An experimental investigation of cavitation surge in draft tube of Francis turbine

Hydro turbines are often used at off-design point to adjust the power output. At off-design condition, a swirl flow occurs downstream of the turbine runner and a cavitation occurs at the vortex core due to the decrease of the pressure. The cavitation in the draft tube can cause a flow instability called draft tube surge. Because the draft tube surge causes the flow rate fluctuation, unsteady fluid forces occurs on the runner, and consequently, the revolution speed of the runner is affected. In the present study, an experimental investigation was carried out. Characteristics of the dynamic fluid forces due to the draft tube surge are examined at first. In addition, the influence of the fluctuation of the revolution speed of the runner on the cavitation surge were examined. The results showed that the cavitation surge caused the hydraulic torque fluctuation on the runner but the fluctuation of the cavity volume were affected by the runner revolution speed.