Analysis Of Steady And Unsteady Control Of Flow Separation In Low-Speed Axial Compressor

Flow separation is a complicate flow phenomenon, which exist in all kinds of practical problems of engineering, such as Astronautics, Aeronautics, low-speed fluid machines and so on. The essence of flow separation lies on the mutual interaction between the viscous and unviscous flow. The closed flow separation and reattachment will result in the significant energy loss, which will bring the remarkably negative effect on the performances of fluid mechines, so the control of separation is regarded as one of the key problems of fluid engineering research. The research in this paper is performed around the flow separation and corrective problems in axial compressor. The mechanism of production of corner separation in axial compressor and the controlling of it are discussed from the view of steady flow, at the same time, some new the research perspectives of unsteady flow are studied to control flow separation and reduce energy loss. All of them are carried out, to explore the performance potential of the low-speed or large-scale axial compressors.The steady numerical simulation in axial cascade is carried out by three-dimensional Reynold-averaged Naiver-Stokes equations (RANS). The comparison between the computational results and experimental ones appears the equal relation about numbers of nodal point and saddle point in the cascade passage without tip clearance. By analyzing the layered motion of low-energy fluid in the end-wall boundary layer, the formation cause of three-dimensional corner separation is lucubrated. That is the end-wall boundary layer streamlines are driven toward the suction surface by the circumferential pressure gradient and reversed because of the adverse streamwise pressure gradient. The flow structures under different incoming boundary layer condition are compared, which also appear that the deposit of end-wall boundary layer toward corner is regarded as the crucial factor of 3D separation.The effects on performance of axial compressor with outlet guide vane from different inlet velocity profiles are carried out, the variation rule of boundary layer through the blade passage is appeared. The results which mentioned above are compared with experimental results, to check the precision of computational method used in this chapter. To improve the deposit of low-energy fluid and reduce the loss of separation, the research of chordwise slot in axial cascade is developed, emphasis on examining the effects from several parameters including the positions of suction point and return point and height of slot. In addition, based on the track of end-wall low-energy layered fluid, the suction mass from the suction surface or end-wall is analyzed, the results appear that a few suction mass from reasonable suction point can remove major separation region.The three-dimensional numerical simulations of large-scale axial induced fan with complicate inlet box are carried out. The results show that complex geometry structures of inlet box and illogical axes cover increase the flow instability, loss and apparent circumferential asymmetry downstream. The effects on performances from different pre-whirl conditions of impeller inlet are compared. The tandem blades using in this kind of compressor and relative circumferential adjustment projects of next blade are brought forward and attempted firstly by the author. The computational results indicate that the stall margin and stability is extended, which provides valuable primary effects for further blade controlling research.The coherent structures of leading separation is studies by Large Eddy Simulation, and the reasonable incoming periodicity conditions for controlling flow separation is explored and colligated. By analyzing the evolution of free shear layer at the leading edge of inclined flat, three kinds of flow visualization forms for the free shear layer flow with wall are compared. The results appear that Q criterion is fit for revealing the coherent structures with incoming fluctuation. By comparing the time-averaged performance, unsteady spectrum and so on, the different types of incoming periodicity conditions for better controlling flow separation are analyzed to explore the optimum effect.To search the unsteady potential of axial compressor, the interaction mechanism between the upstream blade row wake and separation structures of downstream large turning angle axial blade row is studied by LES. The two-dimensional computational results reveal that some reasonable unsteady excitations could reduce the separated bubble to some extent. The vortices corresponding to the external effective frequency are enhanced. As a result, the vortices of other scales are entrained by the strengthened vortices, which will make for the combination of the disordered vortices. In the 3D simulations, different incoming conditions bring extreme differential effects. The comparison between two directional motions of wake generator is carried out. Two external frequencies corresponding to f_shed and f_shear respectively strengthen the K-H rolls structures of the separated shear layer, and the included angle between the streamwise vortices and the streamwise direction is more apparent, but there are no clear coherent structures in other exciting frequencies. Finally, in the case with inlet random small-scale fluctuation, the improvement of separated coherent structures and time-averaged performance is analyses.