Pressure oscillation suppression by air admission in a Francis turbine draft tube

Abstract

When Francis turbine operated at part-load conditions, there is strong vortex rope in the draft tube, which may induce violent pressure oscillation, mechanical vibration and noise. This paper treats the pressure oscillations at a part-load operation by analyzing the unsteady multiphase flow in a Francis turbine. Air admission is used to alleviate pressure oscillation under different cavitation number. In order to improve the numerical accuracy, a modified method including the Level-set based method and FBDCM model is applied in the calculation. The simulation results indicate that the modified method can reasonably predict the pressure oscillations in turbine with good agreement with experimental data. The method can reflect more detailed flow characteristics and capture a clearer phase interface compared to the conventional method. It is noted that air admission with suitable ventilation rate does suppress the pressure oscillation in the turbine. The pressure oscillation in the vaneless area due to rotor-stator interaction cannot be depressed completely. Further, the mechanism on the pressure oscillation suppression in the draft tube is due to the spiral motion depression of vortex rope by air admission. As the increase of air admission, the pressure and its gradient distributions become much homogeneous. The homogeneous distribution of pressure in the draft tube would be favourable for the suppression of pressure oscillation