Research On Flow Excitation Characteristics In Radial Turbine Under Co-action Of Pulsation And Shock Wave

The 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 nozzle brake and low-end operating points),strong shock waves is generated.Consequently,strong rotor-stator interaction between turbine nozzle and impeller is the key factor of the impeller high cycle fatigue and failure.At the same time,due to the discontinuity of the piston engine operation,the turbine inlet is pulsating flow.This low frequency pulsation will have the low frequency influence on the formation,shape and sweeping rule of the shock wave on the nozzle trailing edge.The combination of intake pulsating and shock wave causes the new characteristics of the internal airflow excitation characteristics of the turbine.In this paper,the influence of the nozzle opening and expansion ratio on the formation mechanism of shock wave,the law of sweeping and the perturbation characteristics of impeller upstream airflow parameters of the nozzle under the pulsating condition is studied by numerical simulation means.The complex unsteady flow and shock wave characteristics of the turbine nozzle are discussed under the collective action of low frequency unsteady effect of intake pulse and unsteady effect of rotor-stator interference,the effects of intake pulse on the transient characteristics of the turbine,the frequency characteristics of the flow field and the excitation force of the impeller are studied.And the evolution characteristics of the shock wave trajectories on the linear nozzle are verified by the Schlieren test and the numerical simulation methods.The grooved nozzle vane is proposed to change the shock wave morphology and weaken the shock wave intensity.Meanwhile,the effects of different groove structures on shock wave morphology and shock wave suppression are studied.The main research contents and achievements are as follows:Influence of intake pulse on turbine flow characteristics of VNT.Through the reasonable simplification of the actual exhaust curve of the engine,the sinusoidal pressure pulsation waveform is designed as the turbine inlet pulse condition.The influence of intake pulse on the transient performance,frequency characteristics and flow loss of the turbine is studied by numerical simulation method.The results showed that: The intake pulse leads to a significant change in the transient performance of the VNT.In the pulse period,the transient mass flow curve is ring and the mass flow is less than the steady mass flow under the same expansion ratio.The transient efficiency curve presents a ring and surrounds the steady efficiency curve.The unsteady flow caused by intake pulse is weaker than that of rotor-stator interaction in the leading edge of the impeller,and the fluctuation caused by rotor-stator interaction has a faster dissipation rate in the impeller.The intake pulse has a great influence on the leakage flow of the nozzle blade,which increase the flow loss in the flow channel.The flow loss caused by the attack angle increases with the increase of the intake pulse pressure.The evolution characteristics and influencing factors of the shock wave under uniform air intake is studied.Through the unsteady calculation of the uniform intake of the VNT,the influence mechanism of the rotor-stator interaction,the expansion ratio,the wake and the nozzle opening on the shock wave are determined.The results showed that the formation of shock wave at the trailing edge of the nozzle is closely related to the relative position of the rotor and stator.When the impeller leading edge is rotated to be closer to the trailing edge of the nozzle vane,the shock intensity is enhanced.Otherwise,it will be weakened.The shock wave can produce large pressure gradient on the impeller,leading to the obvious change of the impeller leading edge load,which is the main source of the high cycle fatigue of the impeller.When the shock wave and the wake interfere with each other,the total pressure loss and the entropy increase.The influence of the shock wave on the leading edge of the rotor blade is much greater than that caused by the wake;as the expansion ratio increases,the shock wave intensity increases gradually,and the shock generation position shift to the trailing edge.At the same time,the nozzle opening is smaller,the shock intensity is stronger.The excitation characteristics of airflow in VNT under the coupled effect of the intake pulse and shock wave is studied.Through the study of the static pressure contour and the Mach number contour at different intake pulse times,the shock wave characteristics at trailing edge of the nozzle under inlet pulse condition is clarified.At the same time,the load distribution of rotor blades at different spans are discussed,and the effect of intake pulse on the excitation force of rotor blades is analyzed.The results showed that: There is a significant effect of intake pulse on the shock wave intensity and the formation position.During the pulse period,the position of the shock wave is slightly fluctuated at low frequency,and the fluctuation frequency is the same as the intake pulse frequency.At the same relative position of the impeller leading edge and the nozzle trailing edge,the high pulse pressure causes the shock wave position shifting close to the nozzle trailing edge,and the shock wave strength is enhanced.While the intake pulse pressure is decrease,the shock wave formation position shifts to the upstream of the nozzle trailing edge,and the shock wave strength is decreased.When the pulse pressure is changed about 10 kPa,the shock wave intensity is changed about 7%.The influence of the intake pulse on the shock wave of the nozzle makes the impeller leading edge load fluctuation obviously,and the variation range is more than 20%.The Evolution of Shock Wave Morphology in the cascade nozzles is studied.The influence mechanism of the expansion ratio and the clearance size on the shock wave shape in linear nozzle is clarified by Schlieren test and numerical simulation methods.The results showed that: At the span location adjacent to the end-wall,the shock wave is squeezed and deformed by the clearance leakage vortex,and the shape of the shock wave is bent in the opposite direction of main flow in the interaction region.The shock wave is truncated by the clearance leakage flow and mixed stream-wise with a distorted shock wave structure.With the increase of clearance ratio,the shock wave structure near the end-wall is further distorted by the clearance leakage flow,while the shock wave intensity near mid-span is increased.The interaction of clearance leakage flow and shock wave leads to a strong flow distortion at nozzle vane exit which introduces a high excitation force on the impeller blade.The influence mechanism of grooved structure on shock wave shape and shock intensity suppression is studied.The method of designing the groovse on the suction side of the nozzle is proposed to change the shock wave Morphology.The influence of the groove structure on the shock Morphology is studied by Schlieren test and numerical simulation methods.The results showed that with the grooved vane,the shock wave structure is altered from a normal shock wave to oblique shock waves followed by a normal shock wave.The oblique shock waves help to decrease flow speed upstream the normal shock wave and thus significantly depressed the intensity of the normal shock wave.The pressure distortion across the shock wave downstream of the nozzle can be dramatically decreased by the grooved vane.The shock wave mitigation effectiveness of the grooved vane depends upon the expansion ratio and the distance to the nozzle trailing edge.The shock wave can be more effectively mitigated at higher expansion ratios.The groove geometry,orientation,number as well as the position have a significant influences on the shock wave control.For the grooved surface vane,when the width is constant,the depth is smaller,the effect of grooved surface vane on shock wave mitigation is more effective.When the depth is constant,the width is narrower,the effect of grooved surface vane on shock wave mitigation is more effective.