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-   -   Negative value for static enthalpy calculation in CFD Post (http://www.cfd-online.com/Forums/cfx/134381-negative-value-static-enthalpy-calculation-cfd-post.html)

brajh11 April 29, 2014 02:01

Negative value for static enthalpy calculation in CFD Post
 
Anybody know what exactly means negative value for static enthalpy in CFD Post?

I have seen in material definition and the reference state is 25 C and 1 atm.

For example:
areaAve(Static Enthalpy)@Outlet = -1.53639e+006 [J kg^-1]
areaAve(Static Enthalpy)@inlet = -2.5098e+006 [J kg^-1]
areaAve(Static Enthalpy)@Burner_1_Air_inlet = -2198.38 [J kg^-1]

ghorrocks April 29, 2014 03:33

Negative enthalpy simply means the enthalpy is less than the reference value. It is the differences in enthalpy which are important.

brajh11 April 29, 2014 04:03

Where is possible to check the enthalpy for standard material (for example CO, CO2, H20, N2, O2,...) at reference state if material properties is selected in NASA Format?

ghorrocks April 29, 2014 07:50

You define the reference point in the materials tab.

brajh11 April 29, 2014 07:53

I know. I have defined reference point at 25 C and 1 atm but I need to know the value for enthalpy at defined reference point - which value for standard materials (CO, CO2, H2O,....)if material properties is selected in NASA Format. How is possible to get this value?

evcelica April 29, 2014 17:22

Like Glenn said, the value is arbitrary, it is based off some arbitrary zero point. If you want to match something, NIST RefProp is a great source for fluid properties. I.m sure there are tables in textbooks and online as well.

Opaque April 30, 2014 00:25

Check your material definition, and reference state settings.

Under Material: MyMaterial, there should be a section named Reference State, and you can set, or check the following:

Reference Temperature
Reference Pressure
Reference Specific Enthalpy
Reference Specific Entropy

If your material uses the NASA format to define the specific heat capacity, the reference state is implicit within the coefficients, and you will have to evaluate the NASA expression at the reference state (usually 25 [C] and 1 [atm]..

brajh11 April 30, 2014 01:41

If under Material select reference specific enthalpy: -1.15334e+07 J/kg and reference specific entropy: 0 J/kgK at reference state 25 C and 1 atm, what is the value for specific enthalpy and entropy at 100 C? same value?

Opaque April 30, 2014 02:53

A quick look at the ANSYS CFX Solver Theory Guide / Basic Capability / Documentation Conventions / Variable Definitions / Static Enthalpy will help understand how all the pieces are connected.

It explains how static enthalpy is computed for the different materials, and how the reference value is used.


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