Negative value for static enthalpy calculation in CFD Post

brajh11 · April 29, 2014, 03:01

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, 04:33

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, 05:03

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, 08:50

You define the reference point in the materials tab.

brajh11 · April 29, 2014, 08:53

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, 18:22

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, 01:25

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, 02:41

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, 03:53

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.

April 29, 2014, 03:01	Negative value for static enthalpy calculation in CFD Post	#1
brajh11 New Member Bostjan Join Date: Apr 2009 Posts: 24 Rep Power: 17	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]

April 29, 2014, 04:33		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Negative enthalpy simply means the enthalpy is less than the reference value. It is the differences in enthalpy which are important. njdyck likes this.

April 29, 2014, 05:03		#3
brajh11 New Member Bostjan Join Date: Apr 2009 Posts: 24 Rep Power: 17	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? Last edited by brajh11; April 29, 2014 at 08:39.

April 30, 2014, 03:53		#9
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	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. njdyck likes this.

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April 29, 2014, 08:50		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	You define the reference point in the materials tab.

April 29, 2014, 08:53		#5
brajh11 New Member Bostjan Join Date: Apr 2009 Posts: 24 Rep Power: 17	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?

April 29, 2014, 18:22		#6
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	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.

April 30, 2014, 01:25		#7
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	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]..

April 30, 2014, 02:41		#8
brajh11 New Member Bostjan Join Date: Apr 2009 Posts: 24 Rep Power: 17	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?