Ansys Employee

No to all! 

If you prevent backflow you need to identify why it's happening. What are the wall boundary conditions? I can see a few reasons you are seeing the backflow, but you need to figure that out as I'm bound by a few rules. However, if you answer the various questions you should either get to the answer or provide enough information for us to help. 

Mass in and mass out is a very bad idea. In a steady model in theory you finish up with vacuum or infinite pressure: in practice Fluent will do something and most likely not converge or diverge.