# Wrong total enthalpy values

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February 20, 2017, 15:20
Wrong total enthalpy values
#1
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Join Date: Dec 2016
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Hello all,

I'm simulating a radial inflow turbine (single rotor blade and single stator vane) in Ansys CFX with a R134a as the working fluid (hence real fluid). The boundary conditions are total pressure and total temperature at inlet of stator and static pressure at outlet of rotor. The fluid properties are imported from the library.

To my knowledge, which I also found in the documentation, the enthalpy is expressed as a function of pressure and temperature h(P,T). Since the total pressure and total temperature are given (Po = 1690 KPa, To = 350), the total enthalpy must be 447 KJ/Kg according to REFPROP ( I attached a snapshot of REFPROP screen). However, Ansys CFX gives totally different value which 67.619 KJ/Kg ! Same thing for the other enthalpies with negative value of the isentropic static enthalpy at the rotor outlet!

Could anyone thankfully clarify that difference?

All the best
Attached Images
 R134a.jpg (40.7 KB, 43 views) R134aa.jpg (91.0 KB, 30 views)

February 20, 2017, 15:25
#2
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Attached also the value of the enthalpy from ANSYS CFD post. It is obvious that the total pressure and temperature are slightly different. However, the impact of that on the enthalpy value is negligible.
Attached Images
 R134aaa.jpg (146.7 KB, 30 views)

 February 21, 2017, 06:50 #3 Senior Member   Join Date: Jun 2009 Posts: 174 Rep Power: 17 Go back to Basics of Thermodynamics at college. Enthalpy and entropy are measured relative to any reference. Only the delta is good to see.

February 21, 2017, 07:51
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Quote:
 Originally Posted by turbo Go back to Basics of Thermodynamics at college. Enthalpy and entropy are measured relative to any reference. Only the delta is good to see.
Thank you Turbo.
I'm already at college and learing Thermodymanics now. So, patience and a bit detailed answer would help

That what came to my mind .. In the documentation, I believe the default reference point is 25c and 1 atm
the delta you mean is the difference between the value at state point and the value at the reference, am I correct ?

 February 21, 2017, 09:27 #5 Senior Member   Join Date: Jun 2009 Posts: 174 Rep Power: 17 Turbomachinery analysis is only focused on the delta enthalpy between inlet and exit.

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