# Principal variable in energy equation is total enthalpy or total internal energy?

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 June 9, 2019, 09:30 Principal variable in energy equation is total enthalpy or total internal energy? #1 Member   phd Join Date: Oct 2013 Posts: 76 Rep Power: 11 Hi, there When I use ‘Total energy’ as the heat transfer model, may I know if the principal variable in energy equation is total enthalpy or total internal energy? From the CFX theory guide as shown in the following link: https://www.sharcnet.ca/Software/Ans....html#BGBCHDHC in the energy equation, d(rho ht)/dt is subtracted by dP/dt, which equals to d(rho et)/dt, where et is the total internal energy. Does this mean the principal variable in energy equation is total internal energy? Otherwise, why do we need to subtract dP/dt? PS, I found the majority solvers are all internal energy base, not enthalpy based. Thanks!

 June 10, 2019, 08:34 #2 Senior Member   Join Date: Jun 2009 Posts: 1,662 Rep Power: 29 The total energy equation has 3 variables to represent energy: 1 - Internal energy in the transient term 2 - Total enthalpy in the advection term 3 - Temperature in the conduction term You have a choice to make, the ANSYS CFX Solver decided to solve for total enthalpy; therefore, it is convenient to write the equation in terms of the unknown. As long as the equation represents the total energy conservation in the frame of choice, any variable is OK. lostking18 likes this.

June 10, 2019, 17:09
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phd
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Quote:
 Originally Posted by Opaque The total energy equation has 3 variables to represent energy: 1 - Internal energy in the transient term 2 - Total enthalpy in the advection term 3 - Temperature in the conduction term You have a choice to make, the ANSYS CFX Solver decided to solve for total enthalpy; therefore, it is convenient to write the equation in terms of the unknown. As long as the equation represents the total energy conservation in the frame of choice, any variable is OK.
Hi, Opaque, Thanks! My I know in CFX steady simulation if the pressure transient term (dp/dt) updated with the energy equation at each iteration?

 June 11, 2019, 02:20 #4 Senior Member   Join Date: Jun 2009 Posts: 1,662 Rep Power: 29 For steady state, the dP/dt term would be 0 at convergence; therefore, there is no need to discretize it.

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