Proper use of inStream() in Modelica - modelica

Question

I am trying to build a very simple model of a distributed thermofluid volume in Modelica and am struggling to implement it correctly with the stream operator. This volume uses DryAirNasa as the medium, and I would like it to have no mass storage, no pressure drop, and no energy storage (much like the Modelica.Fluid.Pipes.StaticPipe model). However, I would like to explicitly perform an energy balance so that there can be heat transfer interactions. I would also prefer not to define the mass flow rate in this model, but let it be defined in one of the boundaries attached to an end of the pipe (e.g., Modelica.Fluid.Sources.MassFlowSource_h).
I have created a test implementation of such a model, but this model apparently lacks one equation according to Dymola. I would appreciate any insights into how to fix this model so that it is correct. If I add the equation
port_a.h_outflow = Medium.specificEnthalpy(state_a)
to the equation section, the model has the same number of equations and unknowns, but I don't have any good justification for adding such an equation.
model AirFlowTemp
// Objective: create a component that has no pressure drop, no mass storage,
and no energy storage, but that has a heat input.
import SI=Modelica.SIunits;
final replaceable package Medium=Modelica.Media.Air.DryAirNasa;
AirFlow.AirFlowPort port_a(redeclare package Medium
= Medium);
AirFlow.AirFlowPort port_b(redeclare package Medium
= Medium);
Interfaces.HeatPort heatPort;
Medium.EnthalpyFlowRate[2] H_flow "enthalpy flow";
SI.HeatFlowRate Q_flow "heat flow rate";
Medium.Temperature T_mean;
Medium.ThermodynamicState state_a;
Medium.ThermodynamicState state_b;
equation
// no pressure drop across the component.
port_a.p = port_b.p;
// Assume that there is no mass storage in the volume
0 = port_a.m_flow + port_b.m_flow;
// Energy balance
H_flow[1] = semiLinear(port_a.m_flow, inStream(port_a.h_outflow), inStream(port_b.h_outflow));
H_flow[2] = semiLinear(port_b.m_flow, inStream(port_b.h_outflow), inStream(port_a.h_outflow));
0 = Q_flow + H_flow[1] + H_flow[2];
state_a = Medium.setState_ph(port_a.p, inStream(port_a.h_outflow));
state_b = Medium.setState_ph(port_b.p, inStream(port_b.h_outflow));
T_mean = (Medium.temperature(state_a) +
Medium.temperature(state_b))/2;
heatPort.Q_flow = Q_flow;
heatPort.T = T_mean;
end AirFlowTemp;
connector AirFlowPort
replaceable package Medium = Modelica.Media.Interfaces.PartialMedium;
Medium.AbsolutePressure p;
flow Medium.MassFlowRate m_flow;
stream Medium.SpecificEnthalpy h_outflow;
stream Medium.MassFraction Xi_outflow[Medium.nXi];
end AirFlowPort;
connector HeatPort
extends Modelica.Thermal.HeatTransfer.Interfaces.HeatPort;
end HeatPort;

Answer 1

I had the same problem doing the same thins about 4 weeks ago.
Try if this works out as a step to get closer to right use of inStream.
Remove
Medium.EnthalpyFlowRate[2] H_flow "enthalpy flow"
SI.HeatFlowRate Q_flow "heat flow rate";
Interfaces.HeatPort heatPort;
SI.HeatFlowRate Q_flow "heat flow rate";
Medium.Temperature T_mean;
T_mean = (Medium.temperature(state_a) + Medium.temperature(state_b))/2;
heatPort.Q_flow = Q_flow;
heatPort.T = T_mean;
=> just testing the pipe, without the heatPort
Use
port_a.outflow*port_a.m_flow = inStream(port_b.outflow)*port_b.m_flow;
port_b.outflow*port_b.m_flow = inStream(port_a.outflow)*port_a.m_flow;
instead of:
H_flow[1] = semiLinear(port_a.m_flow, inStream(port_a.h_outflow), inStream(port_b.h_outflow));
H_flow[2] = semiLinear(port_b.m_flow, inStream(port_b.h_outflow), inStream(port_a.h_outflow));
0 = Q_flow + H_flow[1] + H_flow[2];
Change your Test for no heatport
=> Does this work?
Regards
Uwe