Multi-scale leakage vortex interaction analysis in tip clearance of low-head pumped hydroelectric energy storage systems in pump mode

In low-head pumped hydroelectric energy storage systems operating in pumping mode, the tip clearance leakage vortex, emerging from the narrow gap between the impeller's outer perimeter and the pump casing, significantly impacts unit efficiency, head, and energy consumption. This paper uses simulations combined with experiments to study how the leakage vortex forms and changes in the pumping mode. Our methodology incorporates the Rigid vortex technique to perform a detailed three-dimensional structural analysis of the leakage vortex and its developmental stages. The findings indicate that the presence of tip leakage flow initiates the formation of a complex vortex structure. This structure comprises primary and secondary vortices, the dynamics of which are significantly influenced by interactions with the main flow. At low flow conditions, we observed that the primary vortex, originating at the head region on the back of the blade, undergoes premature collapse. This collapse is manifested by a shortened core band, diminished stability, and intensified vortex motion, accompanied by an increased angle relative to the blade posterior region. The leakage vortex separation and shedding take place near the entrance of the adjacent blade. Conversely, as the flow rate increases, the main vortex core band extends, leading to enhanced stability. The angle between the core band and the blade back diminishes, resulting in the leakage vortex separation and shedding occurring further along, towards the leading edge of the neighboring blade. This shift in vortex behavior covers a substantial portion of the flow field, impacting the overall efficiency and stability of the units. The results guide stability improvement in low-head pumped storage pumping mode operation.