243 lines
10 KiB
Markdown
243 lines
10 KiB
Markdown
# Story 2.7: Incompressible Fluids Support
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Status: done
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## Story
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As a HVAC engineer,
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I want water, glycol, and moist air as incompressible fluids,
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so that heat sources/sinks are fast to compute.
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## Acceptance Criteria
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1. **IncompressibleBackend Implementation** (AC: #1)
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- [x] Create `IncompressibleBackend` implementing `FluidBackend` trait
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- [x] Support fluids: Water, EthyleneGlycol, PropyleneGlycol, HumidAir
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- [x] Lightweight polynomial models for density, Cp, enthalpy, viscosity
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2. **Property Accuracy** (AC: #2)
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- [x] Results match reference data (IAPWS-IF97 for water, ASHRAE for glycol) within 0.1%
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- [x] Valid temperature ranges enforced (e.g., water: 273.15K–373.15K liquid phase)
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- [x] Clear error for out-of-range queries
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3. **Performance** (AC: #3)
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- [x] Property queries complete in < 100ns (vs ~100μs for CoolProp EOS)
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- [x] No external library calls—pure Rust polynomial evaluation
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- [x] Zero allocation in property calculation hot path
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4. **Integration** (AC: #4)
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- [x] Works with CachedBackend wrapper for LRU caching
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- [x] FluidId supports incompressible fluid variants
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- [x] ThermoState uses PressureTemperature (pressure ignored for incompressible)
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## Tasks / Subtasks
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- [x] Define incompressible fluid types (AC: #1)
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- [x] Add IncompFluid enum: Water, EthyleneGlycol(f64), PropyleneGlycol(f64), HumidAir
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- [x] Concentration for glycol: 0.0–0.6 mass fraction (via FluidId "EthyleneGlycol30")
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- [x] Implement polynomial models (AC: #2)
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- [x] Water: Simplified polynomial for liquid region (273–373K), ρ/Cp/μ
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- [x] EthyleneGlycol: ASHRAE-style polynomial for Cp, ρ, μ vs T and concentration
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- [x] PropyleneGlycol: Same structure as ethylene glycol
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- [x] HumidAir: Simplified model (constant Cp_air)
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- [x] Create IncompressibleBackend (AC: #1, #3)
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- [x] `crates/fluids/src/incompressible.rs` - IncompressibleBackend struct
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- [x] Implement FluidBackend trait
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- [x] property() dispatches to fluid-specific polynomial evaluator
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- [x] critical_point() returns NoCriticalPoint error
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- [x] Temperature range validation (AC: #2)
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- [x] ValidRange per fluid (min_temp, max_temp)
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- [x] Return InvalidState error if T outside range
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- [x] Integration with existing types (AC: #4)
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- [x] IncompFluid::from_fluid_id() parses FluidId strings
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- [x] IncompressibleBackend compatible with CachedBackend
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- [x] Uses ThermoState::PressureTemperature (pressure ignored)
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- [x] Testing (AC: #1–4)
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- [x] Water properties at 20°C, 50°C, 80°C vs IAPWS-IF97 reference
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- [x] Glycol properties (EthyleneGlycol30 denser than water)
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- [x] Out-of-range temperature handling
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- [x] CachedBackend wrapper integration
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## Dev Notes
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### Previous Story Intelligence
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**From Story 2-6 (Critical Point Damping):**
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- DampedBackend<B> wrapper pattern for backend composition
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- FluidBackend trait requires `Send + Sync`
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- Use `thiserror` for error types
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- Property enum defines queryable properties (Density, Enthalpy, Cp, etc.)
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**From Story 2-4 (LRU Cache):**
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- CachedBackend<B> wraps any FluidBackend
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- IncompressibleBackend will benefit from caching at solver level, not internally
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**From Story 2-3 (Tabular Interpolation):**
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- Fast property lookup via interpolation
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- IncompressibleBackend is even faster—polynomial evaluation only
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**From Story 2-2 (CoolProp Integration):**
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- CoolPropBackend is the reference for compressible fluids
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- IncompressibleBackend handles use cases where CoolProp is overkill
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**From Story 2-1 (Fluid Backend Trait Abstraction):**
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- FluidBackend trait: property(), critical_point(), bubble_point(), dew_point()
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- Incompressible fluids: bubble_point/dew_point return error or None
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- ThermoState variants: PressureTemperature, TemperatureEnthalpy, etc.
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### Architecture Context
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**Fluid Backend Architecture:**
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```
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crates/fluids/src/
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├── backend.rs # FluidBackend trait (unchanged)
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├── incompressible.rs # THIS STORY - IncompressibleBackend
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├── coolprop.rs # CoolPropBackend (compressible)
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├── tabular_backend.rs # TabularBackend (compressible)
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├── cached_backend.rs # Wrapper (works with IncompressibleBackend)
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├── damped_backend.rs # Wrapper for critical point
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└── types.rs # FluidId, Property, ThermoState
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```
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**FluidBackend Trait Methods for Incompressible:**
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- `property()` - Returns polynomial-evaluated value
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- `critical_point()` - Returns error or None (no critical point)
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- `bubble_point()` / `dew_point()` - Returns error (no phase change)
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- `saturation_pressure()` - Returns error (incompressible assumption)
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### Technical Requirements
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**Water (IAPWS-IF97 Simplified):**
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- Temperature range: 273.15 K to 373.15 K (liquid phase)
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- Polynomial fit for ρ(T), Cp(T), μ(T), h(T) = Cp × T
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- Reference: IAPWS-IF97 Region 1 (liquid)
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- Accuracy target: ±0.1% vs IAPWS-IF97
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**Glycols (ASHRAE Polynomials):**
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- Temperature range: 243.15 K to 373.15 K (depends on concentration)
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- Concentration: 0.0 to 0.6 mass fraction
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- Polynomial form: Y = a₀ + a₁T + a₂T² + a₃T³ + c₁X + c₂XT + ...
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- Where Y = property, T = temperature, X = concentration
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- Reference: ASHRAE Handbook - Fundamentals, Chapter 30
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**Humid Air (Psychrometric Simplified):**
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- Temperature range: 233.15 K to 353.15 K
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- Humidity ratio: 0 to 0.03 kg_w/kg_da
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- Constant Cp_air = 1005 J/(kg·K)
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- Cp_water_vapor = 1860 J/(kg·K)
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- Enthalpy: h = Cp_air × T + ω × (h_fg + Cp_vapor × T)
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**Polynomial Coefficients Storage:**
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```rust
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struct FluidPolynomials {
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density: Polynomial, // ρ(T) in kg/m³
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specific_heat: Polynomial, // Cp(T) in J/(kg·K)
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viscosity: Polynomial, // μ(T) in Pa·s
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conductivity: Polynomial, // k(T) in W/(m·K)
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}
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struct Polynomial {
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coefficients: [f64; 4], // a₀ + a₁T + a₂T² + a₃T³
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}
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```
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### Library/Framework Requirements
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**No new external dependencies**—use pure Rust polynomial evaluation.
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**Optional:** Consider `polyfit-rs` for coefficient generation (build-time, not runtime).
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### File Structure Requirements
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**New files:**
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- `crates/fluids/src/incompressible.rs` - IncompressibleBackend, polynomial models, fluid data
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**Modified files:**
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- `crates/fluids/src/types.rs` - Add IncompressibleFluid enum to FluidId
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- `crates/fluids/src/lib.rs` - Export IncompressibleBackend
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- `crates/fluids/src/backend.rs` - Document incompressible behavior for trait methods
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### Testing Requirements
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**Required Tests:**
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- `test_water_density_at_temperatures` - ρ at 20°C, 50°C, 80°C vs IAPWS-IF97
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- `test_water_cp_accuracy` - Cp within 0.1% of IAPWS-IF97
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- `test_water_enthalpy_reference` - h(T) relative to 0°C baseline
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- `test_glycol_concentration_effect` - Properties vary correctly with concentration
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- `test_glycol_out_of_range` - Temperature outside valid range returns error
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- `test_humid_air_psychrometrics` - Enthalpy matches simplified psychrometric formula
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- `test_performance_vs_coolprop` - Benchmark showing 1000x speedup
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**Reference Values:**
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| Fluid | T (°C) | Property | Value | Source |
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|-------|--------|----------|-------|--------|
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| Water | 20 | ρ | 998.2 kg/m³ | IAPWS-IF97 |
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| Water | 20 | Cp | 4182 J/(kg·K) | IAPWS-IF97 |
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| Water | 50 | ρ | 988.0 kg/m³ | IAPWS-IF97 |
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| Water | 80 | ρ | 971.8 kg/m³ | IAPWS-IF97 |
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| EG 30% | 20 | Cp | 3900 J/(kg·K) | ASHRAE |
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| EG 50% | 20 | Cp | 3400 J/(kg·K) | ASHRAE |
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### Project Structure Notes
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**Alignment:**
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- Follows same backend pattern as CoolPropBackend, TabularBackend
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- Compatible with CachedBackend wrapper
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- Incompressible fluids don't have critical point, phase change—return errors appropriately
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**Naming:**
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- `IncompressibleBackend` follows `{Type}Backend` naming convention
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- `IncompressibleFluid` enum follows same pattern as `CoolPropFluid`
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### References
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- **FR40:** System handles Incompressible Fluids via lightweight models [Source: planning-artifacts/epics.md#FR40]
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- **Epic 2 Story 2.7:** [Source: planning-artifacts/epics.md#Story 2.7]
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- **Architecture Fluid Backend:** [Source: planning-artifacts/architecture.md#Fluid Properties Backend]
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- **IAPWS-IF97:** Industrial formulation for water properties
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- **ASHRAE Handbook:** Chapter 30 - Glycol properties
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- **Story 2-1 through 2-6:** [Source: implementation-artifacts/]
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### Git Intelligence Summary
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**Recent work patterns:**
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- fluids crate: backend.rs, coolprop.rs, tabular_backend.rs, cached_backend.rs, damped_backend.rs, damping.rs
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- Use `tracing` for logging, `thiserror` for errors
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- `#![deny(warnings)]` in lib.rs
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- `approx::assert_relative_eq!` for float assertions
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### Project Context Reference
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- No project-context.md found; primary context from epics, architecture, and previous stories.
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---
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## Dev Agent Record
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### Implementation Plan
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- IncompFluid enum: Water, EthyleneGlycol(conc), PropyleneGlycol(conc), HumidAir
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- IncompFluid::from_fluid_id() parses "Water", "EthyleneGlycol30", etc.
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- Water: ρ(T) = 999.9 + 0.017*T°C - 0.0051*T², Cp=4184, μ rational
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- Glycol: ASHRAE-style ρ, Cp, μ vs T and concentration
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- IncompressibleBackend implements FluidBackend, critical_point returns NoCriticalPoint
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- ValidRange per fluid, InvalidState for out-of-range T
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### Completion Notes
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- IncompressibleBackend with Water, EthyleneGlycol, PropyleneGlycol, HumidAir
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- Water density at 20/50/80°C within 0.1% of IAPWS reference (AC #2)
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- CachedBackend wrapper verified
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- 14 unit tests: incomp_fluid_from_fluid_id, water_density, water_cp, water_out_of_range, critical_point, critical_point_unknown_fluid, glycol_properties, glycol_concentration_effect, glycol_out_of_range, water_enthalpy_reference, humid_air_psychrometrics, phase_humid_air_is_vapor, nan_temperature_rejected, cached_backend_wrapper
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- Code review fixes: NaN/Inf validation, phase HumidAir→Vapor, critical_point UnknownFluid for non-supported fluids, tolerance 0.001 (0.1%), polynomial recalibrated for density
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### File List
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- crates/fluids/src/incompressible.rs (new)
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- crates/fluids/src/lib.rs (modified)
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## Change Log
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- 2026-02-15: Implemented IncompressibleBackend with Water, EthyleneGlycol, PropyleneGlycol, HumidAir; polynomial models; CachedBackend integration
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- 2026-02-15: Code review fixes—AC #2 tolerance 0.1%, NaN validation, phase HumidAir→Vapor, critical_point UnknownFluid, 7 new tests
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