Rob
Ansys Employee

OK, so do you want to model the phase change using Fluent's multiphase models, or just look at the displacement of vapour with fresh air? The latter can be done with phase change, or by adding vapour mass into the system based on partial pressures and source terms. 

With CFD, the starting point of any project is to work out what you want to know, and how much time/effort/cpu you're willing to spend doing it. From that, you work out the model definitions, determine (and defend) any assumptions and document. At that point you open the software.