Rob
Ansys Employee

The inlet pressure with a velocty inlet is set to get the flow through the domain against the value of the outlet pressure boundary. So, if you have zero on the outlet, inlet pressure will be some Pascals above zero. 

If the flow is constant density use a velocity boundary. Mass flow is designed for compressible flow. It works, but it's lazy modelling: I'm all for that but not where hand calcs can be used. That also allows you to check the flow rate more easily: if you know the velocity and mass flow and run the model are they correct? That gives you confidence that the model size (scale) is correct and that you've used the correct material. 

So, check domain scale (Domain tab, Mesh section) and in Cell Zone what fluid are you using? If it's single phase try deleting the air material.