I understood that if you want to model a multi-phase flow (for instance a bubble column), creating boundary conditions (only gas out - no liquid flow out) in Fluent gives problems with convergence. Other software (CFX) would be better suited for these kinds of problems. Anybody has experience in Fluent with this and has suggestions?

Thanks

There are no problems. You can either just patch the top surface with air and accordingly adjust the backflow volume fraction, or better still you can use the degassing B/C UDF available on www.fluentusers.com.