Optimization of nozzle vane surface treatment using multi-grooves to mitigate shock wave for a variable nozzle turbine

One of the important concerns for a variable nozzle turbine (VNT) is shock wave generated at nozzle exit while the turbine operates under small nozzle opening. Due to the rotor-stator interaction, the strong excitations at the leading edge of turbine rotor may be a concern of high-cycle fatigue failure of rotor blades. In this paper the unsteady numerical simulations of a VNT were performed to investigate the effect of nozzle vane surface treatment using multi-grooves to weaken the intensity of shock wave. VNT models in which nozzle vanes with different groove parameters were numerically simulated and the internal flow field were analyzed. It shows that the grooves can generate multiple oblique shock waves which can slow down the flow and weaken the intensity of shock wave near the nozzle vane trailing edge. The simulation results also show that with the increase of groove number as well as smaller gap between grooves, the impact on shock wave mitigation is more. Considering the intensity of shock wave is weaker near the nozzle vanes endwall, the simulation of VNT in which nozzle vanes with half-height grooves was also carried out. It shows that it can reduce the shock wave intensity with smaller turbine efficiency penalty. Finally the preliminary experiments using Schlieren photography for a linear turbine with smooth surface and grooved surface were undertaken to validate the generation of the multiple shock wave with this grooved surface treatment.