What are you thinking about a outflow BC as inlet in laminar isotherm (no energy equation) case? Other BCs are Wall and Velocity Inlet (as outlet=negativ velocity)? This is perheaps only a theoretical problem! Thanks for your comments!

I tried Inlet as outlet (=negativ velocity) plus pressure-inlet and it worked fine in my test case. But the handbook says, that the bc of the v-inlet as outlet would not be taken into account (Was is taken instead?). On the other hand my result of the massflow was exact that I conditioned by my v-inlet (as outlet). Hence continuity was not hurted.

From the point of solving differential equations it should be possible to have a pressure-inlet and v-outlet as bc instead of the opposite.

But maybe it causes other numerical problems.

Would be nice for me too to hear contributions from experts concerning this question.