Variable-speed operation of pump-turbines enhances efficiency and grid stability in renewable-rich power systems. However, the internal complex flow and energy dissipation mechanisms during speed regulation, particularly throughout complete speed increase and decrease cycles, are not well understood, posing challenges to operational safety and efficiency optimization. This study systematically investigates and compares the internal flow and energy dissipation under linear speed increase and decrease processes in pump mode using high-fidelity numerical simulation, experimental validation, and entropy generation theory. The results indicate that variable-speed operation significantly affects the internal flow structure and energy distribution: although the speed increase process locally improves the flow pattern at the stay vanes, the total system entropy generation increases by 17.9% compared to constant-speed operation, indicating intensified flow disturbance; during the speed decrease process, velocity distribution at runner outlet becomes more uniform, the velocity gradient decreases significantly, effectively suppressing flow separation and vortex dissipation in areas of runner and guide vanes, leading to a significant 12.6% reduction in total system entropy generation compared to constant-speed operation and improved energy efficiency. The runner and double-row cascades remain the primary sources of energy loss, accounting for over 62.7% of entropy generation. Entropy gradients are strongest near the runner crown, while the spiral casing exhibits nonlinear loss and the draft tube remains stable. These findings, derived from a direct comparison of bidirectional speed regulation, fill a gap in understanding transient loss mechanisms and provide a theoretical basis for the optimized design and safe operation of variable-speed pump-turbines.

Frequent start-stop operations of pumped storage power stations subject pump-turbine units to high-amplitude water hammer pressures at small guide vane openings, thereby compromising operational stability and service life. This study numerically simulates the pump mode start-up process of a prototype pump-turbine under three heads, with a focus on runner forces and internal flow dynamics. Results show that the x-direction component of runner hydraulic thrust exhibits minimal head dependence, whereas the y-component decreases by 28.2 % at 220.05 m relative to 175.4 m. At 220.05 m head, the force magnitude increases significantly, and rotor eccentricity intensifies. During small guide vane openings, reverse flow develops in the vaneless space between stay and guide vanes, driven by reverse blockage and forward blockage. Additionally, significant flow maldistribution persists in stay vane passages throughout the start-up process. The absolute radial hydraulic thrust on guide vanes decreases with increasing guide vane opening and head, achieving a maximum reduction of 75.4 %. These findings establish a dual-blockage theoretical framework for understanding transient instabilities, providing critical guidance for stability optimization in high head pumped-storage power plant designs.

The blade wrap angle of the runner has an obvious influence on the energy and cavitation performance of the pump-turbine, and it is difficult to take into account the performance of both turbine and pump conditions in the design process. In this paper, the blade setting angles of the high-pressure edge, that of the low-pressure edge, and the control coefficients of blade setting angle distribution are taken as optimization variables, and the blade wrap angle control strategy is introduced to optimize the runner for multi-objective and multi-operating condition optimization. Numerical simulations are used to compare and analyze the internal flow field, blade load distribution, and cavitation characteristics of the pump-turbine before and after its optimization. The results show that the optimized runner blades can obviously alleviate the flow separation problem at the middle position of the original runner blades under turbine conditions, as well as the impact and flow separation problems under pump conditions. After optimization, there is neither impact nor flow separation in the runner, and the streamline distribution is uniform; additionally, the cavitation performance of the low-pressure edge of the runner blades under both turbine and pump conditions is greatly improved. The efficiency of the pump at the 1.0Qp discharge condition reaches 91.84%, an increase of 1.84%, and the efficiency of the turbine at rated output and above-rated operating conditions is not reduced. The blade wrap angle control strategy can provide important constraints for the multi-objective and multi-operating condition optimization design of the high-head pump-turbine, and shorten the optimization time while improving speed and efficiency.

To investigate cavitation-induced noite under different tip clearancer, the NACA0009 hydrofoil is analyzed by numerical simulation. To better capture tip leakage vortex (TLV) cavitation, the shear stress transport ke tur-bulence model with the rotation-curvature corrected is modified by introducing the turbulent energy backacatter effect (MSST-CC), and the Schnerr Sauer cavitation model is improved by incorporating vortex vapor in-teractions (VISS). Comparisons with experiments show that the improved model accurately predict: TLV cavi-tation at mall and medium clearances. Results indicate that the overall sound pressure level (OSPL) decreases with increasing tip clearance, with a reduction of approximately 6.75 dB observed at the location IC downstream as the dimensionlest tip clearance increases from 0.5 to 1.5. The reduction in preudo-loading noise generated within the flow field is identified as the primary cause of the OSPL decreare with increasing clearance, whereas variations in pseudo thickness noise induced by bubble dynamics are relatively minor. The evolution of sheet and cloud cavitation is identified as the dominant source of preudo-thicknen noise, while the vortex-cavitation noire associated with the ip separation vortex (TSV) and TLV exhibits more complex variation characteristics. These findings offer theoretical insights for the prediction of cavitation-induced noise and the optimization design of marine propellers and pump-jets.

The presence of tip clearance can generate a leakage vortex, which complicates the cavitating flow characteristics and significantly affects the energy loss. This study numerically investigates the cavitating flow around a National Advisory Committee for Aeronautics 0009 hydrofoil with a tip clearance. Based on velocity gradient tensor (VGT) decomposition and entropy production (EP) theory, the fluid motion patterns and the sources of vortices in hydrofoil cavitating flow are investigated; furthermore, the energy losses and their underlying mechanisms are revealed. The results show that pure shear (PS) is the predominant deformation in hydrofoil cavitating flow, while rigid rotation accounts for a relatively small proportion. The vortices can be divided into three categories based on their causes, including cavity growth induced vortex, cavity shedding induced vortex, and cavity collapse induced vortex. The detached cavity moves downstream with internal rotation rather than a simple translation. With increasing tip clearance, the energy loss presents a declining trend. The turbulence dissipation EP serves as the primary contributor, constituting over 90% of the total. The changes in cavity volume and EP exhibit opposite trends during one cycle. The distributions of EP rates exhibit a strong correlation with those of VGT components. The energy loss predominantly arises from the PS and compression/stretching (CS) deformation. Among them, direct dissipation EP mainly comes from CS, and turbulent dissipation EP is mainly caused by PS.

When a centrifugal pump is used to transport gas-liquid two-phase flow, the surge at high gas volume fraction leads to head and efficiency decrease, as well as cause strong vibrations in the pump system. To improve the transport performance of centrifugal pumps, the closed impeller is redesigned into a semi-open impeller. The effect of the blade tip clearance size on the head and efficiency is explored. The similarities and differences in the internal gas distribution patterns between closed and semi-open impellers are analyzed. The suppression mechanism of the tip leakage flow on surge is clarified. At 5% inlet gas volume fraction, the closed impeller exhibits significant gas accumulation characteristics on the pressure surface, forming gas pocket flow patterns and gas vortex. Additionally, some gas bypasses the blade leading edge, forming gas leading edge overflow, and re-accumulates at adjacent blades, further exacerbating the gas pocket flow patterns. This results in high gas phase distribution zones and low liquid phase velocity zones, inducing surge. There is a proper tip clearance, which provides an additional flow path for the gas in the flow passage. The liquid and gas mixed tip leakage flow impacts and suppresses the gas accumulation. The gas pocket flow pattern and gas vortex are disrupted. The liquid and gas mixed fluids can smoothly flow out of the impeller. The surge is suppressed and the performance of gas-liquid mixed transport is significantly improved. The optimal tip clearance size of 1 mm increased the head and efficiency by 16.2% and 13.1% respectively under surge condition.

Surge in gas-liquid two-phase centrifugal pumps under high inlet gas volume fractions (IGVF) will lead to performance degradation and vibration increase, and threaten safe and stable operation. A novel method is proposed to suppress surge with radial grooves in a semi-open centrifugal pump. A parametric study on the circumferential coverage ratio, depth, and number of radial grooves was conducted, and the gas-liquid mixing characteristics, radial velocity, streamline, and vortex were quantitatively analyzed. The results show that the gas-liquid transportation performance of the centrifugal pump is optimal when the circumferential coverage ratio, depth, and number of radial grooves are 0.125, 3 mm, and 12, respectively. The maximum increases in the head and efficiency were 23.9%, and 6.9%, respectively. Gas accumulation in the flow channel forms high-gas volume fraction (GVF) regions. With an increase in the IGVF, the gas accumulation phenomenon becomes more obvious, and GVF becomes larger. The fluid enters the radial groove from the high-pressure side and flows out from the low-pressure side at a relatively high speed, impacting the gas accumulation and gas pocket flow, improving the flow state at the blade inlet, and reducing GVF in the impeller and the scale of the vortex, thereby increasing the mixing degree of the gas-liquid two phases. The mixing coefficient increased by 9.8%.

The hump phenomenon represents a typical instability in pump-turbines, adversely affecting operational stability during pumping mode in pumped-storage units. This investigation examines hump characteristics through integrated numerical simulation, theoretical analysis, and experimental testing. The energy balance analysis reveals that the hump phenomenon occurs when the partial derivative of output power P-out with respect to flow rate Q is greater than the ratio of P-out to Q (partial derivative P-out/partial derivative Q > P-out/Q), resulting from combined effects of reduced input power and increased hydraulic losses. Frequency-domain analysis demonstrates a transition from blade passing frequency and its harmonics to low-frequency (0.1f(n)) characteristic within the hump region. Complementary ensemble empirical mode decomposition reveals that this component is predominantly concentrated in the seventh to ninth intrinsic mode functions. Partial Transfer Entropy analysis reveals fundamental transformations in causality between power components and low-frequency pressure pulsation within hump region. Pre-hump conditions exhibit weak coupling, while at the hump peak, causality is significantly enhanced through resonance between power fluctuations and vortex shedding frequencies, reaching its maximum. At hump valley, low-frequency pressure pulsation becomes the dominant factor governing power variations. This study comprehensively elucidates the transition from energy-controlled to dynamics-dominated process in pump-turbine, clarifying the intrinsic mechanisms underlying hump formation.

As a core regulating component in multi-energy complementary system, Francis turbines (FTs) are required to frequently undergo variable load processes (VLPs) to mitigate power fluctuations from wind and photovoltaic generation. To clarify the influence of guide vane opening speed (GVOS) on hydraulic stability during VLPs, numerical simulations are conducted for three GVOSes over the load variation from 30% to 100% rated power. The evolution of internal flow structures and pressure pulsation (PP) characteristics in the flow passage components is systematically analyzed. A flow field decomposition framework based on complementary ensemble empirical mode decomposition (CEEMD) is further introduced to reveal formation mechanism of high-amplitude PPs of draft tube (DT) and the evolution of their dominant modes. The results indicate that the influence of GVOS on runner blade PPs is mainly concentrated near the blade inlet edge, and the peak-to-peak value of PP decreases as the opening speed increases. With faster opening speeds, both vortex rope volume and turbulent kinetic energy in the DT are reduced. High-amplitude PPs are primarily concentrated at 0.27fn and its second harmonic 0.54fn, where 0.27fn corresponds to the vortex rope frequency. Pressure field decomposition shows that a higher opening speed delays the moment when the vortex rope reaches its most unstable state and shortens its duration. The formation intensity and energy diffusion of the dominant vortex rope are significantly weakened, resulting in improved hydraulic stability during the VLP. These findings provide useful guidance for enhancing the transient stability of FTs under variable load operation.

Seawater pumped storage stations play a critical role in power and frequency regulation within coastal power systems, where pump-turbines (PTs) are frequently subjected to start-up and load rejection conditions. Under such transient operations, PTs are prone to entering the unstable region of the S-shaped characteristic (SSC), which poses significant risks to operational stability and grid safety. Reliable identification of SSC-related instability is therefore essential for intelligent monitoring and decision support in complex energy systems. In this study, a stability experiment is conducted on a high-head PT operating under different guide vane openings (GVOs), and the vibration and pressure pulsation (PP) signals within the SSC region are systematically analysed. To accurately recognize whether the PT enters the unstable SSC region, a data-driven intelligent recognition framework integrating red-billed blue magpie optimization, bidirectional long short-term memory and a multi-head attention mechanism is developed. This framework enables effective feature extraction and adaptive information fusion from heterogeneous vibration and PP signals. The experimental results indicate that the PP intensity in the vaneless region is significantly higher than that in the draft tube elbow within the SSC region. With the increase of the GVO, the vibration in the vaneless region gradually intensifies. Recognition results demonstrate that the proposed framework achieves an accuracy of 99.75% in identifying SSC-related unstable operating states, confirming its effectiveness. The proposed method provides a practical, measurement-based approach for the online monitoring and early warning of operational instability in PTs and similar energy conversion systems.

Inflow distortion has long been recognized as a critical factor contributing to increased energy consumption in large-scale pumping systems worldwide. When combined with the inherent circumferential asymmetry of double-suction pumps, the resulting unsteady effects further impair operational stability. However, the associated coupling mechanisms under valve-inflow conditions have not been systematically investigated or quantitatively characterized. This study addresses this gap by employing entropy generation theory and vortex dynamics analysis to investigate the energy-loss mechanisms of double-suction pumps subjected to valve-induced inflow distortion. Results show that complex shedding and standing vortices develop downstream of the valve, with dissipation concentrated within 1D0(where D0 is the water supply pipe diameter) of the valve axis and fully dissipated by turbulence at 2.5D0. At high flow rates, these structures reduce efficiency by 3.4% and head by 3.7%, modify the relative flow angle near the impeller leading edge and suction surface, intensify impact losses and induce separation, and generate a new pulsation frequency of 0.13fn in the suction chamber. The findings validate the applicability of entropy-generation analysis for quantifying local energy dissipation, clarify the valve-pump coupling mechanisms, and provide practical guidance for improving the hydraulic efficiency and operational reliability of large pumping stations.

In this article, the particle size of the original coal gasification fine slag (CGFS) sample is mechanically ground to less than 40 microns by mechanical activation. The traditional alkali washing and acid washing pretreatment technology improved by microwave heating method is used to further treat the mechanically activated samples, and three experimental samples were obtained. Mechanical activation sample CGFS-A, alkali washing sample CGFS-B and acid washing sample CGFS-C. The kinetic parameters such as activation energy (E) and pre-exponential factor (A) are calculated by thermogravimetric analysis and Cotes-Redfern model to analyze the combustion process. The results show that the mechanically activated sample CGFS-A has the highest flammability index (c) and comprehensive combustion characteristic index (s), indicating that its comprehensive combustion performance is the best. In this article, through the comparative analysis of the three samples, it is concluded that acid washing and alkali washing remove more than 95% of the metal oxides, resulting in a significant reduction in the combustion catalyst in the CGFS combustion reaction. Therefore, simple mechanical activation can better improve the combustion performance of CGFS.

In pumped storage systems, mixed-flow pumps (MFPs) are frequently subjected to start-up and shutdown operations to meet grid regulation demands driven by the variability of renewable energy sources. The start-up strategy (SUS) significantly affects the transient hydraulic stability of MFPs during the start-up process (SUP). Based on stability experimental data, this research quantitatively analyzed the effect of different power exponential law (PEL) SUSes on the stability of MFP by signal analysis method and technique for order preference by similarity to ideal solution (TOPSIS) method. The results indicate that the power exponent (PE) of PEL SUSes significantly affects the stability of MFP during the SUP. The increase in PE helps to reduce the shock shaft vibration (SV) and the pressure pulsation (PP) amplitude in the pump during the SUP. Moreover, it can also lower the total effective sound pressure level (SPL) of the MFP in the SUP. By comparing the stability index (St) that integrate the characteristic values of multi-source signals, the stability of the MFP started in Strategy 5 (PE is 4) is 62.66 % lower than that in Strategy 3 (Linear). The nonlinear polynomial fitting method is employed to establish the relationship between the PE of SUSes and the St. And then the PE with the optimal St is established to be 3.65, corresponding to the Strategy-P3.65. Experiments have confirmed that Strategy-P3.65 outperforms the other five strategies in terms of SV, noise and PP, verifying the reliability of the fitted model.

The internal flow characteristics in the hump region of a pump-turbine exhibit complex flow structures and multi-scale vortex evolution. This study analyzes flow features in dual-row cascades at typical operating points under different guide vane openings (GVOs) using the rigid vorticity transport equation. Results demonstrate that under small GVO, vorticity structures primarily concentrate on the guide vane surface and stay vane leading edge, while at large GVO, these vortices mainly emerge in the vaneless space at the runner outlet. The pseudo-Lamb term (RCT) and viscous term (RVT) in the rigid vorticity transport equation are identified as key factors governing vorticity transport. At the hump valley point, RVT becomes orthogonal to streamlines, while RCT forms an obtuse angle with them. Frequency domain analysis of pressure pulsations reveals a characteristic frequency of 0.2 f n at GVO=12° when vortices accumulate near the hub and shroud, whereas at GVO=20°, the pressure pulsations exhibit broadband characteristics as the vortex range extends in the vaneless space.

Cavitation in centrifugal pumps leads to significant performance degradation and mechanical damage, making the understanding of losses under cavitation conditions crucial for optimizing pump design. An improved entropy production method considering the effect interphase interactions between water and vapor (EPMI) with computational fluid dynamics (CFD) is employed to study the energy loss caused by cavitation in a centrifugal pump. The shear stress transport (SST) k-omega turbulence model coupled with Schnerr-Sauer cavitation model is used for simulation. The results demonstrate that EPMI offers more accurate predictions of energy losses, particularly under severe cavitation conditions. Cavitation primarily impacts direct, fluctuation, and interaction losses, with wall losses remaining largely unaffected. Direct and fluctuation losses are mainly caused by the flow separation near the pressure surface of blades, and almost account for more than 55 % of the impeller losses. Interaction losses composed of mass transfer and surface tension effects, can account for up to 26.3 % of impeller losses and should not be neglected under severe cavitation. The dominant frequencies of fluctuation and wall losses are governed by the cavity volume evolution in the impeller, while direct and interaction losses are influenced by both impeller rotation and cavity dynamics. The research results can provide reference for predicting the cavitation performance of centrifugal pumps and guiding optimization design work.

Pump-turbine operation in pump mode demonstrates characteristic hump instability in the head curve, accompanied by intricate flow dynamics and compromised hydraulic stability. This investigation integrates numerical modeling with experimental validation, utilizing entropy production analysis and vorticity decomposition methodology to examine the relationship between transient flow patterns and pressure pulsation characteristics within the hump region. Results indicate that energy dissipation predominantly originates from the vaneless space and guide vane channels. Hump region operation induces a significant increase in guide vane inlet angle of attack (AOA), triggering progressive flow separation and rigid vortex accumulation. This hydrodynamic instability leads to periodic separation vortex formation and shedding, with vortex structures exhibiting distinct migration patterns: some maintain circumferential rotation in the vaneless space, while the remainder propagate into guide vane channels. Spectral analysis identifies a frequency shift in vaneless space pressure fluctuations from the characteristic rotor-stator interaction frequency (10fn) to low-frequency components (0.051fn and 0.064fn), accompanied by broadband energy distribution across the 0-10fn range. Quantitative analysis establishes a positive correlation between decreasing flow rates and amplified broadband fluctuation amplitudes, attributed to expanding rigid vortex volumes.

Cavitation results in increased losses, reduced performance, and blade damage, which seriously affects the safe and stable operation of centrifugal pumps. This study introduces a novel bionic blade design aimed at inhibiting cavitation based on the humpback whale's bumpy tubercles. The study investigates the impact of the geometric parameters of the bionic blade on cavitation, pressure, and vortex distribution. Furthermore, the losses inside the impeller are analyzed using entropy generation theory. The research findings indicate that compared to the original model, the bionic blade exhibits superior performance, with the highest efficiency and head achieved when the amplitude of the bionic blade is 0.08 times the blade outlet width. The head and efficiency of the bionic blade increased by 4.4% and by 2.7%, respectively. The bionic structure changes the flow state of the blade leading edge, leading to a reduction in the vortex near the shroud, a decrease in the low pressure region on the pressure surface of the blade, and a substantial increase in the impeller outlet pressure, reaching 13.6%. Moreover, the disappearance of the vortex on the suction surface of the blade inhibits cavitation and high entropy generation regions. As a result, the vapor volume and total entropy generation in the impeller significantly decrease by 38% and 37.4%, respectively, under severe cavitation conditions.

Numerical simulation has become a widely used and effective method for investigating cavitation-induced noise in hydraulic machinery and multiphase flow systems. The accuracy of noise prediction strongly depends on the precise simulation of both the flow field and the cavitation field. However, most commonly used turbulence models are developed based on single-phase flow assumptions, neglecting the compressibility effects introduced by phase interactions in two-phase flows, which can lead to significant computational errors. Therefore, this study takes into account the compressibility effects induced by phase mixing and proposes an improved SST k -ω turbulence model. Comparison with experimental results demonstrates that the improved model more accurately predicts the development and breakup processes of bubbles in the flow field. Compared with the original model, the modified model’s prediction of vapour volume is approximately twice that of the original model. The cavitation-induced sound field demonstrates that the results obtained using the compressible turbulence model show a 1.4% improvement compared to the original model, and align better with experimental data.

The inlet elbow pipe of a double-suction pump in solid–liquid two-phase flow can easily lead to a variety of unstable flow conditions, such as flow asymmetry, turbulence intensity, and vortex formation. Additionally, the inertia and aggregation effects of particle movement complicate this phenomenon further, significantly affecting energy performance and causing erosion damage. This study explores the relationship among elbow curvature, solid–liquid two-phase flow characteristics, entropy production distribution, and wear patterns in a double-suction centrifugal pump. The interaction between inlet vortex evolution and particle movements is elucidated, followed by an exploration of the spatial energy performance and erosion distribution within the double-suction pump. The results indicate that a rise in elbow curvature has a significant effect on the flow field, intensifying flow asymmetry, enhancing turbulence and vortex formation, which, in turn, leads to greater energy loss and entropy production rate, while diminishing pump efficiency. The influence of elbow curvature on energy loss is determined through the entropy production theory. In solid–liquid two-phase flow, the inertia and aggregation effects of the particles intensify wall friction and turbulent dissipation, resulting in a greater localized wear intensity. Under conditions with greater curvature, the effects of particle aggregation are enhanced, resulting in a more pronounced erosion intensity. This study offers a comprehensive insight into the pump performance in terms of the interplay of inlet vortex characteristics and the particle migration process.