I am thinking of a simple ducted fan with a single set of rotating blades, no stator vanes behind. If the effect of the fan is to raise the pressure of the air, how is there flow into the fan against this pressure gradient?
I understand that the rotating fan blades have the effect of increasing the stagnation enthalpy of the flow. For an incompressible flow, this is equivalent to the stagnation pressure increasing via the fan blades imparting kinetic energy into the flow. I know that static pressure and velocity increase across the fan, so how can there be flow against this pressure gradient?
I think the explanation will have something to do with the different frames of reference chosen to analyse the problem.