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vitmalin August 27, 2007 03:05

Mass conservation law violation in stationary case
Dear colleagues,

I have got very surprising result trying to simulate air ideal gas heating process in closed volume (stationary case). The matter is that instead of expected pressure rise due to ideal gas heating CFX decreases ideal gas density! From my point of view in accordance with mass conservation law in closed system the air ideal gas density = mass/volume is constant!

Problem setup: closed volume with wall at 450K. Initial conditions: 300K and relative pressure = 0 Pa. Reference pressure = 1e5 Pa. Simulation type: Steady State.

P.S. The same model in transient case provides physically adequate result: final pressure = 1.5e5 Pa.

Could anyone advise me something to fix this problem?

Best Regards, Vitmalin

Glenn Horrocks August 27, 2007 18:15

Re: Mass conservation law violation in stationary

CFX can simulate this type of physics so you have got the set up wrong somewhere. What heat transfer model are you using? Is there an outlet somewhere in the box? Is your simulation fully converged, including imbalances?

Regards Glenn Horrocks

vitmalin August 31, 2007 07:59

Re: Mass conservation law violation in stationary
Dear Mr. Horrocks,

Thank you very much for reply. I use total energy heat transfer model together with laminar flow approach. There is no outlets (closed volume) in the sustem and walls are at fixed temperature 450K (no slip bounadry condition). RMS residuals converged below 7e-6 and MAX residuals below 2e-5.

Best regards, Vitmalin

V. Kumar September 3, 2007 13:32

Re: Mass conservation law violation in stationary

Are you taking volume expansion term in the energy equation into account? Seems to be a strange problem. I guess something is wrong with the modelling of your problem. I think the solver is not understanding that in your system density is constant probably because you are applying the ideal gas law and hence decreasing the density with the increasing temperature. Try applying const. density fluid.

Anyway, If you find the solutin please let me know. I am still interested to know if it was or was not a CFX Problem.

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