Numerical investigation on the high-cycle pressure fluctuation mechanism of VNT rotor

Abstract

Due to the rotor-stator interaction of Variable Nozzle Turbine (VNT), the strong excitations at the leading edge of turbine rotor can be created during engine braking down or transient acceleration. This may be a concern of high-cycle fatigue failure of VNT rotor blades. In this paper the 3-D unsteady numerical simulation of a VNT was performed to investigate the effects of nozzle vanes clearance leakage, wake and shock wave on the rotor blade pressure fluctuations, which can specify where and how these three factors influence the excitations of the turbine rotor blade. The results show that the shock wave and nozzle vanes clearance leakage are the main reasons for rotor blade surface pressure fluctuation. The shock wave only affects the suction surface at the leading edge of rotor blade because of the shape of rotor blade. When the nozzle vanes opening was closed small, the shock wave and the amplitude of the pressure fluctuations on the rotor blade surface were greatly increased compared to that for full open condition. In the regions near the hub and shroud side on the rotor blade suction surface, the magnitude of the pressure fluctuation is increased significantly by the influence of the nozzle vanes clearance leakage. Through FFT analysis of the pressure fluctuation at the rotor blade leading edge, it can be seen that the fluctuation amplitude increased significantly on the rotor passing frequency and this also implies the possibility of VNT high-cycle fatigue failure with the high-frequency excitations.