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。

在大负荷高膨胀比或导叶制动工况下，可变几何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。在高频转静干涉非定常作用和低频脉冲进气的共同作用下，激波扫掠叶轮叶片前缘表面并形成周期性突变载荷，易导致叶片高周疲劳失效。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并寻求导叶内激波抑制方法对于可调涡轮增压器的涡轮结构强度设计、提高其可靠性具有重要指导意义。项目以抑制导叶内激波为最终目标，首先对研究了可变几何涡轮内激波形成机理以及导叶开度、膨胀比对涡轮内激波扫掠规律以及其对叶轮上游气流参数扰动特征额影响；在此基础上，考虑了涡轮低频进气脉冲与高频动-静干涉高频非定常效应共同作用下涡轮导叶内部的复杂非定常流动以及激波特征，研究了脉冲进气对涡轮瞬态特性、流场频域特性、激波行为以及叶轮上游气流激振力的影响。结果表明在可变几何涡轮中小开度、大膨胀比工况，涡轮导叶吸力面尾缘侧产生较强激波，激波的强度决定于叶轮前缘与导叶尾缘的相对位置，当叶轮前缘旋转至距离导叶尾缘较近时，激波强度增强，反之减弱；脉冲进气对导叶内激波强度和位置存在较为明显的影响，进而影响叶轮前缘载荷的强烈变化；针对可变几何涡轮内部激波形成特点，研究了导叶叶片表面开置凹槽以改变激波形态、弱化激波的激波抑制方法，并采用试验和数值相结合的方法研究了不同凹槽结构对激波形态和激波抑制效果的研究。研究结果表明，导叶吸力面开置合理尺寸凹槽的方法可以改变导叶内的激波形态并有效抑制激波强度。与光滑叶片相比，凹槽结构叶片可以将正激波转变为多道斜激波，从而降低了激波强度；凹槽尺寸对激波抑制效果影响明显，在槽宽固定时，过大和过小的槽深都使得其对激波的抑制效果较弱；在槽深固定时，槽宽的减小有利于激波的抑制。