Numerical Investigation Of The Reduction Of Shock Wave For Variable Nozzle Turbine

Turbocharging technology has become an important method for internal combustion engine to save energy and protecte environment.Using a turbocharger can increase engine output power and maximum torque without changing engine’s capacity.Varible nozzle turbine(VNT)is widely used because it has the advantage of reducing the engine fuel consumption and emissions and achieving good matching with the engine in the whole operating range.However,under certain small nozzle opening conditions(such as engine brake),strong shock waves are generated in the turbine nozzle.The strong shock wave sweeps through the leading edge of turbine rotor and generates strong unsteady force on the rotor blade surface.Consequently,this shock wave induced rotor-stator interaction is the key factor of the rotor high cycle fatigue failure.This thesis was mainly focused on weakening VNT shock wave by using groove structure on the nozzle vanes.Firstly,the shock wave characteristics is investigated under a small nozzle opening condition by numerical method,and the load of rotor blades at different blade span is calculated.The results show that the intensity of shock wave is strong at mid-span and weak near the end-wall.Meanwhile,the load of rotor blade has the same characteristic as the shock wave.Then,the reason why using groove structure can weaken shock wave intensity was analyzed.The groove structure not only can significantly reduce the nozzle vanes trailing edge shock wave but also can make shock wave appear in advance.Six models with different groove structure on nozzle vane surface are analyzed.The result shows that for the grooved surface vane,the number of grooves is more,the arrangement of grooves is closer,the width of grooves is smaller,the effect of grooved surface vane on shock wave mitigation is more effective.Because there is no shock wave near endwall,the model with half height grooves structure can reduce shock wave intensity with very small turbine efficiency penalty.Then,the model with four equal-height grooves and with for unequal-height grooves are investigated.The results show that groove structure can change the form of shock wave and make oblique shock waves appear before the normal shock wave and form multiple shock waves together with the original shock wave.Both two models can decrease excitation loading on rotor blade and minimize the risk of high cycle fatigue failure.Compared with the model with equal-height grooves,nozzle vanes with unequal-length grooves is better to weaken shock wave intensity and has a smaller turbine efficiency penalty.