# how fluent calculate energy equation?

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May 19, 2019, 18:16
how fluent calculate energy equation?
#1
Senior Member

Weiqiang Liu
Join Date: Feb 2018
Posts: 278
Rep Power: 9
Hi all,

I am doing a porous media combustion with fluent. In the literature, I found the energy equation is solved with enthalpy change. I also know that energy equation sometimes can be represented with specific heat.

The author mentioned that both enthalpy and specific heat is temperature dependent. I defined temperature function of specific heat. However, I cann't see any reason I should defined temperature function of enthalpy.

I wonder does the author mean enthalpy is a function of temperature because specific heat is temperature dependent and enthalpy is a function of specific heat. Therefore, enthalpy is temperature dependent indirectly?

I put the energy equation in the paper below:
It's more a question of thermodynamics than CFD. Can anybody help me?
Best regards.
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 May 20, 2019, 00:36 #2 Senior Member   Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34 is you assume, that specific heat is constant, than enthalpy is not a function of temperature best regards

 May 20, 2019, 09:14 #3 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,679 Rep Power: 66 What are you even discussing... Enthalpy depends on temperature or we'd violate at least one law of thermodynamics.... Enthalpy is heat (or heat potential) and temperature (from kinetic theory) is the average kinetic energy of the molecules in the volume. More temperature = more kinetic energy = more enthalpy.

May 20, 2019, 10:35
#4
Senior Member

Weiqiang Liu
Join Date: Feb 2018
Posts: 278
Rep Power: 9
Quote:
 Originally Posted by AlexanderZ is you assume, that specific heat is constant, than enthalpy is not a function of temperature best regards
specific heat is not constant. it's temperature dependent. However, even specific heat is constant, enthalpy is temperature-dependent, right?

Best.

May 20, 2019, 10:57
#5
Senior Member

Weiqiang Liu
Join Date: Feb 2018
Posts: 278
Rep Power: 9
Quote:
 Originally Posted by LuckyTran What are you even discussing... Enthalpy depends on temperature or we'd violate at least one law of thermodynamics.... Enthalpy is heat (or heat potential) and temperature (from kinetic theory) is the average kinetic energy of the molecules in the volume. More temperature = more kinetic energy = more enthalpy.
I know enthalpy is temperature dependent. I mean how fluent calculate temperature dependent enthalpy. I have defined temperature dependent specific heat. Do I have to define temperature dependent enthalpy or fluent will calculate temperature dependent enthalpy automatically according to specific heat?

 May 20, 2019, 12:05 #6 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,679 Rep Power: 66 You should state specifically your setup because it varies. For combustion modeling for example, you normally use the nasa 7-polynomials which specifies the specific heat + a reference enthalpy and you are done.

May 20, 2019, 12:17
#7
Senior Member

Weiqiang Liu
Join Date: Feb 2018
Posts: 278
Rep Power: 9
Quote:
 Originally Posted by LuckyTran You should state specifically your setup because it varies. For combustion modeling for example, you normally use the nasa 7-polynomials which specifies the specific heat + a reference enthalpy and you are done.
is it because anthalpy is an integration of specific heat plus standard state enthalpy. if I know temperature dependent specific heat, then I know enthalpy under different temperatures. By mass fraction weighted calculation, I know enthalpy of the whole system?

 Tags enthalpy, specific heat, temperature dependent

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