UDF for boundary condition setting

Luke

Hellow~ In a tank, I want to do the water face which have the effects of the thermal convection out and the internal gas released through the water face. (I don't care the air grid outside the tank, just care about the tank filled with water. )

But no boundary type is suit for these two conditions. If the water face is setting to "WALL", it can be set to "CONVECTION" or "MIXED" and it has the effect of thermal convection out off the wall (water face). But if considering to gas released through water face from the internal tank, the "WALL" can't be used. (Because the gas will be stop by the water face wall.) How can I hold these two phenomenon? (thermal convection out and gas released through this boundary of water face)

Somebody tell me that I can set "WALL" to water face and specify some species like H2 gas can pass through the wall by UDF used. Is it really?

My problem is very big to me. If anybody understand how to do my problem, please tell me ! Thank you~