Complex energy conversion and energy dissipation occur in pump-turbines during the load rejection process. However, the underlying fluid mechanism is not clear. In order to solve these problems, in this study, a three-dimensional (3D) transient turbulent flow in a pump-turbine, with clearance during the load rejection process, was simulated using the method of coupling of the rigid rotor motion with flow and dynamic mesh technology. The simulated rotational speed shows good agreement with the experimental data. Most of the differences of rotational speed between simulations and experiments are very small and lower than 5%. Based on the numerical simulation, the energy conversion process, loss distribution, and flow mechanism in a pump-turbine were analyzed using the method of coupling of the entropy production analysis with the flow analysis. The results indicate that the load rejection process of a pump-turbine is an energy-dissipation process where the energy is converted among various energy forms. After load rejection, the hydraulic loss in the reverse pump process distributes primarily in the stay/guide vanes (GV), the vaneless space, and near draft tube inlet. While the hydraulic losses in the runaway process and the braking process are distributed mainly in the elbow section of the draft tube, the clearance of runner (RN), and the vaneless space, the hydraulic losses are mainly caused by viscous dissipation effects of the vortex flows, including the flow separation vortices, the shedding vortices of flow wake, the secondary flow, and the backflow.

At present, pumped-storage power technology is the only available and effective way for the load balancing and energy storage in the grid network scale. During the frequent switch back and forth conditions, there are severe pressure pulsation and cavitation in pump-turbines. However, their generation mechanism has not been determined yet. This work contributes to the numerical simulation of the transient behaviors in a prototype pump-turbine during the load rejection process with special emphasis on cavitation effect. In this study, the two-dimensional dynamic remesh and variable speed slide mesh methodologies were employed to perform the simulation of the transient single-phase flow and cavitation flow in a pump-turbine. The simulation results of single-phase flow and cavitation flow were both consistent with the experimental data except in local regions based on the experimental validation of prototype tests. However, the numerical results considering cavitation effects have a better behavior than those of single-phase flow in the predictions of pressure pulsation and rotational speed. Then, the cavitation flow simulation results were analyzed deeply, especially in pressure pulsation and cavitation flow field. Analysis revealed that three typical complex frequency components of pressure were captured in the cavitation flow, which significantly affect the axial hydraulic thrust on the runner. And it is validated that they are primarily induced by the cavity collapse near the trailing edges of the runner blades in reverse pump mode and the interaction between cavitation and vortex rope in draft-tube in turbine mode.

To demonstrate the influence of cavitation on pressure fluctuation in a pump-turbine, one- and three-dimensional coupling numerical simulations were conducted on a typical runaway process that occurs when the turbine rejects the load. Transient flow in the pipelines was simulated using a one-dimensional single-phase flow model. The unsteady three-dimensional turbulence flow in the pump-turbine was calculated using the single- and two-phase flow models. The numerical method was validated with available experimental data. The maximum simulation errors of the rotational speed and fluctuating pressure were 3 and 10%, respectively. Local backflow vortices leading to the cavitation were identified at the high-pressure runner inlet. The cavitation interacted with the vortices during the runaway process when the turbine rejected the load. These interactions increased the existing duration and action range of the backflow vortices at the runner inlet. Under the excitation of the local cavitation backflow vortices and the shock pressure induced by the instantaneous cavity collapse, the amplitudes of the pressure and hydraulic thrust fluctuations using the two-phase flow model increased significantly to approximately thrice that obtained using the single-phase flow model. While using the single-phase flow model, severe fluctuations occurred near the no-load condition. The occurrence instants of severe fluctuations while using the two-phase flow model were delayed by a specific duration compared to the no-load condition; this is attributed to the duration of cavitation development from the inception of cavitation to the maximal cavity volume. This finding is valuable for the accurate prediction and optimization of pressure fluctuations in the turbine runaway process.

Both foil structure and surface texturing have been widely used to improve bearing performance. However, there is little research on their combination, namely, textured gas foil bearing. This paper adopts the Reynolds equation as the pressure governing equation of bump-type foil journal bearing to study the influence of textures located on the top foil. The Newton-Raphson iterative method and the perturbation method are employed to obtain static and dynamic characteristics, respectively. Thereafter, based on three texture distribution types, further analysis about the effect of the relative texture depth and the textured portion is carried out. The results indicate that an appropriate arrangement of textures could improve the performance of gas foil bearing. For #1 texture distribution, the maximum increment of load capacity could exceed 10% when ω = 1.4 × 10⁵ r/min, ε = 0.2.