A Study On The Endwall Unsteady Flow And Loss Of Highly-Loaded Turbine

Highly-Loaded Turbine designe is one of the key solutions for advanced aero-engine to realize the goal of high performance and high thrust-weight ratio.Three-dimensional flow in endwall region is a main source of aerodynamic loss in highly-loaded turbine.And the unsteady flow and interaction problems in endwall region become more serious due to the design of higher pressure ratio,larger turning angle and smaller aspect ratio,which increases the loss in endwall region.Therefore,this paper focuses on the development of highly-loaded turbine,and carries out a study on the mechanism and loss control of endwall unsteady flow in axial turbine by numerical and experimental methods.The investigation covers four aspects of this problem: the complex three-dimensional flow structures in endwall region of turbine rotor,the instability mechanism of tip-leakage vortex,the interaction between endwall-region vortex and downstream bladerow,the interaction between endwall secondary flow and suction-surface separation bubble in low-pressure turbine.The main work and conclusions of this investigation are as follows:(1)Numerical simulation is performed in a high-pressure turbine rotor to figure out the three-dimensional flow structures in endwall region.The result shows that the formation and structure of rotor's casing passage vortex is very different to the hub passage vortex which agrees well with the classical secondary flow theory.Scrapping effect plays an dominant role in the formation of caseing passage vortex.It makes casing boundary layer fluid roll up to passage vortex from inner side to outer side.The strength of leading-edge horseshoe vortex and passage vortex is up to the ratio of tip gap size and incoming boundary layer thickness,which means the depth of blade penetrating into the boundary layer.And these two vortices would almost disappear when the ratio is higher than 1.0.Studies have also been done to analysis the influences of tip caving with cooling air,casing with backward step,rotor outer with casing sealing air,which will be encountered in the real operating environment of turbine.(2)For the breakdown problem of tip-leakage vortex,LES is carried out to study the instability mechanism of leakage vortex.According to the analysis of leakage vortex's formation,structure,breakdown feature,development of flow purtubation,and unsteady spectra,a knowledge of leakge vortex's stability is obtained.And a more complete model for the instability and breakdown of leakge vortex is figured out: the instability mechanism of leakage vortex is charged by two part,the self-induced helical instability of vortex core and the Kelvin-Helmhotz instability of vortex's outer shear layer;under the centrifugal instability,the self-induced helical structure and vortex node induced by K-H instability expand,and ultimately breakdown into smaller vortex.Centrifugal instability takes the dominant role in leakge vortex breakdown.However,even the breakdown do not happen,the K-H instability and vortex core helical instability would result in the unsteady flow in leakage vortex.The shockwave has an important impact on leakage vortex's stability,but it is not the essential reason of vortex breakdown: as a strong discontinuity surface,shockwave leads to flow deceleration and strong negative pressure gradient in vortex core region.It thus triggers off the centrifugal instability and induces vortex breakdown.The “negative jet” effect of upstream wake reduces the leakage flow,but it may enhance the K-H instability and lead to spanwise vortex.Besides,the vortex breakdown can be suppressed or delayed by moving forward the tip profile loading distribution or employing straight profile on the suction surface after passage throat.(3)The transport of endwall-region secondary flow through downstream bladerow and the interaction effect are investigated numerically.Entropy generation is used to locate loss source caused by the interacton between the incoming streamwise vortex and bladerow.It suggests that the stretching of vortex tube near leading-edge suction surface is the main source of interaction loss and the extruding between two legs leads to high dissipation in their connection region.The result also shows that the high loss region at middle span is caused by the suction-side leg of incoming shed vortex.Some instructive conclusions for suppressing streamwise vortex/bladerow interaction loss are obtained based on lean/bow blade to control the interaction between the streamwise vortex and blade row.(4)Numerical and experimental investigation is carried out for the interaction between endwall secondary flow and suction surface separation bubble in highly-loaded low-pressure turbine.The color oil flow visualization has clearly shown the interaction region.Numerical result shows the power for the fluid staying and rotating in interaction region,and there are two reasons for the earlier separation transition of boundary layer: the increased negative pressure gradient on suction surface;the increased original perturbation in boundary layer by endwall secondary flow.But the endwall secondary flow doesn't change the mechnisim of boundary-layer's separation transition.Finally,the design method of endwall leading-edge fillet based on teardrop curve is presented,and its effect on the interaction of secondary flow and separation bubble is investigated.