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Pumped-storage power plants change operating conditions frequently, which seriously affects the safe operation of the unit operating under non-design conditions. In this study, the relationship between the flow structure with different guide vane openings (GVOs) of pump-turbine under the runaway condition is discussed. Moreover, the internal flow characteristics and the pressure fluctuation under the runaway condition are investigated. The dominant pressure fluctuation with 7 f n is obtained in the vaneless and double-row cascade space of pump-turbine under runaway conditions. The pressure fluctuation of large GVOs is more serious than that of low GVOs in general. The pressure fluctuation amplitude exists to the most significant amount in the vaneless space of pump-turbine to compare with the other component. The rotating stall is observed with a large guide vane opening under the runaway conditions. Nevertheless, the rotating stall disappears with a small GVO of 6 mm. [ABSTRACT FROM AUTHOR]

To understand the pressure fluctuations occurring in S-shaped region of the pump-turbine and their relationships with rotor-stator interaction, rotating stall and vortex rope, the numerical and experimental studies of a reversible pump-turbine were conducted under turbine, runaway, turbine brake, small flow rate and reverse pump operating modes. The experimental results show that pressure fluctuations under turbine mode were most affected by the guide vane opening. This effect was weaker under turbine brake mode and weaker still under reverse pump mode. At a guide vane opening of 19 mm, the maximum fluctuation peak in the vaneless space is 5.7 times, 6.9 times and 2.0 times greater than that in the spiral casing inlet, draft tube cone and draft tube elbow, respectively. Unsteady simulation results for the whole flow passage of the pump-turbine show that asymmetric flow occurred in the runner under turbine, runaway and turbine brake operating modes. Meanwhile, an obvious rotating stall vortex with a frequency of 0.65fn was generated in turbine brake mode, resulting in relatively large pressure fluctuations in the vaneless space. At the small flow rate, flow separation in the vaneless space became very obvious, while that in the runner weakened, resulting in decreased pressure fluctuations in the vaneless space. Under reverse pump mode, pressure fluctuations in the vaneless space were relatively small because of obvious symmetrical flow separation in the runner. Pressure fluctuations in the draft tube were mainly attributed to the combined actions of rotating stall and vortex rope phenomena. What's more, the fluctuation amplitudes at the rotating stall frequency of 0.65fn and the vortex rope frequencies of 0.30fn and 0.17fn were relatively large. • The fluctuation and flow instability under different operating modes are investigated. • The pressure fluctuations in turbine mode is most affected by the guide vane openings. • The asymmetric internal flow in runner is closely related to the fluctuation in vaneless space. • Fluctuation in draft tube is attributed to the combined actions of rotating stall and vortex rope. [ABSTRACT FROM AUTHOR]

The flow in the draft tube of a Francis turbine is highly complex and unstable when the turbine operates far from its design point. To extend the Francis turbine operation range to meet the variable energy demand, a J-groove technology is introduced on the turbine draft tube wall to stabilize the unsteady flow in the draft tube. However, J-groove contributes to energy losses in the draft tube. In this study, the J-groove shape optimization process is proposed based on steady flow analysis using the response surface method. The energy loss and swirl intensity in the draft tube are selected as objectives for the optimization design. The flow characteristics in the draft tube without J-groove and with initial J-groove and optimized J-groove shape are compared. Results show that the swirl intensity in the draft tube with the optimized J-groove is suppressed effectively with the low energy loss in the draft tube. Moreover, the pressure fluctuation in the draft tube with the optimized J-groove is mitigated significantly to stabilize the Francis turbine operation under the off-design point condition. [ABSTRACT FROM AUTHOR]

The pressure fluctuations of the turbines are caused by the swirl component in the draft tube, which is an undesired phenomenon that needs to be improved when the turbine operates in the off-design condition. In order to avoid these kinds of unsteady flow phenomena, a technology of J-Groove on the draft tube wall is investigated by numerical method. The validation test of performance is conducted to compare the result of numerical and experimental methods. The steady and unsteady state analyses are conducted to investigate the internal flow of a Francis hydro turbine model draft tube with and without J-Groove. The swirl flow in the draft tube is significantly suppressed by the J-Groove installation. Moreover, according to the unsteady state analysis, the amplitude of pressure fluctuation in the draft tube is reduced effectively by the J-Groove. [ABSTRACT FROM AUTHOR]

Korea has a myriad of islands in the south western coast that extensively rely on diesel generators for power production, which increase cost and environment pollution. The small hydro pump-turbine system for ocean renewable energy storage system is a kind of hybrid system that can reduce the usage of diesel generators and help to contribute to the environment in a positive manner by helping to reduce carbon emissions. The study focuses on initial hydraulic design and numerical analysis of a 30 kW-class pump-turbine system for energy independent islands in South Korea. The purpose of the study is to propose an ocean renewable energy storage system using a small pump-turbine system working with seawater. A 30 kW-class pump-turbine does not require a large head; approximately 30 m is sufficient for the design and application. Several other renewable energy systems like wind turbines, tidal turbines, wave energy converters and solar energy could be used to make a hybrid system with pump-turbine. The initial design achieved more than 85 % efficiency in both pump and turbine modes. However, further optimizations of the impeller blade shape and number of guide vane and stay vanes could improve the overall efficiency of the system. [ABSTRACT FROM AUTHOR]

South Korea has huge renewable energy potential that has been to some extent and can be further extracted to reduce its heavy reliance on fossil fuels. This project focuses on ocean and coastal wind resources available in the south-western regions of Korea that are currently being monitored and found to have huge potential. The project will propose a new concept of wind, tidal current and wave energy converting systems combined together that can boost output power capacity at a single location also making it easier for grid connection. Wind turbines nowadays have been quite efficiently developed and implemented throughout the world both onshore and offshore. However, tidal current and wave energy have yet more room for improvement. The current study will combine the offshore wind turbine (HAWT) system with two tidal current turbines (HATCT) both in horizontal axis classification on a single tower. It will further discuss on adding a wave energy converter in between the wind and tidal current turbines. Optimization of the tower is also a minor part of this study. The structural analysis of the main tower, which is the main focus of the study, is investigated by dynamic response analysis to check its feasibility for combining the three types of renewable energy converters on one support. Actual extreme conditions of the wind, ocean currents and waves are simulated using ANSYS CFX fluid flow analysis. The results are used for conducting a one way FSI analysis assuming a rigid structure to verify the safety of the system. [ABSTRACT FROM AUTHOR]

As a core component of a hydropower station, hydro turbines play a vital role in the integration of a power station. Research on the technology of hydro turbine is continuously increasing with the development of water electricity. It is effective and successful to design a Francis turbine runner blade with good performance by one-dimensional hydraulic design method. For the one-dimensional hydraulic design, the runner blade angle at leading and trailing edges can be defined by calculation of Euler's head. Design of the runner blade profile at several cross sections is needed to design a runner shape. In this study, there are three different blade loadings conducted to compare the internal flow characteristics and performance. The result shows that the front loading achieves the best efficiency in comparison to other loadings, which is good at suppressing the loss at draft tube. [ABSTRACT FROM AUTHOR]

Elastic metamaterials with double negativity can manipulate the propagation of elastic waves at sub-wavelength scales by inducing multiple resonances to achieve different negative effective parameters. Numerous efforts have been made to control the dynamic behavior by directly tuning the frequency range of double negativity in elastic metamaterials. This study examines the impact of tensile loads on double negativity in relation to ligament inclination angle. The findings will aid in the direct tunability of double negativity in single-phase chiral metamaterials. The study examines a simple single-phase four-ligament chiral unit with low-order double negativity. The presence of double negativity was verified through band structure analysis and calculation of the four effective dynamic parameters. Additionally, the effects of two geometrical factors on the frequency ranges of negative parameters were investigated through parametric scanning. The results indicate that the frequency range of double negativity reaches its maximum at a ligament inclination angle of around 45° and disappears as the angle approaches 65°. Given that the ligament inclination angle of the unit cells can be easily altered by external tension, this intriguing outcome is leveraged to achieve the emergence and vanishing of double negativity. This characteristic is confirmed through the examination of negative refraction phenomena via simulation examples. [ABSTRACT FROM AUTHOR]

The internal flow field of the hydrogen circulation pump becomes more complex and flow process becomes more intense during the stopping process, strong vibration is generated to the pump and the working life of the pump is shortened. Therefore, the pressure pulsation at the inlet and outlet section, the radial force characteristics of the active rotor and the transient flow mechanism of the internal flow under different stopping modes are studied in this paper. The results show that the intake pressure and exhaust flow change greatly in exponential stopping mode, while the pressure pulsation amplitude decreases with the increase of stopping time in linear stopping mode. In addition, the radial force value of the active rotor in the exponential stopping mode is significantly smaller than that in the linear stopping mode, and the direction of the radial force F x and F y point to the negative direction of their respective axes. In this paper, the dynamic mesh technology is used to design the corresponding rotating speed UDF according to different stopping functions, and then the transient simulation of the hydrogen circulation pump under different stopping modes is completed. Finally, the change law of pressure and flow pulsation, radial force characteristics and internal flow field under different stopping modes are analyzed. [Display omitted] • Effects of different stopping modes on flow characteristics are studied. • Pressure variation under different stopping modes are discussed. • Radial force of the rotor under different stopping modes are analyzed. • Working process is researched from the pressure and velocity distribution. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to investigate the flow and pressure pulsation characteristics of hydrogen circulation pump under varying pressure differences. A dynamic mesh technology was employed to conduct an unsteady numerical simulation of the interior of the hydrogen pump. The pressure pulsation within the pump cavity area was analyzed using fast Fourier transform, and the unevenness of pressure and flow inside the pump under varying pressure differential conditions was specifically studied. The results reveal that interference between dynamic and static in the pump cavity, caused by meshing clearance between rotors and radial clearance between the rotor and pump cavity, leads to pressure pulsation peaks occurring at twice the rotational frequency at all monitoring points. Additionally, as the pressure difference increases, the exhaust flow rate at the outlet of the pump cavity decreases from 49.237 m3/h to 12.392 m3/h, while volumetric efficiency decreases from 85.95% to 32.3%. The article utilizes Fluent dynamic mesh technology to conduct transient simulations on four distinct pressure differential conditions of hydrogen circulation pumps, thereby obtaining the variation principle of the internal flow field under varying pressure differences and elucidating the influence of pressure differences on both the non-uniformity of pressure and flow rate. [Display omitted] • Analyzed the reasons for two pressure pulsations occurring in the pump chamber. • Studied the pressure field to perform a total of 4 suction and exhaust in one cycle. • The pressure unevenness coefficient descents with the rise of pressure difference. [ABSTRACT FROM AUTHOR]

Nearly 71% of Earth's surface area is occupied by the ocean, which is abundant in energy, with wave energy being one of its essential components. As a result, the investigation of wave energy generation devices is a crucial matter. This paper analyzes the performance of the oscillating water column (OWC) wave energy conversion device under the influence of unidirectional regular waves and optimizes the OWC device accordingly. The impact of different structural dimensions on the first-stage conversion efficiency of the OWC device is determined. Moreover, to evaluate the performance of the OWC device in real sea conditions, the wave climate at Sanmen Gorge is employed as the experimental parameter. The study examines the performance and characteristic response of the OWC device regarding the air chamber width, thickness of the front wall, and draught of the front wall. The findings reveal that the width of the OWC air chamber and the draught of the front wall significantly affect the OWC performance. Decreasing the width of the air chamber results in a stable horizontal plane, eliminating the occurrence of standing wave phenomena. As the draft of the front wall increases, the reflection coefficient of the air chamber gradually increases, and it becomes difficult for the wave to penetrate into the air chamber section. The Latin hypercube design experiment method was used to select experimental points and establish quadratic polynomial response functions. The OWC device was optimized based on multi-island genetic algorithm. The simulation and fault tolerant optimization methods presented in this paper can be widely used to improve system performance. • The effects of different structure sizes on the primary conversion energy efficiency of OWC units were compared. • Optimize OWC devices for real sea conditions. • The feasibility analysis of the installation is carried out for the specific sea area. [ABSTRACT FROM AUTHOR]

Pressure fluctuations in the draft tube mainly arise from the rotational motion of the vortex rope. The formation and development of the vortex rope are significantly influenced by cavitation. However, the precise mechanism underlying the progression of cavitation from its initial weak state to a more severe level remains unclear. The objective of this study is to investigate cavitation in a pump-turbine operating under part load turbine conditions. Numerical simulations were conducted to analyze the impact of the cavitation coefficient on various parameters, including vapor volume, efficiency, and head. With increasing degree of cavitation, the vortex rope's pattern in the draft tube undergoes a transition from a helical type to a torch-like type. This transition consequently leads to a shift in the dominant frequency of pressure fluctuations within the draft tube. Furthermore, the helical vortex rope exhibits a higher rotational speed compared to the torch-like vortex rope, thereby causing a more pronounced effect on the pressure field. The factors contributing to the morphological transition of the vortex rope were explored, with particular emphasis on the accelerating reverse axial flow and the reduction in the draft tube's circumferential velocity. Additionally, this study examined the effect of the cavitation coefficient on the swirl number and flow pattern, while also assessing its impact on entropy production. These findings provide valuable insight into the control of cavitation flow in pump-turbines operating under part load conditions. Moreover, they carry significant implications for the design and optimization of pump-turbine systems. [ABSTRACT FROM AUTHOR]

Pumped storage hydropower is perhaps the most reliable solution to the energy crisis, but its development and application are limited by hydraulic instability, such as S-shaped characteristics. In this paper, based on the S-shaped region pressure fluctuation test of a model pump turbine, the connection between the guide vane opening and S-shaped pressure fluctuation is established, and a guide vane opening of 28 mm is selected for numerical analysis to expand the discussion of the S-shaped pressure fluctuation characteristics to large guide vane openings. The frequency components of the vaneless space are extracted by fast Fourier transform analysis, and the rotational stall frequency is identified. Among them, the blade passage frequency and its multiplier are induced by the rotor–stator interaction, and the low-frequency component of pressure fluctuations is induced by the rotational stall and vortex rope. Based on this, the internal flow and flow rate along the S-shaped characteristic curve and their correlation with the rotational stall are analyzed. In addition, the percentage of and variation in the entropy production rate in the S-shaped region are also analyzed. [ABSTRACT FROM AUTHOR]

Pumped hydro energy storage systems are a reliable solution for maintaining grid stability. However, the pump turbines utilized in these systems often display instability due to pressure fluctuations, and the draft tube experiences additional fluctuations from the formation of a cavitation and vortex rope. This paper introduces a novel J-groove design method and presents four different designs of J-grooves. Unsteady analysis is performed.The J-grooves reduce the magnitude and alter the distribution of axial and tangential velocities inside the draft tube, which results in the disruption of vortex rope through the generation of jet flow from J-grooves. The grooves exhibit noticeable suppression effects on cavitation, vortex rope and pressure fluctuations, while this comes at the expense of efficiency. J-grooves with uniform thickness significantly suppress pressure fluctuations, while thickening the leading and rear edges respectively benefits the suppression of runner and draft tube cavitation. The cavitation suppression effectiveness of the long-short type J-groove is inferior to the other models, but it incurs lower efficiency losses. These findings provide insights into the suppression of cavitation and vortex rope and are of significant importance for the optimization of J-grooves. • A design approach for controlling the frontage and rear edges of the J-groove has been proposed. • The J-groove mitigates cavitation, vortex rope and pressure fluctuations effectively. • The jet flow induced by the J-groove impacts the axial and tangential velocities. [ABSTRACT FROM AUTHOR]