The Aerodynamic Design And Optimization On Large Expansion Ratio Axial Turbine Of Marine Turbocharger

China's marine low-speed diesel engine industry has achieved rapid development in recent years,but marine low-speed diesel engine technology has long relied on imports,resulting in lack of independent research and development capabilities,and there is a certain gap with foreign advanced level.Therefore,the state launches the "Innovation Project for Marine Low-Speed Machine" to improve the independent research and development capabilities of marine low-speed diesel engine and supporting components.Turbocharger is a key component of low-speed machine.The design of high efficiency and high pressure ratio is the development trend of future marine turbocharger.As the core component of the turbocharger,turbine is gradually developing towards large expansion ratio and high efficiency.The flow field in the cascades is more complicated.The flow field can appear transonic flow.So the high-efficiency design theory and design methods of the large expansion ratio transonic turbine,as well as the methods of improving its performance are the hotspots of special attention at home and abroad.This project relies on the "Innovation Project for Marine Low-Speed Machine" to carry out the aerodynamic design and optimization on the large expansion ratio axial tubine of marine turbocharger.Firstly,this paper studies the design characteristics of large expansion ratio turbine of meridional expansion flow and meridional non-expansion flow.Under the condition of meridional non-expansion flow,the flow coefficient,load coefficient and reaction degree are studied and the turbine blade design on the twisting law of the control vortex is discussed.And the layout of the blade flow path is studied for the wide variation of the Mach number of the radial exit of the blade.The results show that the meridional non-expansion flow of rotor leads to increase in the expansion capacity of the blade and the Mach number at the exit,which in turn increases the shock strength and increases the loss of the exit residual velocity,thereby reducing the performance of turbine.For the transonic blade which the radial exit Mach number spans 1.3,the combination of the divergent-convergent flow path and the convergent flow path can improve turbine performance and improve turbine efficiency.Aiming at the transonic blade designed by the combination of the divergent-convergent flow path and the convergent flow path,the influence of the throat position of the divergent-convergent flow path and the radial distribution range of the divergent-convergent flow path on the performance of the turbine blade is studied in depth.After the position of throat is moved,the uncovered section forms multiple shock wave.The shock strength decreases in medium-diameter,and the radial Mach number distribution changes the radial distribution of the stage reaction degree more unevenly.While the radial extent of the divergent-convergent flow path increases,the blade root gradually changes from multiple shock structures to shock and reflection shock structures.The divergent-convergent flow path is distributed in the radial range where the Mach number is greater than 1.2,and the flow loss is smaller in the range of0% to 30%.The distribution interval is more reasonable.On this basis,the studies of shock loss and secondary flow loss control are carried out from the perspective of control for internal flow loss.The research indicates that controlling the blade profile and reducing the unguided turning improves the curvature of the throat and uncovered.It reduces the strength of the shock and reflection shock in the blade root and changes the shock structure in medium-diameter.The total pressure loss coefficient is reduced by 0.37%.Under variable conditions,the optimized blade has smaller total pressure loss than the original blade in the high expansion ratio range.For the secondary flow loss control,the design of concave curvature end-wall profile weakens the reverse pressure gradient of the trailing edge region of the blade and the accumulation of low energy fluid in the end region.While delaying the formation position of the horseshoe vortex,flow of the pressure surface branch and the suction surface branch of horseshoe vortex within the blade is shortened,thereby reducing secondary flow loss.The selection of turbine aerodynamic design parameters,the spatial layout of turbine blade flow area,the control methods of turbine internal flow loss and the turbine design scheme can provide a reference for the development of large expansion ratio turbine of marine turbocharger.