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October 8, 2009, 02:29 
Diff bet total energy & thermal energy model??

#1 
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Vijesh Joshi
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Hello everyone,
What is the diff bet total energy model and thermal energy model (HEAT TRANSFER MODELS) ?? when to use total energy model and when to use thermal energy model?? what are the parameters on which it depends on?? plz reply... thank you.. 

October 8, 2009, 05:17 

#2 
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Glenn Horrocks
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The "thermal energy" option activates the enthalpy/temperature equation, but does include compressibility effects. It is used for low speed thermal models like AC flows, combustion, heat exchangers, etc.
The "total energy" option activates the full enthalpy equation, including compressibility effects. It is used for high speed flows (Ma>0.3) or anywhere else where compressibility is important. 

October 8, 2009, 15:53 

#3 
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Hi Vijesh,
a further way to think of when thermal energy or total energy option should be selected is to look at the equations solved. In thermal energy we are advecting h. In total energy we are advecting h + 0.5*V^2. If you look at the processes taking place say it is flow and heat transfer on some surface you can compare the size of the terms. If you solve for total energy there will be a temperature recovery as you stagnate the flow. How big is this relative to the heat transfered? Also note that is always correct to solve total energy.....so why do we not solve it all the time? Unfortunatly it adds complexity and might affect robustness due to additional coupling! That is life with our current CFX techology.....if you can neglect something or make it constant...the solver always runs more robustly. Regards.............Bak_Flow PS Glenn I don't really like your term "full enthalpy equation" better is Total Enthalpy or the Full Energy Equation ie D/Dt(h + 0.5*V^2), where h + 0.5*V^2 is the Total or Stagnation Enthalph! Last edited by Bak_Flow; October 8, 2009 at 16:35. 

October 8, 2009, 17:58 

#4  
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Glenn Horrocks
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Quote:


October 9, 2009, 09:56 
which book gives details of models??

#5 
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Vijesh Joshi
Join Date: Mar 2009
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Dear friends,
thanks for the reply... Which text book covers details of heat transfer models?? 

October 9, 2009, 10:31 

#6 
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George
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i think you need a basic thermodynamics book, and after then look into how the models work.
a good undergraduate book is fundamentals of heat and mass transfer by Frank P. Incropera and David p. DeWitt
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