Mitigation of BLI Circumferential Distortion Using Non-Axisymmetric Fan Exit Guide Vanes

This paper presents an assessment of the effect of non-axisymmetric 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 non-uniformities is characterized, for two-dimensional flow, in terms of the variations in incidence angle and unsteady rotor forces. FEGV designs with non-axisymmetric exit flow angle perturbations can eliminate either of these. More importantly, non-axisymmetric stator row designs can reduce both non-uniformities relative to the influence of an axisymmetric stator row. This suggests that design for distortion need not involve a trade-off between performance and aeromechanics; a non-axisymmetric 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 inlet distortion effects can be mitigated with realizable non-axisymmetric FEGVs. The magnitude of the geometry variations required decreases with decreased axial rotor-stator spacing and increased flow coefficient.