Study On The Characteristic And Control Of Secondary Flow In Subsonic Highly Loaded Turbine Cascades

The development of aero engines with high thrust-to-weight ratio is the need of the development of the aviation industry.The highly-loaded design of the cascade,which could reduce the number of blades by increasing the its load,can increase the engine thrust-to-weight ratio by reducing the weight of turbine.However,it also showed that the highly-loaded design would lead to stronger secondary flow that was more difficult to control.Therefore,based on the blade profile at the root of the first rotor in the low-pressure turbine of a turboshaft engine,studies were carried out to learn the characteristic and controlling method of secondary flow.In detail,the study analyzed the relationship between the blade profile parameters and its load,discussed the influences of variable pitch design and variable turning angle design on blade load,analyzed the main flow field characteristics of a large-pitch highly-loaded turbine cascade and a large-angle highly-loaded turbine cascade,analyzed and verified the effect of the blade compound bowing design in the large turning angle highly-loaded turbine cascade as well as the effect of the endwall partitioned profiling design in the large pitch highly-loaded turbine cascade,and studied the joint application of the two control methods based on the two highly-loaded turbine cascades.Besides,an improvement of the experimental system to improve the periodicity of the flow field with the sidewalls kept intact was also presented in the study.The blade load adjustment research studied the relationship between the blade profile parameters and its load,introduced the variable pitch design and variable turning angle design during the load adjustment,and discussed the characteristics and control ideas of secondary flow based on types of highly-loaded turbine cascades.The results showed that increasing turning angle,increasing channel width and the shortening axial chord would all lead to higher blade load,but only the variable pitch design and the variable turning angle design were selected for the consideration of strength and some other aspects.In detail,the variable turning angle design mainly adjusts the load by changing the pitch-wise static pressure gradient,so the large turning angle highly-loaded cascade showed a strong interaction between the passage vortex and the fluid near the suction surface;the variable pitch design mainly adjusts the load by changing the pitch-wise channel width,so the large pitch highly-loaded cascade showed a strong tendency for large-scale suction surface flow separation.The design and application of the compound bowing design were showed based on a large turning angle highly-loaded cascade.The compound bending design applied a positive bowing design on the suction surface of a negative bowed blade near the endwall region in this study.The result showed that the negative blade bowing design weakened the suction surface boundary layer near the mid-span as well as the accumulation and secondary migration tendency of the low-momentum fluid near the shedding vortex by adjusted the static pressure distribution near the mid-span;it also showed that the positive conner bowing design limited the enhancements of the passage vortex and corner vortex during the turning process of the cross flow by increasing the included angle between the suction surface and the cross flow,and weakened the suction surface boundary layer near the endwall as well as the accumulation and secondary migration tendency of the low-momentum fluid in the endwall-suction corner.As a result,the compound bowing design further improved the flow field of the turning angle highly-loaded cascade based on the conventional negative bowing design.Taking the cascades with-20° blade bowing angles as an example,the reduction of the energy loss coefficient that caused by the compound bowing design is 1.66 times that of the conventional negative bowing design.The design and application of the endwall partitioned profiling design was carried out based on the large-pitch highly-loaded cascade.The partitioned profiled is mainly composed of the pressure-side convex structure near the leading edge and the suction-side convex structure at the midstream and downstream side of the channel.The result showed that the pressure-side convex structure suppressed pan-wise and pitch-wise migration tendencies of inlet flow by adjusting the static pressure distribution of the flow field,so that effectively limited the development of the pressure leg of the horseshoe vortex;the suction-side convex structure adjusted the static pressure distribution of the flow field and increased the include angle between the cross flow and the suction surface,so the accumulation tendency of lowmomentum fluid,the development of corner vortex and the flow separation at the trailing edge of the suction surface were all suppressed near the endwall-suction corner.As a result,the energy loss coefficient of the large-pitch highly-loaded cascade was reduced from 0.0564 to 0.0485,of which the secondary flow loss was reduced by 25%.The joint application of the blade compound bowing design and endwall partitioned profiling design was carried out based on both of the two types of highly-loaded cascades,and the mutual influence of the control methods as well as the adjustment of parameters were both discussed.The result showed that the suction surface bowing design was not suitable for the large-pitch highly-loaded cascade because of its high adverse pressure gradient;it also showed that the joint application could improve the comprehensive control effect of the secondary flow: when the compound bowing design and the partitioned profiling design reduced the energy loss coefficient from 0.0988 to 0.0796 and 0.0769,respectively,the joint application can further reduce the energy loss coefficient to 0.0708.