SIMULATING STALL INCEPTION IN A HIGH-PERFORMANCE FAN WITH CLEAN AND DISTORTED INLETS

The effect of distorted inflow on jet engine fan stability is an important consideration in both commercial and high performance aircraft applications. Novel commercial aircraft configurations offer significant reductions in fuel burn, but require engine fans to continuously operate with severe amounts of inlet distortion. Complex inlet ducts in high performance aircraft also result in distorted flow at the fan inlet. This paper investigates and compares the mechanisms of stall inception for cases of both clean and distorted inlet flow. Unsteady RANS simulations of stall inception in a high performance transonic fan were obtained for both cases. In both instances, the mechanism for stall inception is shown to be interactions between the detached bow shock and the tip clearance vortex, which causes the formation of two vortices within the blade passage. The location and strength of these vortices both affect the LE spillage into the adjacent blade rows. When inlet distortion is present, mass redistribution upstream of the fan results in variations in rotor incidence. An area of high incidence is located outside the circumferential extent of the applied distorted sector. In the high incidence region, the shock is detached 3.9%-8.1% chord more than the clean inlet case, making LE spillage more likely. The same mechanisms of stall inception are observed when distortion is present. However, the severity of the stalling phenomena is more pronounced within the high incidence region of the annulus. A rotating stall cell grows out of the stalled passages present at the near-stall operating point and ultimately extends 59% chord upstream of the LE and 18.7% span radially. Understanding the effect of distortion on the mechanisms of stall inception will allow appropriate steps to be taken to extend the stable operating range of modern commercial and high performance fans.