Research on hydrodynamic noise in the Francis turbine using large eddy simulation and acoustic analogy

Abstract

For Francis turbines, frequent operations under extremely low load conditions result in significant noise and pressure fluctuation issues. These issues may cause vibration and fatigue damage to the unit, accompanied by difficulties in connecting to the grid and reductions in the power generation efficiency of renewable energy. However, there is limited research on the relationship between pressure fluctuations and the induced noise of Francis turbines during extreme operations. In the present study, an acoustic numerical simulation based on the Ffowcs Williams-Hawkings equation and large eddy simulation is used to analyze the acoustic performances of Francis turbines. In the current study, for evaluating the acoustic characteristics under such terrible conditions, the results of variable flow rate and guide vane opening conditions are compared. Results indicated that Francis turbine noise is mostly due to pressure fluctuations brought on by rotor-stator interference and corkscrew-shaped vortices. The blade passing frequency (BPF) of 130.00 Hz and the low frequency of 0.33 f_n (where f_n denotes the rotating frequency) are the key factors affecting pressure and noise fluctuations. The influence of low frequency is reduced as the flow rate rises, whereas the influence of BPF gradually increases. Besides, the hydrodynamic noise of Francis turbines is primarily low-frequency, with discrete and broad-band features. The rotating noise with distinct peak values and the turbulence noise produced by large-scale vortices (corkscrew-shaped vortices) make up the majority of low-frequency noise. Therefore, reducing pressure fluctuations is a key strategy for lowering flow-induced noise radiation.