proper equations of state
+        match fluid {
+            "Nitrogen" | "Oxygen" | "Air" => self.ideal_gas_property(property, state, 29.0),
+            "Water" => self.water_property(property, state),
+            _ => {
+                // For refrigerants, use simplified correlations
+                self.refrigerant_property(fluid, property, state)
+            }
+        }
+    }
+
+    fn ideal_gas_property(
+        &self,
+        property: Property,
+        state: FluidState,
+        _molar_mass: f64,
+    ) -> FluidResult<f64> {
+        let (p, t) = match state {
+            FluidState::PressureTemperature(p, t) => (p.to_pascals(), t.to_kelvin()),
+            _ => {
+                return Err(FluidError::InvalidState {
+                    reason: "TestBackend only supports P-T state for ideal gases".to_string(),
+                })
+            }
+        };
+
+        // Simplified ideal gas: R = 8.314 J/(mol·K), approximate
+        let r_specific = 287.0; // J/(kg·K) for air
+
+        match property {
+            Property::Density => Ok(p / (r_specific * t)),
+            Property::Enthalpy => Ok(1005.0 * t), // Cp * T, Cp ≈ 1005 J/(kg·K) for air
+            Property::Entropy => Ok(r_specific * t.ln()), // Simplified
+            Property::Cp => Ok(1005.0),           // Constant pressure specific heat
+            Property::Cv => Ok(718.0),            // Constant volume specific heat
+            Property::Temperature => Ok(t),
+            Property::Pressure => Ok(p),
+            Property::ThermalConductivity => Ok(0.025), // W/(m·K) for air
+            Property::Viscosity => Ok(1.8e-5),          // Pa·s for air
+            _ => Err(FluidError::UnsupportedProperty {
+                property: property.to_string(),
+            }),
+        }
+    }
+
+    fn water_property(&self, property: Property, state: FluidState) -> FluidResult<f64> {
+        let (p, t) = match state {
+            FluidState::PressureTemperature(p, t) => (p.to_pascals(), t.to_kelvin()),
+            _ => {
+                return Err(FluidError::InvalidState {
+                    reason: "TestBackend only supports P-T state for water".to_string(),
+                })
+            }
+        };
+
+        // Simplified water properties at ~1 atm
+        if p < 1.1e5 && t > 273.15 && t < 373.15 {
+            match property {
+                Property::Density => Ok(1000.0),                 // kg/m³
+                Property::Enthalpy => Ok(4200.0 * (t - 273.15)), // Cp * ΔT
+                Property::Cp => Ok(4184.0),                      // J/(kg·K)
+                Property::ThermalConductivity => Ok(0.6),        // W/(m·K)
+                Property::Viscosity => Ok(0.001),                // Pa·s
+                Property::Temperature => Ok(t),
+                Property::Pressure => Ok(p),
+                _ => Err(FluidError::UnsupportedProperty {
+                    property: property.to_string(),
+                }),
+            }
+        } else {
+            Err(FluidError::InvalidState {
+                reason: "Water property only valid in liquid region".to_string(),
+            })
+        }
+    }
+
+    fn refrigerant_property(
+        &self,
+        _fluid: &str,
+        property: Property,
+        state: FluidState,
+    ) -> FluidResult<f64> {
+        let (p, t) = match state {
+            FluidState::PressureTemperature(p, t) => (p.to_pascals(), t.to_kelvin()),
+            _ => {
+                return Err(FluidError::InvalidState {
+                    reason: "TestBackend only supports P-T state for refrigerants".to_string(),
+                })
+            }
+        };
+
+        // Simplified refrigerant properties
+        match property {
+            Property::Density => {
+                // Rough approximation for liquid (~1000 kg/m³) vs vapor (~10-50 kg/m³)
+                if p > 1e6 {
+                    Ok(1000.0) // Liquid
+                } else {
+                    Ok(30.0) // Vapor
+                }
+            }
+            Property::Enthalpy => {
+                if p > 1e6 {
+                    Ok(200000.0) // Liquid region
+                } else {
+                    Ok(400000.0) // Vapor region
+                }
+            }
+            Property::Cp => Ok(1500.0), // Approximate
+            Property::Temperature => Ok(t),
+            Property::Pressure => Ok(p),
+            Property::ThermalConductivity => Ok(0.015),
+            Property::Viscosity => Ok(1.5e-5),
+            _ => Err(FluidError::UnsupportedProperty {
+                property: property.to_string(),
+            }),
+        }
+    }
+
+    fn determine_phase(&self, fluid: &str, state: FluidState) -> Phase {
+        let (p, t) = match state {
+            FluidState::PressureTemperature(p, t) => (p.to_pascals(), t.to_kelvin()),
+            _ => return Phase::Unknown,
+        };
+
+        // Get critical point if available
+        if let Some(cp) = self.critical_points.get(fluid) {
+            let pc = cp.pressure_pascals();
+            let tc = cp.temperature_kelvin();
+
+            if p > pc && t > tc {
+                return Phase::Supercritical;
+            }
+            if (p - pc).abs() / pc < 0.05 || (t - tc).abs() / tc < 0.05 {
+                return Phase::Supercritical;
+            }
+        }
+
+        // Simplified phase determination
+        if p > 5e5 {
+            Phase::Liquid
+        } else {
+            Phase::Vapor
+        }
+    }
+}
+
+impl Default for TestBackend {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl FluidBackend for TestBackend {
+    fn property(&self, fluid: FluidId, property: Property, state: FluidState) -> FluidResult<f64> {
+        if !self.is_fluid_available(&fluid) {
+            return Err(FluidError::UnknownFluid {
+                fluid: fluid.0.clone(),
+            });
+        }
+
+        self.ideal_property(&fluid.0, property, state)
+    }
+
+    fn critical_point(&self, fluid: FluidId) -> FluidResult<CriticalPoint> {
+        self.critical_points
+            .get(&fluid.0)
+            .copied()
+            .ok_or(FluidError::NoCriticalPoint { fluid: fluid.0 })
+    }
+
+    fn is_fluid_available(&self, fluid: &FluidId) -> bool {
+        self.available_fluids.iter().any(|f| f == &fluid.0)
+    }
+
+    fn phase(&self, fluid: FluidId, state: FluidState) -> FluidResult<Phase> {
+        if !self.is_fluid_available(&fluid) {
+            return Err(FluidError::UnknownFluid { fluid: fluid.0 });
+        }
+
+        Ok(self.determine_phase(&fluid.0, state))
+    }
+
+    fn list_fluids(&self) -> Vec<FluidId> {
+        self.available_fluids
+            .iter()
+            .map(|s| FluidId::new(s.clone()))
+            .collect()
+    }
+
+    fn full_state(&self, fluid: FluidId, p: entropyk_core::Pressure, h: entropyk_core::Enthalpy) -> FluidResult<crate::types::ThermoState> {
+        let t_k = self.property(fluid.clone(), Property::Temperature, FluidState::from_ph(p, h))?;
+        Err(FluidError::UnsupportedProperty {
+            property: format!("full_state for TestBackend: Temperature is {:.2} K", t_k),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_backend_available_fluids() {
+        let backend = TestBackend::new();
+
+        assert!(backend.is_fluid_available(&FluidId::new("CO2")));
+        assert!(backend.is_fluid_available(&FluidId::new("R134a")));
+        assert!(!backend.is_fluid_available(&FluidId::new("R999")));
+    }
+
+    #[test]
+    fn test_list_fluids() {
+        let backend = TestBackend::new();
+        let fluids = backend.list_fluids();
+
+        assert!(fluids.len() > 0);
+        assert!(fluids.iter().any(|f| f.0 == "CO2"));
+    }
+
+    #[test]
+    fn test_critical_point() {
+        let backend = TestBackend::new();
+
+        let cp = backend.critical_point(FluidId::new("CO2")).unwrap();
+        assert!((cp.temperature_kelvin() - 304.13).abs() < 0.1);
+        assert!((cp.pressure_pascals() - 7.3773e6).abs() < 1e4);
+    }
+
+    #[test]
+    fn test_critical_point_not_available() {
+        let backend = TestBackend::new();
+
+        let result = backend.critical_point(FluidId::new("UnknownFluid"));
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_property_nitrogen() {
+        let backend = TestBackend::new();
+
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(101325.0),
+            Temperature::from_kelvin(300.0),
+        );
+
+        let density = backend
+            .property(FluidId::new("Nitrogen"), Property::Density, state)
+            .unwrap();
+
+        assert!(density > 1.0); // Should be ~1.16 kg/m³
+    }
+
+    #[test]
+    fn test_property_water() {
+        let backend = TestBackend::new();
+
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(101325.0),
+            Temperature::from_celsius(25.0),
+        );
+
+        let density = backend
+            .property(FluidId::new("Water"), Property::Density, state)
+            .unwrap();
+
+        assert!((density - 1000.0).abs() < 1.0);
+    }
+
+    #[test]
+    fn test_property_unknown_fluid() {
+        let backend = TestBackend::new();
+
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(101325.0),
+            Temperature::from_kelvin(300.0),
+        );
+
+        let result = backend.property(FluidId::new("R999"), Property::Density, state);
+
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_phase_co2_supercritical() {
+        let backend = TestBackend::new();
+
+        // Above critical point
+        let state =
+            FluidState::from_pt(Pressure::from_pascals(8e6), Temperature::from_kelvin(320.0));
+
+        let phase = backend.phase(FluidId::new("CO2"), state).unwrap();
+        assert_eq!(phase, Phase::Supercritical);
+    }
+
+    #[test]
+    fn test_phase_liquid() {
+        let backend = TestBackend::new();
+
+        let state = FluidState::from_pt(
+            Pressure::from_pascals(10e5),
+            Temperature::from_celsius(25.0),
+        );
+
+        let phase = backend.phase(FluidId::new("Water"), state).unwrap();
+        assert_eq!(phase, Phase::Liquid);
+    }
+
+    #[test]
+    fn test_thermo_state_is_mixture() {
+        let mix = Mixture::from_mass_fractions(&[("R32", 0.5), ("R1234yf", 0.5)]).unwrap();
+
+        let state_pure =
+            FluidState::from_pt(Pressure::from_bar(1.0), Temperature::from_celsius(25.0));
+        assert!(!state_pure.is_mixture());
+
+        let state_mix = FluidState::from_pt_mix(
+            Pressure::from_bar(1.0),
+            Temperature::from_celsius(25.0),
+            mix,
+        );
+        assert!(state_mix.is_mixture());
+    }
+}

crates/fluids/src/types.rs

@@ -0,0 +1,369 @@
+//! Types for fluid property calculations.
+//!
+//! This module defines the core types used to represent thermodynamic states,
+//! fluid identifiers, and properties in the fluid backend system.
+
+use crate::mixture::Mixture;
+use entropyk_core::{Enthalpy, Pressure, Temperature};
+use std::fmt;
+
+/// Difference between two temperatures in Kelvin.
+#[derive(Debug, Clone, Copy, PartialEq, PartialOrd)]
+pub struct TemperatureDelta(pub f64);
+
+impl TemperatureDelta {
+    /// Creates a new TemperatureDelta from a difference in Kelvin.
+    pub fn new(kelvin_diff: f64) -> Self {
+        TemperatureDelta(kelvin_diff)
+    }
+    
+    /// Gets the temperature difference in Kelvin.
+    pub fn kelvin(&self) -> f64 {
+        self.0
+    }
+}
+
+impl From<f64> for TemperatureDelta {
+    fn from(val: f64) -> Self {
+        TemperatureDelta(val)
+    }
+}
+
+/// Unique identifier for a fluid.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct FluidId(pub String);
+
+impl FluidId {
+    /// Creates a new FluidId from a string.
+    pub fn new(name: impl Into<String>) -> Self {
+        FluidId(name.into())
+    }
+}
+
+impl fmt::Display for FluidId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+impl From<&str> for FluidId {
+    fn from(s: &str) -> Self {
+        FluidId(s.to_string())
+    }
+}
+
+impl From<String> for FluidId {
+    fn from(s: String) -> Self {
+        FluidId(s)
+    }
+}
+
+/// Thermodynamic property that can be queried from a backend.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum Property {
+    /// Density (kg/m³)
+    Density,
+    /// Specific enthalpy (J/kg)
+    Enthalpy,
+    /// Specific entropy (J/(kg·K))
+    Entropy,
+    /// Specific internal energy (J/kg)
+    InternalEnergy,
+    /// Specific heat capacity at constant pressure (J/(kg·K))
+    Cp,
+    /// Specific heat capacity at constant volume (J/(kg·K))
+    Cv,
+    /// Speed of sound (m/s)
+    SpeedOfSound,
+    /// Dynamic viscosity (Pa·s)
+    Viscosity,
+    /// Thermal conductivity (W/(m·K))
+    ThermalConductivity,
+    /// Surface tension (N/m)
+    SurfaceTension,
+    /// Quality (0-1 for two-phase)
+    Quality,
+    /// Temperature (K)
+    Temperature,
+    /// Pressure (Pa)
+    Pressure,
+}
+
+impl fmt::Display for Property {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Property::Density => write!(f, "Density"),
+            Property::Enthalpy => write!(f, "Enthalpy"),
+            Property::Entropy => write!(f, "Entropy"),
+            Property::InternalEnergy => write!(f, "InternalEnergy"),
+            Property::Cp => write!(f, "Cp"),
+            Property::Cv => write!(f, "Cv"),
+            Property::SpeedOfSound => write!(f, "SpeedOfSound"),
+            Property::Viscosity => write!(f, "Viscosity"),
+            Property::ThermalConductivity => write!(f, "ThermalConductivity"),
+            Property::SurfaceTension => write!(f, "SurfaceTension"),
+            Property::Quality => write!(f, "Quality"),
+            Property::Temperature => write!(f, "Temperature"),
+            Property::Pressure => write!(f, "Pressure"),
+        }
+    }
+}
+
+/// Input specification for thermodynamic state calculation.
+///
+/// Defines what inputs are available to look up a thermodynamic property.
+#[derive(Debug, Clone, PartialEq)]
+pub enum FluidState {
+    /// Pressure and temperature (P, T) - most common for single-phase
+    PressureTemperature(Pressure, Temperature),
+    /// Pressure and enthalpy (P, h) - common for expansion/compression
+    PressureEnthalpy(Pressure, Enthalpy),
+    /// Pressure and entropy (P, s) - useful for isentropic processes
+    PressureEntropy(Pressure, Entropy),
+    /// Pressure and quality (P, x) - for two-phase regions
+    PressureQuality(Pressure, Quality),
+    /// Pressure and temperature with mixture (P, T, mixture)
+    PressureTemperatureMixture(Pressure, Temperature, Mixture),
+    /// Pressure and enthalpy with mixture (P, h, mixture) - preferred for two-phase
+    PressureEnthalpyMixture(Pressure, Enthalpy, Mixture),
+    /// Pressure and quality with mixture (P, x, mixture) - for two-phase mixtures
+    PressureQualityMixture(Pressure, Quality, Mixture),
+}
+
+impl FluidState {
+    /// Creates a state from pressure and temperature.
+    pub fn from_pt(p: Pressure, t: Temperature) -> Self {
+        FluidState::PressureTemperature(p, t)
+    }
+
+    /// Creates a state from pressure and enthalpy.
+    pub fn from_ph(p: Pressure, h: Enthalpy) -> Self {
+        FluidState::PressureEnthalpy(p, h)
+    }
+
+    /// Creates a state from pressure and entropy.
+    pub fn from_ps(p: Pressure, s: Entropy) -> Self {
+        FluidState::PressureEntropy(p, s)
+    }
+
+    /// Creates a state from pressure and quality.
+    pub fn from_px(p: Pressure, x: Quality) -> Self {
+        FluidState::PressureQuality(p, x)
+    }
+
+    /// Creates a state from pressure, temperature, and mixture.
+    pub fn from_pt_mix(p: Pressure, t: Temperature, mix: Mixture) -> Self {
+        FluidState::PressureTemperatureMixture(p, t, mix)
+    }
+
+    /// Creates a state from pressure, enthalpy, and mixture (preferred for two-phase).
+    pub fn from_ph_mix(p: Pressure, h: Enthalpy, mix: Mixture) -> Self {
+        FluidState::PressureEnthalpyMixture(p, h, mix)
+    }
+
+    /// Creates a state from pressure, quality, and mixture.
+    pub fn from_px_mix(p: Pressure, x: Quality, mix: Mixture) -> Self {
+        FluidState::PressureQualityMixture(p, x, mix)
+    }
+
+    /// Check if this state contains a mixture.
+    pub fn is_mixture(&self) -> bool {
+        matches!(
+            self,
+            FluidState::PressureTemperatureMixture(_, _, _)
+                | FluidState::PressureEnthalpyMixture(_, _, _)
+                | FluidState::PressureQualityMixture(_, _, _)
+        )
+    }
+}
+
+/// Entropy in J/(kg·K).
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Entropy(pub f64);
+
+impl Entropy {
+    /// Creates entropy from J/(kg·K).
+    pub fn from_joules_per_kg_kelvin(value: f64) -> Self {
+        Entropy(value)
+    }
+
+    /// Returns entropy in J/(kg·K).
+    pub fn to_joules_per_kg_kelvin(&self) -> f64 {
+        self.0
+    }
+}
+
+impl From<f64> for Entropy {
+    fn from(value: f64) -> Self {
+        Entropy(value)
+    }
+}
+
+/// Quality (vapor fraction) from 0 (saturated liquid) to 1 (saturated vapor).
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Quality(pub f64);
+
+impl Quality {
+    /// Creates a quality value (0-1).
+    pub fn new(value: f64) -> Self {
+        Quality(value.clamp(0.0, 1.0))
+    }
+
+    /// Returns the quality value (0-1).
+    pub fn value(&self) -> f64 {
+        self.0
+    }
+}
+
+impl From<f64> for Quality {
+    fn from(value: f64) -> Self {
+        Quality::new(value)
+    }
+}
+
+/// Critical point data for a fluid.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct CriticalPoint {
+    /// Critical temperature in Kelvin.
+    pub temperature: Temperature,
+    /// Critical pressure in Pascals.
+    pub pressure: Pressure,
+    /// Critical density in kg/m³.
+    pub density: f64,
+}
+
+impl CriticalPoint {
+    /// Creates a new CriticalPoint.
+    pub fn new(temperature: Temperature, pressure: Pressure, density: f64) -> Self {
+        CriticalPoint {
+            temperature,
+            pressure,
+            density,
+        }
+    }
+
+    /// Returns critical temperature in Kelvin.
+    pub fn temperature_kelvin(&self) -> f64 {
+        self.temperature.to_kelvin()
+    }
+
+    /// Returns critical pressure in Pascals.
+    pub fn pressure_pascals(&self) -> f64 {
+        self.pressure.to_pascals()
+    }
+}
+
+/// Phase of matter.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Phase {
+    /// Liquid phase.
+    Liquid,
+    /// Vapor/gas phase.
+    Vapor,
+    /// Two-phase region.
+    TwoPhase,
+    /// Supercritical fluid.
+    Supercritical,
+    /// Unknown or undefined phase.
+    Unknown,
+}
+
+impl fmt::Display for Phase {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Phase::Liquid => write!(f, "Liquid"),
+            Phase::Vapor => write!(f, "Vapor"),
+            Phase::TwoPhase => write!(f, "TwoPhase"),
+            Phase::Supercritical => write!(f, "Supercritical"),
+            Phase::Unknown => write!(f, "Unknown"),
+        }
+    }
+}
+
+/// Comprehensive representation of a thermodynamic state.
+///
+/// This struct holds a complete snapshot of all relevant properties for a fluid
+/// at a given state. It avoids the need to repeatedly query the backend for
+/// individual properties once the state is resolved.
+#[derive(Debug, Clone, PartialEq)]
+pub struct ThermoState {
+    /// Fluid identifier (e.g. "R410A")
+    pub fluid: FluidId,
+    /// Absolute pressure
+    pub pressure: Pressure,
+    /// Absolute temperature
+    pub temperature: Temperature,
+    /// Specific enthalpy
+    pub enthalpy: Enthalpy,
+    /// Specific entropy
+    pub entropy: Entropy,
+    /// Density in kg/m³
+    pub density: f64,
+    /// Physical phase of the fluid
+    pub phase: Phase,
+    /// Vapor quality (0.0 to 1.0) if in two-phase region
+    pub quality: Option<Quality>,
+    /// Superheat (T - T_dew) if in superheated vapor region
+    pub superheat: Option<TemperatureDelta>,
+    /// Subcooling (T_bubble - T) if in subcooled liquid region
+    pub subcooling: Option<TemperatureDelta>,
+    /// Saturated liquid temperature at current pressure (Bubble point)
+    pub t_bubble: Option<Temperature>,
+    /// Saturated vapor temperature at current pressure (Dew point)
+    pub t_dew: Option<Temperature>,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_fluid_id() {
+        let id = FluidId::new("R134a");
+        assert_eq!(id.0, "R134a");
+    }
+
+    #[test]
+    fn test_fluid_state_from_pt() {
+        let p = Pressure::from_bar(1.0);
+        let t = Temperature::from_celsius(25.0);
+        let state = FluidState::from_pt(p, t);
+        match state {
+            FluidState::PressureTemperature(p_out, t_out) => {
+                assert_eq!(p.to_pascals(), p_out.to_pascals());
+                assert_eq!(t.to_kelvin(), t_out.to_kelvin());
+            }
+            _ => panic!("Expected PressureTemperature variant"),
+        }
+    }
+
+    #[test]
+    fn test_quality_bounds() {
+        let q1 = Quality::new(0.5);
+        assert!((q1.value() - 0.5).abs() < 1e-10);
+
+        let q2 = Quality::new(1.5);
+        assert!((q2.value() - 1.0).abs() < 1e-10);
+
+        let q3 = Quality::new(-0.5);
+        assert!((q3.value() - 0.0).abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_critical_point() {
+        // CO2 critical point: Tc = 304.13 K, Pc = 7.3773 MPa
+        let cp = CriticalPoint::new(
+            Temperature::from_kelvin(304.13),
+            Pressure::from_pascals(7.3773e6),
+            467.0,
+        );
+        assert!((cp.temperature_kelvin() - 304.13).abs() < 0.01);
+        assert!((cp.pressure_pascals() - 7.3773e6).abs() < 1.0);
+    }
+
+    #[test]
+    fn test_property_display() {
+        assert_eq!(format!("{}", Property::Density), "Density");
+        assert_eq!(format!("{}", Property::Enthalpy), "Enthalpy");
+    }
+}