Study on the Interaction of Clearance Flow and Shock Wave in a Turbine Nozzle

Radial flow Variable Nozzle Turbine (VNT) enables better matching between the turbocharger and engine. At partial loading or low-end engine operating points, the nozzle vane opening of the VNT is decreased to achieve higher turbine efficiency and transient response, which is a benefit for engine fuel consumption and emission. However, under certain small nozzle opening conditions (such as nozzle brake and low-end operating points), strong shock waves and strong nozzle clearance flow are generated. Consequently, strong rotor-stator interaction between turbine nozzle and impeller is the key factor of the impeller high cycle fatigue and failure. In present paper, flow visualization experiment is carried out on a linear turbine nozzle. The turbine nozzle is designed to have single-sided clearance, and the Schlieren visualization method is used to describe the formation and development process of clearance flow and shock wave under different clearance and expansion ratio configurations. Numerical simulations are also performed to investigate the flow structure and the interaction behavior between shock wave and clearance flow in details. Results indicate that for the investigated turbine nozzle, the shock wave is squeezed and bent in the opposite direction of the main flow in the interaction region. In the location close to the end-wall, the shock wave is truncated by the clearance flow and mixed downstream-wise with a distorted shock wave structure. Furthermore, increasing the clearance size causes the distortion of the shock wave structure near the end-wall, while the shock wave intensity near mid-span is increased. Meanwhile, the clearance leakage flow and shock wave can cause the static pressure of the nozzle vane exit to fluctuate violently.