[Kucki]

Hi FOAMers,

i try to simulate the heat transfer in a rectangle duct with different cooling structures but i have the problem that my energy balance in the system fails. As BC for the heating i use "compressible::turbulentHeatFluxTemperature" with a definded power and the other not couppled faces are adiabatic. But when i calculate the energy in the fluid (nearly like water) via Q_dot = m_dot*cp*(T_out-T_in) my solution ist always round about 50% too high. With a refinement of my boundary layers in the fluid mesh the solution looks a little bit better but i think my mesh is already very fine with a resolution of 0.5mm and a first cell high of about 0.05 in aspect to the resolution. The convergence of the simulation is also good and the solutions looks not wrong. As solversettings for the fluid i use the following:

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.3.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

ddtSchemes
{
default steadyState;
}

gradSchemes
{
default Gauss linear;
grad(U) cellLimited Gauss linear 1;
}

divSchemes
{
default none;
div(phi,U) bounded Gauss upwind;
div(phi,h) bounded Gauss upwind;
div(phi,e) bounded Gauss upwind;
div(phi,Ekp) bounded Gauss upwind;
div(phi,K) bounded Gauss upwind;
div(phi,k) bounded Gauss upwind;
div(phi,omega) bounded Gauss upwind;
div((muEff*dev2(T(grad(U))))) Gauss linear;
}

laplacianSchemes
{
default Gauss linear limited 0.333;
}

interpolationSchemes
{
default linear;
}

snGradSchemes
{
default limited 0.333;
}

fluxRequired
{
default no;
p_rgh;
}

// ************************************************** *********************** //

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.3.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

solvers
{
p_rgh
{
solver GAMG;
tolerance 1e-7;
relTol 0.05;

smoother DIC;

cacheAgglomeration true;
nCellsInCoarsestLevel 10;
agglomerator faceAreaPair;
mergeLevels 1;

maxIter 100;
}

"(U|h|e|k|omega)"
{
//solver PBiCG;
//preconditioner DILU;
//tolerance 1e-6;
//relTol 0.05;

solver GAMG;
tolerance 1e-7;
relTol 0.05;

smoother DILU;

cacheAgglomeration true;
nCellsInCoarsestLevel 10;
agglomerator faceAreaPair;
mergeLevels 1;

maxIter 100;
}
}

SIMPLE
{
nNonOrthogonalCorrectors 2;
rhoMax rhoMax [ 1 -3 0 0 0 ] 1100;
rhoMin rhoMin [ 1 -3 0 0 0 ] 900;
}

relaxationFactors
{
fields
{
rho 1;
p_rgh 0.7;
}
equations
{
U 0.5;
"(h|e)" 0.99;
k 0.3;
omega 0.3;
}
}


// ************************************************** *********************** //

As BC for the fluid i use:

inlet, outlet, couppledwall
U: flowRateInletVelocity, inletOutlet, fixedValue
T: fixedValue, zeroGradient, compressible::turbulentTemperatureCoupledBaffleMix ed
p_rgh: fixedFluxPressure, fixedValue, fixedFluxPressure

If sb know this problem and could help me i would be very glad.

Best regards
Steffen

[GDTech]

Hi,

Energy balance can be checked using wallHeatFlux utility :

wallHeatFlux -region <region>

Thermal convergence is reached when thermal fluxes are equal on both region for any couple patch pair in your domain.

If thermal convergence is not reached, restart you computation until fluxes are balanced.

Regards,
Laurent.

[Kucki]

Hi Laurent,

thanks for your quick reply. You are right with the wallHeatFlux utilitiy and it works. But when I say thermal convergence is reached when the wallheatflux is equal in both regions my average temperature at the outlet is much too high -.- so I think i have still a mistake in my settings. Or is there a mistake in my chain of thoughts

Best regards
Steffen

[GDTech]

Would you please post your T file ?

[Kucki]

Fluid:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      T;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 0 0 1 0 0 0];

internalField   uniform 363.15;

boundaryField
{
    inlet
    {
        type            fixedValue;
        value           $internalField;
    }
    
    outlet
    {
        type            zeroGradient;
    }
    
    wasser_zu_alu       //coupled
    {
        type            compressible::turbulentTemperatureCoupledBaffleMixed;
        value           $internalField;
        Tnbr            T;
        kappa           fluidThermo;
        kappaName       none;
    }
          
}

// ************************************************************************* //

Solid:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      T;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //


dimensions      [0 0 0 1 0 0 0];

internalField   uniform 363.15;

boundaryField
{
    heizung        //heater
    {
        type            compressible::turbulentHeatFluxTemperature;
        heatSource      power;        // power [W]; flux [W/m2]
        q               uniform 500;  // heat power or flux
        kappa           solidThermo; // calculate kappa=alphaEff*thermo.Cp
    kappaName    none;
        Qr              none;        // name of the radiative flux
        value           $internalField; // initial temperature value
    }
        
    adiabat          //adiabatic
    {
        type            zeroGradient;
    }
    
    alu_zu_wasser         //coupled
    {
        type            compressible::turbulentTemperatureCoupledBaffleMixed;
        value           $internalField;
        Tnbr            T;
        kappa           solidThermo;
        kappaName       none;
    }
        
}

// ************************************************************************* //

OF version is 2.3.0 and solversettings for the fluidregion are described in the first post

[GDTech]

I'm not sure you are allowed to use "compressible::turbulentHeatFluxTemperature" on a solid boundary.
Would you please post the output of "wallHeatFlux -region solid" to check if this bc fixes correctly the heatFlux ?

[Kucki]

Yes this BC works fine if I want a heatflux of 500 I get via wallHeatFlux +500 on the heater -500 from the solid to the fluid (in solid region) and +500 from the solid to the fluid (in fluid region).

[GDTech]

Good !
So energy is conserved and you results are correct ;-)

[Kucki]

yes this might be right and thanks for your support. but it doesn't solve my problem with the too high temperature at the outlet

[hcl734]

Did you solved that issue?