Design And Analysis Of High Performance Multistage Low Pressure Turbine Under Low Reynolds Number

In order to pursue higher performance of aeroengine,reducing weight and increasing blade load becomes an important development direction of low pressure turbine.However,the risk of boundary layer separation for high-load blades at high altitude and low Reynolds number increases significantly,which leads to the rapid decay of low-pressure turbine efficiency and affects the thrust and fuel consumption of the entire aeroengine.Therefore,it’s significative to study the flow characteristics of low pressure turbines in multi-stage environment with low Reynolds number,to analysis the effects of blade boundary layer transition,separation and re-attachment on performance,and to develop high performance multi-stage low pressure turbine aerodynamic design methods.In this paper,two transition models,γ and γ-Reθ,are further developed in the CFD solver NUAA-Turbo.They are suitable for numerical calculation and analysis of boundary layer transition in turbomachinery under steady and unsteady conditions.Under low Reynolds number conditions,the accuracy of transition model is discussed through numerical simulation of T106-EIZ cascade with uniform inflow and upstream wake sweep.The separation correction model is applied to the γ model to solve the problem of excessive laminar separation bubble predicted by γ model,which provides a reliable tool and basis for numerical verification in the design process of multistage low pressure turbine.On the other hand,based on curvature control profile modeling method,a three-dimensional aerodynamic design program for multi-stage low pressure turbine is developed and perfected,which can realize the design of blade profiles with different load distributions and ensure that curvature is continuous and derivable.The radial distribution law includes the design of equal circulation,equal alpha angle and variable power.Then,the T106D-EIZ profile was redesigned by different curvature distributions,and the pre-loaded and post-loaded profile T106D-F and T106D-A were obtained.Under low Reynolds number conditions,the resistance of T106D-F to separation is enhanced,and the separation bubble is further suppressed by the upstream wake.However,the boundary layer of T106D-A is still in an open separation state at the same wake passage frequency due to the increase of the reverse pressure gradient in the diffusion zone.The total pressure loss coefficient of T106D-A is about three times of that of T106D-F.Then,the numerical analysis of typical high-performance low pressure turbine GE-E3 is carried out by using SST-γ model.The results show that the γ transition model with separation correction could capture secondary flow induced transition,tip leakage induced transition and separation transition in multistage low pressure turbine.The isentropic efficiency of the low pressure turbine calculated is 2.9% higher than that of the full turbulent boundary layer assumption.Further reducing the working Reynolds number,the SST-γsep model predicts that the suction surface of the blade will be separated in different degrees,resulting in a 3.6% reduction in the isentropic efficiency of GE-E3 LPT compared with the design point condition.And the GE-E3 is redesigned by the three-dimensional design program,which can improve the isentropic efficiency of the turbine by about 0.8% at lower Reynolds number when the blade load is moved forward,but at the same time,the secondary flow in the end zone will be enhanced.Finally,numerical studies are carried out for typical aft-loaded high-performance five-stage low-pressure turbines,and the design characteristics and flow characteristics of high-load blades are discussed.By reducing the number of blades in each row by 20%,the flow field characteristics of multi-stage low pressure turbine with super-high load profile at high altitude cruise point are studied,and the guide vane which is the main source of loss is redesigned.The results show that when the throat position is moved to the blade channel,the aft-loaded blade still has good aerodynamic performance,and has the advantages of small coverage area of turbulent boundary layer and weak secondary flow intensity in the enwalls.When the blade number is further reduced and the blade profile load is increased,laminar separation bubbles appear in the uncovered area of the blade and could induce boundary layer transition and reattachment.The isentropic efficiency of the five-stage low-pressure turbine decreases by only about 0.9% when the number of blades is reduced by 20%.