Hey, thanks for the quick response.
There are severeal issues with this. First I cant keep it under one phase when using inhomogeneous PBM. All Air Phases represent different bubble size classes with different velocities. Therefore multiple Air Phases are needed. Secoundly Degassing Boundary is just for one continous Phase and one disperse Phase. I wrote UDF fixing that that works well so I am able to use degassing for several disperse phases. But Air Volume Fraction is getting up to nearly 30% so if I would use degassing at initial water level I would ignore a enormous part of the column filled with water-air system.