Optimization design of low-head reversible pumped storage units in pump mode based on linear circulation distribution

• Propose the influence of circulation on the hydraulic performance of low-head reversible pumped storage units. • Optimize low-head reversible pumped storage units via parametric design and orthogonal experimental method. • Apply linear circulation modification at impeller outlet and guide vane inlet. • Improve hydraulic efficiency in pump mode and alleviate excessive blade twist angle through circulation optimization. • Analyze flow field changes and establish evaluation criteria for circulation distribution. Improving hydraulic efficiency of low-head pumped hydroelectric energy storage systems enables energy savings, shortens charging times, and enhanced round-trip efficiency. This study employs a linear distribution strategy for circulation at the impeller outlet and guide vane inlet of a low-head reversible pumped storage unit in pump mode. To determine the optimal design based on numerical simulations, a combined approach of parametric design and orthogonal experiments was used. Linearizing circulation distribution significantly improved hydraulic efficiency of the low-head reversible pumped storage unit under both low-flow and rated conditions in pump mode, with rated efficiency increasing by 1.03% to 85.48%. The circumferential uniformity of absolute velocity at the impeller outlet and guide vane inlet was improved, resulting in a more uniform flow field. High-pressure zones at impeller leading edges decreased, suction-side pressure became more uniform, and flow separation was restrained. Vortex intensity and extent at trailing edges diminished, lowering turbulent entropy production. Flow at the guide vane inlet and inter-vane channels became more consistent. These coupled improvements reveal the intrinsic mechanism by which the optimized circulation enhances hydraulic performance. This study provides theoretical guidance for efficiency enhancement of low-head reversible pumped storage units in pump mode.