In commonly applied one-dimensional choking models for radial turbines, choked flow is assumed to appear in the geometrical throat of each stator and rotor. Coupled and complex three-dimensional effects are not considered. In order to analyze the internal aerodynamic in a radial turbine at off design conditions and before carrying out experimental tests, which in the case of automotive turbocharger are limited by their compact size, computational fluid dynamics (CFD) simulations stand out as a useful tool. This paper presents the study of a variable geometry turbine (VGT) of a commercial turbocharger at off design conditions reaching choked flow, analyzing the presence of this limiting conditions in the stator and rotor under different operation points and VGT positions. Reynolds-averaged Navier-Stokes (RANS) and unsteady RANS simulation have been performed to obtain the flow structures in stator and rotor. The results reveal that the choked effective area mostly depends on the stator vane position and pressure ratio. For the closed VGT position a standing shock wave appears on the stator suction side and expands through the vaneless space. For the opened VGT position the flow is choked at the rotor outlet. However, the evolution of the choked area highly depends on the rotational speed and the secondary flow. A strong interaction with the tip leakage vortex has been identified.

在用于径向涡轮机的常用一维节流模型中，节流被假定为出现在每个定子和转子的几何喉部中。没有考虑耦合和复杂的三维效应。为了在非设计条件下以及在进行实验测试之前分析径向涡轮的内部空气动力学特性，在汽车涡轮增压器受其紧凑尺寸限制的情况下，计算流体动力学（CFD）模拟是一种有用的工具。本文介绍了一种商用涡轮增压器的可变几何涡轮机（VGT）在非设计条件下达到节流的研究，分析了在不同工作点和VGT位置下定子和转子中存在这种极限条件的情况。雷诺平均的Navier-Stokes（RANS）和非稳态RANS模拟已经执行，以获得定子和转子中的流动结构。结果表明，节流有效面积主要取决于定子叶片的位置和压力​​比。对于关闭的VGT位置，定子吸力侧会出现驻波，并在无叶片空间中扩展。对于打开的VGT位置，流量在转子出口处阻塞。然而，阻塞区域的演变高度取决于旋转速度和二次流。已经确定了与尖端泄漏涡的强烈相互作用。对于关闭的VGT位置，定子吸力侧会出现驻波，并在无叶片空间中扩展。对于打开的VGT位置，流量在转子出口处阻塞。然而，阻塞区域的演变高度取决于旋转速度和二次流。已经确定了与尖端泄漏涡的强烈相互作用。对于关闭的VGT位置，定子吸力侧会出现驻波，并在无叶片空间中扩展。对于打开的VGT位置，流量在转子出口处阻塞。然而，阻塞区域的演变高度取决于旋转速度和二次流。已经确定了与尖端泄漏涡的强烈相互作用。