[en] Conventional parameters based on CFD methodology for the investigation on hump characteristics of a pump turbine cannot reflect the dynamic interaction mechanism between the runner and the fluid. This research presents a dynamic interaction mechanism of a pump turbine operating in the hump region. First, vorticity dynamic parameters were obtained based on the theory of vorticity dynamics. Second, 3-D unsteady flow simulations were performed in a full pump turbine model using the SST k-ω turbulence model, and numerical results have a good agreement with the experiments. Then, analysis was carried out to determine the relation between the vorticity dynamic parameters and hump characteristics. The results indicate that the theory of vorticity dynamics has an advantage in evaluating the dynamic performance of a pump turbine. The energy transfer between the runner and the fluid is through vorticity dynamic parameters-pressure and friction terms, in which the pressure term accounts for the most. Furthermore, vortex generation mainly results from the skin friction. Combining vorticity dynamic analysis with the method of Q-criterion indicates that hump characteristics are related to the reduction of the surface normal pressure work and vortex motion on the suction surfaces close to the leading edges in the runner, and the increase of skin friction work in the stay-guide vanes

[en] Highlights: • An interesting hysteresis phenomenon was analyzed using entropy production theory. • A function was used to calculate the entropy production in the wall region. • Generation mechanism of the hump and hysteresis characteristics was obtained. - Abstract: The hydraulic loss due to friction and unstable flow patterns in hydro-turbines causes a drop in their efficiency. The traditional method for analyzing the hydraulic loss is by evaluating the pressure drop, which has certain limitations and cannot determine the exact locations at which the high hydraulic loss occurs. In this study, entropy production theory was adopted to obtain a detailed distribution of the hydraulic loss in a pump-turbine in the pump mode. In the past, the wall effects of entropy production were not considered, which caused larger errors as compared with the method of pressure difference. First, a wall equation was proposed to calculate the hydraulic loss in the wall region. The comparison of hydraulic loss calculated by entropy production and pressure difference revealed a better result. Then, through the use of the entropy production theory, the performance characteristics were determined for a pump-turbine with 19 mm guide vane opening, and the variation in the entropy production was obtained. Recently, an interesting phenomenon, i.e., a hysteresis characteristic, was observed in the hump region in pump-turbines. Research shows that the hysteresis characteristic is a result of the Euler momentum and hydraulic loss; the hydraulic loss accounts for a major portion of the hysteresis characteristic. Finally, the hysteresis characteristic in the hump region was analyzed in detail through the entropy production. The results showed that the hump characteristic and the accompanying hysteresis phenomenon are caused by backflow at the runner inlet and the presence of separation vortices close to the hub and the shroud in the stay/guide vanes, which is dependent on the direction of discharge.

[en] Highlights: • Positive slope characteristics of pump-turbines were reviewed. • System instability analyses, numerical and experimental methods were summarized. • Influences of geometric and operating unit parameters were discussed. • Precautions and countermeasures for the improvement of positive slopes were proposed. -- Abstract: Positive slopes in the performance characteristics of pump-turbines are an indicative feature of unstable behaviour in pump mode operation, and are additionally conducive to instability problems during pump start-up. With the rapid development of pumped-storage power plants, this instability has become increasingly critical to the safe and stable operation of the entire unit. Thus, this topic has been attracting attention in the pump-turbine design and research fields. In this study, the relative achievements with regard to positive slopes, which have been obtained by various investigators over the last two decades, were reviewed. Such studies, including flow characteristics, and influence parameter, related to positive slopes were summarized. The generating mechanism, accompanying unsteady phenomena, and corresponding hysteresis phenomena, are also discussed. Investigations on the influence of geometric and operating unit parameters are presented. Finally, precautions and countermeasures for the improvement of positive slopes, and various perspectives regarding future work are proposed. The present review provides theoretical guidance for the design and operation of pump-turbines, in terms of the stability of the pumped-storage powers plants.