Mechanisms of flow-induced runner forces in a giant Francis turbine under wide load

With the rapid increase in the penetration of wind and solar power in new-type power system, hydropower units are frequently required to serve as grid-regulation machines. Consequently, Francis turbines are compelled to operate over a much wider load range, which poses significant challenges to unit's safety and stability. However, the runner forces and flow characteristics that govern turbine stability remain insufficiently understood. In this study, three-dimensional computational fluid dynamics simulations were performed to investigate wide operating conditions ranging from 5% to 110% of the rated power Pr. The flow patterns, vortex structures, pressure distributions, and runner forces were systematically analyzed. The results show that the loads lower than 40%Pr and higher than 100%Pr have degraded characteristics. The blade-channel vortices and the central backflow in the straight cone are the dominant factors responsible for the increase in radial and axial runner forces, respectively. The main regions contributing to radial and axial forces are located near the short blade inlet and the long blade outlet, respectively. The maximum radial and axial forces occur at 40%Pr and 5%Pr, respectively, while large-scale cavitation is developed at 5%Pr and 110%Pr. These findings provide reference for the safe and stable operation of giant Francis turbines under wide-load operating conditions.