Comprehensive hydraulic performance assessment of a novel coaxial surge tank with shaft in pumped storage power plants: Stability, head loss characteristics, and refined transient simulation

With the rapid expansion of pumped storage power plant (PSPP) construction, a novel coaxial surge tank with shaft (CSTWS), featuring compact structure, shorter construction period, and easier maintenance, is proposed and shows great potential for applications. Nevertheless, this configuration alters the spatial relationship between the surge chamber and shaft, resulting in more complex hydraulic performances that differ markedly from those of conventional surge tanks (CSTs). In this study, comparative physical model tests, precise head-loss measurement, three-dimensional computational fluid dynamics simulations, and one-dimensional (1D) refined transient modeling was employed to assess comprehensive performance of the CSTWS. The results indicated that under steady-state operation, the flow in the CSTWS remained smooth without vortex or flow separation; yet, the pressure pulsation amplitude at the chamber bottom was approximately 1.38–1.75 times higher than that of the CST. Under load-disturbance conditions, the water-level oscillation amplitude of the CSTWS was 1.44–2.95 times larger, and the recovery time to stability was approximately 1.2–2.0 times longer, indicating slightly worse stability but eventual steady recovery. The flow coefficient of the CSTWS ranged from 0.75 to 1.17, i.e., significantly higher than that of the CST (0.6–0.8), implying stronger flow capacity. Incorporating dynamic head-loss coefficients into the 1D transient simulations revealed that the extreme surge in the CSTWS was more pronounced—the maximum water level was 2.14 m higher and the minimum level 2.66 m lower than that in the CST—suggesting that adequate safety freeboard and submergence depth must be ensured to prevent overtopping or air entrainment accidents. In conclusion, the findings provide theoretical insights and experimental evidence for the hydraulic design, stability assessment, and engineering application of CSTWS in large-scale PSPPs.