Effect of Nozzle Vane and Rotor Clearance on Performance of Variable Nozzle Turbine

In a variable nozzle turbine (VNT), the nozzle vane and rotor clearance can generate complicated tip leakage flow, which produces a large flow loss and results in a noticeable reduction in the VNT performance. Therefore, it is necessary to study the influence of the nozzle vane and rotor clearance on the VNT performance to reveal the underlying mechanisms. In this study, the variation of VNT performance resulting from the nozzle vane and rotor clearance was studied numerically using the commercial CFD code. The numerical results were in good agreement with the experimental data. VNT performance under three varied clearance conditions under different clearance sizes, rotating speeds, and expansion ratios was analyzed. Clearance conditions included the individual effect of nozzle vane clearance, the individual effect of rotor clearance, and the combined effect of nozzle vane and rotor clearance. The results showed that the rotor clearance (clearance size equal to 0.4 mm) resulted in a certain reduction in thermal efficiency (−3.98%) and torque (−4.48%). Meanwhile, the nozzle vane hub and shroud clearance also affected the thermal efficiency (−3.66% and −13.37%, respectively) and torque (12.54% and −0.05%, respectively). Compared with the rotor clearance, the nozzle vane clearance dominated the variation in the VNT performance, and the values of the mass flow ratio, thermal efficiency, and torque increased by 12.15%, 5.43%, and 8.01%, respectively (The clearance on both sides of the vane was equal to 0.2 mm.). Under the combined effect of the nozzle vane and rotor clearance, the deviation of the thermal efficiency and mass flow ratio was smaller than the sum of the values of their individual effects; when the clearance on both sides of the vane was equal to 0.2 mm and the rotor clearance was equal to 0.6 mm, the value was reduced by 2.77% and 1.71%, respectively. This study also explains the influence mechanisms of clearance at both ends of the nozzle vane and the vane/rotor clearance interaction on the VNT performance in detail.