cp values for really low temperature

Henning86 · May 13, 2013, 07:41

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

May 13, 2013, 07:41	cp values for really low temperature	#1
Henning86 Member Join Date: Nov 2012 Posts: 83 Rep Power: 13	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

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May 14, 2013, 03:15		#2
Chris Lucas Senior Member Christian Lucas Join Date: Aug 2009 Location: Braunschweig, Germany Posts: 202 Rep Power: 17	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

May 14, 2013, 08:56		#3
Henning86 Member Join Date: Nov 2012 Posts: 83 Rep Power: 13	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).

May 15, 2013, 03:17		#4
Chris Lucas Senior Member Christian Lucas Join Date: Aug 2009 Location: Braunschweig, Germany Posts: 202 Rep Power: 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