Flow does not support two phase flow. My best suggestion is to model the air as particles. I have done this specifically using small spheres which I gave teh attribute of polystyrene foam, or any other user defined solid that would have a density much less than water.
This will not compute any interaction that may exist in an actual water/air flow, because the particle calculation is done after the fluid flow is solved. But I think it works very well if you are investigating water/air separation.
Like Daniel said, no 2 phase flow in flow simulation. But can you describe your application? There might be some things it can do.
where I will find example of use of Two-phase (fluid + particles) Flows - it's taken from technicalreference.pdf of most recent version.
Or just please give me some hints how to proceed with that in creating a study.