Aerodynamic Mechanism Analysis And Experimental Research Of A Counter-rotating Compressor With Boundary Layer Suction

As the improvement of thrust-weight ratio in the design of modern aero-engine,the axial distance of compressor turns to be shorter,loading of single stage becomes heavier and flow turning angle of designed blade is larger than before.Traditional design methods can not reach the requirements in the design of future highly loaded compressor blades.At the same time,the aero-engine should be more efficient to decrease the kerosene consumption.The well control or elimination of losses in the compressor can make a contribution to the improvement of efficiency.In the past years,the active flow control technology–boundary layer suction（BLS）,which is an effectively method in the design of high efficiency and highly loaded compressor,shows great potential in the improvement of blade loading and decreasing the flow separation losses.Theoretical study and experimental investigation on BLS can enrich the design methods and provide technical solutions for the aero-engine with high thrust-weight ratio.The axial dual-stage counter-rotating compressor which locates in the national key lab of aerodynamic design and research,Northwestern Polytechnical University has been studied using BLS in this paper by numerical calculation as well as experimental research at the 100%and 70%corrected rotating speed.This compressor contains inlet guide vanes（IGV）,the first rotor（R₁）,the second rotor（R₂）with the opposite rotating direction and outlet guide vanes（OGV）.Numerical and experimental researches on the designed and 70%corrected rotating speeds reveal that there is high entropy flow near the shroud casing with complex flow pattern at the near stall condition and tip leakage vortex（TLV）contributes to this phenomenon.TLV not only affect the flow near the rotors’tip region but also worsens the incoming flow condition of OGV,makes the OGV worke in a high incidence and undertake serious flow separation on the suction surface.BLS has been employed in the counter-rotating compressor to control the high entropy flow zone,decrease related flow losses and improve the working efficiency.Results of numerous numerical calculations incidate that the flow field having strong TLV and downstream influnence at the near stall condition with 70%corrected rotating speed is very typical and representative.BLS on shroud casing locates between the core district of high entropy flow zone and its 20%chord distance downstram can effectively decrease the high entropy flow zone,improve the flow capacity near the tip region and finally improve the compressor’s efficiency.Flow control measures taken in this paper are as followings:1)BLS on the shroud casing:suciton holes have been designed on the shroud casing over the rotors’tip region and locate at 77%,31%relative chordwise separatively for R₁ and R₂.2)Blade optimization of OGV’s tip zone has been carried out to match the incoming flow near shroud casing and two slots was designed on the suction side so as to furtherly control the flow separation.One slot starts from 28%chord position near the shroud casing,develops with an angle of 10 degrees along the radial direction and covers 40%span.The other one starts from 65%chord position with the same direction and it is half the length of the first one.Several kinds of suction schemes are numerical analysed with unsteady simulations at the near stall condition with 70%corrected rotating speed,the results of post-processing show detailed informations about the development of high entropy zone in the compressor and its interaction with blade row.Research reveals that BLS can not only influence the internal flow but also change the unsteady aerodynamic forces on the blade surface.It will strengthen the vibration on the surface of blade near the suction position,enlarge the range of solid static pressure fluctuation.Through the fast fourier transform（FFT）treatment of these static pressure data,it can be concluded that BLS can change the dominant frequency of static pressure fluctuation,improve its magnitude and creat a series of frequencies with high magnitude.BLS on the OGV has little effect on the pressure fluctuation.Aspirated modification on the counter-rotating compressor have been carried out according to the numerical simulations,hollow outlet guide vanes were manufactured,related equipments such as the vacuum power system,buffer tank,flowmeter and suciton pipeline have been set up or be organized.It has been the first time to do experiments on the aspirated counter-rotating compressor in our country.Experiments were carried out at the representative 70%corrected rotating speed.Large experiments with single position suciton and kinds of two-position combined suction have been finished under this rotating speed.Results show that the effect of single position suciton is very limited and it should use BLS at R₂ position when the compressor works at near stall condition.Two-position combined suction can imprvoe the isentropic efficiency and total pressure ratio in a wider range of massflow,the combination of suction at R₂ and OGV location with a certain suction rate will be the best suction scheme.