脉冲与激波耦合作用下涡轮内气流激振特性研究

Turbocharger with VNT (variable nozzle turbine) has a very clear prospect of being widely used in high efficiency and low emission diesel engine, with the ability of increase the power output and decrease in oil consumption and emission. Under operation points of high expansion ratio or nozzle brake, the flow in radial turbine is of strong transonic behavior and the flow at exit of nozzle is normally accompanied by shocks with strong intensity. Integrated in the effect of rotor-stator interaction, the shock sweeps through the inlet region of turbine impeller and generates strong alternating force on impeller blade surface. The force has a very high frequency and can easily lead high-cycle fatigue for the impeller blade. Meanwhile, the working procedure of the diesel engine decides the truth that the flow upstream turbine is pulsating, which can lead obvious pulsation in turbine inlet flow parameters. The flow pulsation leads to extra low frequency alternations on the intensity and position of the nozzle exit shock. Hence, the behavior of the shock sweep as well as interaction upon impeller decides upon the coupling effects of these two alternating excitation forces which are of high frequency and low frequency separately. The research tends to investigate the behavior of nozzle exit shock, including the law of shock sweeping and intensity changing, and make clear the distribution and variation characteristics of excitation force under pulsation and shock co-action. The research achievements will be of key significances to the structure design of pulsation restraining radial turbine as well as improving the durability of VNT.

喷嘴导叶可调的涡轮增压器可有效改善车用内燃机的动力性、经济性和排放性能，在未来高效低排放车用内燃机中具有非常明确的应用前景。在大负荷高膨胀比或导叶制动工况下，涡轮内部流动具有跨音特性，在喷嘴导叶出口常伴随强激波产生。在转静干涉作用下，激波扫掠叶轮叶片前缘表面并形成周期性强高频突变载荷，极易导致叶片高周疲劳断裂；同时活塞式内燃机的工作过程决定了脉冲进气是车用发动机废气涡轮增压器涡轮的工作特点，此低频脉冲特征导致涡轮进气参数发生明显变化，使得喷嘴出口激波强度和位置产生低频交变。喷嘴和叶轮之间的激波扫掠及干涉规律决定于高频和低频两种强脉冲载荷作用。探讨脉冲进气及激波形成工况下喷嘴出口激波强度变化及扫掠规律、研究激波与脉冲耦合作用下径流涡轮内部气流激振力分布及变化特性，对于可调涡轮增压器的涡轮叶轮结构强度设计、抑制气流激振力、提高涡轮可靠性具有重要指导意义。