Learning Forum

[joshua.wong1196]

I am currently modelling mass transfer between two phases: Water vapour in humid air mixture (phase 1) to ice (phase 2). Currently I'm using the DEFINE_SOURCE UDF. My question is if I need to hook it onto both the species and mass or just the species (h2o)?

I've seen both methods so I am not sure which is the correct one and I couldn't find the answer in the ANSYS guide files (or not looking at the right place).

[aitor.amatriain]

Mass = Phase continuity equation
h2o = Specie (h2o) continuity equation
What do you think is the right answer?

[joshua.wong1196]

If you put it that way, then my guess would be both? I'm not sure if I hook it to the species only, does Fluent automatically apply this change to mass as well?

[aitor.amatriain]

Yes, you must apply source terms in both species and phase continuity equations. You also must define all terms corresponding to the mass transfer.

[joshua.wong1196]

I see, thank you very much for your help.

[Rob]

Don't forget to account for the heat due to phase change.

[Amine Ben Hadj Ali]

You require Two Phase Source UDF's, one bulk mass and the last one for heat transfer wrt to Tref=298.15 K.

[joshua.wong1196]

Thanks for the advice. Right now I'm still working on the mass transfer part but will include energy and momentum too. By the way, since I'm modelling phase change using DEFINE_SOURCE, how will Fluent treat any user-defined heat sources (in this case, mass rate * latent heat)? Will it "know" that its latent heat or will it take it as sensible heat which will cause temperature change when it shouldn't.

[Rob]

It'll change the temperature. If you drip acetone on your skin it'll cool the skin as it evaporates.

[Amine Ben Hadj Ali]

You should include the heat source relative to Tref=298.15 so you need to account for any standard state enthalpies and latent heat.