Unsteady flow characteristics during runaway process in Francis turbine: Insights from numerical investigation

The runaway process in hydraulic turbines is characterized by unstable flow that results in the formation of vortex structures, pressure fluctuations, and energy dissipation. This study focuses on the unsteady flow characteristics of a Francis turbine during the runaway process using numerical simulations. The obtained runaway speed and discharge align well with the experimental results. The findings reveal that larger openings lead to more rapid attainment of the runaway speed. During the runaway process, extensive flow separation at the runner blade generates a columnar vortex, which obstructs the channel and dissipates energy. High-amplitude pressure fluctuations, with a frequency below 0.5 times the blade frequency, are observed in the flow passage components. These pressure fluctuations are attributed to forming a columnar vortex structure at the hub and a sheet vortex band at the trailing edge of the runner blade. A large opening leads to an earlier occurrence of high-amplitude pressure fluctuations, a gradual increase in the amplitude of low-frequency fluctuations, and a more intense force in the runner. An analysis of the energy dissipation characteristics using the energy balance equation reveals that turbulence plays a dominant role in energy transfer and dissipation during the runaway process. Additionally, the dissipation is caused by the formation of a columnar vortex structure induced by flow separation at the blade hub and the presence of a sheet vortex band at the trailing edge. Furthermore, the findings observe that energy conversion and dissipation within the runner channel intensify with increasing guide vane opening.