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Medison November 8, 2012 04:15

Enthalpy of mixing(heat of mixing)

Can someone tell me is Ansys CFX takes into account the enthalpy of mixing(heat of mixing) when two different fluids are mixed in the process of calculating? If yes, in witch equation?

Thank you very much for your attention.

evcelica November 8, 2012 07:09

I'm guessing you mean some sort of chemical reaction where the mixing process produces heat energy? CFX will not just do this by default. You would have to set up the chemical reaction process to make this happen.

Medison November 8, 2012 07:50

Thank you for your reply evcelica.

I mean, this enthalpy of mixing:

Originally Posted by Wikipedia
"The enthalpy of mixing (also called heat of mixing) is the heat that is taken up or released upon mixing of two (non-reactive) chemical substances. When the enthalpy of mixing is positive, mixing is endothermic while negative enthalpy of mixing signifies exothermic mixing."

For example: I am modeling mixing between two different pure substances(refrigerant vapor). They flow from two different inlets and then mix. Im using "Variable Composition Mixture" and "Ideal mixture" when im defining mixture properties in "Materials". The heat of mixing is taken into account during calculation?

Thank you for your attention.

evcelica November 9, 2012 20:57

I think I understand this "enthalpy of mixing" but I'm not a chemist. Is it like when you mix ammonia and water, and it creates heat and the temperature will rise as the two mix?

If so, then no, CFX will not do this by default, it will just interpolate the material properties between the two; and the temperature would just be an interpolation of the two temperatures as well. If you want to simulate the "enthalpy of mixing" then you will have to set this up in CFX as a reaction process.

Medison November 10, 2012 03:55

As i remember the ammonia and water reacts when mixing forming ammonium hydroxide:
(NH3 + H20 -> NH4OH)
and this reaction is exothermic ideed. But what happens when two non-reactive fluids mix? I suppose, if you choose "Ideal mixture" you just get interpolation of enthalpies with no additional enthalpy change. Am i right?

evcelica November 10, 2012 19:26

Yes I would believe that is correct. Why would CFX ever just add or subtract anything else to the enthalpy? The solver theory guide would have more information on the mixing models.

Medison November 11, 2012 05:35

Thank you very much for your answers! It helped me a lot.

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