Aerodynamic Mechanism Analysis And Experimental Research Of Non-axisymmetric Endwall In A High Pressure Turbine

With the preformance of modern aero-gas turbine engines improving,the gas turbine as one of the key components of the core engine,has been proposed more stringent requirements,so it is critical to develop a new aerodynamic design for high-performance gas turbines.However,the internal flow field of gas turbines is extremely complex,and the complexity becomes further aggravated as a result of fewer stages,higher blade loads,greater pressure and temperature gradients,the metal material cooling and the cooling system of rim seal required by the development of high-performance gas turbines.Conventional methods of turbine aerodynamic design have struggled to meet the higher demands for the flow field organization by highperformance gas turbines,therefore,it is necessary to discuss pointedly on the fine flow organization and to research on the appropriate flow control technology in turbines.At present,the non-axisymmetric endwall method as an effective technology to control secondary flow has become one of the promising technical reserves for the development of high-performance gas turbines.Thus it is important to carry out the research on the non-axisymmetric endwall method and to explore in great depth the influence rule of non-axisymmetric endwall on the internal flow field in turbines and the flow mechanism of improving the flow field quality by nonaxisymmetric endwall,which is of great significance to the design of high-performance gas turbines.The research works for the non-axisymmetric endwall method in this paper are summarized as follows:Firstly,the research on the profiling method of non-axisymmetric endwall was carried out.According to the basic principle of non-axisymmetric endwall profiling technology,a novel design method of non-axisymmetric endwall profiling was developed based on the Bezier curves.Through the modification and integration of several typical non-axisymmetric endwall profiling methods,a set of “non-axisymmetric endwall profiling system” with multiple profiling methods was established,which combined mesh generation,flow field calculation and other automatic operations for the numerical simulation of the turbine cascade with profiled endwall.The effect of each profiling method on improving flow field quality and reducing flow loss was compared and analyzed.The research on the influence rule of the profiling parameters on the aerodynamic performance and the internal flow feild for each profiling method was carried out systematically.The research results show that the influence rules of the profiling parameters on flow structures and loss in the cascade are different for different profiling methods,but most of them have the best match between the profiling parameters,which is that the convex(concave)surface of non-axisymmetric endwall should be away from the blade surface and the value of peak(valley)is generally within 2%~4% height of blade.Secondly,the optimization design of non-axisymmetric endwall was studied,which adopted an improved genetic algorithm based on the approximation model and was applied to the shroud and hub of an high-subsonic high-pressure turbine stator.The influence of the optimized nonaxisymmetric endwalls on the flow structures in the endwall region of stator was analyzed in detail.The flow mechanism was revealed that the non-axisymmetric endwalls improved the flow structures and reduced the flow loss in the endwall region.The research results show that the non-axisymmetric endwalls readjust the migration rule of boundary layer and the behavior of vortex system in the endwall region by objectively changing the distribution of partial static pressure field,as a result,the effects of entraining the surrounding low-momentum fluid by pressure side leg of horseshoe vortex and passage vortex are weakened and the low-momentum fluid that climbs to the suction surface is reduced.Consequently,the purpose of reduction of the secondary flow loss in the endwall region can be achieved.For the optimized hub endwall,the acceleration of flow field by the convex surface near the suction surface and downstream the throat increases the momentum of local low-momentum fluid and reduces the momentum thickness near the trailing edge of suction surface,which delays the generation of the separation vortex and reduces the scale and intensity of the separation vortex,thereby reduces the secondary flow loss in the hub region.Moreover,when both the non-axisymmetric shroud endwall and hub endwall are applied to the stator,the total pressure loss coefficient is reduced by more than 10%.Thirdly,based on the continuous high-subsonic cascade wind tunnel which locates in the Key Laboratory of Science and Technology for National Defense——Aerodynamic of Cascade and Airfoil Profile Laboratory in Northwestern Polytechnical University,the original linear cascade wind tunnel was transformed into sector cascade wind tunnel and a set of measurement and control system for the sector cascade wind-tunnel test was established.For the optimized nonaxisymmetric endwalls,wind-tunnel test with the sector cascade was carried out,and the influence on the aerodynamic performance on blade surface and the flow loss at cascade exit was analyzed.The test results show that the total pressure loss coefficients of two set of sector cascades with the optimized non-axisymmetric endwalls are reduced 2.11% and 5.88% respectively,and the benefit of the reduction in exit flow loss from the non-axisymmetric upper endwall can also be gained.Finally,the numerical research of the high-pressure turbine stator in the stage condition was carried out for the optimized non-axisymmetric endwalls,and the effect of the stator with optimized endwalls on the performance of the high-pressure turbine was discussed,the aerodynamic performance of the stator with optimized endwalls in the stage condition was investigated,the influence of the stator with optimized endwalls on the internal flow field and the aerodynamic performance of the rotor was analyzed in depth.The research results show that the non-axisymmetric shroud endwall optimized in the stator condition can more significantly reduce the total pressure loss at stator exit in the stage condition and the peak values of entropy at rotor exit caused by the tip leakage vortex and the passage vortex in shroud endwall region are remarkably reduced,at the same time the flow loss is also decreased between the tip leakage vortex and the passage vortex because the interaction between both of them becomes weakened,and the average of entropy at rotor exit is reduced by 0.33~0.45%.Therefore the isentropic efficiency of the high-pressure turbine is improved by using the stator with the nonaxisymmetric shroud endwall.