Study On Flow Separation Control And Mechanism Of Low Pressure Turbine Cascade

High altitude long endurance unmanned aerial vehicle(HALE UAV)is an important complement and enhancement method for reconnaissance satellite.HALE UAV can carry on strategic reconnaissance and acquire strategic information,which plays an important role in information warfare.HALE UAV is highly valued by all countries,and is one of the key technologies in the United States Air Force in twenty-first Century.However,when HALE UAV is cruising at high altitude,Reynolds number in HALE UAV low pressure turbine(LPT)can drop below 0.25×105 due to the decrease of atmospheric density.This may easily lead to flow separation on the suction side of the blade,and decrease the overall performance of the engine.In order to reduce the engine manufacturing and operation cost,and improve the thrust-weight ratio,the load of LPT will increase.It is of great significance to control the flow separation and increase the blade lift.In order to control the flow separation in LPT cascade,the effects of increasing the trailing edge thickness(IT)and deflected trailing edge(DT)on the total pressure loss and flow turning angle of LPT cascade were studied.The flow control mechanism of trailing edge thickness and deflected trailing edge was revealed.The results show that increasing trailing edge thickness and deflected trailing edge can increase the flow angle of LPT cascade,and only reduce the cascade loss at low Reynolds number(Re),improve the aerodynamic performance.However,trailing edge modification reduces the ratio of throat and pitch,and reduces the mass flow of each stage.When the pitch increase and maintain the mass flow,the trailing edge modification can also reduce total pressure loss of the cascade,increase the flow angle.Deflected trailing edge and increasing the trailing edge thickness can deflect the main flow,accelerate the surface boundary layer flow on the suction surface,inhibit the flow separation,and reduced high turbulence area and intensity on the suction surface,On the other hand,deflected trailing edge and increasing the trailing edge thickness enhances the flow mixing.The effects of deflected trailing edge and increasing the trailing edge thickness enhances on the loss of LPT cascade are the composite result of these two aspects.Considering that increasing trailing edge thickness and deflected trailing edge increase the loss of LPT cascade at high Re,the effects of vortex generator jets(VGJs)and trailing edge jets on the aerodynamic performance of LPT cascade were studied and compared.The results show that VGJs can use less mass flow to control boundary layer separation.However,VGJs increase high turbulence area,and the combination of active and passive control was used to improve the aerodynamic performance of VGJs LPT cascade.The effects of the combination of VGJs and Gurney flap(VGJs-GT)and the combination of VGJs and deflected trailing edge(VGJs-DT)on the boundary layer flow separation were studied and compared.The flow control mechanism of VGJs-GT and VGJs-DT were revealed.The results show that VGJs-GT and VGJs-DT can decrease the total pressure loss of LPT VGJs cascade,increase the flow turning angle.Compared with VGJs alone,VGJs-GT and VGJs-DT can use less mass flow rate to inhibit the flow separation,and improve the aerodynamic performance.In order to increase the blade load and decrease the solidity of LPT,trailing edge modification(deflected trailing edge,Gurney flap),VGJs,VGJs-GT and VGJs-DT were used to control the flow separation in the low solidity LPT cascade,and improve the aerodynamic performance.Trailing edge modifica tion can only control flow separation in the low solidity LPT cascade at high Re.VGJs can inhibit the flow separation in the low solidity LPT cascade,however the loss of LPT cascade was still large.VGJs-DT and VGJs-GT can inhibit the flow separation,and decrease the VGJs low solidity LPT cascade.VGJs-DT and VGJs-GT can decrease the solidity 12.5%,without significant increase in loss.Considering endwall secondary flow in the LPT cascade,the effects of boundary layer control devices in the straight LPT cascade were studied.The effects of endwall jets on the endwall secondary flow were studied.The flow control mechanism of endwall jets was revealed.The results show Gurney flap and deflected trailing edge increase the transverse pressure gradient,an d enhanced the secondary flow.Gurney flap,deflected trailing edge,VGJs-GT,and VGJs-DT were only applied in the middle of blade,which can improve the aerodynamic performance of LPT cascade,and avoid increasing endwall loss.The endwall jets can produce vortice contrary to passage vortices,and make the passage vortex away from the suction surface,weakens the interaction between the passage vortex and boundary layer,and decrease the endwall loss.The combination of endwall VGJs and endwall jets can be further reduce the enwall loss of the cascade,compared only with enwall VGJs.The endwall VGJs increase the boundary layer energy and decrease the boundary layer thickness,avoid passage vortex involved in more low-energy fluids,and further reduce the endwall loss.