The influence of runner cone perforation on the draft tube vortex in Francis hydro-turbine

In Francis hydro-turbine, the pressure oscillation in the draft tube is the main source of flow-induced vibration and noise. The structural design of runner cone exerts significant influence on the tangential flow velocity within the draft tube and the pressure fluctuations downstream. In this work, we propose a simple and effective method by perforating through-holes on the runner cone to weaken or eliminate pressure fluctuations induced by the vortex ropes in draft tube. An 2 and 4 holes are perforated on the lower half of runner cone, and numerical simulations and experiments are conducted to understand the changing of phenomenology and features with regard to vortex ropes and pressure levels, comparing with the original unperforated case. The analyses are done for two different guide vane openings, α, and the results were validated by experiments with high-speed photography. It is found that at the two openings, the runner cone with perforations can reduce the volume of vortex ropes, and decrease the pressure fluctuation level. The motion of vortex ropes tends to be less violent with more perforated holes. Therefore, the runner cone perforation has the potential of reducing the vortex rope strength.