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static enthalpy calculation in CFD Post

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Old   March 21, 2014, 23:53
Default static enthalpy calculation in CFD Post
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According to ANSYS documentation, static enthalpy is defined as hs=specific heat capacity at constant pressure (cp) * static temperature. (there is a second term in the equation but it is neglected due to the use of ideal gas according to documentation)

I defined a variable of hs by using the formula stated above and calculate the mass flow average of it on a isosurface. The answer that I got is as below.
(cp=1.0044e-3 Jkg^-1K^-1 for ideal gas)

Mass Flow Average of hs on iso

massFlowAve(hs)@iso
319276 [J kg^-1]

However, if calculate the mass flow average by using the pre-defined static enthalpy in CFD post, I got a lot lower value.

Mass Flow Average of Static Enthalpy on iso

massFlowAve(Static Enthalpy)@iso
19814.4 [J kg^-1]

I calculated the mass average static temperature ad the result is as below.

Mass Flow Average of Temperature on iso

massFlowAve(Temperature)@iso
317.878 [K]

I then use the static temperature and the answer for the pre-defined static enthalphy in CFD post to calculate cp. The cp is only 62.33 Jkg^-1K^-1 and it is a lot smaller than the cp of the ideal gas (defined in cfx-pre). Therefore, I think the pre-defined static enthalpy in CFD Post is wrong. Can anyone advise on this matter? Is it because the pre-defined static enthalpy in CFD Post uses different formula?

Thank you in advance.
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Old   March 22, 2014, 01:09
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If you evaluate the following

( 319276 [J kg^-1] - 19814.4 [J kg^-1] )
---------------------------------------- = 298.1497 [K]
1.0044e+3 [J kg^-1 K^-1]


Does it look familiar ? Check the documentation on how the reference state in the material definition is used when computing properties.

Also, CFD-Post does not compute any properties. Properties are passed from the ANSYS CFX Solver to CFD-Post through the results file.
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Old   March 22, 2014, 08:28
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Thank you so much for your comments! You are right. 298.1497K is the reference static temperature for the material in my case, which is the idea gas. Now I know that the static enthalpy in CFD Post is the relative static enthalpy. I have to add the reference static enthalpy to get the absolute static enthalpy. I just missed this important point in the documentation.
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