i am trying to solve one problem having both compressible (air-ideal gas) and incompressible (water) fluids and the solution is not converging but when we have taken both the fluids (air &water) as incompressible fluids, the solution is converging. Can you tell what the problem in giving conditions was and how to handle the problem if we are having compressible and incompressible fluids using simultaneously?

You can try a density based solver. I think you are using a pressure based solver

what do your boundary conditions for your outlet ? try to use pressure out-let.

So long MAch number for air (the compressible) media is < 0.3, you can always treat it Incompressible without loss of acccuracy!

What phenomena (application) are you trying to solve!

Dude, try a pressure based coupled solver. It might work in your case. Give air as compressible and water as incompressible phases