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

Abstract

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.

Analysis of Channel Vortex and Cavitation Performance of the Francis Turbine under Partial Flow Conditions

“…Trivedi Chirag [27] took the extreme operating conditions of the turbine into consideration, and he found that the cavitation phenomenon in hydraulic turbine gradually disappeared with the increase of rotation speed; however, when running at high rotating speed, the intensity of cavitation reached its maximum. Sun Longgan et al [28] found that the position of channel vortexes in Francis turbines shows a clearer correlation to rotating speed than that to guide vane opening. Finally, Shi Guang-tai et al [29] found that the cavitation on the blade surface of Francis turbine under BEP conditions is only subtly affected by the water head, but with the increase of the guide vane opening, cavitation on the blade surface is more likely to occur.…”