I am also interested in this as well. I typically analyze flow through pressure regulators and most of the time I have a fixed supply pressure and a known outlet pressure, but the flow rate through the regulator is unknown. I have the same issue with mismatched inlet/outlet flow rates when I specify pressure boundary conditions at both the inlet and outlet.
In addition, depending on how the mesh is consustructed, I may also see a pressure at the outlet (surface parameter - average pressure) that differs considerably from that specified as the outlet boundary condition. I don't understand this, since I assumed that the boundary condition would be a fixed value. The calculated pressure resulting at the outlet should exactly match the boundary condition, right?
I have been searching for other information on this example without success so far.
Can pressure be used as an inlet condition? Yes, but most commonly it is used when you have a pressure on both ends. (you're driving flow rate based on 2 pressure conditions)
The more likely cause of your issue is the outlet volume flow rate, the developers do not recommend using that in your BC
definition. However, I have seen it work as long, you just need to be sure you understand the assumptions that it makes and that they are valid for your situation.
When you say it isn't able to converge, can you elaborate on what you mean? What goals do you have defined? What criteria are you using for stopping? What causes the solution to stop...etc. It may just be that you need to remove the default option to stop the solution when it reaches a max of 4 iterations or choose the option "all criteria have been acheived".
Andrew, I'd recommend opening a separate thread for your question. Mismatched flow rates doesn't make sense. Have you checked mass flow rate vs volume flow rate? Regarding the pressures if you're using environmental pressure you might need to look at total pressure if it becomes an inlet. Would need more information to understand the issue. It could be similar to Michal's (stopping criteria) but it sounds different.
Yes, it does sound like a different issue altogether. My example has pressure conditions specified at both boundaries. And by the end of the analysis, the resulting outlet pressure is vastly different from the boundary condition that I specified as an input.
I'll start another thread and attach some results plots.