Control Researches To Flow In Compressor Cascade Caused By Bowed Blade And Endwall Jet

A compression system with high loading is always the objective of the modern aircraft engine design.Therefore,it is the key issue to explore how to control the flow separations of large sizes caused by the blade high loading.The combination of bowed blade and endwall vortex generator jet can apply a potential for the application of blade with large camber angle to the real design of compressor.In this study,the flow field structures with/without endwall vortex generator jet were studied by numerical simulation method,and the jet actuation types were divided into steady actuation and pulsed actuation.Firstly,under the inlet condition of 0° incidence and reference boundary layer(the uniform total pressure distribution along the radial direction at cascade inlet),by the optimization design method based on the genetic algorithm,it was obtained that the optimal jet parameter combination projects which could get the largest loss reductions in bowed compressor cascades.The results show that the endwall steady jet can improve the flow fields in bowed compressor cascades by three ways: firstly,the jet fluid can inject momentum into the low energy fluid near the side walls;secondly,a separation line is produced on the endwall when the streamwise vortex caused by the jet develops downstream,which prevents the low energy fluid on the endwall from reaching the suction side,and hence the three-dimensional separations in compressor cascade are suppressed from the source;thirdly,the rotation direction of the streamwise vortex is opposite to that of the passage vortex,and hence the passage vortex is weakened.In positively bowed blade,the flow separation on the suction side almost disappears;in negatively bowed blade,the size of flow separation in corner region also decreases effectively.The largest loss reductions in positively and negatively bowed blades are 2.7% and 9.1%,respectively.With the changes of incidences and inlet boundary layers,the effectiveness of endwall steady jet on the flow field in bowed blades were also analyzed.The results show that the flow separations in bowed cascades are aggravated with the increase of incidence or inlet boundary layer thickness,which applies larger space for the improvement to the flow fields and hence the loss reductions caused by endwall jet also increase.As the inlet conditions change,the improvement mechanisms to flow fields in bowed cascades induced by endwall jet are similar with that at 0° incidences.At negative incidences,due to the weak flow separations in bowed cascades,the improvement to flow fluid caused by endwall jet is limited.In positively bowed blade,because of the large mixing loss induced by endwall jet,the total loss increases by 1%;in negatively bowed blade,the total loss only decreases by 2.9%.At positive incidences,in positively bowed blade,the flow separations on suction sides are all effectively suppressed under all conditions,and the loss reductions increases with the rise of incidence.The largest total loss reduction is 9.27%,which corresponds to the inlet condition of +10° incidence.In negatively bowed blade,as the incidence increases from-5° to +7°,the loss reduction caused by endwall jet increases,and the total loss decreases by 19.75% at most;at +10° incidence,because that the size of flow separation in corner region is too large so that the separation almost occupies the whole blade height,the endwall jet hardly has positive effects on flow field and leads to a loss reduction of 1.37%.As the inlet boundary layer thickness or velocity defect into the boundary layer increases,in positively bowed blade,the loss decrease degree caused by endwall jet increases and the largest reduction value is 8%.In negatively bowed blade,compared to the inlet condition with reference boundary layer,as the inlet boundary layer thickens,the loss reduction caused by endwall jet rises from 9.1% to 12.5%.With the further increase of inlet boundary layer thickness or velocity defect into the boundary layer,the loss reduction almost keeps unchanged,which means that the improvement to flow field in negatively bowed blade caused by endwall jet has reach its limit under the given jet parameters in this study.At the same time,for explaining the mechanisms caused by the combination of bowed blades and steady endwall jet more clearly,the topological theory was introduced to analyze the flow fields in cascades.The results show that the flow control technique can make the separation structure on suction side or endwall transform from saddle point-spiral point type to saddle point-spiral point form or to regular point-spiral point type,which can suppress the formations of flow separations with large sizes and vortices in compressor cascades and hence the total losses are reduced.That is the essential reason why the concentrated shedding vortex on the suction side can be suppressed by applying the flow control technique with the combination of steady endwall jet and bowed blades.At last,it were discussed that the effects and mechanisms to control the flow separations in bowed blades caused by endwall pulsed jet at 0° and +7° incidences.The results show that,under the jet parameters given in this research,the endwall pulsed jets also can cause the loss reductions in bowed cascades at both 0° and +7° incidences.The mechanisms to control flow separations caused by pulsed jet are similar with that of steady case,which is that the pulsed jet can produce streamwise vortices and inject momentum into the boundary layer to improve the flow fields in bowed cascades,but the endwall pulsed jet generates a pair of streamwise vortices with the opposite sense of rotation.In positively bowed blade,the development of passage vortex is effectively suppressed;in negatively bowed blade,the strength and size of concentrated shedding vortex also decreases.