Stability Expansion Mechanism Of Coupled Casing Treatment For Axial Compressor At Variable Speed

Modern aero-engine requires the compressor to have the characteristics of high load,high efficiency and high stall margin.Casing treatment is an important means to improve the stall margin of compressor.However,there is a problem that the effect of stabili ty expansion decreases when the speed changes,that is,the "robustness" is insufficient.Therefore,it is of great significance to explore the structure which can effectively expand stability at different speeds and to study the corresponding stability expansion mechanism.In this paper,NASA rotor 37 axial-flow compressor is taken as the research object.Firstly,the stability expansion mechanism of slotted casing with back cavity and tip injection is studied.On this basis,the parametric study of coupled casing treatment is carried out,and the unsteady numerical simulation of the optimized coupled casing treatment is carried out,The improvement mechanism of the flow field inside the compressor and the stall mechanism of the compressor are revealed.The main research contents and conclusions are as follows:(1)Three dimensional unsteady numerical simulation of NASA rotor37 axial compressor was carried out to study the stall mechanism of solid wall casing at different speeds.The results show that:at 100%and 80%speed,the leakage vortex at the top of the compressor blade is broken when the solid wall casing is near stall,leading to a large area of flow blockage at the top of the rotor blade,which is the main cause of compressor stall;However,at 60%speed,the tip leakage vortex is not broken,and the tip blockage caused by boundary layer separation on blade suction surface induces compressor stall.(2)Based on the stall mechanism of a compressor,the effects of slot casing treatment and tip injection on the internal flow of the compressor at different speeds were studied,and the new stall mechanism of the compressor was also studied.The results show that the slotted casing with back cavity can effectively restrain the tip leakage vortex at high speed,which improves the stall margin of the compressor,but it has weak effect on the blade boundary layer at low speed,which leads to insufficient stability expansion.Because the high-speed jet can restrain the tip leakage vortex and the boundary layer separation,the tip jet can improve the stall margin of the compressor at different speeds,but the improvement effect is not as good as the slotted casing treatment at high speeds.(3)A coupled casing treatment structure(CCT)was designed by coupling the slot casing treatment with back cavity and tipjet,and the unsteady numerical simulation was carried out.The results show that the stall margin increases by 9.31%,8.26%and 8.68%at 100%,80%and 60%respectively.Through the quantitative analysis of flow field,it is found that CCT can increase the tip load of compressor at different speeds,but reduce the tip leakage intensity.At high speeds,CCT can improve the stall margin by suppressing the channel blockage caused by tip leakage vortex.At low speeds,CCT can effectively suppress the channel blockage caused by boundary layer separation and greatly improve the stall margin.At 100%speed,the stall induced by CCT is no longer caused by tip leakage vortex,but by channel blockage caused by boundary layer separation.(4)Multi channel unsteady numerical simulation was carried out for the optimized CCT structure,and the influence of CCT circumferential coverage ratio on compressor performance at different speeds was studied.The results show that CCT has the best performance at 66%circumferential coverage ratio.The stall margin increases by 10.45%,8.63%and 15.38%respectively at 100%,80%and 60%circumferential coverage ratio,and the design point efficiency decreases by 0.53%,0.23%and 0.18%respectively.Through the flow field analysis,it is found that CCT has an unsteady effect on the blockage caused by tip leakage vortex and boundary layer separation,which makes CCT still have a good effect of stability expansion under partial circumferential coverage ratio.Therefore,CCT has the ability to improve the stall margin at different speeds.