Investigation On The Coupling Relationship And Mechanism Of Curved Blade Parameters In Subsonic Compressor Cascade

With the increase of thrust-weight ratio of aero-engine,the flow and loss of highload single stage compressor becomes a research hotspot.The increase of cascade load means that the viscous effect,adverse pressure gradient,unsteady characteristics and complex geometric configuration dominate the flow field with complex separated flows and vortex structure as the main characteristics,which fundamentally affect the compressor performance.Therefore,under the premise of improving the cascade load,how to effectively organize the secondary flows in the endall corner to achieve efficient and stable flows by reasonably selecting design parameters and the matching relations is the key problem to be solved to improve the performance of the high-load cascade.Curved blade can reasonably and effectively organize the three-dimensional endwall corner flow to improve the performance of the cascade,by changing the pressure field distribution in the endwall corner region.The basic research and engineering design work of curved blade in the range of conventional cascade design parameters have shown that curved blade has a significant effect on the overall performance improvement of compressor cascade and multistage compressor.However,within the range of unconventional cascade parameters,the influence of the coupling effect of curved blade and cascade design parameters on the corner pressure field,boundary layer development mechanism,vortex composition and its development mechanism,separation flow control method,flow field characteristics under cascade choke and stall conditions remains to be discussed.In this paper,by means of the effect of curved blade on the pressure field reconstruction in the endwall corner region,a large number of curved cascade programes are calculated by numerical simulation method validated by experimental data.The variation of the flow field structure in curved blades at different cascade parameters is analyzed,the development mechanism of boundary layer and vortex motion and the highload flow control mechanism under unconventional design parameters are discussed,and the relationship between curved blade design parameters,traditional critical cascade design parameters and cascade end wall corner flow and flow losses is established.Firstly,the influence of blade curving on cascade flow field structure evolution and cascade flow loss was analyzed.Results show that in the cases of loa-load cascade with open cornenr separation flow,the main source of endwall flow loss is caused by the lowenergy fluid in the end-wall boundary layer suction by passage vortex.Positive curved blade enhances the radial pressure gradient of the blade and effectively weakens the boundary layer accumulation in the suction surface separation vortex(SSV)and concentrated shedding vortex(CSV),thus reducing the loss.However,the leading edge horseshoe vortex are increased,the inlet boundary layer at the leading edge is thickened and the endwall loss is increased.At the same time,the airflow transverse motion on the endwall of the cascade is enhanced,which leads to the enhancement of the passage vortex and the starting position of the passage vortex moves forward and conforms to the position of the lowest pressure point on one side of the suction surface.The high loss caused by the low-energy fluid convolution in the passage vortex and the profile loss caused by the accumulation of low-energy fluid in the middle of the blade span are dominant,so curved blade cannot achieve positive effects.With the increase of blade load,horseshoe vortexes and passage vortex increase,corner separation flow changes from open separation to closed separation,suction surface separation vortexes and concentrated shedding vortexes increase,and separation starting point moves forward.The high loss of cascade is mainly caused by the suction surface separation vortex and the concentrated shedding vortex.After the blade being curved positively,although transverse pressure gradient of endwall continues to be increased,transverse secondary flow is increased,whose scale and strength are increased,and trailing edge severse flow strength and range are increased,the separation flow in the corner region is effectively weakened,the separation vortex on the suction surface and the concentrated shedding vortex are reduced,the cascade loss is effectively reduced,and the diffusing capacity of the cascade is improved.With the increase of incidence angle,the corner separation flow breaks down into corner stall or even cascade stall.Positively curved blade delays the occurrence of corner stall.With incidence increasing,the separation form changes from closed separation on corner region to trailing edge separation on the suction surface,and the low-energy fluid in the middle of the blade blade seriously accumulates,resulting in an increase of total loss.After the stall happening,the endwall backflow moves forward to the leading edge and the passage vortex disappears.But columnar streamwise vortex appears at the outlet of the trailing edge,and the convolutional low-energy fluid flows out of the cascade,and the cascade loss continues to increase.The loss is mainly caused by the mixing of the suction surface separation vortex and the streamwise vortex with the boundary layer in the middle of the blade blade.Secondly,the influence of different parameters on the flow structure and performance parameters of cascade are analyzed,including the impact of the minimum loss incidence and the accordingly performance parameters,as well as the critical incidence and performance parameters.(1)Solidity decreases,cascade load increases,transverse pressure gradient increases.Under a large negative incidence angle,the separation flow in the corner area is weak,when decreases,the distance between adjacent blades decreases,the corner separation flow intensity weakens,the passage vortex onset moves dowstream slightly,and the cascade loss decreases.As the incidence increases,the cascade load increases further,the corner separation flow increases.When the solidity decreases,the separation flow increases and the suction surface separation vortex concentrated shedding vortex increases,the cascade loss increases.(2)The decrease of aspect ratio significantly increases the scale of suction surface separation vortexes and concentrated shedding vortexes,but the vorticity strength is slightly reduced.The main source of cascade flow loss is the accumulation of low-energy fluid on suction surface,which leads to the decrease of cascade load and static pressure rise.In straight blade,the passage vortex onset moves forward while the intensity and scale decrease slightly.The dissipation position of the suction surface branch of horseshoe vortex moves downstream,while the dissipation position of the pressure surface branch moves forward.Positively curved blade reduces the intensity and scale of suction surface separation vortex and concentrated shedding vortex,while enhances the passage vortex.With the aspect ratio decreases,the passage vortex onset moves backward,the strength and scale decrease,and the loss caused by the passage vortex decreases.(3)With the increase of inlet Mach number,the development speed of boundary layer decreases,but the separation flow in cascade corner region is enhanced,so there is an inlet Mach number that can minimize the loss of cascade.The optimal angle increases with the increase of inlet Mach number.(4)With camber anlge increasing,the cascade load increases,the corner separati on flow strengthens,the corner separation transforms from the open separation to the closed separation.Positively curved blade enhances the radial pressure gradient and reduces the separation flow in the corner region,so that the separation flow changes from closed separation to open separation.In particular,under the condition of large-load cascade(low solidity,low aspect ratio,and large camber angle),positively curved blade can effectively reduce secondary flow loss in the end area of cascade,restore the diffuser capacity of endarea cascade,and improve the performance of cascade,but at the same time,blade profile loss in the middle part of blade development is increased.Finally,the influence of the geometric and aerodynamic parameters of the cascade and the positively curved blade on the minimum loss incidence angle of the cascade,the diffusion factor and the total pressure loss coefficient under the minimum loss incidence angle,the performance parameters under the critical incidence angle and the stable working range of the positive incidence angle were analyzed.Using regression analysis method to establish the correlation of minimum loss incidence,corresponding diffusion factor and total pressure loss coefficient,and establish the variable relationship between performance parameters and curved blade design parameters as well as the traditional key cascade design parameters,which can more accurate predict the minimum loss condition.Curved blade breaks the balance of parameters in straight blade cascade and enriches the design scheme of blade cascade.Under the condition of minimum loss incidence,curved blade can obtain higher diffuser pressure,lower loss and fewer blade number.Based on the analysis of cascade performance parameters of at design conditions and the positive stable working range,the parameter matching relation of curved blade design scheme with high load and wide working range is revealed.Results show that the use of positively curved blade in the high-load cascade with low solidity and low aspect ratio can not only reduce the cascade loss and blade number in the design condition,but also expand the stable working range of the cascade,providing some technical support for the design of high-load diffuser cascade.