Gravity-induced virtual clocking effect in large-capacity/low-head pumped hydro energy storage system with horizontal shaft

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Significant gravity effects in a large-capacity/low-head pumped hydro energy storage system with horizontal shaft can destroy the axisymmetric inner flow structures in pump mode compared with its hydraulic model. For the stationary guide-vane region, this makes the inflow condition of each vane change with its circumferential installation position and thus causes a virtual clocking effect between the visible guide-vanes and the invisible gravity. The virtual clocking effect may change the hydraulic performance and induce instability risks, but its specific influence is still unclear. In this article, for a low-head pumped hydro energy storage prototype system with horizontal shaft, four virtual clocking schemes, namely the top guide-vane inlet edge and the gravity direction staggering 0/4, 1/4, 2/4 and 3/4 passage interval angle, were established, and a comparative investigation on hydraulic performance in the optimal pump mode was carried out. Firstly, the pressure polarization oscillation is observed in the impeller region under different schemes. The differences in the polarization degree do not exceed 1%, but the circumferential distributions of the peak-to-peak values of pressure fluctuations at the impeller outlet show a phase difference, which should be considered in the circumferential support structures. Secondly, the partial flow separation is observed in the guide-vane region under different schemes, and the flow deviation ratio between the left and right sides at the guide-vane outlet exhibits differences of up to 10%, which cannot be ignored for a huge system. The scheme with staggering 1/4 passage interval angle exhibits the least flow deviation ratio while the scheme with staggering 3/4 passage interval angle exhibits the greatest flow deviation ratio. Overall, to ensure the hydraulic stability of the large-capacity/low-head pumped hydro energy storage system with horizontal shaft, the virtual clocking scheme with staggering 1/4 passage interval angle is recommended for the energy engineering.