Impact of Endwall Clearances on Off-Design Performance of Boundary Layer Ingesting Variable Pitch Fans

Abstract

Aircraft propulsion research in recent years, driven by challenging emission targets, has focused on hybrid-electric propulsion systems such as electrically powered boundary layer ingesting (BLI) aft-fans. Variable pitch BLI fans have performance benefits at low speed, low altitude conditions over conventional designs. However, blade variability necessitates additional nonuniform endwall clearances to avoid intersections during repitch, which act as drivers of secondary flows. This paper investigates, numerically using 3D single passage Reynolds-averaged Navier–Stokes (RANS), the impact of hub and tip gaps on the aerodynamic performance of electrically powered BLI fans with variable pitch at takeoff conditions. It validates how restaggering the rotor leads to better utilization of the electric motor capabilities and improves overall performance while decreasing the relative mass flow across the tip. Comparing the performance at the aerodynamic design point (ADP) between a 14 deg variable blade and a fixed blade with a constant gap height equaling the average of the former reveals a 10.7% higher tip leakage mass flow rate across the fixed blade tip. Correspondingly, the 14 deg variable blade has a 17.7% greater flow rate with respect to a nonvariable blade with the same nominal gap height. Two blade pitch variability ranges of 10 deg and 14 deg were considered, which corresponds to an increase in maximum tip gap at design stagger by 0.2% of the blade span. Evaluating speedlines for the takeoff operating point at a rotor pitch setting of 10 deg resulted in an additional stability margin (SM) of 6.8% for the less variable blade.