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-   -   cp values for really low temperature (http://www.cfd-online.com/Forums/openfoam-solving/117665-cp-values-really-low-temperature.html)

Henning86 May 13, 2013 07:41

cp values for really low temperature
 
My problem is that i need them for really low temperature. I have max Mach Number of about 8 so may temperture are pretty low about 20 K which model should i use?

is janaF still valid in this temperatures?

and how to get the values?

Best Regards

Henning

Chris Lucas May 14, 2013 03:15

Hi,

have a look at the paper where the janaF data comes from. In the paper, a temperature range is stated.

Are you sure that the fluid is still a gas (perfect gas) at 20 K?

Regards,
Christian

Henning86 May 14, 2013 08:56

Thanks for the response


the pressure is pretty low as well about 700 Pa, which is under the triple point for nitrogen. these conditions are only reached for a really short period about 0.1 ms so only a small part of the fluid will become solid.

So the perfect gas should be still valid

what do you think?

the problem is that the janaf coefficients are usually valid until 200 K (i think haven't seen a different case).

Chris Lucas May 15, 2013 03:17

Hi,

at the working conditions your describing, I would also say that the perfect gas law can't be used anymore. You can find a real gas implementation in OpenFOAM 1.6 ext. here, which might be better suited near the vapor dome.

http://www.cfd-online.com/Forums/ope...l-gas-eos.html

The question is, if you really need to use "correct" thermodynamics in this small local area where the low temperatures occurs or if you should only insure that your thermodynamics return reasonable results (e.g. positive specific heat).

Kind Regards
Christian


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