Future civil turbofans are moving towards lower fan pressure ratios to improve propulsive efficiency. However, the cost of a low pressure ratio fan is a large engine diameter, which increases the system weight and drag. A way to mitigate these penalties is to shorten the axial length of the engine intake. A key disadvantage of a short intake is the flow will likely separate during sharp climb manoeuvres when the aircraft angle of attack (AOA) is high. Current fans are not designed to ingest a separation at high AOA and their capacity to deliver thrust deteriorates when acutely necessary.Low pressure ratio fans that are designed to maintain pressure rise with a separation at high AOA can exploit the cruise benefit of a short intake. To date, a separation-tolerant fan design methodology is underdeveloped because conventional engines with long intakes could operate with attached flows at high AOA. This paper addresses this opportunity by understanding how the fan sweep design controls the fan-intake aerodynamics at the separated high AOA condition.This paper compares two low pressure ratio fans, radial and swept, coupled to the same short intake at high AOA. Analysis is conducted using unsteady, full annulus CFD. In the first part of the paper, we show the sweep design influences the upstream intake flow, the fan face distortion pattern and in turn the rotor work input. In the second part, we focus on two key sources of loss at high AOA: rotor midspan choking and rotor-separation interaction (RSI). Positive sweep is shown to incline the passage shock and reduce midspan choking losses compared to the radial fan. However, the losses associated to RSI worsen indicating the swept fan is approaching the stall boundary.

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

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

Turbo Expo 2021

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