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forced convection of air inside a heated rectangular channel

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Old   June 3, 2013, 08:11
Default forced convection of air inside a heated rectangular channel
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Ank
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Hey,

I am simulating the forced convection of air inside a heated rectangular channel. The air velocity is 1 m/s at the inlet. The temperature of the all four walls is 400 K and air is at 300 K. Initially I have taken the case of pitzdaily in simpleFoam solver and included the energy equation to incorporate the heat transfer. Then I used buoyantBoussinesqPimpleFoam solver but I am not able to get the convergence in pressure. I have given the velocity in x direction, but it does not show any velocity in that direction, instead it is showing the effect of buoyancy only, and show the circulating cells. I have tried the totalPressure, fixedPressure boundary conditions at the inlet and zeroGradient at the outlet, but no improvement.
I am using kEpsilon model with first order upwind. For testing purpose the grid is a bit coarser. I am able to run the case without energy equation successfully with the simpleFoam solver, but problem comes whenever I include the energy equation.

Please help me, it seems an easy problem but I am really stuck...


Thanks
Ankur
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Old   June 4, 2013, 03:23
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Hey

I just checked my results. The velocity vectors are showing that the velocity reaches to zero value near the outlet. It is non physical as velocity should be 1 m/s everywhere.
I am attaching the snapshot. I dont know what is happening.

Please have a look.
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Old   June 5, 2013, 05:31
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Hi Ankur,

could you please upload case files.

thanks
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Old   June 5, 2013, 06:38
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Hey

I found the solution to my earlier problem, Now the problem is different and the velocity which is 1 m/s at the inlet is going above to 1.14 m/s at the outlet. I am posting the case files and snapshot.

I have made a silly mistake in the calculation of epsilon. I will get back to you once fix it. To get the pure forced convection my gravity and expansion coefficients are both zero.
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Old   June 5, 2013, 06:50
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The problem remains same with correct calculations also. I am attaching more files now. The velocity is going beyond its correct value.
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Old   June 5, 2013, 07:19
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Perhaps you should try buoyantPressure BC for the pressure at the inlet and outlet. This is the "perturbation" pressure that you are referring to, correct?
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Old   June 5, 2013, 07:42
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Hey Deji

I do not understand your reply. I have taken zero gravity so do you still think It will work?
secondly I tried it but it is diverging.
Now pressure is also converging but the velocity is going beyond its value as I posted above.

Thanks
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Old   June 5, 2013, 08:10
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Quote:
Originally Posted by Ank View Post
Hey,

For testing purpose the grid is a bit coarser. I am able to run the case without energy equation successfully with the simpleFoam solver, but problem comes whenever I include the energy equation.

Thanks
Ankur

What do you mean to say by the mentioned Quote? Are you running your own solver "simpleFoam" with included energy equation in to it?
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Old   June 5, 2013, 08:13
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Hey Tushar

Good to see your reply again..

No I am using my own solver. Sorry for the language. It meant when I shifted from simpleFoam to buoyantBoussinesqSimpleFoam. It seems to be a bit easy problem but I am getting strange velocity increment at the outlet.

And the case I have taken is with very coarse grids, dimensions 0.1 X 0.05 X 0.5 with 17000 hexa cells.
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Old   June 5, 2013, 08:16
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My mistake. Since g is set to zero, that BC would be invalid.
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Old   June 5, 2013, 08:19
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It seems like your flow is accelerating inside the channel due to a negative pressure gradient, -dP/dx. What kind of pressure boundaries do you have at the inlet and outlet? And is this pressure the "perturbation pressure"?
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Old   June 5, 2013, 08:22
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Hey Deji

Thanks for the reply..

I am using zeroGradient at the inlet and fixedvalue (uniform 0) at the outlet. I am sorry but I dont know the meaning of perturbation pressure.

Normally the simulation runs fine with these BCs when heating is not there.
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Old   June 5, 2013, 08:26
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Do me a favor, look in solver and paste the pressure equation and how the pressure is solved. You are probably solving a pressure equation, I would think.
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Old   June 5, 2013, 08:33
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Hey

I think I got the perturbation pressure, is it the pressure correction term in the SIMPLE algorithm?

Here is the solver


00002 volScalarField rUA("rUA", 1.0/UEqn().A());
00003 surfaceScalarField rUAf("(1|A(U))", fvc::interpolate(rUA));
00004
00005 U = rUA*UEqn().H();
00006 UEqn.clear();
00007
00008 phi = fvc::interpolate(U) & mesh.Sf();
00009 adjustPhi(phi, U, p_rgh);
00010
00011 surfaceScalarField buoyancyPhi = rUAf*ghf*fvc::snGrad(rhok)*mesh.magSf();
00012 phi -= buoyancyPhi;
00013
00014 for (int nonOrth=0; nonOrth<=nNonOrthCorr; nonOrth++)
00015 {
00016 fvScalarMatrix p_rghEqn
00017 (
00018 fvm::laplacian(rUAf, p_rgh) == fvc::div(phi)
00019 );
00020
00021 p_rghEqn.setReference(pRefCell, getRefCellValue(p_rgh, pRefCell));
00022
00023 // retain the residual from the first iteration
00024 if (nonOrth == 0)
00025 {
00026 eqnResidual = p_rghEqn.solve().initialResidual();
00027 maxResidual = max(eqnResidual, maxResidual);
00028 }
00029 else
00030 {
00031 p_rghEqn.solve();
00032 }
00033
00034 if (nonOrth == nNonOrthCorr)
00035 {
00036 // Calculate the conservative fluxes
00037 phi -= p_rghEqn.flux();
00038
00039 // Explicitly relax pressure for momentum corrector
00040 p_rgh.relax();
00041
00042 // Correct the momentum source with the pressure gradient flux
00043 // calculated from the relaxed pressure
00044 U -= rUA*fvc::reconstruct((buoyancyPhi + p_rghEqn.flux())/rUAf);
00045 U.correctBoundaryConditions();
00046 }
00047 }
00048
00049 #include <finiteVolume/continuityErrs.H>
00050
00051 p = p_rgh + rhok*gh;
00052
00053 if (p_rgh.needReference())
00054 {
00055 p += dimensionedScalar
00056 (
00057 "p",
00058 p.dimensions(),
00059 pRefValue - getRefCellValue(p, pRefCell)
00060 );
00061 p_rgh = p - rhok*gh;
00062 }
00063 }
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Old   June 6, 2013, 00:02
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Hello Ank,

Since, Boussinesq approximation is used for the pressure, as you are using buoyantBoussinesqSimpleFoam. So, you need to provide the BC for p_rgh something like this....

{
type buoyantPressure;
rho rhok;
value uniform 0;
}

see the tutorial on heatTransfer/buoyantBoussinesqSimpleFoam/ you will get an idea.
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Old   June 6, 2013, 01:42
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Hey Tushar

I tried that also, but it didnt work. Another thing is that its totally forced convection and my gravity is also zero so buoyant does not mean anything here as I have understood. I do not know what else can go wrong in this simple problem.

I will try more things now, lets see how it goes. You can also think what else can be wrong.

Thanks
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Old   June 6, 2013, 01:54
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Hello,

I mean to say by putting the mentioned BC for "p_rgh" and setting g=0, your case will be of forced convection only (i.e., independent of buoyancy).
I have a doubt with pressure "p" BC. Try setting the BC for Outlet & inlet.
Check:
If you are not getting solution.
Also check:
Are you getting convergence?
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Old   June 6, 2013, 02:19
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I am posting my p_rgh and U files, kindly suggest me what I should change,

P_rgh
dimensions [0 2 -2 0 0 0 0];

internalField uniform 0;

boundaryField
{
inlet
{
type zeroGradient;

}
pipe
{
type zeroGradient;


}

outlet
{
type fixedValue;

value uniform 0;

}

wall
{
type zeroGradient;
}



U

dimensions [0 1 -1 0 0 0 0];

internalField uniform (0 0 0);

boundaryField
{

inlet
{
type fixedValue;

value uniform (1 0 0);
}

pipe
{
type fixedValue;
value uniform (0 0 0);
}

outlet
{
type zeroGradient;


}

wall
{
type fixedValue;
value uniform (0 0 0);
}

I have tried changinp_rgh to buoyantPressure it was not converging.
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Old   June 6, 2013, 02:25
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Tushar Chourushi
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Check my earlier post for "p" BC...
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Old   June 6, 2013, 02:45
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I changed the p_rgh to buoyantPressure and p file to zeroGradient at the inlet and fixedValue at the outlet.

It is showing this error



--> FOAM FATAL ERROR:
Continuity error cannot be removed by adjusting the outflow.
Please check the velocity boundary conditions and/or run potentialFoam to initialise the outflow.
Total flux : 0.000771162
Specified mass inflow : 0.005
Specified mass outflow : 0
Adjustable mass outflow : 2.34371e-110


From function adjustPhi(surfaceScalarField& phi, const volVectorField& U,const volScalarField& p
in file cfdTools/general/adjustPhi/adjustPhi.C at line 118.

FOAM exiting
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