I'm still using free surface and I put back the buoyancy reference density to the one of the lighter phase.
Now I add a short piece of conduit at the very bottom and I set the boundary to opening so that I specified the pressure and the fluid values: WATER volume fraction to 1.00 and AIR volume fraction to 0.00 with component details for AIR to 0.233 O2 mass fraction and 0.767 N2 mass fraction. (before the drainage was set as outlet and only the bulk mass flow rate was specified)
Regarding the fluid-dynamic I think the solution is pretty much converged. At least by seeing the gas volume fraction distribution in the column that remain stable over the time. The absolute pressure at the drainage, on the sparger and at the opening on the top (101325 Pa) are all three stable as well. Same situation for mass flow of WATER and AIR on the sparger at the drainage boundary and on the opening at the top of the column.
Checking the molar fraction of O2 at the sparger is 0.21 as it should be.
But again the domain does whatever it wants in terms of composition, I really don't understand and I'm a little bit contrariated about this. Now is quite some time that I can't get ahead of this. Below some screenshot in chronological order from left to right and from up to down.
You can see the molar fraction of O2 going from roughly 0.21 (at least in the region where air is present) to 0.00, then to values very close to 1.00, and then again some pockets at O2 molar fraction of 0.00 appears. I don't understand where that AIR with that composition come from (now all the opening are set as: component details for AIR to 0.233 O2 mass fraction and 0.767 N2 mass fraction).
Additional information that may be problematicin my setting are:
-the disperse phase is compressible (AIR is defined as: material group>user, mixture property>ideal mixture, thermodynamic property>equation of state>ideal mixture)
-At the sparger the volume fraction of water and air are set as zero gradient. This boundary is set as inlet>fluid dependent and then I specified separately the mass flow of AIR and WATER.