static enthalpy calculation in CFD Post

 Register Blogs Members List Search Today's Posts Mark Forums Read

 March 21, 2014, 22:53 static enthalpy calculation in CFD Post #1 Member   Join Date: Mar 2013 Posts: 68 Rep Power: 13 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.

 March 22, 2014, 00:09 #2 Senior Member   Join Date: Jun 2009 Posts: 1,799 Rep Power: 31 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. SunnyHere and falsc233 like this.

 March 22, 2014, 07:28 #3 Member   Join Date: Mar 2013 Posts: 68 Rep Power: 13 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. jyothsna k likes this.