[mezomatic]

Hi there,
I have a case that I can't share in it's entirety but it's not really complex.
Basically it's a large electrical component in a very large room (warehouse), that releases heat (69 kW overall). The boundary condition i've used for the component is externalWallHeatFluxTemperature, see the following definition:

    
     electrical_component
    {
        type            externalWallHeatFluxTemperature;
        mode            power;
        Q               69000;
        kappaMethod     fluidThermo;
        kappa           none;
        value           $internalField;
    }

I've run two simulations with buoyantSimpleFoam (realizeable k-epsilon model) as a first test of the case. One with radiation turned on (fvDOM), one with radiation off, leaving everything else untouched.
I've gotten some reasonable results flowwise (in both cases the results look quite similar) but the temperatures between the two cases differ a lot. I've tracked the mean temperature inside the room over the tiemsteps and in the radiation case it converges to about 90°C, in the case without radiation it converges to about 60 °C.

Then I've checked the heat fluxes over the components patch with the wallHeatFlux utility and integrated them in paraView to see if they matched the 69 kW I specified in the BC.

In the simulation without radiation it looks quite good:
wallHeatFlux_no radiation.jpg

In the other simulation including radiation I was a bit surprised about the really high values:
wallHeatFlux_with radiation.jpg
Somehow it's now 1,3 MW heat coming off the patch. But if you add the -1,28 MW from qr (radiative flux) you end up with 69 kW as the difference.

So it seems like the 69 kW specified in the BC is a 100% convective heat load and in the case of activated radiation, there is a huge amount of radiative heat added to that (I've also got very high surface temperature in both cases of over 800 °C mean surface temp, when it should be somewhere around 200 °C more or less. The resulting mean heat transfer coefficient is 2,3 W/m²K in both cases, which seems a bit low).

Now my questions are:
1. Why is the surface temperature that high?
2. Is there a way to specify the total heat load (convective and radiative) and let OF sort out the proportion? (That's what I thought it would do...)

Thanks in advance!
Let me know if you need any more info...

[Yann]

Hello,

I don't know if it will solve your issue, but you need to specify the name of the radiative field in the boundary condition definition. If not, it is not taken into account in the condition: https://doc.openfoam.com/2306/tools/...uxTemperature/

Property	Description			Type	Required	Default
qr		Name of radiative field		word	no		none

I hope this helps,
Yann

[mezomatic]

Hey Yann, thanks for the hint. I didn’t think it was necessary, but I‘ll try another run with the radiative field specified.

[mezomatic]

ok if i only add

qr				qr;

to the externalWallHeatFlux BC then the simulation blows up in the second timestep because of negative temperatures:

[1] --> FOAM FATAL ERROR: (openfoam-2306)
[1] Negative initial temperature T0: -34760.71479

[Yann]

You might want to try some relaxation, and/or the limitTemperature fvOptions.