Effect of inlet circulation on energy consumption of outlet conduit of low-head pump system for pumped storage and its optimization

Many large low-head pump stations can operate in reverse and utilize upstream and downstream lakes and rivers for pumped storage. The hydraulic loss in the outlet conduit accounts for a significant proportion of the pump head, greatly affecting the pump system efficiency. To investigate the effect of inlet circulation distribution on the hydraulic loss of the outlet conduit and the overall pump system efficiency, numerical simulations were conducted to analyze the flow patterns and energy fields of an isometric circular pipe and a siphon outlet conduit. Applying dynamic mode decomposition (DMD), the evolutionary properties of entropy production in the circular pipe were examined under non-rotating flow, swirling flow, and secondary flow, respectively. By distributing circumferential velocity at the inlet, the hydraulic characteristics in the outlet conduit were quantified and analyzed. Additionally, the effect of the guide vane outlet angle on the internal flow and energy performance of the pump system was evaluated. The results reveal that a proper swirling flow can reduce entropy production, which was confirmed in the experiment. A high pump system efficiency is achieved when the guide vane outlet angle is 101°-99° from hub to shroud, with the flow deflection angle at the guide vane outlet of 4°-7°, which reduces flow separation on the guide vane, unfavorable flow patterns in the outlet conduit, and rotational kinetic energy losses in the outlet sump. In this way, the pump system efficiency was improved by 14.57 percentage points. The findings provide valuable insights for improving the efficiency of rotating machinery systems with guide vanes.