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Old   February 5, 2015, 18:39
Default buoyantSimpleFoam: fixed wall heat flux
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Dear all cfd-online members,

my question involves two parts. First of all I am using buoyantSimpleFoam solver and I would like:
1) to include a fixed heat flux wall boundary condition. Is that possible in OF and if yes how?
2) in case that is possible to have a fixed wall heat flux boundary condition, I would like that I dont have to specify the heat transfer coefficient, so that I calculate it in post-processing using the temperature difference solution field. Is it possible?

thanks and cheers,
TaPantaRei
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Old   February 6, 2015, 07:12
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Hi,

Yes, it is possible. I think what you want something is a boundary condition of type fixedGradient (see sec. 5.2 of OF Users Guide).
What you want then is to calculate a convection heat transfer coeficient h [W/mēK]?
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Old   February 6, 2015, 08:07
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So thiagopl you mean that in the file T for the temperature, for this wall, I should set a fixedGradient boundary condition? Yes my aim is to calculate a heat transfer coefficient, assuming a heat flux value.
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Old   February 6, 2015, 17:45
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Yes. You'll need to define your Nusselt number and then calculate the h coeficient.
I don't know how to do this in OF. I usually export data and do all the post processing outside OF.
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Old   February 8, 2015, 07:16
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Thiago there is a misunderstanding, I didn't clearly mentioned it, but I want to calculate the material's conductivity k [W/mK]. So, from the equation: q = -k*dT/dx, I need k. I was thinking if it was possible to fix a value q, and then from the temperature solution field I compute dT/dx and then I just find k. In Ansys i know thats possible. I don't know if I fix a gradient dT/dx how can i find k?
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Old   February 8, 2015, 09:12
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You already gave the answer
You know the flux and you calculated the temperature field, so, use some discretisation scheme for dTdx and then you can find k.
It is still (for me) a post processing issue.
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Old   February 8, 2015, 17:37
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hmm i don't see how exactly I gave the answer because you say that I have to use fixedGradient in the T file, so this means I will give a value for dT/dx which is -q/k. Then from the temperature field how I get k? what I wanted is that I give in T file a fixedValue for q if that's possible? and then from the temperature field I can solve for k.
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Old   February 5, 2016, 10:01
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Quote:
Originally Posted by TaPantaRei View Post
hmm i don't see how exactly I gave the answer because you say that I have to use fixedGradient in the T file, so this means I will give a value for dT/dx which is -q/k. Then from the temperature field how I get k? what I wanted is that I give in T file a fixedValue for q if that's possible? and then from the temperature field I can solve for k.
Dear TaPantaRei,

Did you ever solve this problem? Like you I just want to specify q but don't know k or T... What do you think?

Cheers,
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Old   February 5, 2016, 14:56
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Maybe what you need is externalWallHeatFluxTemperature or turbulentHeatFluxTemperature BC's.

Hope it helps.

Best regards

Alex
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The case I talk about in this site was solved with chtMultiRegionSimpleFoam solver and involves radiation. Some basic tutorials are also resolved step by step in the web. If you are interested in these matters, you are invited to come in!
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Old   May 26, 2017, 22:41
Default externalWallHeatFluxTemperature with buoyantPimpleFoam
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Hi,
I have to use heat flux at the boundary and convective (robin) boundary condition at the other one as heat loss.
I tried using externalWallHeatFluxTemperature, but it doesn't seem to work with buoyantPimpleFoam but turbulentHeatFluxTemperature works. Can anyone explain the reason. In principle both are same and during debugging I found both are getting solved and both are giving same output, but externalWallHeatFluxTemperature fails with some library error message:

Code:
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
PIMPLE: iteration 1
DILUPBiCG:  Solving for Ux, Initial residual = 0.99845272, Final residual = 4.411321e-07, No Iterations 11
DILUPBiCG:  Solving for Uy, Initial residual = 1, Final residual = 3.8992528e-07, No Iterations 11
#0  Foam::error::printStack(Foam::Ostream&) at ??:?
#1  Foam::sigFpe::sigHandler(int) at ??:?
#2  ? in "/lib/x86_64-linux-gnu/libc.so.6"
#3  Foam::heThermo<Foam::rhoThermo, Foam::pureMixture<Foam::constTransport<Foam::species::thermo<Foam::hConstThermo<Foam::perfectFluid<Foam::specie> >, Foam::sensibleInternalEnergy> > > >::he(Foam::Field<double> const&, Foam::Field<double> const&, int) const at ??:?
#4  Foam::mixedEnergyFvPatchScalarField::updateCoeffs() at ??:?
#5  Foam::fvMatrix<double>::fvMatrix(Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> const&, Foam::dimensionSet const&) at ??:?
#6  Foam::tmp<Foam::fvMatrix<double> > Foam::fv::optionList::operator()<double>(Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> const&, Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh>&, Foam::word const&) at ??:?
#7  ? at ??:?
#8  __libc_start_main in "/lib/x86_64-linux-gnu/libc.so.6"
#9  ? at ??:?
Floating point exception (core dumped)
It shows that Ux and Uy are getting solved but during T solution there is some mistake.
Kindly suggest

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Old   May 27, 2017, 06:54
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I am little bit confused what the problem is but, using fixedGradient you can define constant heat flux. So, as I know if you define q = 1000 then you need to give OpenFoam the result of dT/dx by dividing your q into k and you will define k in your thermophysical properties file.
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Old   May 28, 2017, 08:30
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That is a way I am solving the problem right now but further ahead, my k will change with temperature and also during turbulence, k will be replaced by k+keff.
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boundary condition, buoyanboussinesqsimple, buoyant solver, heat flux, heat flux coefficient

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