Flow mechanism of self-recirculating casing treatment in a low-reaction transonic compressor rotor

Abstract

Casing treatment is used to ensure the stability of an aeroengine compressor when it is operated away from its design point. However, currently, there is little published data about the effect of casing treatment on highly loaded axial compressors. In this study, unsteady numerical simulations are applied to explore the flow mechanism responsible for the stability enhancement of a low-reaction (LR) transonic compressor rotor with ultra-high loading owing to self-recirculating casing treatment (SRCT). A special SRCT configuration is designed based on the suction effect, which can simultaneously improve the stall margin and aerodynamic performance. Radial skew in the direction of blade rotation is employed to enhance the control outcomes further. The results show that the best skew angle for stall margin and performance improvement is 45 degrees. Further analysis of the instantaneous flow structures reveals that for the smooth casing, the mechanism by which tip leakage vortex (TLV) breakdown, TSV, and blade loading interact induces periodic oscillations of the tip flow field. As the unsteady suction effect introduced by the SRCT dominates the evolution processes of vortical structures, the original oscillation frequency is converted into the SRCT-induced excitation frequency. Furthermore, a substantial amount of double-leakage flow is inhaled into the recirculation channel without rolling up into the adjacent TLV, which dampens the shock/TLV interaction and the formation of downstream TSVs. The blockage along the pressure side caused by the aforementioned large-scale vortices is effectively eliminated.