Pumped-storage hydropower requires pump-turbines to operate safely and efficiently under off-design conditions, where stall-induced unsteady flows can redistribute hydraulic losses and reduce operational stability. Unlike previous analyses focused mainly on spatial correlations, this study develops a spatiotemporal framework to clarify how hydraulic loss (HL) and vortex evolution (VE) co-vary under different stall states at the valley point of the pump-mode hump region in a low-specific-speed, ultra-high-head pump-turbine. Detached eddy simulations (DESs) were performed for an original-runner scheme (ORI) and an optimized-runner scheme (OPT), with identical stationary components, boundary conditions, and numerical settings. The comparative cases cover four representative flow states: non-stall, fixed stall, rotating stall, and mixed stall. The local hydraulic-loss rate (LHLR) was decomposed into dissipation (DIS) and transport (TRANS) terms, and Liutex-based vorticity decomposition was used to distinguish shear- and rigid-rotation-related vortex quantities. Pearson correlation analysis was then applied in both space and time. The results show that DIS is consistently associated with shear enstrophy ΩS, whereas the spatiotemporal correlation associated with TRANS and VE parameters exhibits stronger regional and stall-state dependence. These findings provide a quantitative basis for identifying loss-sensitive vortex features and support flow-control and runner-optimization strategies for improving pump-turbine efficiency and stability.
Unsteady wake dynamics and associated energy dissipation mechanisms in mixed-flow pumps remain critical challenges detrimentally affecting operational stability and efficiency, particularly under off-design operating conditions. This study employs high-fidelity numerical simulations incorporating a modified Wray-Agarwal turbulence model to systematically investigate the spatiotemporal evolution of wake vortex structures and their coupling with energy loss across various flow regimes—ranging from design to deep stall. A multimethod framework integrating Q-criterion-based vortex identification, Dynamic mode decomposition, and entropy production analysis is utilized to characterize flow instabilities and hydraulic losses. Findings indicate that under design conditions, the wake structures exhibit periodicity and dominated by rotor-stator interactions; conversely under stall conditions, large-scale coherent vortices induce significant flow-field disruptions, leading to flow blockage and intensified shear-layer instabilities. The vortex morphology evolves from a stable "T-shaped" structure to a distorted "V-shaped" vortex, ultimately manifesting a multiscale turbulent band during deep stall. Entropy production analysis delineates the tip clearance and mid-span regions as the predominant sites of dissipation, with energy losses increasing by over 80% under deep stall conditions. This work elucidates a quantitative correlation between unsteady vortex dynamics and hydraulic inefficiency, providing a novel diagnostic framework for optimizing pump performance through targeted flow control strategies.
Y. Chen, W. Li, L. Ji et al.· Journal of Applied Fluid Mec...· 0 citations
During load rejection in pumped-storage power stations, the rotational speed of the pump-turbine increases abruptly. The consequent structural deterioration of the internal flow induces high-amplitude hydraulic excitations, posing a serious threat to the operational stability of the unit. This study investigates a Francis pump-turbine to elucidate its flow evolution and instability mechanisms during load rejection. The fluid is modeled as weakly compressible water to capture finite pressure wave propagation. Dynamic mesh simulates guide vane closure, while vortex identification and short-time Fourier transform analyze transient pressure pulsations. The results indicate that the transient process can be sequentially divided into four typical stages—turbine mode, turbine-braking mode, reverse-pump mode, and return-to-turbine mode—to account for the most critical periods during the load rejection transient. The unit exhibits the poorest stability near the maximum rotational speed (443.34 r/min), where flow reversal and the full development of vortex structures significantly amplify fluctuations in hydraulic thrust. The vaneless space is identified as the primary source of pressure pulsations, whose characteristics are dominated by rotor–stator interaction mechanisms, and such disturbances decay rapidly in the downstream direction. Under turbine-braking and reverse-pump conditions, vortex rings, backflow, and asymmetric vortex structures generated within the spiral casing collectively contribute to the severe deterioration of the internal flow field quality.
Lei Deng, Wenfu Han, Yuhao Yan et al.· Water· 0 citations
Pump-turbines often experience performance deterioration under high-load conditions beyond their best efficiency point, while the underlying flow mechanisms remain insufficiently understood. In this study, we investigate the relationship between internal flow structures and energy performance in a pump-turbine operating at a rated head of 202 m over a range of guide vane openings. Energy losses are evaluated using an average kinetic energy-based method and compared with an entropy production approach. A threshold-independent rigid vorticity method is adopted for vortex identification, and a streamline-based coordinate system is introduced for spatial quantification of energy loss and blade loading. The results show that hydraulic losses are mainly concentrated in the draft tube (66–75%) and runner (25–30%) under high-load conditions. A coupled vortex system formed by separation vortices and horseshoe vortices governs localized dissipation in the runner. In the draft tube, a columnar vortex rope generates strong shear layers that dominate energy loss in the cone and elbow regions. At high flow rates, negative incidence induces pressure-side separation, forming negative torque regions that reduce net runner torque and lead to output power deterioration. These findings highlight the dominant role of coupled vortex structures and pressure redistribution in performance degradation under high-load operation.
Lingkai Zhu, Kai Liang, Yunkuan Yu et al.· Applied Sciences· 0 citations
The grid volatility caused by the integration of wind and solar power poses challenges to power systems, where Pumped Storage Hydropower (PSH) plays an irreplaceable role. During start-up, shutdown, and mode transition of pump turbines, near-zero flow conditions frequently occur, leading to severe hydraulic instability, guide vane vibration, and abnormal noise. This review synthesizes field observations from multiple high-head pumped storage stations together with recent experimental, numerical, and theoretical studies. The review indicates that hydraulic instability is primarily associated with the coupled effects of clearance leakage flow, bi-stable flow, and Rotor–Stator Interaction (RSI). The review suggests that self-excited vibration, rather than forced resonance, dominates guide vane vibration and abnormal noise under near-zero flow conditions. Four mainstream regulation strategies are summarized, including Misaligned Guide Vanes (MGVs), start-up/shutdown sequence optimization, structural-parameter adjustment, and operating range avoidance. The applicability and limitations of each strategy are discussed. These findings provide support for the design and operation of high-head, large-capacity pump turbines.
Hui Zeng, Yuhao Yan, Bin Wang et al.· Machines· 0 citations
The transition process from synchronous condenser pump (SCP) mode to pumping mode determines the response rapidity of the startup procedure and operational stability of pump-turbines; however, the complex gas–liquid interaction and transient hydraulic characteristics during this process remain insufficiently understood. To address this, this study develops a numerical framework for the SCP-to-pumping transition process, incorporating the full-passage system, a multiscale mesh strategy for coupling mainstream and clearance flow regions, and a gas–liquid two-phase flow model based on the Volume of Fluid (VOF) method. The reliability of the numerical model is verified through comparison with model experiments, demonstrating good agreement between simulations and experimental data. Based on the validated model, the transient evolution of hydraulic forces, pressure pulsations, and internal flow structures is systematically analyzed. Axial force analysis reveals a significant internal equilibrium; the crown bears a maximum instantaneous fluctuation of approximately 2800 kN. Conversely, the radial force is primarily dominated by blade hydraulic thrust (1294 kN), showing distinct anisotropic behavior. The runner blade channels and the upper draft tube region are identified as critical areas with intense pressure fluctuations, with peak-to-peak pressure amplitudes reaching 45~48 m and 54 m head, respectively. Furthermore, reducing the duration of the exhaust process constitutes the main strategy for accelerating the transition and mitigating prolonged high-amplitude force and pressure fluctuations. The findings provide new insights into the transient hydraulic mechanisms of SCP-to-pumping transitions and offer guidance for optimizing transition control strategies in pumped-storage units.
Lei Deng, Longxiang Chen, Hai-Chao Feng et al.· Applied Sciences· 0 citations
Owing to its efficient energy recovery capability, the pump as turbine (PAT) has attracted considerable attention and has been widely applied in micro hydropower systems. However, under off-design conditions, large-scale helical vortex ropes are readily induced in the draft tube, causing severe hydraulic losses and flow instability. Because existing theoretical models do not account for the slip effect at the impeller outlet, this study combines vortex dynamics theory with numerical simulation and introduces a correction coefficient to develop a Burgers vortex-based analytical wake vortex model for a PAT with splitter blades. The model is verified by its ability to predict the peak tangential velocity and radial decay trend of the vortex core. In addition, the influence of draft tube configuration on vortex rope evolution is revealed using the Liutex vortex identification method and enstrophy analysis. The results show that the geometric curvature of the elbow draft tube induces vortex rope breakup and high energy dissipation. Finally, entropy production theory is used to quantitatively evaluate the vortex suppression benefit and hydraulic loss caused by deflector plates. The results indicate that the transverse deflector plate (TDP) provides a significantly better suppression effect than the longitudinal deflector plate (LDP) by disrupting the circumferential continuity of the vortex rope. Although increasing the insertion depth of the deflector plate improves vortex suppression, it induces non-negligible local high-entropy production on the upstream-facing surface (US). This study clarifies the physical mechanism of wake vortices in a PAT with splitter blades and provides theoretical guidance for efficient PAT operation and wake vortex control.
Chenguang Wang, Wang Zheng, Yingxiao Shi et al.· Water· 0 citations
