Determination of heat flow [W]

#1

Dear community,

I’ve built a simulation of an air conditioner in an office:
https://www.simscale.com/projects/Sternin/ventilation_of_the_workplace1/

Within the scope of post-processing I want to identify the heat flow rate from a warm surface “human skin” into the room interior (air).
In the simulation setup I used the Analysis Type “Natural convective heat transfer” defining a constant temperature for the skin surface.

For post-processing I’m using the ParaView-Tool. I’ve already identified the relevant surfaces. Now I just need to know which tool I have to use in order to determine the energy transfer [W] (heat flow) from these surfaces into the air. Of course heat flux [W/m²] values would be acceptable too.

Thanks in advance
Andrew

#2

Hi Andrew (@Sternin),

Glad to have you here in the community. Great project idea - I quite enjoyed the description of the problem “At the afternoon your department is going to meet some important customers, so nobody can afford unattractive sweat patches under the armpits today” :smile:

Now to answer your question:

In order to calculate the fluxes of heat coming away from a body, you need to do the following in ParaView:

  1. load the InternalField and the surfaces (together)
  2. use filter: GradientOfUnstructuredDataSet using CellData
  3. use filter: ExtractBlock choosing the surfaces
  4. Integrate variables and read cell data.

The results will be the heat flux going through the selected face. Hope this helps and I look forward to seeing the final results.

Best,
Anna

#3

Hi Anna,

Thank you for the quick response. I follwed your instructions.
At the final step the “integrate variables”-filter opens a SpreadSheetView1 which gives me the following values:
CellData, Area, CellType, T, U, alpha, k, mut, omega, p, p_rgh

Neither of them seems to represent the heat flux.
Did I maybe look at the wrong place?

Thanks
Andrew

#4

@Sternin,

We’ve had a deeper look - and it seems like determining the energy transfer is slightly more complicated than was initially assumed.

We are still looking into it, in the meantime another place to ask could be

http://www.cfd-online.com/Forums/openfoam-paraview/

Hope this helps,

Best,
Anna

#5

Dear all,

calculating the heatflux within paraview is somehow a bit tricky and hard. I would suggest you (if possible) to use the openfoam inhouse programm wallHeatFlux. This will return exactly the heat flux in [W/m^2] or in [W] (do not remember). The problem calculating the flux in paraview is as follow:

we need to calculate the temperature gradient T normal to the surface. As far as I know there is no function available that is calculating this one (we need the distance from face center to cell center and the magnitude of the face normal vector). Nothing else is done in the wallHeatFlux tool within Openfoam. How you could calculate this within simscale environment, I don’t know even with paraview.

I always use the tool wallHeatFlux or build my own field that calculates the heat flux within the whole internal field (but normally it is sufficient to know the heat flux only on the walls).

Kind regards,
Tobias Holzmann

#6

Further Info @Sternin

The fact that you do not see a field that correlates to the flux is because:
a) You do not calculate any flux in the method mentioned by Anna
b) The Spread Sheet can switch between cell and point values, the Gradient field you calculate is a point field
c) I also tried to get the flux with PV but without success but maybe it will inspire you:

  1. Activate all patches
  2. GradientOfUnstructedDataSet (I am not sure what this filter do, the gradients look really terrible and for me it does not seem to be physical. One should be sure what this filter is finally doing)
  3. ExtractBlock for the patch of interest (if you check the gradients now, it looks even worse)
  4. Calculator -> mag(gradients) * lambda (name is Field1)
  5. Filter -> Point to cell
  6. IntegrateVariables
  7. Calculator -> Field1 * Area

But that is not correct because I am not sure about the gradient calculation and furthermore about lambda because FOAM is using alpha for calculation and I already forgot how it is calculated :slight_smile: