# Ideal Gas/Total Energy gave Joule Thomson (JT) effect

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 March 11, 2014, 23:36 Ideal Gas/Total Energy gave Joule Thomson (JT) effect #1 New Member   Join Date: Oct 2012 Posts: 8 Rep Power: 12 this is a comment, but thoughts are welcome. I realize ideal gas with "total energy" is the easiest way to model compressibility effects in high speed flow in CFX (im using v15), but i am surprised it also accounts for Joule Thompson (JT) cooling - which i observed flow flow in a pipe. Indeed it was similar to OLGA JT cooling (a 1D pipe simulator). In hindsight maybe that is to be expected, even though an ideal gas is being used, it is still compressible, so JT cooling is still allowed for. Initially I thought a real gas would be required to see JT cooling in CFX

 March 12, 2014, 04:01 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,366 Rep Power: 139 JT cooling is simply the cooling effect associated with the expansion of a compressible gas. An ideal gas is compressible so can show the effect, so can a real gas. Why is this surprising?

May 12, 2020, 03:55
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 Originally Posted by cfdonlineuser this is a comment, but thoughts are welcome. I realize ideal gas with "total energy" is the easiest way to model compressibility effects in high speed flow in CFX (im using v15), but i am surprised it also accounts for Joule Thompson (JT) cooling - which i observed flow flow in a pipe. Indeed it was similar to OLGA JT cooling (a 1D pipe simulator). In hindsight maybe that is to be expected, even though an ideal gas is being used, it is still compressible, so JT cooling is still allowed for. Initially I thought a real gas would be required to see JT cooling in CFX
No, Ideal gases do not show the joule thomson effect. This is simply because the joule thomson coefficient for an ideal gas is zero. The cooling associated with rapid decompression is not JT cooling. In a JT process, the enthalpy has to be constant. You can examine this in fluent by allowing any compressible gas to expand completely and at the end of the expansion, note the temperature. Here it is important to note the temperature at the end of the expansion. For an ideal gas, you will see that the temperature is constant, however for a real gas some increase or decrease in temperature is seen. If you want to simulate the JT effect in fluent, set the gas to soave redlich kwong or peng robinson (preferably soave redlich kwong)

https://www.simulationscience.net/po...n-ansys-fluent

 May 12, 2020, 06:25 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,366 Rep Power: 139 You are correct: https://en.wikipedia.org/wiki/Joule%...Thomson_effect You should be able to model the JT cooling in CFX using one of the real gas models in CFX as well. But you have correctly pointed out that ideal gas will not show the effect. Thanks for the correction. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 May 13, 2020, 15:02 #5 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,141 Rep Power: 22 So it seems what you may be seeing in your simulation would be termed "adiabatic cooling" and not JT cooling? Ideal gas would have a JT coefficient of zero of course. But Ideal gases can still experience a local temperature change due to pressure change. These are not answers, just discussion and clarification questions. cfdonlineuser likes this.

 Tags compressibility, ideal, joule thomson