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How is the Heat Flux calculated in CFD?

Hi,

I am working with StarCCM and I am wondering, how the boundary heat flux at a wall is calculated:

So I know that in the background I have an energy equation which needs to be fullfilled for each cell.

To calculate the heat flux for convection, I need a heat transfer coefficient, which I can more or less define random, depending on the reference temperature with htc=q/(T_Wall - T_Ref) .

So StarCCM let's me calculate the htc by this. But I am wondering, how StarCCM is calculating q and T_Wall .

According to my understanding, to fullfill the energy equation at a wall, the energy equation must work with some pre-defined htc, which is a term of definition (Nusselt, Prandtl - number) ?

So all in all, I don't know how I can calculate the convective heat flux (and htc) because it's all about of my own definition?

I always thought the energy equation works without assumptions fot htc and could generate me real heat fluxes

Does anyone understand my problem?

Thank you!

It's going to depend on your boundary conditions and whether you have conjugate heat transfer between the fluid and the body. Simple case - adiabatic wall, no heat flux. Next boundary condition would be a constant temperature wall, in which case I can compute a temperature gradient at the wall and determine the heat flux knowing the fluid properties. This also applies if I have known temperature distribution on the wall, since the constant distribution is just a special case of this. Another boundary condition is where the heat flux through the wall is specified - then the temperature gradient is known and this is a Neumann BC (of which the adiabatic wall is a special case). Finally, you can have coupled heat transfer between the fluid and the body and you will have other equations in your simulation, with the heat flux conducted in/out of the body by conduction being equal to the heat flux in/out of the fluid.

Somewhere in your problem setup you are specifying a boundary condition on temperature/energy equation. Go back and understand how that BC is driving your solution, and see if that doesn't help to clarify the situation.

Calculating incident heat flux on a block

I have a question on the process of calculating heat flux in OpenFOAM. In my case, I am using laplacianFoam to calculate the 1D heat equation in the z-direction (see image). Based on the codedFixedValue boundary condition I am using, the surface temperatures are fixed in the y-direction and vary in the x-direction over time.

The image attached depicts the internal mesh in paraview. See that there are ~30 cells in the z-direction.

I want to know the work flow to calculate the heat flux by taking the gradient of the scalar field T please. In particular, I only want to calculate the heat flux incident on the first cell, meaning that the gradient (dT/dz) would only consider the uppermost cells. I am certain that I will need to include the grad() function in my controlDict file, but I how do I specify the region of interest? It will not be the same as my boundary patch from my codedFixedValue BC? Please advise, thank you!

Code:

functions

{

    grad1

    {

        // Mandatory entries (unmodifiable)

        type            grad;

        libs            (fieldFunctionObjects);



        // Mandatory (inherited) entries (runtime modifiable)

        field           <T>;



        // Optional (inherited) entries

        result          <fieldResult>;

        region          ???? //region0;

        enabled         true;

        log             true;

        timeStart       0;

        timeEnd         1000;

        executeControl  timeStep;

        executeInterval 1;

        writeControl    timeStep;

        writeInterval   1;

    }

}