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Old   October 19, 2005, 18:21
Default Enthalpy for Energy
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Hello all.

We have lots of different forms of the energy equation we could use. But i read recently that invariably the engineering field use the Enthalpy of Specific Enthalpy in a simulation with chemical reaction.

I would like to know why this form is chosen rather than any other form of the internal energy equation? Is it just based on tradition, or are there logical reasons why this method is best suited to a particular flow problem.

Any thoughts? Thankyou.
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Old   October 21, 2005, 14:12
Default Re: Enthalpy for Energy
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I think it is tradition. I have done lots of combustion simulations using the thermal internal energy equation, and it works just fine. The big numerical advantage of not using the enthalpy equation is that there is no dP/dt term to deal with. But the enthalpy equation has been used successfully by a lot of people, so take your pick.
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Old   October 21, 2005, 17:51
Default Re: Enthalpy for Energy
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There are a number of factors influencing the choice of which 2 thermodynamic variables to use. Pressure or density is usually one depending on low or high speed flow but other factors may shift the choice. The other energy-related quantity is often enthalpy but other common choices are internal energy, entropy and temperature. It often depends on which quantity is the best to conserve numerically given the overall solution procedure. It will vary depending on the solution strategy and "invariably" is not correct except, possibly, within a particular solution procedure.
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