Runner cone optimization to reduce vortex rope-induced pressure fluctuations in a Francis turbine

TLDR: In this paper, two new types of runner cones (i.e., abnormally shaped and long straight cones) were proposed to optimize the pressure distribution in the draft tube, and unsteady numerical simulations were performed to determine their mechanism of action.

Abstract: Pressure fluctuations induced by a vortex rope are the major causes of hydraulic turbine vibration in partial load operating conditions. Hence, an effective control strategy should be adopted to improve rotating characteristics of the vortex rope and reduce the corresponding pressure fluctuation. In this study, two new types of runner cones (i.e., abnormally shaped and long straight cones) were proposed to optimize the pressure distribution in the draft tube, and unsteady numerical simulations were performed to determine their mechanism of action. Numerical results were validated using flow observation and pressure fluctuation experiments. Detailed analyses were conducted to understand the effects of the helical vortex rope operating conditions. The results indicated that pressure fluctuations in the draft tube at partial load operation result primarily from low frequency fluctuations induced by the rotation of the helical vortex rope, whose amplitudes are related to the rotating radius of the helical vortex rope. Both runner cone types could effectively reduce the pressure-fluctuation amplitude. The long straight type could reduce the amplitude of low-frequency fluctuation induced by vortex rope to a maximum of 74.08% and the abnormal-shape type to 38.31%. Thus, the effective optimization of the runner cone can potentially reduce pressure-fluctuation amplitudes. Our research findings were applied to a real hydraulic plant in China.