Heat exchanger time constant - update default value

[RaphaelGebhart]

Change the default value of ThermofluidStream.HeatExchangers.Internal.PartialNTU.TC to be more realistic. (Or remove the default value).

Will be part of the next major release.

[tobiasreischl]

Make TC flow-dependent via optional nominal mass flow scaling (backwards-compatible)

Motivation

PartialNTU currently uses a fixed first-order time constant TC. A constant TC is only valid near one operating point; across load changes it becomes unrealistic:

• lower mass flows → should respond slower (larger effective time constant)
• higher mass flows → should respond faster (smaller effective time constant)

This aligns with the documentation note that the default TC=0.01 s is not realistic and can increase stiffness/simulation time.

Proposal

Interpret TC as a nominal time constant at a user-defined nominal operating point and scale it with the actual heat capacity flow rates:
[
TC_\mathrm{eff} = TC \cdot \frac{C_{A,nom}+C_{B,nom}}{C_A+C_B}, \quad C=\dot m,c_p
]
Clamp the scaling factor to avoid extreme values at very low flows.

Backwards compatibility

• New parameters default to 0 → scaling disabled → current behavior unchanged.
• No existing parameter names changed → no conversion script required.

How to use

• Set TC for the desired time constant at nominal conditions.
• Set m_flow_nom_A and m_flow_nom_B to design/typical mass flows (kg/s).
• TC_eff adapts automatically with load (slower at low flows, faster at high flows).

Patch (only changes)

@@
   parameter Modelica.Units.SI.Time TC=0.01 "Heat exchanger time constant (increase as recommended in the documentation)";
+  // Optional: interpret TC as nominal time constant and scale with actual capacity rates
+  parameter Modelica.Units.SI.MassFlowRate m_flow_nom_A=0
+    "Nominal mass flow rate side A for TC scaling (set >0 to enable scaling)"
+    annotation (Dialog(tab="Advanced"));
+  parameter Modelica.Units.SI.MassFlowRate m_flow_nom_B=0
+    "Nominal mass flow rate side B for TC scaling (set >0 to enable scaling)"
+    annotation (Dialog(tab="Advanced"));

@@
 protected
@@
   SI.HeatFlowRate q_max "Maximum heat flow rate A to B (Q_flow_max)";
   SI.HeatFlowRate q_flowA "Heat flow rate A to B (Q_flowA)";
   SI.HeatFlowRate q_flowB "Heat flow rate B to A (Q_flowB)";
+
+  // Effective time constant (scaled around nominal operating point)
+  Modelica.Units.SI.Time TC_eff;
+  Real C_A_nom(unit="J/(K.s)") "Nominal heat capacity flow rate of Medium A";
+  Real C_B_nom(unit="J/(K.s)") "Nominal heat capacity flow rate of Medium B";
+  Real scaleTC(unit="1") "Scaling factor for TC";
+  constant Real scaleMin(unit="1") = 0.05 "Lower clamp for TC scaling";
+  constant Real scaleMax(unit="1") = 20   "Upper clamp for TC scaling";

@@
 equation
@@
   //Heat capacity rates
   C_A = (abs(inletA.m_flow) + eps)*cp_A;
   C_B = (abs(inletB.m_flow) + eps)*cp_B;
+
+  // --- TC scaling (enabled only if both nominal flows are set > 0) ---
+  if noEvent(m_flow_nom_A > 0 and m_flow_nom_B > 0) then
+    C_A_nom = (abs(m_flow_nom_A) + eps)*cp_A;
+    C_B_nom = (abs(m_flow_nom_B) + eps)*cp_B;
+    scaleTC = (C_A_nom + C_B_nom)/max(C_A + C_B, 1e-3);
+    scaleTC = min(scaleMax, max(scaleMin, scaleTC));
+    TC_eff  = max(TC, 1e-6)*scaleTC;
+  else
+    TC_eff  = max(TC, 1e-6);
+  end if;

@@
-  der(q_flow_dyn)*max(TC, 1e-6) = q_flow - q_flow_dyn;
+  der(q_flow_dyn)*TC_eff = q_flow - q_flow_dyn;