Mitigation of Boundary Layer Ingestion Circumferential Distortion Using Nonaxisymmetric Fan Exit Guide Vanes

Abstract

This paper presents an assessment of the effect of nonaxisymmetric fan exit guide vanes (FEGVs) to mitigate, or even reduce to zero, the impact of circumferential flow distortion—such as created by boundary layer ingestion (BLI)—on fan rotor efficiency and vibratory structural forcing. The fan response to circumferential nonuniformities is characterized, for two-dimensional flow, in terms of the variations in incidence angle and unsteady rotor forces. FEGV designs with nonaxisymmetric exit flow angle perturbations can eliminate either of these. More importantly, nonaxisymmetric stator row designs can reduce both nonuniformities relative to the influence of an axisymmetric stator row. This suggests that design for distortion need not involve a tradeoff between performance and aeromechanics; a nonaxisymmetric aerodynamic design approach can improve both simultaneously. Analysis of the impact of stage design parameters has been carried out to determine FEGV exit flow angle variations that shield the rotor from the adverse effects of circumferential distortions representative of BLI. The results indicate that inlet distortion effects can be mitigated with realizable nonaxisymmetric FEGVs. The magnitude of the geometry variations required decreases with decreased axial rotor–stator spacing and increased flow coefficient.