Investigations Of Corner Separated Flow Loss Mechanism And Its Flow Control Techniques For Axial-compressors

As a unique flow instability phenomenon in the compressor, the three-dimensional corner separation is one of the most critical factors for the high-loaded compressor design in the future, thereby restricting the further improvement of compressor aerodynamic performance. In order to well understand the flow mechanism of 3D corner separation and the related flow control strategies, three parts of the research work are conducted in this thesis: the physical mechanism of 3D corner separation in high-performance compressor cascades, the boundary layer transition mechanism in the formation of 3D corner separation and flow control techniques on the compressor corner separation. The main research work and contributions are as follows:(1) Physical mechanism of three-dimensional corner separation in high-performance compressor cascades was studied firstly. Based on the previous progress on secondary flow behaviors, numerical simulations of a linear high-speed compressor cascade were performed to analyze the flow mechanism of compressor secondary flow development. Then, a more comprehensive topology of endwall secondary flow is explored for this compressor cascade, in order to well understand the formation mechanism of 3D corner separation: The leading-edge vortex system is formed by the layering of incoming boundary layer flow; The concentrated shed vortex(CSV) is formed by the suction leg of leading–edge corner vortex(LECV); The suction branch of leading-edge horse-shoe vortex(LE-HSV) meets the pressure branch of the adjacent blade in the compressor passage, rolling up the inflow endwall boundary layer, and then forms the passage horse-shoe vortex(P-HSV). Influnced by the cross flow pressure gradient, the newly formed P-HSV is forced to the corner of blade suction trailing edge/endwall, where it coils the corner reverse flow. This develops a local strong blockage in the blade passage, namely 3D corner separation. Included, the interaction between the corner reverse flow and the passage horse-shoe vortex forms a spanwise vortex, 3D corner separation initial vortex(CSIV), which substantially influences the occurrence and development of 3D corner separation in compressor cascades.(2) Then the thesis numerically analyzed what influnce the development of 3D corner separation, and what are the determiners. The development of LE-HSV to the P-HSV is found to be the intuitive factor, but the fundamental cause of 3D corner separation phenomenon are the streamwise adverse pressure gradient and cross-flow pressure gradient in the compressor cascade. Based on this, the paper developed a new criterion for determining the size of 3D corner separation, and analyzed the influence of incidence angle and inflow boundary layer thickness on the size of corner separation by the new criteria.(3) Exploratory study of boundary layer transition mechanism in the formation of 3D corner separation was carried out on the same compressor cascade with LES method. Three existing transition types, the laminar-turbulent(LT) transition on the blade suction surface, the bypass transition and the reverse transition, play essential roles on the origination and development of the compressor corner separation: a) The occurrence of reverse transition determines the initial location of 3D corner separation; b) although the LT transition and bypass transition can affect the development of 3D corner separation and its radial range, however, by their nature, no matter where they exist, they cannot determine the occurrence of 3D corner separation. Based on the study of the transition model of numerical method, free stream turbulence and the turbulence length scales, the rules between the parameters and their influence on the transition mechanism and corner separation are concluded, to benefit the high-performance compressor blade flow control techniques design.(4) Then, the design of flow control methods on the compressor corner separation are addressed as the most important part of this work. In this part, the designing parameters of four flow control methods are discussed numerically to make benefits for the further engineering field: For the endwall boundary layer single suction slot, the axial- and circumferential- slot locations, slot size and suction flow ratio are analyzed; For the endwall boundary layer segment suction slots, the design concept and suction flow ratio are presented; For the blade suction-surface boundary layer suction slot, the axial slot location, slot aspect ratio and suction flow ratio are discussed; And for the endwall corner MTE jet, the jet flow angle and jet flow ratio are investigated. On this basis, through the contrast of controlling mechanism and effects of the four flow control methods, Two flow control schemes, namely MTE suction(an endwall single suction slot) and SS3 suction(a suction-surface suction slot) are found to optimally inhibit the 3D corner separation and its negative effect on the compressor blade aerodynamics. Finally, the experiment method is adopted to optimal flow control scheme, MTE suction slot, at the design condition and proves the reliability of the numerical simulation method used in this thesis. Then, the compressor cascade performance is experimentally studied under the positive and negative incidence angles with MTE suction flow ratio variations, to show the its ablility to improve the corner flow status in compressor cascades and the related cascade aerodynamic performance.