Investigation Of Shock Control Methods For Transonic Turbine Cascades

The stage loading level of a high pressure turbine is gradually increasing to achieve higher work extraction with the development of aero-engine technology.Increasing the expansion ratio is an effective way to improve the stage loading which,however,will result in severe shock induced aerodynamic losses under transonic operating conditions with supersonic exit Mach numbers,and thus the significant decrease of turbine efficiency.Therefore,advanced blade design method and flow control technique are required to achieve a breakthrough in the aerodynamic design of a transonic turbine.In this paper,investigations on the trailing edge flow characteristics,loss mechanisms,and the corresponding shock control methods for typical highly-loaded transonic turbine vane and rotor cascades are presented.In the first place,numerical and experimental investigations on the flow characteristics with various exit Mach numbers were performed for both transonic turbine cascades.The shock/ boundary-layer /wake interaction and the reflected shock were analyzed in detail,providing a theoretical basis for the development of shock control methods.The blade surface curvature distribution was found to have significant influences on the boundary layer development and the trailing edge shock strength.Hence,a curvature based controlled expansion blade design model was developed for highly-loaded transonic turbines.A continuous,smooth and even controlled curvature distribution could be ensured with this blade design model,which was proved to be effective with excellent agreement between baseline and design profiles for both vane and rotor.Characteristics of a shock-less blade profile can be achieved from aerodynamic optimization methods.Therefore,an optimization procedure,based on Design of Experiment technique and Response Surface Method,was performed to optimize both vane and rotor cascades.Increasing stager and decreasing unguided turning were found to have the most impact on the suction side shock strength,which was reduced by 30.90% for the optimized rotor cascade,with total pressure loss reduced by 6.95% as well.Comparisons between baseline and optimized profiles indicated that the suction surface curvature distribution could be reasonably adjusted to create a flow expansion that was beneficial to control the trailing edge shocks and to improve the loading level at the same time.The controlled expansion concept,combined with the curvature based blade design model,was thereby proposed to reduce the suction side shock strength,the shock/wake interaction loss,and the pressure field non-uniformity at the cascade outlet with decreased pre-shock Mach number at the trailing edge.Finally,the reduced shock method based on a bump designed on the suction surface was developed to control the pressure side shock effects.The shock/boundary-layer interaction and the reflected shock strength were obviously weakened with the pre-compression effect and the weakening effect as a result of the bump induced compression waves and expansion waves,respectively.In addition,analysis of combined controlled expansion and reduced shock methods indicated that their controlling effects on the suction side shock and the pressure side shock were independent,and thus could be adopted solely.With combined shock control methods,numerical results showed a 29.68% reduction in the suction side shock strength,a 29.26% reduction in the pressure field non-uniformity at the cascade outlet and a 12.07% reduction in the total pressure loss coefficient for the rotor cascade.The developed shock control methods were further confirmed with off-design performance analysis.