Transient characteristics analysis in variable-speed process of pump-turbine based on rothalpy change and vortex method

Variable-speed (VS) operation enables rapid and flexible power adjustment in pump-turbines (PT), while operational stability during transient processes is critically dependent on internal flow dynamics. Thus, the transient flow field inside the PT during the VS transition process is investigated by numerical simulation. Based on rothalpy change and vortex methods, the changes in energy loss, vortex structure, and pressure pulsation during the VS process with the three openings are investigated. The results show that in the initial stage of the VS condition 1 (VSC1) that corresponds to small guide vane opening condition, the rothalpy change is significant in the vaneless region of the guide vane and the runner flow passage. Moreover, aggregated vortices are observed in the initial stage of VSC1, leading to significant power oscillations with an amplitude reaching 37.2% of the stable output. During the middle and late stages of VSC1, vortex intensity diminishes significantly, leading to the disappearance of oscillations. Increasing the guide vane opening mitigates vortex aggregation in the initial stage of VSC1, resulting in improved power stability.