Hi
I suggest you to follow the above suggestion that Mfix need to extend the geometry above the inlet and outlet as Gbotha. The other suggestion from me is the quality of STL file that you export from the software. You need to mesh surface to create more domain in STL file before exporting to the STL file. I use Spaceclaim from Ansys to mesh and then export to STL in my case. I hope that my suggestion can help you to solve your problem.