Abstract: Hydropower units are forced to operate in part load conditions more frequently to balance the power grid parameters due to tremendous development of renewable energy resources; as a consequence, the characteristics of inter-blade vortices under such conditions become a significant index for evaluating hydraulic design and optimization of hydraulic turbines. This paper analyzes systematically the formation mechanism of inter-blade vortices in a Francis turbine through theoretical analysis and scale model experiment, and examines new effective measures for alleviating the incipient vortices and their development. Results show that breaking the pressure gradient that would otherwise be continuous and steady, due to the runner tending to adapt to the wide range of flow rate changes, could be one of the factors affecting the vortex formation. We find that design modification of the runner blades–shortening the runner flow passage, adopting a design of bionic streamline, or controlling the twist degree of the blades–can effectively suppress the incipient vortices and reduce their developing rate, achieving a useful result for expanding the stable operation range of Francis turbines.

Key words: Francis turbine, inter-blade vortex, formation analysis, design modification, alleviation