Diff bet total energy & thermal energy model??

vijeshjoshi23 · October 8, 2009, 03:29

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..

ghorrocks · October 8, 2009, 06:17

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.

Bak_Flow · October 8, 2009, 16:53

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!

ghorrocks · October 8, 2009, 18:58

Quote:

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!

Point taken, thanks.

vijeshjoshi23 · October 9, 2009, 10:56

Dear friends,
thanks for the reply... Which text book covers details of heat transfer models??

ckleanth · October 9, 2009, 11:31

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

October 8, 2009, 03:29	Diff bet total energy & thermal energy model??	#1
vijeshjoshi23 New Member Vijesh Joshi Join Date: Mar 2009 Posts: 10 Rep Power: 17	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, 06:17		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,844 Rep Power: 144	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. sasanghomi, 86682164, hassan1201 and 1 others like this.

October 8, 2009, 16:53		#3
Bak_Flow New Member Join Date: Jun 2009 Posts: 8 Rep Power: 17	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 + 0.5V^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 + 0.5V^2), where h + 0.5V^2 is the Total or Stagnation Enthalph! Mazze[ITA], yuvarajmdesign, k.vimalakanthan and 2 others like this. Last edited by Bak_Flow; October 8, 2009 at 17:35.*

October 9, 2009, 10:56	which book gives details of models??	#5
vijeshjoshi23 New Member Vijesh Joshi Join Date: Mar 2009 Posts: 10 Rep Power: 17	Dear friends, thanks for the reply... Which text book covers details of heat transfer models??

October 9, 2009, 11:31		#6
ckleanth Senior Member George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 18	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 __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

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