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Old   May 15, 2013, 14:39
Default [THEORY] Species equation
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Dear CFD folks,

Hi have a simple, fundamental question;

For the mass fraction equations, (or species equations), are they representing only the gas-phase species? What about the liquid-phase species?

For example, if I am to model the two-phase flow of the fully humidified hydrogen (hydrogen gas + water vapour) and include the condensation/evaporation processes as the source/sink terms to the species equations, the MASS FRACTION OF H2O equation will represent gas or liquid water?

For example, there will be one equation for hydrogen gas with no source/sink term
another equation for water vapour with source/sink term

if evaporation occurs, more water vapour is added to the system as the positive source term

On the other hand, if condensation occurs, some water vapour will be taken out through the negative source term (sink term)

Am I understanding this correctly? Any comments would be very much appreciated.


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Old   May 16, 2013, 03:48
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Hi, in a multispecies formulation like that, the H2O mass fraction represents the water GAS phase.
Apart from the hydrogen and water gass mass fractions you will need a third transport equation for the liquid water phase with, as you said, the appropriate source and sink terms to account for the mass exchange processes (evaporation and condensation) between the liquid and gaseous water phases.

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