Fix BPHX mass flux per-side channels so pressure drop is visible.

Mass flux used all plate gaps instead of half the pack per fluid, underestimating G and making ΔP look like zero in the UI (0.000 bar). Also show ΔP in kPa/Pa and sync the web example.

Co-authored-by: Cursor <cursoragent@cursor.com>

crates/components/src/heat_exchanger/bphx_exchanger.rs

@@ -498,7 +498,8 @@ impl BphxExchanger {
         let dp_hot = self.side_channel_dp(state[m_h], rho_hot, z_dp)?;
         let dp_cold = self.side_channel_dp(state[m_c], rho_cold, z_dp)?;
 
-        let a_flow = self.geometry.channel_flow_area() * self.geometry.n_channels() as f64;
+        let a_flow =
+            self.geometry.channel_flow_area() * self.geometry.n_channels_per_side() as f64;
         if a_flow <= 1e-30 {
             return Err(ComponentError::InvalidState(
                 "BPHX channel flow area too small for pressure-drop Jacobian".into(),

crates/components/src/heat_exchanger/bphx_geometry.rs

@@ -170,20 +170,29 @@ impl BphxGeometry {
         self.channel_spacing * self.plate_width
     }
 
-    /// Returns the total number of channels (n_plates - 1).
+    /// Returns the total number of inter-plate gaps (n_plates − 1).
     pub fn n_channels(&self) -> u32 {
         self.n_plates.saturating_sub(1)
     }
 
-    /// Returns the mass flux for a given mass flow rate.
+    /// Channels available to **one** fluid in a single-pass counterflow pack.
     ///
-    /// G = m_dot / (A_channel × n_channels)
+    /// Alternating hot/cold channels split the gaps roughly in half:
+    /// `⌊n_plates / 2⌋` (at least 1). Using the full `n_channels` for mass flux
+    /// underestimates G by ~2× and ΔP by ~4×.
+    pub fn n_channels_per_side(&self) -> u32 {
+        (self.n_plates / 2).max(1)
+    }
+
+    /// Returns the mass flux for a given mass flow rate on one fluid side.
+    ///
+    /// G = m_dot / (A_channel × n_channels_per_side)
     ///
     /// # Arguments
     ///
-    /// * `mass_flow` - Total mass flow rate (kg/s)
+    /// * `mass_flow` - Total mass flow rate on that side (kg/s)
     pub fn mass_flux(&self, mass_flow: f64) -> f64 {
-        let n_channels = self.n_channels() as f64;
+        let n_channels = self.n_channels_per_side() as f64;
         if n_channels < 1e-10 {
             return 0.0;
         }
@@ -460,8 +469,12 @@ mod tests {
             .unwrap();
 
         let mass_flow = 0.1;
+        // 10 plates → 5 channels per side; A_ch = 0.002 × 0.1 = 2e-4 m²
+        // G = 0.1 / (2e-4 × 5) = 100 kg/(m²·s)
         let g = geo.mass_flux(mass_flow);
-        assert!(g > 0.0);
+        assert!((g - 100.0).abs() < 1e-9, "got G={g}");
+        assert_eq!(geo.n_channels_per_side(), 5);
+        assert_eq!(geo.n_channels(), 9);
     }
 
     #[test]