For an overall fuel burn benefit, a short intake is needed to mitigate the weight and drag penalties associated to a large diameter, low pressure ratio fan. This weakens the ability of the intake to deliver uniform and attached flow to the rotor when the aircraft angle of attack (AOA) is high, threatening engine thrust. Using unsteady CFD, this paper aims to explain the rotor work and loss mechanisms that set thrust at high AOA, in both the attached and separated intake regimes. Following this, the effect of the rotor radial vortex design on key aspects of the flow is explored.

High AOA imposes a three-dimensional inlet condition on to the rotor, which is broken down into eleven key features that alter the local loss generation and work input around the annulus and up the span, relative to operation in clean flow. Overall, the mass averaged rotor pressure ratio and flow reduces for all designs considered. However, the thrust penalty is mitigated as the rotor-intake system adds axial momentum to the flow as the captured streamtube is turned towards the engine centreline.

The key rotor loss sources in both attached and separated flows is due to high flow acceleration over the bottom intake lip leading to passage choking in the lower half of the annulus. For the separated case, flow is redistributed around the intake separation to high Mach numbers leading to negative incidence at the rotor midspan causing strong shock-boundary layer interaction and flow separation inside the passage. Additional loss is generated during the rotor-separation interaction process. Flow deficit inside the intake separation drives strong positive incidence at the rotor tip, which triggers further separations of the passage and casing boundary layers. These separations merge into a corner separation, which eventually decays without incepting rotating stall. The tip velocity triangle design is shown to be a key design lever in this recovery time after exiting the separation.

High AOA leads to radial mass flow redistribution through the rotor, which is captured using a time averaged particle tracking technique. Radial flows are shown to exchange work input between effective blade speed and exit relative swirl velocity mechanisms. In the lower half of the annulus, streamlines follow a hubward trajectory and exit the rotor at highly cambered sections with high relative swirl velocity leading to a net increase in work input to the benefit of thrust delivery.


Fan Aerodynamics With a Short Intake at High Angle of Attack

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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