Investigation Of Boundary Layer Suction On The Influence Of Flow Control And Performance Of Axial Flow Compressor

The progress of aero-engine generates a heavier demand on the performance ofits key component, such as compressor. As the stage load of compressor increases, thephenomena of separation flow and secondary flow in the blade passage greatlydecrease the efficiency and stability of the compressor, which in turn, hinders theadvancing of stage load. Further exploitation of potential performance of axialcompressor benefits from the investigation of flow mechanism and flow controlmethod within compressors. In axial compressors, the separation flow of boundarylayer, corner stall, passage vortex, leakage vortex, etc. of both rotors and stators havegreat influence on the performance of whole compressor; the aerodynamic excitingforce caused by shedding vortex from blade surface largely affects the stableoperation of compressors. Meanwhile, as one of the several flow control methods,boundary layer aspiration has proven to be effective to control the separation ofboundary layer and increase stage load. Therefore, a deep study of flow field controlusing boundary layer aspiration is very important to further enhance the performanceof axial compressors.To validate the improvement of compressor cascade flow field using boundarylayer aspiration, the author designed two aspirated compressor cascade and measuredthe performance and the wake curves of the cascade before and after the boundarylayer suction. Study showed a significant decrease of cascade loss. Under variousexperiment conditions, the width and depth of the wake curve obviously narrowedand decreased, and the wake curve moved nearer to the blade trailing edge, whichindicated a weaker separation and a larger flow turning angle.Blade surface shedding vortex has essential impact on inter-stage interference andstable operation of axial compressors. Employing the approach of large eddysimulation, an investigation was made on a single airfoil and an aspirated cascadebefore and after boundary layer aspiration. This work successfully captured theshedding vortexes from blade surface on the single airfoil and the aspirated cascadeunder various conditions. In the numerical experiments without suction, sheddingvortexes mainly comes from blade suction surface and randomly comes into beingalone rather than in couple, which is commonly seen in Kármán Vortex. Afterboundary layer suction, shedding vortexes of the single airfoil is controlledsignificantly. The boundary layer along both pressure and suction surface shed toform typical Kármán Vortex, the principal frequency and the amplitude of the Kármán Vortex remarkably increases and decrease respectively under variousoperating condition; and so is the aspirated cascade under small incidence anglecondition. Under large incidence angle, on the aspirated cascade, the sheddingvortexes cannot be control totally and no Kármán Vortex is found to exist at thetrailing edge. However, when more mass flow is removed, the structure of sheddingvortex become more regular and the amplitude is notably decreases.As for the linear cascade, different schemes of suction surface suction, endwallsuction, and combined suction surface and endwall suction were designed; for thebowed cascade, different schemes of suction surface suction and endwall suction weredesigned respectively.Numerical simulation results show, for linear cascade localaspiration on blade suction surface can improve the flow field at the aspiratedspanwise position, nevertheless, the flow field is even worsen at spanwise positionwithout suction; and full span suction can improve the whole flow field. For linearcascade, the phenomenon of separation is totally eliminated using suctionsurface/endwall combined aspiration and a more uniform static pressure distributesalong spanwise.Based on the full comprehension of bow blades, attention was put on the bowedcascade boundary layer aspiration.When the positive bowed blade was combined withlocal boundary layer suction, improvement was seen at the suction surface and nearcorner region; when combined with full-span aspiration, mid-span flow was obviouslyimproved with a less-improved flow field at near corner region on suction surface,resulting from the spanwise static pressure distribution. Without boundary layersuction, negative bowed blade controls the mid-span separation but worsen the flowfield near endwall. When combined with endwall suction, the flow field near endwallnotably improved, but the separation region at mid-span was remarkably enlarged.Based on the understanding of influence of tip gap size on flow field, various tipendwall suction slots were designed and numerical simulation was carried out toinvestigate the impact of endwall suction on blade tip leakage flow. Results show that,for cascade without suction, larger tip gap and incidence angle cause stronger tipleakage vortexes. After employ endwall suction, tip leakage vortex is weakened bylarger suction mass flow, and the blade tip passage vortex’s loss is the tip leakagevortex’s gain. In various schemes, slot right above the blade tip gain most benefits interms of tip leakage vortex and slot located near suction surface comes second; slotnear pressure surface comes last. When located right above the blade tip, long slotalmost controls the leakage vortex. The near tip region is improved and spanwise location away from the tip was worsen in terms of loss coefficient. Static pressureratio is also increased when more mass flow is removed.Moreover, efforts were made to investigate the boundary layer suction in amultistage environment in a dual-stage counter-rotating axial compressor. Accordingto the flow field, the suction slot was located at different location at front rotor (R1)and rear rotor (R2) to consider the influence of boundary layer suction of R1or R2alone on the performance and flow field details of the counter-rotating compressor. Inaddition, extra discussion was made to combine the suction on R1and R2at thecondition where efficiency of whole compressor and R1peaked respectively usingsuction on the front rotor alone. Results indicates that, when R1aspirated alone,efficiency of R1increases as suction mass raises; however, efficiency of R2decreasesbecause of the inlet aerodynamic parameter mismatch the condition of R2. As a whole,the efficiency of the counter-rotating compressor first increases then decreases withthe suction mass flow increases thus resulting a peak efficiency; and as much as0.9%of efficiency increase is seen. When R2aspirated alone, efficiency of R2firstincreases then decreases and R1is barely affected by downstream condition. As awhole, the efficiency of the counter-rotating compressor first increases then decreasesand as much as1.4%of efficiency increase is seen. Combined suction on R1and R2further increase the efficiency.Finally, a preliminary design of super high pressure ratio aspirated compressorwas designed. The rotor employed the approach of super large turning angle, impulsetype rotor and low aspect ratio. Front section of the rotor blade deployed apre-compression airfoil and the turning is focused on the rear section, which evenexceeds90%at all spanwise position. The pressure ratio of the rotor can reach as highas5.7and the corresponding efficiency is87%. The effect of boundary layer suctionon the aerodynamic performance of the compressor is verified.