This paper considers the impact of a damaged leading edge on the stall margin and stall inception mechanisms of a transonic, low pressure ratio fan. The damage is representative of a heavily degraded engine fan blade and takes the form of a blunt, squared-off leading edge over the upper half of the blade span. The ability to predict and understand the fan response to this type of damage could be used to guide mitigation, maintenance and test strategies for rig tests and engines.Full-annulus, unsteady CFD simulations are used to explain the different stall inception mechanisms for the fan at low- and high-speed operating points. A combination of single-passage steady and full-annulus unsteady simulations are further used to show that the fan is predicted to be sensitive to leading edge damage at low speed, but insensitive at high speed. These predictions, along with the hypothesised mechanism described below, were carried out blind in advance of a detailed experimental campaign to evaluate how well a blind prediction approach could match the test data. The results were found to be an accurate prediction of the measured impact on stability.The difference in response is found to be caused by the change between subsonic and supersonic flow regimes at the fan leading edge. At low rotational speeds, the inlet relative flow is subsonic. Rotating stall is initiated by the growth and propagation of a subsonic leading edge flow separation. This separation is triggered at higher mass flow rates when the leading edge is damaged, leading to a reduction in stable flow range. At high rotational speeds, the inlet relative flow is supersonic. The flow undergoes a supersonic expansion around the leading edge, creating a supersonic flow patch which is terminated by a shock on the suction surface. At this condition, rotating stall is triggered by boundary layer separation at the location of this shock, which is well downstream of the blade leading edge. The stall inception mechanism at high speed is therefore shown to be insensitive to leading edge shape, creating a marked difference in fan leading edge damage sensitivity at low- and high-speed operating points.

Fan Stability With Leading Edge Damage: Blind Prediction and Validation

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

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