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-   -   about energy equation (http://www.cfd-online.com/Forums/phoenics/51215-about-energy-equation.html)

helen June 28, 2001 03:56

about energy equation
 
I use the newest edition of Phoenics 3.3 to simulate the temperature and velocity fields of steelí»s Solidification process. The energy equation is enthalpy equation. The variable I solved is H1, then I name (H1)=TMP1. I want to know whether the result is temperature or enthalpy. If the variable I solved is TEMP, the result of course is temperature. But what is the difference between the two methods? I use the two methods to solve the temperature field but the result is not same. I do appreciate you if you can explain these.

Mike Malin July 9, 2001 08:20

Re: about energy equation
 
In PHOENICS the energy equation may be solved in terms of enthalpy (H1) or temperature (TEM1). Both these equations have units of Watts when integrated over the contol volume.

If you solve enthalpy H1 then you must specify an enthalpy-temperature relationship (H-T) via TMP1=GRND, for example. If you then STORE(TMP1), H1 is the specific enthalpy and the specific heat is implied in the H-T relationship from which PHOENICS will deduce the temperature TMP1. The boundary conditions must reflect the fact that you are solving for H1 by means of specifying the incoming values of H1 according to your H-T relationship.

If you want to solve for temperature directly, then I would not recommend your current practice of converting H1 to temperature, as you will have to be very careful with your implementation to ensure consistency. It is simply not worth the effort because you can solve for the energy equation in terms of the temperature variable TEM1. This means you will have to specify the specific heat (CP1) and thermal conductivity PRNDTL(TEM1).

If you are dealing with solidification then the evolution of latent heat can appear directly as a source term when using H1 or TEM1, or with TEM1 one can use the alternative approach of an effective specific heat. However, The latter may lead to slow convergence and so I would not recommend that initially.

The boundary conditions and source terms for TEM1 are specified in terms of energy, except in so far when the temperature is known, for then temperature can be specified directly.


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