Fluent 14.5 can simulate evaporation and condensation in a closed thermosyphon without programming any UDF ( you should only define your primary and secondery phase than you active evaporation and condensation). You Will find attached an important article in which the author validated VOF + some UDFSs for mass and heat transfer model with the experimentale results.
In this numerical study, and when I use the model in fluent pricising two mass transfer : one from liquid to vapor and the other one from vapor to liquid. The liquid film in the condenser section is not appearing ( I think that Iam doing something wrong ).
Is it possible to simulate phase change (evaporation/condensation) phenomena in FLUENT? -