To start i think you should simulate it with comsol

I don't think this is what would generally be considered a "two-phase" problem unless there is condensation in your steam/air flow or boiling/evaporation in the water phase as both parts of the flow seem to be separated by a solid boundary.  (The two phases have to coexist in the same space to be truly two phase.)  If you have access to either COMSOL or FLUENT, these are fine and very complete tools to start your investigation.  However, if you are wondering which package to invest your money in, could I suggest you first try OpenFOAM (http://www.openfoam.org/), an open source (and hence free) CFD package which also includes multiphase modules as well as a large suite of other physics modules and a large, and very active online community (e.g. http://www.cfd-online.com/Forums/openfoam/).  Your problem should be quick and simple to code up in any of these packages.

Thank you guys for your replies.

David, yes it has condensation in the side of mixture ( steam/air ). I have full access to  FLUENT and I think I can do the same with COMSOL. In fact I would like to focus on one of them. So, if there is any materials, files or links to help me starting my problem that will be great and appreciated. 

I would suggest you should go with FLUENT. It is a classic problem, where you can use mixture model with evaporation and condensation mechanism. This model is available readily in FLUENT (No need of UDF's). To start with, please go through the user manual and also check in the online community http://www.cfd-online.com. This should give a head start for you in the problem.

Thank you Shailesh for your reply.

Mohammed,

I don't have an exact answer to your problem, all what I can say....is that I tried both COMSOL and ANSYS in another field.....COMSOL was superior.....I guess it is the best choice

Thank you  Sallal for adding your opinion.