With the increasing adoption of renewable energy sources globally, hydropower contributes significantly to energy generation through various schemes ranging from big to small-scale plants. In ...small-scale hydropower plants, the preference for reverse-operated pumps (known as pump as turbines or PATs) over small-scale hydroturbines has increased. However, apart from the associated economic advantages, PATs, like any other hydraulic machinery, are not free from common problems such as cavitation. Cavitation is a phenomenon in which air bubbles are formed within the fluid medium due to substantial local pressure drop and their eventual collapse causes material erosion and degrades the overall machine efficiency. Several studies have focused on PAT conventional operating mode, while its reverse mode just begun to gain research interest. Nevertheless, cavitation remains a common problem in PATs at various hydro-sites. Therefore, to analyze PAT cavitation performance and highlight the differences between its two operating modes in terms of their development mechanisms, this article presents a thorough review of PAT cavitation dynamics and influencing parameters, as well as the future research directions. It is found that PAT reverse mode is more prone to cavitation, but more damages would occur in the conventional mode. Nevertheless, modifying the PAT geometric design parameters can considerably improve its cavitation performance. However, this approach has not been sufficiently investigated for PAT reverse operating mode and hence requires further research. Note that the terms “PAT conventional mode,” “PAT pumping mode,” and “pump” are equally used throughout this paper.
•Studies on cavitation performance in hydraulic pumps as turbines (PATs) are reviewed.•Both pump and turbine modes are considered in the review.•PATs are found to have gained importance due to their applicability in remote areas.•PAT reverse mode operations require high heads and flows; prone to cavitation.•Geometric design modification is widely used for PAT cavitation performance improvement.
The transient characteristic of the power-off process is investigated due to its close relation to hydraulic facilities’ safety in a pumped storage hydropower (PSH). In this paper, power-off ...transient characteristics of a PSH station in pump mode was studied using a three-dimensional (3D) unsteady numerical method based on a single-phase and volume of fluid (SP-VOF) coupled model. The computational domain covered the entire flow system, including reservoirs, diversion tunnel, surge tank, pump-turbine unit, and tailrace tunnel. The fast changing flow fields and dynamic characteristic parameters, such as unit flow rate, runner rotate speed, pumping lift, and static pressure at measuring points were simulated, and agreed well with experimental results. During the power-off transient process, the PSH station underwent pump mode, braking mode, and turbine mode, with the dynamic characteristics and inner flow configurations changing significantly. Intense pressure fluctuation occurred in the region between the runner and guide vanes, and its frequency and amplitude were closely related to the runner’s rotation speed and pressure gradient, respectively. While the reversed flow rate of the PSH unit reached maximum, some parameters, such as static pressure, torque, and pumping lift would suddenly jump significantly, due to the water hammer effect. The moment these marked jumps occurred was commonly considered as the most dangerous moment during the power-off transient process, due to the blade passages being clogged by vortexes, and chaos pressure distribution on the blade surfaces. The results of this study confirm that 3D SP-VOF hybrid simulation is an effective method to reveal the hydraulic mechanism of the PSH transient process.
Abstract
The ultra-low head pump stations often have bidirectional demand of water delivery, so there is a risk of runaway accident occurring in both conditions. To analyze the difference of the ...runaway process under forward runaway condition (FRC) and backward runaway condition (BRC), the whole flow system of a horizontal axial flow pump is considered. The Shear-Stress Transport (SST)
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model is adopted and the volume of fluid (VOF) model is applied to simulate the water surface in the reservoirs. Meanwhile, the torque balance equation is introduced to obtain the real time rotational speed, then the bidirectional runaway process of the pump with the same head is simulated. In addition, the vortex transport equation and swirl number are proposed to reveal the flow characteristics during the runaway process. The results show that the runaway process can be divided into five stages: the drop, braking, rising, convergence and runaway stages, according to the changing law of torque curve. In the rising stage, the pressure difference on the blade surface continues to increase, which contributes to the abnormal torque increase. In this stage, the flow hits the pressure surface (PS) at a faster speed enlarging the pressure on PS, and the flow separation takes place on the suction surface (SS) weakening the pressure on SS. During the convergence and runaway stage, the pulsation amplitude of torque and axial force under FRC is obviously larger than those under BRC. This is because the rotation frequency of the vortex rope is the same as main pressure fluctuation frequency in impeller under FRC, which enhances the pulsation amplitude. Whereas the vortices are broken due to the inhibitive effect from guide vanes under BRC.
The dynamic characteristics of the pumped storage unit (pump-turbine runner) make it highly susceptible to vibrations. Previous studies seldom addressed the clearance variation due to runner ...vibration, largely because of two challenges: the integration of the moving grid with the clearance and governing equations, and the intricacy of factoring in the entire shaft system's influence on vibration. By employing user-defined functions (UDF), kinematic equations for the pump-turbine runner's rotation and translation were formulated and integrated with computational fluid dynamics results. This paper introduces a computational method to simulate clearance-induced vibration displacement. The study examines the impact of clearance vibration displacement on the pump-turbine's transient flow field and runner vibrations under various operating conditions. Notably, deviations from the rated load condition led to an expansion-contraction trajectory of the runner axis, culminating in chaotic behaviour in the "S" characteristic zone. The correlation between runner vibrations and pressure fluctuations strengthened with the clearance displacement model, especially in narrow labyrinth seal clearances. Simulations also provided a refined estimation of the secondary rotational speed increase during load rejection. The method's reliability is confirmed through field test data comparison, offering a fresh perspective to identify the vibration characteristics of the pump-turbine runner.
The existence of vortex ropes inside the draft tube significantly impacts hydraulic efficiency and operational stability, and few studies on the formation mechanism of vortex ropes and hydraulic loss ...problems have been explored. Hence, in this paper, we build an inherent correlation between the local entropy production rate (LEPR) in the draft tube and the dynamics of vortex motion, by incorporating the vortex identification method Ω~R with entropy production theory, using the OpenFOAM-v2212 software. From the analysis of the entropy production theory, the entropy production rate caused by turbulence dissipation (EPTD) is responsible for the majority of energy loss in the form of entropy production rate, accounting for about 87% of the total entropy production rate (TEPR) in different load operations. Comparatively, the entropy production rate caused by wall shear stress (EPWS) can account for up to 12%, while the entropy production rate due to direct dissipation (EPDD) plays a minor role in TEPR. The rotating vortex rope movement of the unit at part load conditions leads to more intense LEPR. Therefore, to determine the hydraulic loss caused by the vortex rope, the TEPR at the cross-section can be used to assess the hydraulic characteristics of the draft tube.
► Thirty urban parks in developed and new growth areas in Hong Kong were studied. ► Bird species composition in the new growth areas was more even and nested than that in the developed areas. ► ...Composition of species groups was very different regarding residence status, food and habitat type. ► Bird communities were very different between the two development types. ► Multi-scale habitat factors strongly affected bird distribution in Hong Kong.
Urban development affects avian community structure dramatically, and is considered a major cause of native species extinction. Hong Kong is a highly urbanized city, but there is a paucity of data on its urban avian community or the differences between areas with different development history and human population density. We assessed the impacts of urban development on bird community, and the effects of landscape-scale habitat factors at two scales. We selected 30 urban parks from areas of two development types as sampling sites. We censused bird communities in the breeding and wintering seasons in 2010–2011. Communities in the developed areas were more heterogeneous than those in the new growth areas. Regarding species groups, residents dominated urban parks in both development types, and introduced species were only recorded in the developed areas. More granivores, but fewer insectivores and insectivore–frugivores appeared in the developed areas than in the new growth areas which supported more forest-specialist birds when compared with the developed areas which had more open-habitat species. Contrastingly, bird communities were more nested in the new growth areas. Although there were no significant differences in species richness, diversity and composition between the two development types, bird communities were very different in terms of species evenness, species groups and nestedness. Based on Nonmetric Multidimensional Scaling, bird distribution pattern were strongly associated with habitat evenness and largest patch index for woodland at the 400-m scale and contagion index at the 400- and 1000-m scales in the two development types.
To obtain the flow mechanism of the transient characteristics of a Kaplan turbine, a three-dimensional (3-D) unsteady, incompressible flow simulation during load rejection was conducted using a ...computational fluid dynamics (CFD) method in this paper. The dynamic mesh and re-meshing methods were performed to simulate the closing process of the guide vanes and runner blades. The evolution of inner flow patterns and varying regularities of some parameters, such as the runner rotation speed, unit flow rate, unit torque, axial force, and static pressure of the monitored points were revealed, and the results were consistent with the experimental data. During the load rejection process, the guide vane closing behavior played a decisive role in changing the external characteristics and inner flow configurations. In this paper, the runner blades underwent a linear needle closure law and guide vanes operated according to a stage-closing law of “first fast, then slow,” where the inflection point was t = 2.3 s. At the segment point of the guide vane closing curve, a water hammer occurs between guide vanes and a large quantity of vortices emerged in the runner and the draft tube. The pressure at the measurement points changes dramatically and the axial thrust rises sharply, marking a unique time in the transient process. Thus, the quality of a transient process could be effectively improved by properly setting the location of segmented point. This study conducted a dynamic simulation of co-adjustment of the guide vanes and the blades, and the results could be used in fault diagnosis of transient operations at hydropower plants.
Axial-flow pumps with siphon outlet are widely used in South-to-North Water Transfer Project of China for the excellent characteristics and good performance in stopping period. The reverse rotational ...speed and the head of impeller are the key parameters for the system’s safety in stoppage. In this article, the computational fluid dynamics method was used for three-dimensional unsteady numerical simulations. The method based on volume of fluid model was executed on geometrical model of the whole flow system to get the variation laws of internal and external characteristic parameters. Through numerical calculation, the variation laws of the parameters were got in the pump operating condition, braking condition, and turbine condition, and the maximum reverse speed of the impeller was 203.0 r/min (−0.947 times the rated speed). Compared with the designed mode, when the air valve was opened in advance, the pump system would go through the weir flow state first, and a higher head of impeller would be got as a result. When the air valve refused to open, the pump system would get into runaway state. Comparisons of the calculated and measured results indicate the proposed computational fluid dynamics method is reliable in the simulation of the transient flow in axial-flow pump system.
Cavitation is a complex multiphase flow phenomenon that is usually involved in marine propulsion systems, and can be simulated with a couple of methods. In this study, three widespread cavitation ...models were compared using experimental data and a new modified simulation method. The accuracy of the three cavitation models was evaluated regarding their steady and unsteady characteristics, such as the flow field, re-entrant jet, vortex-shedding, and so on. Based on the experimental data and numerical results, the applicability of different cavitation models in different conditions was obtained. The Kunz model can accurately capture both the adverse pressure gradient and the action of the re-entrant jet in sheet cavitation, while the full cavitation model (FCM) has an accurate prediction for the flow field structure and the shedding characteristic of cloud cavitation. Through comparing the results, the optimal selection of cavitation models for further study at different conditions was obtained.
Marine and hydrokinetics (MHK) represent an emerging industry with hundreds of potentially viable technologies, such as potential extractable energy from plain area rivers where the water level ...differences are very small and the traditional water turbine pump (WTP) cannot be used. A suitable WTP, composed of a tubular turbine directly driving a centrifugal pump, was designed and developed based on computational fluid dynamics (CFD) and model tests. Two general design schemes of such river-current (RC)-driven WTP are presented here, obtaining the desired operating parameters of discharge and pump head. A CFD analysis of Scheme B, which employs a radial outlet, allowing additional degrees of freedom for the dimensions of the centrifugal pump, was carried out and verified experimentally by model tests. The minimum deviation of pump head is within ±5%, and the trend of other working conditions is consistent, so the results of the numerical simulation and model tests show good agreement, demonstrating the feasibility of the CFD method for practical applications. Then, using the CFD method, the optimum rotational speed for the turbine was determined, and the turbine draft tube was improved further. With a turbine runner diameter of 0.5 m, the results show best performance at n = 350 r/min. The straight conical draft tube was changed to an elbow draft tube with multiple exits. Additionally, four different cross-sectional shapes were designed for the pump volute, and their effects on the performance of the WTP were analyzed. Finally, the round shape was selected, because of its best performance. The turbine unit has the highest efficiency of 81.2%, at an inlet velocity v = 2.4 m/s, while the pump exhibits the best efficiency of 90.2% at the design discharge and head of 30 l/s and 4.45 m respectively. Overall, the RC-driven WTP makes good use of the kinetic energy of the river current as a power source, solving the inapplicability of traditional WTP in plain areas.
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