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Initialisation - equations partly in submodel

#11168  Permalink Top

Initialisation - equations partly in submodel

Oct-28-21 17:35:28

Hi!

I want to model a thermodynamic system.

This is what I have tried:

Code:

model IdealGas

  // Natural Constants

  parameter Real R(unit="J/molK") = 8.31446261815324 "Gas Constant J/molK";

  // Values of Air used

  parameter Real f = 5 "Degrees of Freedom";

  parameter Modelica.Units.SI.MolarMass MG = 28.949e-3 "kg/mol";

  // Variables

  Modelica.Units.SI.Temperature T(start=273.15) "Temperature (K)";

  Modelica.Units.SI.AbsolutePressure p(start=1.015e5) "Absolut Pressure (Pa)";

  Modelica.Units.SI.SpecificEnthalpy u "spezific energy (J/mol)";

  Modelica.Units.SI.SpecificEnthalpy h "spezific Enthalpy (J/mol)";

  Modelica.Units.SI.Density rho "Density (kg/m3)";

  Modelica.Units.SI.Concentration c "mol/m3";

equation

  rho = MG * p / (R * T);

  c = p / (R * T);

  u = 0.5 * f * R * T;

  h = u + R * T;

end IdealGas;



model OpenGasVolume

    import Modelica.Units.SI;

    parameter  SI.Temperature T0=273.15+55 "Start Temperature (K)";

    parameter  SI.Pressure p0=5e5  "Start pressure (Pa)";

    parameter  SI.Volume V=1  "Volume (m3)";

    SI.Mass m(min = 0.0, start = 1) "Mass in Volume (kg)";

    SI.Energy H(min = 0.0) "Enthalpie in Volume (J)";

       

    IdealGas media;   

   



initial equation

  media.T = T0;

  media.p = p0;

  H = media.h * media.c * V;  // <--- Somehow this initialization does not work as i intend

 

equation

  H = media.h * media.c * V;

  media.rho = m / V;

  der(m) = 0;    // No Mass transfer

  der(H) =  0;   // No energy transfer

   

end OpenGasVolume;

Somehow I do not get the  initialization right.
It is mandatory that 'H' is set to a proper start value in 'OpenGasVolume'. Part of the equations are captured in the Model 'media'.
My desired result would be media.T == T0 and media.p=p0 over the whole simulation time.

However I get:

Code:

Simulation process failed. Exited with code 255.

Matrix singular!

The initialization problem is inconsistent due to the following equation: 0 != 55 = T0 - media.T

Error in initialization. Storing results and exiting.

Use -lv=LOG_INIT -w for more information.

How to init the model correctly?

Thanks, Bernd

Edited by: BerndTA - Oct-30-21 16:35:27
#11172  Permalink Top

Re: Initialisation - equations partly in submodel

Oct-30-21 07:59:51

Question also posted at: https://stackoverflow.com/questions/697 … n-submodel
Answer on Stackoverflow prefered. Thanks.

#11173  Permalink Top

Re: Initialisation - equations partly in submodel

Nov-01-21 18:13:59

I could solve it. Now I understand the meaning of 'fixed=true'.

Solution is:

Code:

model OpenGasVolume

    import Modelica.Units.SI;

    parameter  SI.Temperature T0=273.15+55 "Start Temperature (K)";

    parameter  SI.Pressure p0=5e5  "Start pressure (Pa)";

    parameter  SI.Volume V=1  "Volume (m3)";

    SI.Mass m(min = 0.0, start = 1) "Mass in Volume (kg)";

    SI.Energy H(min = 0.0) "Enthalpie in Volume (J)";

       

    IdealGas media(T(start = T0, fixed=true), p(start=p0, fixed=true));   <<<=== Correction done here.

   



initial equation



  H = media.h * media.c * V;  

does not work as i intend



equation

  H = media.h * media.c * V;

  media.rho = m / V;

  der(m) = 0;    // No Mass transfer

  der(H) =  0;   // No energy transfer

   

end OpenGasVolume;

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