Endwall Flow Mechanisms in Electric Ducted Fans

Endwall flow structures near the rotor casing constrain performance and operating range of electric ducted fans. This paper investigates fluid dynamic mechanisms within this unsteady, three-dimensional flow field using steady and unsteady Reynolds-averaged Navier-Stokes simulations. Three fan stages are designed for use with electric aircraft. They are driven by either a conventional hub motor, resulting in a cantilevered rotor, or by a rim motor using a shrouded rotor. The designs share identical nacelle geometry and mission requirements, but their design flow coefficient is varied, resulting in different detailed blade designs. First, the five main sources of loss near the endwall are identified and isolated: flow separation in the tip or shroud gap, mixing of leakage with mainstream flow, endwall and blade shear, and mixing of the blade wake. A parametric study into the effect of clearance on loss mechanisms shows that for cantilevered fans, mainstream mixing dominates loss generation. In shrouded rotors, the relative contribution of loss components depends on clearance size. Second, fan stability is assessed by analysing the interactions between the unsteady flow features. In cantilevered rotors, tip leakage jet spillage triggers rotating stall, whereas for shrouded blades, open corner separations on suction surface initiate the process. The flow structures depend on the clearance size and design flow coefficient. For low design flow coefficient, the fan enters stall but, within a few revolutions, reaches a new equilibrium point characterised by axisymmetric flow. For high flow coefficient designs, the fan stalls more severely, exhibiting rotating stall.