Instability characteristics regarding the saddle-shaped region in a reversible mixed-flow pump applied to the low-head pumped storage

Abstract

The expanding renewable energy has eased the energy crisis but hampers grid stability, which can be offset by scaling up pumped hydro storage. Low-head hydropower resources can be utilized efficiently and economically by a reversible mixed-flow pump (RMFP). However, the saddle-shaped region, a representative unstable feature in the pump mode, severely limits the flexibility and reliability of the RMFP. In this study, external characteristics of the RMFP were experimentally investigated based on a bidirectional test rig. A saddle-shaped region was discovered in the pump head curve formed by the near-rated discharge hump (0.92Qd), valley (0.88Qd), and low-discharge hump (0.71Qd). Transient flows under these feature conditions were simulated using CFD method and compared with the rated discharge condition (1.0Qd). The flow under 0.92Qd was as stable as 1.0Qd but worsened with decreasing discharge. When the discharge decreased to 0.88Qd, the pre-swirl emerged, and the rotating stall occurred and adhered to the blade suction side near the runner shroud. The inflow was blocked by stall vortices, which increased the hydraulic loss and triggered the head drop. When the discharge decreased to 0.71Qd, the inflow collided with stall vortices at the leading edge. This interaction led to the recirculation around the runner inlet, which endowed the inflow with extra energy and triggered the head rise. Overall, this study unveils flow instabilities in the RMFP, which explain the formation of the saddle-shaped region.