Non-Linear Set of Equations on Model using Modelica.Media

Question

I'm trying to model a natural convection case using Modelica.Media package and I've set up a simple grid of 6x6 fixed-volume zones (code attached) which implements mass and energy conservation. Those zones are connected to a no-flow boundary condition model and communicate through a flow model.

But when simulating it, I got a set of non-linear equations during all the steps, related to the calculation of the state p, h, Xi of each zone (figure attached).

Do you know I can set it up to avoid that? Maybe a different set of initial conditions? Any help is much appreciated!

model Zone

    replaceable package Medium = Modelica.Media.Interfaces.PartialMedium annotation(choicesAllMatching = true);
                        Medium.BaseProperties medium(p(start=101325, fixed=false),
                                                   T(start=293.15, fixed=false));

    Test.Port portT(redeclare package Medium = Medium);
    Test.Port portB(redeclare package Medium = Medium);
    Test.Port portL(redeclare package Medium = Medium);
    Test.Port portR(redeclare package Medium = Medium);

    parameter Medium.AbsolutePressure P_ambient = 101325;
    Medium.Temperature T_ambient = Units.Conversions.from_degC(20);
    Medium.MassFraction X_ambient[Medium.nX] = Medium.X_default;

    ...

initial equation

    medium.p = P_ambient;
    medium.T = T_ambient;
    medium.Xi = X_ambient[1:Medium.nXi];

equation

    ...

    m = V * medium.d;
    U = m * medium.u;
    mXi = m * medium.Xi;

    der(m) = portL.mf + portR.mf + portT.mf + portB.mf;
    der(U) = portL.hf + portR.hf + portT.hf + portB.hf + portL.q + portR.q + portT.q + portB.q;
    der(mXi) = portL.mXif + portR.mXif + portT.mXif + portB.mXif;

On the flow model, I`m using

    portI.mf = mf;
    portI.hf = semiLinear(portI.mf, portI.h, portJ.h);
    portI.mXif = semiLinear(portI.mf, portI.Xi, portJ.Xi);
    portI.q = q;

    portI.mf + portJ.mf = 0;
    portI.hf + portJ.hf = 0;
    portI.q + portJ.q = 0;
    portI.mXif + portJ.mXif = zeros(Medium.nXi);

Connector

    Medium.Temperature T;
    flow Units.HeatFlowRate q;
    Medium.AbsolutePressure p;
    flow Medium.MassFlowRate mf;
    Medium.SpecificEnthalpy h;
    flow Medium.EnthalpyFlowRate hf;
    Medium.MassFraction Xi[Medium.nXi];
    flow Medium.MassFlowRate mXif[Medium.nXi];

Non Linear Warning Image

Answer 1

I've just found the problem, my set of equations was not complete and initialization was not helpful at all.