CFD Online Discussion Forums - Static enthalpy in CFX with IAPWS

Hello all,

I have the following problem: I calculate steam turbine in ANSYS CFX and when performing the post-processing I encountered large discrepancies between correlating variables.
Suppose I want to derive static enthalpy at my inlet boundary using the static entropy, temperature, pressure and density with the following formula (all parameters are massflow-averaged except pressure which is surface-averaged):

$H_{in} = T_{in} \cdot S_{in} + \frac{p_{in}}{\rho_{in}}$

but when I calculate the static enthalpy using the formula massFlowAve(Static Enthalpy)@INLET, I get the value which is two times lower than that of the expression above.

One of my ideas was that I didn't take into account the reference entropy and temperature to subtract it from the expression, but in CFX Theory Guide it is written that reference enthalpy and entropy for IAPWS library are set to 0 [J/kg]:

Quote:

The reference state for the IAPWS library is the triple point of water. Internal energy, entropy and enthalpy are all set to zero at this point.

But I found from IAPWS database that the reference conditions for steam are Sref = 9.155 [kJ/kg/K] and Tref = 273.16 [K]. However, even when I substracted these values (Sref*Tref) from the first expression I didn't get the same numbers.

Maybe someone encountered such problem and can help me with that.

Thanks in advance,

Delcraft