Study on pressure pulsation and induced noise under low load conditions of giant Francis turbine

Abstract With the transformation of the energy structure, hydropower units assume the important task of deep peak regulation of the power grid. The hydropower turbines often operate under eccentric load conditions. The problems of pressure fluctuation and induced noise under the small flow conditions of Francis turbines need to be solved urgently. In this paper, based on the DES turbulence model, the three-dimensional numerical simulation of the whole flow channel of a giant mixed-flow unit is carried out, and the noise test is carried out. The results show that in the 218MW and 322MW output conditions, the scale of the leaf channel vortex decreases significantly when the suction surface is fully developed and the load is increased to 425MW output condition. With the increase of load, the low-pressure area of the straight cone section of the tailpipe shifts from the center to the pipe wall. A relatively obvious eccentric vortex band is formed under the 425MW output condition, which induces a higher pressure fluctuation amplitude. Under the influence of the tailwater vortex ring, the low-frequency pressure pulsation with a main frequency of 0.2 times the rotation frequency was detected at the tailpipe inlet and transmitted upstream. A similar pulsation frequency was observed at the volute inlet, but the pulsation amplitude attenuated to 14.8%-31.9% of the fluctuation amplitude of the tailpipe inlet pressure. Comparing the numerical simulation and noise test, the results show that the leaf channel vortex is the main reason for the induced noise of the Francis turbine under low load conditions.