Study on the influence of pre-guide vanes on the bladeless region of water pump turbine at small opening

To address the “S” characteristic issue of pump–turbines under small opening conditions, this study investigates the impact of pre-guide vanes on the bladeless region dynamics, a critical yet underexplored area. This study investigates the changes in the bladeless region caused by the addition of pre-guide vanes in detail, providing insights for future theoretical research and engineering applications. Using a prototype from a Chinese pumped storage plant, three-dimensional unsteady flow simulations with the shear stress transport k–ω turbulence model were validated against experimental data. The results show that adding pre-guide vanes improves the “S” characteristic under small opening conditions and significantly influences the bladeless region. The arrangement and opening of the pre-guide vanes increase the opening of the synchronous guide vanes, which in turn raises the velocity within the bladeless region. Turbulent kinetic energy is concentrated in the bladeless region, where pre-guide vanes at positions 1#, 6#, 11#, and 16# disrupt the flow, generating strong turbulence. The maximum velocity is typically located at 0.005 m relative to the length of A–B, with velocity first increasing and then decreasing. High-frequency pressure pulsations dominate the bladeless region, with amplitude generally decreasing as the synchronous guide vane opening increases. Overall, the addition of pre-guide vanes significantly alters the flow structure and turbulence characteristics in the bladeless region, as well as the high-frequency pressure pulsations.