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-   -   BC wallHeatTransfer, alphaWall: Thermal Diffusivity or Conductivity (http://www.cfd-online.com/Forums/openfoam-solving/119481-bc-wallheattransfer-alphawall-thermal-diffusivity-conductivity.html)

Johnston81 June 18, 2013 07:37

BC wallHeatTransfer, alphaWall: Thermal Diffusivity or Conductivity
 
Hi Foamers

I want to use the wallHeatTransfer BC and I have a question about the input value alphaWall:

Is alphaWall the themal diffusivity (m2/s) or the thermal conductivity (W/(mK))? Or something entirely different?

Looking at the documentation for the wallHeatTransferFvPatchScalarField it states that alphaWall is a required input and that it is indeed the Thermal Diffusivity. However, if you look at people's posted examples, many refer to alphaWall as the Heat Transfer Coefficient. Also, virtually all examples of values given for alphaWall are of the order of magnitude 1e0 where thermal diffusivities are usuallly on the order of 1e-5 to 1e-6.

So what is alphaWall? Documentation says Diffusivity but empirical data suggets Conductivity. Please help me!

Regards/Christopher

PS. I cannot run the BC wallHeatTransfer without the additional input 'value'. Is this some hidden clue to the answer to my question? A different BC? Or old/outdated documentation?

Johnston81 July 25, 2013 14:40

Nobody knows?
 
Hi Foamers,

Nobody knows the answer to my question? Is it to 'heat transfer' specific?

Anyway, I would like to ask one more time. I am now in the situation where I actually need to use the wallHeatTransfer BC - and for reasons I imagine everybody can sympathise with, I would like to know what alphaWall really is: a Thermal Diffusivity or a Heat Transfer Coefficient?

I can easily settle for a unit if anybody has one lying around? ... And what use would a thermal diffusivity be for a boundary condition? Wouldn't you need a volume/body of material and a specific heat capacity to be able to use it for anything?

Argh! Head ache...!

Regards/Christopher


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