Numerical investigation of inter-blade cavitation vortex for a Francis turbine at part load conditions

Inter-blade cavitation vortex at part load has raised significant concerns about operation stability for Francis turbines due to tremendous development and integration of renewable energy resources into power grid. The principal objective of this study is to investigate the flow characteristics of inter-blade cavitation vortex and its influence on pressure fluctuations for a low-head Francis model turbine. Unsteady numerical simulations are carried out using shear stress transport turbulence model and Zwart cavitation model. Numerical results yield to a good validation with the experimental data. The findings show that inter-blade cavitation vortices highlighted by cavitation structure and vortex structure are extremely different at the points adjacent to the incipient line of inter-blade cavitation vortex. The location of inter-blade cavitation vortex development in the runner is more sensitive to rotating speed than that to guide vane opening. The formation mechanism of inter-blade cavitation vortex is attributed to flow separation in the vicinity of the runner hub. Pressure fluctuations induced by inter-blade cavitation vortex are directly associated with the evolution of vapour volume in the runner and feature wide-band and low-frequency characteristic, and the fluctuations on the suction side of the runner blade are significantly magnified because of the presence of inter-blade cavitation vortex.