| The design of high-lift low pressure turbines(LPT)is an effective way to reduce the weight,and increase the thrust-to-weight ratio of military aero-engines and the operation economics of civil aero-engines.However,the increase of blade loading will inevitably strengthen the cross-passage pressure gradient,resulting in increasing secondary flow and the corresponding losses.Especially for the ultra-high-lift LPT with Zweifel lift coefficient more than 1.4,the flow separation becomes more serious,which greatly limits the application of ultra-high-lift LPT in high performance aero-engines.For trying to solve this important technical bottleneck,the present study focuses on the unsteady flow mechanism and flow control issues in LPT cascade through numerical calculation and experimental measurement.The experimental investigation is conducted in low-speed large scale cascade wind tunnel with wake generator equipment.Following four aspects of the subject are addressed in this dissertation:(1)Evolution mechanism of secondary flow in LPT: Discussions are focused on the mechanisms of endwall unsteady flow under incoming wakes in a typical moderate-lift LPT cascade.The effects of unsteady wakes on endwall secondary flow,boundary layer on suction surface and flow losses are mastered.Furthermore,the influence mechanisms of Reynolds numbers and endwall boundary layer thickness on secondary flow are discussed in detail,and it also attempts to use upstream wakes to suppress secondary flow.The results show that periodic wakes can improve blade incidence characteristic in terms of reducing blade loading near the leading edge,the interaction between passage vortex and radial vorticity of wakes results in decreasing time-averaged secondary kinetic energy in a wake passing period,and secondary flow is suppressed.(2)The time-space evolution process of secondary flow under periodic wakes passing and the establishment of secondary flow model: Front-loaded and aft-loaded ultra-high-lift LPT(Zw=1.58)are designed on the basis of the typical moderate-lift LPT blade.The interaction mechanism between incoming wakes and secondary flow in ultra-high-lift LPT is emphatically analyzed.The effects of inflow Reynolds number,wakes passing frequency and blade loading distribution on secondary flow structures and corresponding losses are obtained under steady and unsteady conditions.On above basis,steady secondary vortices structure model is improved and developed,and unsteady secondary vortices structure model is preliminarily established for ultra-high-lift LPT cascade,which enhances understanding the formation and development process of secondary flow in ultra-high-lift LPT.(3)Coupling mechanism between periodic wakes and contoured endwall: The design of contoured endwall should be consider the influence of unsteady wakes,otherwise,contoured endwall designed under steady condition is likely to have negative effect on aerodynamic performance of LPT if it is applied in real turbine stage.In present study,optimization of contoured endwall is based on the endwall flow characteristics inside typical LPT cascade under wakes passing,and reveal the influence mechanism of contoured endwall on secondary flow.For ultra-high-lift LPT,the changes of secondary flow structure under the action of flat endwall and contoured endwall are compared in the presence and absence of unsteady wakes.The coupling mechanism between incoming wakes and contoured endwall is clarified preliminarily.The results indicate that the combined effect of the contoured endwall and periodic wakes can further suppress the strength of endwall secondary flow.(4)Combined control mechanism of fluidic oscillator jet blowing and endwall boundary layer suction on boundary layer and secondary flow: At low Reynolds number,new flow control methods are explored based on the research results of flow mechanism inside ultra-high-lift LPT.The control mechanism of fluidic oscillator to suppress open separation bubble on suction surface is studied through Large Eddy Simulation(LES)method for aft-loaded ultra-high-lift LPT blade.Then,the influence mechanism of endwall boundary layer suction on secondary flow is analyzed in detail under unsteady wakes passing.Finally,a global approach to reduce flow loss using endwall boundary layer suction and fluidic oscillator jet blowing on suction surface is proposed,and underlying physical mechanisms are also analyzed in ultra-high-lift LPT.Mass flow of endwall boundary layer suction is provided to fluidic oscillator,in consequence,both secondary flow and boundary layer are suppressed,the aerodynamic loss decreases by about 66.8%,and the aerodynamic performance of ultra-high-lift LPT is significantly improved.Which provides a new idea for controlling endwall secondary flow in ultra-high-lift LPT. |
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