Investigation on the hump region generation mechanism of pump mode in low-head pumped hydro-storage unit

The pump mode of the low-head pumped hydrostorage unit (pump-turbine) may operate in the hump region under extreme conditions due to the influence of water level variation, and the resulting energy conversion instability will seriously threaten the safety of the unit. However, the generation mechanism of the hump region is still not sufficiently understood, which is mainly due to two reasons: the dominant unstable flow structures that induce the formation of the hump region have not been uniformly recognized, and the influence of the dominant unstable flow structures on the impeller's working capacity has not been effectively revealed. In this study, experiments and numerical simulations were carried out on the low-head pumped hydrostorage unit in the pump mode, and the following results were obtained. It is found that the dominant unstable flow structures that induce the formation of the hump region are the leading edge backflow on the blade inlet shroud side and the horn-like vortex on the blade outlet hub side. The leading edge backflow reduces the blade loading and limits the impeller's working capacity, and the horn-like vortex increases the blade loading and increases the impeller's working capacity. The analysis revealed that the formation of the hump region is the result of the mutual restriction of the horn-like vortex and the leading edge backflow.