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-   -   Temperature dependent Cp: JANAF tables vs Debye model (http://www.cfd-online.com/Forums/openfoam-solving/120698-temperature-dependent-cp-janaf-tables-vs-debye-model.html)

 antb July 12, 2013 10:40

Temperature dependent Cp: JANAF tables vs Debye model

Hello,

I'm working with an expanding gas from a high pressure to a low pressure cavity. With this expansion comes an extreme cooling of the gas to sub 50 K temps from an initial 300 K (Joule-Thomson effect). I am using for this the rhoCentralFoam solver.

Now I remember reading somewhere that solvers start experiencing problems at very low temperatures, though there didn't seem to be any clear stated reasons as to why. One area I can imagine where there may be a problem is the constant Cp (specific heat) characteristic from most thermophysical models.

To help with this, I know there are JANAF tables to specify a temperature dependant Cp. These tables however usually go from values like 200K to 6000K, while we know that Cp starts to drop dramatically at low temps (<< 200K), although this is substance dependent. Therefore, JANAF tables aren't really of a any use. What I want to know, is how to use a Debye model (http://en.wikipedia.org/wiki/Debye_model), which seems to be perfect at modeling low temps Cp. One way I could imagine going about this is fitting the JANAF polynomial onto the Debye curve, but this seems a bit backward.

If I am not required to play with the Cp value and there exists a simpler method for making simulations more stable at very low temp, please let me know.

 immortality August 26, 2013 16:57

Quote:
 In thermodynamics and solid state physics, the Debye model is a method developed by Peter Debye in 1912 for estimating the phonon contribution to the specific heat (heat capacity) in a solid
Hi
it seems Debye be related to solids,are you sure it can be used for gases?

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