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explanation about radiation model coefficients

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Old   December 14, 2023, 12:18
Default explanation about radiation model coefficients
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nicolas Claverie
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Hello !



Stupid question:
What is the physical definition of the coefficient of the radiation model constantAbsorptionEmission :
(for example in the multiRegionHeaterRadiation tutorial)



constantAbsorptionEmissionCoeffs
{
absorptivity absorptivity [0 -1 0 0 0 0 0] 0.0; //opaque
emissivity emissivity [0 -1 0 0 0 0 0] 0.1;
E E [1 -1 -3 0 0 0 0] 0;
}

They canít be the absorption and emission coefficient of the material since they are in [1/meter] units. Also, people online recommend values as high as 1e8 for them in some cases but the coefficient i know should be between 0 and 1.
E is apparently the emission contribution but I see no physical equivalent for this. How can I find those value for a specific material (like aluminum for example).



PS: install information if relevant:
Windows 11 pro 22H2
WSL 2.0.9.0
Ubuntu 22.04.3 LTS
Openfoam 2306
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Old   December 15, 2023, 04:25
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Hey,


I have been struggling too with this!


As far as I know, these values are indeed different from the absorptivity and emissivity of surfaces which are between 0 and 1 as you said. I believe they are the absorption and emission coefficients which can be really high for opaque solids. It basically tells you how much energy from a beam will be dissipated/scattered as it goes through the medium.

For non participating medium, these values should be near 0, meaning the radiation goes through unhampered. For opaque solids, you should just set the radiation model to opaqueSolid, and let your boundaryRadiationProperties take care of the heat transfer, the coefficients don't matter. And if you have semi-transparent media, you are out of luck because OpenFoam does not support it currently.


Hope it helps though!
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Old   December 15, 2023, 11:25
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Thanks a lot for your answer! Any idea what E is supposed to be?
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