Investigation Of Flow Separation Control In Compressor Cascade Using Endwall Vortex Generator Jet

The compressor is one of the main components of a modern aircraft engine.As the engine’s performance keeps increasing,it is a trend that the compressor has higher total pressure ratio,lower weight,and larger blade load.As a result,it is easy to generate large-scale of flow separation in the compressor cascade.Flow separation control method becomes a key technology to improve the compressor performance.Vortex generator jet(VGJ)falls into the category of the active flow control method and has a simple structure and good flexibility.Via a relatively small fraction of the main flow flux,the jet interacts with the main flow and a dominant vortex forms,which will have an influence on the entire flow field.There are a great potential and significance to apply VGJ on the flow separation control in the compressor cascade.Based on a compressor cascade with the NACA65 blade profile and a camber angle of 50°,the effect of leading-edge endwall VGJ on the flow separation control in the cascade were studied.By locating the jet before the cascade,there is a chance to control the secondary flow in the cascade at the initial stage of its formation.Meanwhile,because the jet is on the endwall,holes or slots on the blade can be avoided.Therefore,the blade can be maintained integrated and will be easy to manufacture.A theoretical study on the endwall VGJ was carried out first.The endwall VGJ is essentially an issue of an inclined jet in cross-flow.An analytical model was developed to predict the trajectory and circulation of the dominant vortex(streamwise vortex hereinafter)in the counter-rotating vortex pair.On the basis of the theoretical study,the effect of different endwall VGJ parameters on the flow separation control in the compressor cascade was investigated numerically at 0°,-5° and +5° angles of attack.The aerodynamic performances and flow field of the compressor cascade were analyzed.The theoretical study of the endwall VGJ shows that similar to the issue of a normal transverse jet,in an inclined transverse jet case,the traveling distance(penetration)and circulation of the streamwise vortex are determined by the initial jet impulse component in the transverse plane.From the proposed analytical model,the penetration and circulation of the streamwise vortex are proportional to powers of 1/3 and-1/3 of the downstream station respectively.They are also proportional to powers of 2/3 and 4/3 of the jet velocity ratio(the ratio of jet velocity to freestream velocity)respectively.With increasing jet pitch angle(with respect to the wall),the components of the penetration and circulation of the streamwise vortex in the transverse plane both increase.With the increase of the jet skew angle(with respect to the freestream direction),they both increase first and then decrease,reaching their maximums when the skew angle is 90°.Experimental data from literature were compared to this analytical model.The results show that the trajectory and circulation of the streamwise vortex are predicted well by the model under different jet conditions.Next,the endwall VGJ was applied on the flow separation control in the compressor cascade.Under 0°(design)angle of attack,the numerical simulation was carried out by changing the location,angle and velocity ratio of the jet.The results show that the corner flow separation is weakened evidently by adding the endwall jet.The total pressure loss coefficient at cascade exit is reduced up to 11.5%.The exit flow angle around 25% blade height is increased,which indicates an improvement on the diffusion ability of the compressor cascade.By analyzing the cascade flow field,it is found that the trends of trajectory and circulation of the streamwise vortex are consistent with the analytical model,which proves indirectly that the numerical simulations on the compressor cascade with endwall VGJ are credible.The functioning mechanism of endwall VGJ controlling the flow separation is present as follows.The streamwise vortex jet inducing prevents the endwall low energy fluid migrating to the suction surface and entrains high energy fluid to the corner region,resulting in the increase of the corner fluid momentum and then the capacity against flow separation.However,there is a slight loss increase in the region below 10% blade height due to the interaction between the streamwise vortex and the secondary flow near the endwall,causing intensity reduction and rapid dissipation of the streamwise vortex.Summarizing all the numerical cases under design condition,the reasonable range of effective jet parameters is obtained,with which the streamwise vortex plays the advantageous function fully while its negative influence is suppressed.The numerical investigations were carried out under off-design conditions as well.The results show that by applying the endwall VGJ,the flow separation in the compressor cascade is weakened markedly.Under-5° angle of attack,flow separation near the trailing edge of the blade suction surface disappears completely and the total pressure loss coefficient is reduced by up to 4.1%.Under +5° angle of attack,the flow separation area reduces largely whereas there still is a small fraction of backflow,and the total pressure loss coefficient is reduced by up to 17.3%.As the angle of attack rising,the flow separation area increases due to a higher blade load.By increasing the pitch angle or the velocity ratio of the jet,a streamwise vortex with more strength is generated,which is coincident with the theoretical results.The stronger streamwise vortex can influence the quickly adding secondary flow in the cascade effectively while the additional loss near the endwall doesn’t change very much.Thus the endwall VGJ improves the aerodynamic performance of the compressor cascade even more notably(more reduction of the total pressure loss coefficient at cascade exit).The functioning efficiency of the endwall VGJ in a higher loaded compressor cascade was studied numerically under by a transonic condition of the cascade.Similar to the results under positive angle of attack condition,increasing the strength of the streamwise vortex with higher jet velocity ratio can improve the aerodynamic performance of the cascade further.However,the additional loss near the endwall rises rapidly when the streamwise vortex gets too strong.The reasonable range and trend of choosing the jet parameters are obtained under different flow conditions before and inside the compressor cascade.