Design and Analysis of a Novel Split Sliding Variable Nozzle for Turbocharger Turbine

Abstract

Variable geometry turbine (VGT) has been widely applied in internal combustion engines to improve the engine transient response and torque at low speed. One of the most popular variable geometry turbine is the variable nozzle turbine (VNT), in which the nozzle vanes can be rotated along the pivoting axis and thus the flow passage through the nozzle can be adjusted to match with different engine operating conditions. One disadvantage of the VNT is the turbine efficiency degradation due to the leakage flow in the nozzle endwall clearance, which is needed to allow the nozzle vanes to rotate without sticking. Especially at small nozzle open condition, there is large loading on the nozzle and high pressure gradient between the nozzle pressure and suction side. Strong leakage flow exists inside the nozzle endwall clearance from pressure side to suction side, leading to large flow loss and turbine stage efficiency degradation. In the present paper, a novel split sliding variable nozzle turbine (SSVNT) has been proposed to reduce the nozzle leakage flow and to improve turbine stage efficiency. The idea is to divide the nozzle into two parts: one part is fixed and the other part can slide along the partition surface. The mechanism of nozzle flow passage variation in SSVNT is different from that of the traditional pivoting VNT. The sliding vane and the fixed vane together form an integrated vane. The flow of the turbine is determined by the passage of the integrated vanes. When moving the sliding vane to large radius position, the nozzle flow passage opens up and the turbine has high flow capacity. When moving the sliding vane towards small radius position, the nozzle flow passage closes down and the turbine has low flow capacity. As the fixed vane doesn’t need endwall clearance, there is no leakage flow inside the fixed vane and the total leakage flow through the integrated vane can be reduced. Based on calibrated numerical modeling, the analysis results showed that there is up to 12% turbine stage efficiency improvement with the SSVNT design at small nozzle open condition while maintaining the same flow capacity and efficiency at large nozzle open condition, compared to the conventional VNT. The mechanism of efficiency improvement in the SSVNT design has also been discussed.