Fan aerodynamics with a short intake at high angle of attack

Abstract

Fans that are designed to maintain thrust at the high angle of attack (AOA) flight condition could exploit the cruise fuel burn benefit of a shorter intake design. This article considers how the fan rotor radial pressure ratio distribution and tip velocity triangle can be designed to improve thrust when coupled to a short intake operating at high AOA. Two AOA values are investigated using unsteady computational fluid dynamics: 20 deg (attached flow) and 35 deg (separated flow). Thrust at high AOA is governed by three key loss and work input mechanisms. (i) Rotor choking loss: Flow is accelerated around the intake bottom lip and enters the rotor with high Mach numbers. Fans designed with a tip-high radial pressure ratio distribution reduced choking loss with a separated intake compared to a mid-high design, particularly when the tip velocity triangle was designed with high diffusion instead of high camber. (ii) Rotor-separation interaction loss: The rotor ingests low mass flow when operating inside the separation and the casing boundary layer separates. High diffusion tip designs strengthened the casing separation, but this penalty did not outweigh improved choking loss. (iii) Work input in radial flows: High AOA generates strong radial flows through the rotor, which alter both the amount and the way work is imparted on the flow. Fans designed with a mid-high radial pressure ratio distribution imparted high work on streamlines migrating toward the hub. Consolidating these findings, we propose two design philosophies for improved thrust at high AOA: High work (mid-high radial pressure ratio distribution) or low loss (tip-high radial pressure ratio distribution with high diffusion tip velocity triangle).