Runner cone drilling and slotting to reduce vortex rope - induced pressure fluctuations in a Francis turbine

Pressure fluctuations caused by helical vortex rope in the draft tube is one of the main reasons for the instability of Francis turbines. Previous studies showed that drilling holes on runner cone can reduce the amplitude of pressure fluctuation. In this study, based on the runner cone which has been drilled one hole, two kinds of new runner cones are designed by means of drilling two more holes (2 holes runner cone) and drilling a slot (2 holes and 1 slot runner cone). Pressure fluctuation and flow pattern observation experiments as well as numerical simulations at partial load operation condition in a Francis turbine were carried out for the three types of runner cones. Analyses were performed to reveal the generation mechanism of the low frequency pressure fluctuations. The results showed that low frequency fluctuations are related to the cyclical rotation of helical vortex rope. Holes and slots on the runner cone have little impact on the efficiency of the Francis turbine. Slotting on the runner cone can reduce the amplitude of pressure fluctuation induced by vortex rope to a maximum of 29.41%. Hence, proper slotting on the runner cone can effectively reduce the low-frequency fluctuation caused by vortex rope in Francis turbines. These findings provide a new idea to improve the hydraulic instability of Francis turbines induced by vortex rope.