Investigation On Stability Enhancement Mechanism Of Transonic Compressor Utilizing Self Recirculation Casing

The analysis of compressor instability mechanism and stability control is an important issue accompanying the development of aero engine.In this paper,a transonic rotor Rotor35 is used as the research object.Three-dimensional numerical simulation is conducted to study the flow field characteristics of this rotor at different working conditions,so that the instability mechanism of the compressor is analyzed.Then,the stability enhancement capacity and mechanism of a new-type self recirculating casing which bleeds upon rotor blades are analyzed.Lastly,in order to explore the best casing structure form,the self recirculation casing with different structure is installed at tip region.In addition,the stability enhancement mechanism is studied further,and the reason why the stability enhancement capacity is difference among different casing structure is analyzed.First,three channels calculation model is established to analyze the flow field characteristics and the instability mechanism of the prototype.With the deepening of the rotor throttling,the deterioration trend of flow field at tip region is the most obvious,so the rotating stall begins at the top of blade.In the process of reducing flow rate,the starting position of tip leakage vortex moves forward,and the shock gradually detaches.These lead to the leakage vortex gradually losing its normal velocity resisting the strong inverse pressure gradient of the shock,which results in the leakage vertex breaki down behind the shock surface finally.The accumulation of low energy fluid at the pressure side of the tip,induced by interference between shock and vortex,prevents air coming into the channel,and then the compressor is characterized as blockage instability.The development of low energy cells at tip is unsteady,which affects the pressure distribution at pressure side of the passage periodically,and then the leakage flow and low energy cells coming from the interference between shock and vortex also present the similar development characteristics to the previous one in adjacent passage.This leads to the circumferential propagation mechanism of the unsteady development of the leakage vortex and the low energy cells.Secondly,the self recirculating casing model bleeding air at tip is established.The improvement of stall margin is achieved by 3.33% using only 0.36% of the main flow.It hardly affects the distribution of the total pressure ratio and efficiency characteristic lines before the near stall condition,and the peak efficiency is reduced by only 0.23%.On the one hand,the incidence angle can be periodically reduced through the periodic jet excitation.On the other hand,the leakage flow and low energy cells can be directly excited,and the leakage and low energy cells can be pushed down to the downstream channel.Among the two factors,the second one plays a major role,and the effects of exciting the low energy cells behind the shock surface is better than that of exciting leakage vertex in front of shock.The self recirculation casing enhances the separation on the suction surface at the same time,but overall,it still plays a positive role in the tip flow field,and delays the occurrence of instability.The bearing capacity of the rotor can be improved by the self recirculation casing,and the time average diffusion factor of the rotor can be increased.But when the throttling degree is further deepened,the benefit coming from periodic jet excitation can't maintain the flow capacity of the tip,and then the rotor characterizes as blockage instability similar to the prototype.Finally,several self recirculating casing models with different air bleeding structure forms are established.The difference of the stability enhancement capacity among the three kinds of self recirculating casing with different axial bleeding positions,but without circumferential bias is determined by the magnitude of jet momentum.Through comparing the positive bias casing treatment form which bleeds from tip with the two kinds of casing treatment forms without circumferential bias which bleed behind rotor,it is found that the unsteady process of the coupling between the bridge and the tip flow field can significantly affect the stability enhancement capacity of the self recirculation casing.When the jet is concentrated on the region near pressure surface at the inlet of the channel,the capacity of stability enhancement can be stronger.