# Heat transfer in a channel: solving for T; questions about Pe and solving

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 July 11, 2013, 09:27 Heat transfer in a channel: solving for T; questions about Pe and solving #1 Member   Join Date: Jan 2011 Posts: 45 Rep Power: 7 Hi, I'm doing CFD for a wavy channel, using simpleFoam and the k-omega-SST model to solve the velocity field. I'd now like to solve for T, and did some intermediate processing (post-U, pre-T): For my first attempt at solving T (scalarTransportFoam) I used Code: `div(phi,T) Gauss upwind;` which worked well and converged quickly. However, when switching to Gauss linear, the solution diverged. A colleague suggested to check the Peclet number first, which I calculated with the following code snippet: Code: ``` // characteristic length of each cell: cbrt(V) volScalarField CLen ( IOobject ( "CLen", runTime.timeName(), mesh, IOobject::NO_READ, IOobject::AUTO_WRITE ), mesh, dimensionedScalar( "CLen", dimLength, 0. ) ); forAll (CLen, i) { CLen.internalField()[i] = Foam::cbrt(mesh.V()[i]); } // Pe number volScalarField Pe ( IOobject ( "Pe", runTime.timeName(), mesh, IOobject::NO_READ, IOobject::AUTO_WRITE ), mesh, dimensionedScalar( "Pe", dimless, 0. ) ); Pe = CLen*mag(U)/(alpha + turbulence->nut()/Prt);``` The results are somewhere between 6e-5 and 300. The books say that I'd need Pe < 2 for more accurate schemes. My main question now is: what can I do to get a usable temperature field? While Pe > 2 does not necessarily mean that the solution will be wrong, I could increase alpha and nu (in order to maintain Pr = 7), but that would mean that I have to increase U or the domain size as well to adjust Re. Wouldn't that just lead to the same problem, but on a different scale? Another possibility is of course that my code for Pe is just plain wrong. Maybe someone could comment on this; OF's Pe tool just calculates Pe at the boundary, using a different equation. I can not refine the mesh by a factor of 150, as I already have about 2 million cells. Regards