If the rotational speed of the rotor is known, then pressure boundary conditions can usually be specified. In many cases, the purpose of the analysis is to determine the flow rate for a given pressure. Apply a pressure rise across the device. This will impose the resistance faced (head).
We recommend starting such an analysis with equal pressures assigned to both the inlet and outlet. As the impeller starts rotating and moving flow, the pressure rise can be gradually imposed. This can be done either manually or with a time-varying boundary condition.
Another situation involving a known rotational speed is that the flow rate is known, and the pressure drop is the desired output quantity. For such a model, specify a pressure of 0 gage at the inlet and the flow rate at the outlet. This method will often solve faster than specifying a pressure on both the inlet and outlet.
If the rotational speed of the rotor is unknown (as in the case of the torque-driven or the free-spinning scenarios), then a specified velocity or flow rate is most often appropriate. Recall that a pressure MUST be assigned to at least one opening in the model unless the model is fully enclosed.
Heat transfer boundary conditions can be applied as appropriate to conduct a heat transfer analysis.