Investigation On Application Of Dihedral/Swept Blade And Boundary Layer Suction To Control Vortex Configurations In Compressor Cascades

The development of larger thrust-weight ratio of aero-turbine engine has put a further demand on the aerodynamic performance of compressor component. Since the endwall loss constitutes the major part of cascade energy loss, it is of high value to study the inner flow structure and loss increase mechanism inside compressor cascades. Meanwhile, better controlling of the boundary layer flow shall be the key step to improve the compressor performance.It is well known that flow separation and vortex development directly affects not only the energy loss distributions and the efficiency in three-dimensional compressor cascade, but also the stability. It is important to discuss the separation structures and the vortex model in compressor cascade. Based on the experimental results, therefore, from steady to unsteady and from two-dimensional cascade to three-dimensional rectangular cascade, numerical investiagtions was performed to analyze the rules of flow separation and the vortex development in compressor cascade. The results show that the passage vortex in a compressor is different from the passage vortex in a turbine, the passage vortex in a compressor is not votical and is not a primary role in flow field, and that the concentrated shedding vortex on the suction surface and its mixing with the surrounding fluid play a key role for the aerodynamic performance of the compressor cascade. The results also show that the topology of the critical points on the suction surface varies with the incidence, and that the separation type changes to closed separation from open separation. Finally, a new model of the vortex structures in a compressor cascade was achieved.Based on the knowledge of the separation structures in a compressor cascade, two differert methods including active control such as dihedral or swept, and passitive control such as boundary layer suction, were chosed to control the boundary layer development. At first, a numerical study was performed to analyze the effect of the dihedral/swept with different geometrical parameters, such as dihehral angle, swept angle, blade profile as well as turning angle, and different aerodynamic parameters such as incidence using the topological principles, and to get the aerodynamic mechanism of dihedral/swept blade on different conditions.Positive dihedral cascade has no great effect on the static pressure distribution on the pressure surface, but alters distinctly the static pressure distribution on the suction surface. The spanwise C-shape static pressure distribution is established or intensified on the suction surface, and furthermore, the axial static pressure distribution is changed: the reverse pressure gradient is strengthened at the midspan, and the reverse pressure gradient is weakened near the endwall. Consequently the flow at the corner region changes to open separation from closed separation, contrarily the flow at the midspan changes to closed separation from open separation. Negative dihedral cascade has an opposite effect on flow. Forward swept cascade has established or intensified the spanwise C-shape static pressure distribution both on pressure and suction surface, which weakened the spanwise movement of the boundary layer on the pressure surface. Furthermore, the separation type also changes to open separation from closed separation. The flow performance of the cascade is improved remarkably.Through the discussion of the mechanism of the dihedral/swept cascade on flow, a deeply expatiation on the boundary layer movement theory was carried. Boundary layer movement consists of the movement of surface eddy layers and the movement of free eddy layers which is affected by the separation range and the separation type. In a compressor cascade, the movement of free eddy layers, especially the variation of separation type has a more important effect on flow.To achieve better control of flow separation in high turning compressor cascade, the feasibility and control effect of boundary layer suction was simulated. The variations of flow structure and separation type were analyzed based on the surface patterns by using the topology theorem, to discuss the aerodynamical mechanism of the application of boundary layer suction to control the flow separation and reduce the secondary loss. The results show that boundary layer suction alters the separation structure in highly turning cascades, especially the bouandary layer suction along the span weakens dramatically the flow separation on suction surface, and reduces the critical points. Furthermore, the separation type changes to open separation from closed separation which makes the flow structure simple.Flow loss was reduced by boundary layer suction (BLS) at all kinds of position on the suction surface. However, the optimal position is different with different separation type. According to the criterion suggested by author on estimating energy loss and mass flow, the optimal suction mass flow is 1% of the inlet mass flow. For the cascade with the main character of closed separation, the optimal suction position is just on the staring point of the closed separation. For the cascade with the main character of open separation, the optimal suction position is on the upward of the separation region.When the suction slot on endwall is near suction side, an active effect was achived. The topological structure as well as the separation configuration varies due to boundary layer removal, which restrains the flow separation at the corner and delays or depresses the separation on the suction surface. Compared with the original cascade, the BLS cascade has a higher blade loading along the most span. Furthermore the flow loss decreases and distributes uniformly along the span.