Sweep Effects on Fan–Intake Aerodynamics at High Angle of Attack

Abstract

Sweep in a transonic fan is conventionally used to reduce design point losses by inclining the passage shock relative to the incoming flow. However, future low pressure ratio fans operate to lower Mach numbers meaning the role of sweep at cruise is diminished. Instead, sweep might be repurposed to improve high Mach number off-design conditions such as high angle of attack (AOA). In this paper, we use unsteady computational fluid dynamics to compare two low pressure ratio transonic fans, one radially stacked and one highly swept, coupled to a short intake design, at the high AOA flight condition. The AOA considered is 35deg, which is sufficient to separate the intake bottom lip. The midspan of the swept fan was shifted upstream to add positive sweep to the outer span. It was hypothesised the swept fan would reduce transonic losses when operating at high AOA. However, the swept fan increased the rotor loss by 24% relative to the radial fan. Loss was increased through two key mechanisms. i): Rotor choking: flow is redistributed around the intake separation and enters the rotor midspan with high Mach numbers. Sweeping the fan upstream reduced the effective intake length, increased the inlet Mach number and amplified choking losses. ii): Rotor-separation interaction: the rotor tip experiences low mass flow inside the separation, which increases the pressure rise across the casing to a point where the boundary layer separates. The swept fan diffused the casing streamtube, which increased the size of the casing separation.