Investigation on the shock control using grooved surface in a linear turbine nozzle

Radial flow Variable Nozzle Turbine (VNT) enables better matching between a turbocharger and engine, and can improve the engine performance as well as decrease the engine emissions, especially when the engine works at low-end operation points. With increased nozzle loading, stronger shock wave and clearance leakage flow may be generated. The shock wave consequently introduces strong rotor-stator interaction between turbine nozzle and impeller, which is also a key concern of impeller high cycle fatigue failure. With the purpose of developing a shock wave free or low shock wave intensity turbine nozzle, the influence of grooved vane on the shock wave characteristics is investigated in present paper. A Schlieren visualization experiment was first carried out on a linear turbine nozzle with smooth surface and the behavior of the shock wave was studied. Numerical simulations were also performed on the turbine nozzle. The predicted shock wave shape, position and intensity were compared against the Schlieren images. Guided by the visualization and numerical simulation, grooves were designed on the nozzle surface where the shock wave was originated and numerical simulations were performed to investigate the influence of grooves on the shock wave characteristics. Results indicate that for a smooth nozzle configuration, the intensity of the shock wave increases as the expansion ratios increase, while the onset position is shifted downstream to the nozzle trailing edge. For a nozzle configuration with grooved vane, the position of the shock wave onset is shifted upstream compared to the one with a smooth surface configuration, and the intensity of the shock wave as well as the static pressure distortion at the nozzle vane exit plane are significantly depressed.