Research On Hydrodynamic Characteristics Of Hydrofoil In Cascade Based On CST Parameterization

The axial flow pump,which has the characteristics of larger flow rate and lower head,is widely used in many fields,such as water conservancy project,chemical industry,ship propulsion and so on.The hydrofoil and cascade are the basic elements of axial impeller and guide vane,its hydraulic characteristics directly affect the performance of axial flow pump.The research on characteristics of unsteady flow around hydrofoil and cascade under different conditions and optimization of shape parameters are one of hot issues in fluid machinery research field.The hydrofoils at five different spans of a certain axial impeller and the inline cascade formed by it are taken as the research objects,and then the multi-condition optimization based on the built optimization platform is carried out.The internal flow optical measurement and LES numerical calculation are used to clarify the characteristics of unsteady flow around hydrofoil and cascade under different conditions.Finally,the differences before and after optimization are also analyzed.The main research work and results are as follows:1.Through comparative analysis of fitting accuraciy and drag reduction optimization effect,the applicability of CST parameterization method and improved Hicks-Henne type function method are compared.The automatic optimization platform composed of CST parameterization method,Matlab,ICEM,Fluent and genetic algorithms is set up,and then the multi-condition optimization of lift-to-drag ratio for the selected five schemes hydrofoil in cascade are carried out.The optimization results are applied to the design of axial flow model pump,and the effectiveness of its application is verified through performance comparison.The research shows that the fitting accuracy and optimization effect of CST parameterization method are better than improved Hicks-Henne type function method within the same design space.The lift-to-drag ratio under different schemes after optimation are increased effectively.The greater the maximum thickness of hydrofoil,the higher the increase.The camber near the leading edge is reduced and near the trailing edge is increased for hydrofoil with larger thickness after optimation,and the camber is increased overall for hydrofoil with smaller thickness after optimation.The optimization results are applied to axial flow model pump,the head and hydraulic efficiency of the optimized pump are increased by 3.4% and 1.7%,respectively.2.The DN50 closed water tunnel experiment platform is set up,and the characteristics of flow around hydrofoil and cascade for scheme a under different Reynolds numbers are measured by PIV and LDA test systems.Comparative analysis of the flow characteristics before and after optimization are carried out.The research shows that the distribution zone of low-velocity region in wake flow of hydrofoil and cascade after optimization are both reduced,and the negative vorticity structure in wake flow is weakened significantly.The normal scale of shear flow region near pressure side and the velocity gradient in this region of hydrofoil in cascade are less than that of hydrofoil.The normal scale of shear flow region near the wall and the velocity gradient in this region after optimization is reduced,and the decrease of velocity gradient at the trailing edge after optimization will reduce the strength of wake vortex.The reduction of shear flow and wake flow is the main hydraulic incentives of lift-drag ratio increasing for hydrofoil and cascade after optimization.3.The LES numerical calculation method is verified and used to simulate the unsteady flow around hydrofoil and hydrofoil in cascade for different schemes.The variation law of flow characteristics of hydrofoil in scheme a with the Reynolds number is analyzed,and then the difference of flow characteristics for schemes a,c,e before and after optimization is revealed.The research shows that the critical point of the transition from reverse pressure gradient to positive pressure gradient on pressure surface will move forward to leading edge,while the critical point on suction surface has the opposite trend.The main characteristic frequency,which value will increase with the increase of Reynolds number,is induced by the evolution of separation vortex in suction side,and the broadband discrete signals are induced by the evolution of multi-scale vortex formed by the break of separation vortex in pressure side.The value of characteristic frequency of flow around hydrofoil is greater than that of hydrofoil in cascade,but its pulsation energy is less.After optimization,the corresponding lift-to-drag ratios of different schemes are improved respectively,and the intensity of wide-frequency discrete signals in low-frequency domain is significantly weakened.There is no obvious flow separation phenomenon near the pressure side and suction side of scheme c and e.The wake of scheme c is a typical Karman vortex street,and the wake of scheme e is composed of Karman vortex,strip wave vortex and strip stable vortex.As the thickness and camber of the hydrofoil decrease,the excitation level induced by the flow shows a decreasing trend.