Research Of Compressor Cascades Flow Instability Mechanism

Three-dimensional spatial separation in compressor cascades has great influence on loss distribution and working stability of compressor.It essentially decides the loss mechanism and instability process of compressor cascades,thus being a challenging research direction and research priorities in aerodynamics field.Three-dimensional spatial separation reflects the inherent property of flow(viscosity,compressibility & unsteady et al)under adverse pressure gradient.Corner preparation and tip leakage flow are the most important 3D separation structures in compressor cascades,with the cross-passage unsteady properties,directly affects flow losses and working stability of compressor.In this paper,3D separation phenomenon and instability process of both rectangular cascades and rotor cascades are studied numerically based on SST turbulence model.The research includes mainly the following two aspects:First,the paper investigates the spiral structure and flow separation in rectangular cascades based on multi-cascade unsteady calculation.The evolution processes of corner separation and clearance leakage are studied by variable attack angles(from small to near stall)calculation.The results show that a strong twister-like vortex is formed near the rear part of the blade suction surface and the unsteady characteristics of corner separation and clearance leakage increases with the increase of angles of attack.Corner radial flow increases rapidly at near stall angle and interacts with tip leakage vortex at the blade tip flow field which will lead to the breakdown of leakage vortex.The paper gives the evolution processes of 3D separation structures and stage description of instability process.Second,on the basis of rectangular cascades study,3D flow separation(corner separation and clearance leakage)is studied on an isolated compressor rotor by unsteady numerical simulation.The results show that radial flow also exists at the corner of rotor cascades and the intensity of radial flow will increase with the reduction of incoming flow and it shows significant axial fluctuation at near stall condition.The interaction between radial flow and leakage flow will generate a tip secondary vortex near the tip of the blade and force tip clearance flow to form "leading edge spilling flow".The movement of radial flow fluctuation along circumference will lead to the movement of low energy cell and leading edge spilling flow along the same path.The intensity and numbers of leading edge spilling flow will increase and it will become an important factor for causing compressor flow instability.The results of the study show that low energy fluid is formed near the rear part of the blade suction surface under positive angles of attack at both rectangular cascades and rotor cascades.The low energy fluid will move to the tip of the blade at near stall conditions and promote the interaction between radial flow and leakage flow.The cross-passage propagation of radial flow will lead to leakage vortex breakdown in rectangular cascades and generate a tip secondary vortex near the tip of the rotor blade and thus affect the flow stability.