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Martin January 20, 2001 15:37

Energy equation and Enthalpy
Hi. I would like to know if anyone has experienced this and can explain it to me: In Fluent 5.4, when dealing with the plain air flow at around atmosferic conditions, the reported enthalpy of the fluid is calculated based on cp and some small temperature difference (probably from some reference temperature somewhere, because the reference values should be used for postprocessing only), instead of cp and absolute temperature, how it does it for solids, for instance. The enthalpy is supposed to be h= i+pv = cv*T+pv = cp*T , where i stands for the internal energy. The energy E[J/kg] solved in the energy equation in Fluent is E= h-pv+u2/2 = i+u2/2 . As a result of the above the enthalpy of air is computed very low, thus the reported internal energy and total energy are highly negative. Have I missed anything? I also fiddled with the reference value for enthalpy, but it obviously had no effect. My attention to this problem has been drawn due to problems I am trying to resolve with conjugate heat transfer in 3D (a few messages down). Thanks.

Martin January 25, 2001 18:11

Re: Energy equation and Enthalpy
Just responding to myself here ...

The fact that I neglected is that there is a convention about assigning enthalpy equal to zero to gases at a reference temperature (298K). As a result, the reported internal energy will not be equal to cv*T, but will be highly negative as term p*v is subtracted from enthalpy at atmospheric conditions.

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