bluuebunny/crossref_metadata_2025_split huggingface dataset:
Experimental study of the flow of the Tr-Francis turbine along the no-load curve
Authors: Gilis Aubin , Coulaud Maxime , Munoz Anthony , Maciel Yvan , Houde Sébastien 
Published: October 2022

Abstract
Emergence of new renewable energies, such as solar or wind power, has introduced intermittency on the grid. Hydraulic turbines, initially designed to perform close to their best efficiency point, are more and more at off-design operating conditions, such as no-load regimes, when the runner rotates without energy extraction. These no-load regimes are characterized by complex flow phenomena inducing fluctuations of the blade loading and resulting in a reduced lifetime of the runner. Understanding the origins of pressure fluctuations in no-load conditions is consequently essential to prevent fatigue damage to the turbine. This paper presents the study, with experimental flow visualization, pressure measurements and numerical simulations, of the flow behavior at speed no-load in a medium head Francis turbine model. Evolution of the different flow structures along the no-load curve is then assessed. At speed-no-load, flow visualizations show the presence of a cavitating zone at the blade trailing edge. According to the results of the numerical simulations, this zone seems to correspond to the region where a backflow in the draft tube enters the runner. This interaction results in a backflow inside the runner and in a blockage of the upper part of the inter-blade channel. Sensitivity of this cavitating region to test parameters (discharge and rotation speed) is investigated for different no-load operating points with high-speed visualization at a constant cavitation number. Decrease of the energy coefficient tends to intensify the phenomena and increase the volume of cavitation.
Determination of the natural frequencies of a prototype Kaplan turbine
Authors: Valero C , Egusquiza M , Valentin D , Presas A , Moraga G 
Published: October 2022

Abstract
The natural frequencies of a turbine can be calculated from numerical methods. By comparing these natural frequencies with excitation sources, one can know the danger of a resonance and a possible failure in a component of the turbine. Therefore, it is often very important to have an accurate numerical model of the turbine to determine these natural frequencies. There are not many publications on the determination of the natural frequencies of reduced-scale models of Kaplan turbines. More papers exist for pump turbines or Francis turbines. For real Kaplan turbines, very few experiments can be found to determine mode shapes and natural frequencies. In this paper a Kaplan turbine of 37MW (maximum power), 12.5m (maximum head) and 50 m3/s (maximum flowrate) was tested. The turbine was equipped to determine the natural frequencies of the runner in air. For this purpose, one accelerometer in each blade of the runner was installed and a total of 16 impacts were done in each blade. Frequencies and mode shapes were obtained. In parallel, a numerical model was obtained. Numerical and experimental results were compared and an accurate numerical model is presented. With this numerical model the natural frequencies of the runner in water were calculated.
Investigation of flow behavior causing extraordinary stress on blade during start-up of the Francis turbine
Authors: Mukai T , Nakazono M , Hashidate T , Uchida T , Tezuka K , Koyama H , Matsui J 
Published: October 2022

AbstractThis paper describes flow behavior which causes extraordinary transient stress on runner blade that occurs during start-up of a Francis turbine. Firstly, transient computational fluid dynamics (CFD) was conducted to evaluate hydraulic similarity for the condition of extremely low rotation speed and small guide vane opening. CFD results show that the pressure is below saturation vapor pressure all over the inside of the runner at extremely low rotation speed conditions. Secondly, model test was carried out to measure and evaluate pressure and strain on the blade. The test results revealed that there is a tendency that the phenomenon becomes more severe when rotation speed is lower, when guide vane opening is larger, and when suction pressure is lower. It is concluded that the extraordinary phenomena are not ‘extraordinary’ but commonly arisen as the transient hydraulic behavior and the impact load causing extraordinary stress on runner blades is related to the instantaneous condensation of vapor bubbles such as steam hammer accompanied by the explosive pressure rise.
Mechanism of extraordinary stress on blade during start-up of the Francis turbine
Authors: Mukai T , Shima R , Nakazono M , Tezuka K , Koyama H , Matsui J 
Published: October 2022

Abstract
This paper describes the mechanism of extraordinary transient stress that occurs during start-up of a Francis turbine by investigating the detailed analysis with measured stress of prototype runner and transient response calculation. According to the dynamic stress measurement with high sampling frequency, it has been clear that a unique phenomenon occurs during start-up especially at the low rotation speed. From the macro perspective, impulsive stress occurred at blade outlet instantaneously and sporadically. Time interval of the event can be considered close to random, and there is no synchronization with the rotation speed. From the micro perspective, the strain occurs owing to the entire vane vibration, and the impulsive loads bump blade outlet from the side of the pressure surface. A Blade shows dumped free vibration in its natural mode in water. As the conclusion, the phenomenon that runner blades are struck by impulsive load on the pressure surface during start-up is compared to a drum beaten by drumsticks randomly in short time, and we named the unique phenomenon “Hydro Drum”
Optimization design of the elbow draft tube of the hydraulic turbine
Authors: Semenova A V , Chirkov D V , Skorospelov V A , Ustimenko A S , Turuk P A , Rigin V E , Germanova I A 
Published: October 2022

Abstract
The draft tube of the hydraulic turbine serves to recover the kinetic energy of the water flow that leaves the runner. The amount of this energy depends on the turbine type, its specific speed and flow capacity. For high specific speed turbines, influence of the draft tube on the turbine efficiency is more essential. The height of the elbow-type draft tube has a considerable influence on its performance. Statistics show that increasing the height of the draft tube leads to a considerable increase of the turbine efficiency, but it also increases the scope and cost of civil works. The present paper presents the solution of the multi-objective optimization problem directed to minimization of the height of the toroidal elbow-type draft tube and maximization of the overall efficiency of the turbine. During optimization, 11-19 parameters that determine the shape and dimensions of the draft tube were subject to variation. In optimization, efficiency was maximized in two operating points: best efficiency point and full-load point. Cavitation qualities were defined as constraints. In order to correctly assess the turbine efficiency and cavitation, the computational domain included one channel of the distributor, one channel of the runner and draft tube; whereas losses in the spiral case and stay ring were determined using empiric formulae. Within the optimization the flow analysis was performed using 3D steady-state Reynolds averaged Navier Stokes equations closed by the k-ε turbulence model. On solid walls the method of wall functions was used. The turbine head, being the difference of specific energies in the inlet and outlet cross sections, was pre-set as a constant value, while the discharge was determined in the course of solution of the problem. A multi-objective genetic algorithm was used to solve the optimization problem. For the draft tube shapes obtained in the course of optimization, their performance characteristics and flow character were compared by means of CFD. It was found that it is possible to significantly reduce the height of the classical toroidal draft tubes without considerably affecting their performance characteristics.
Flow Characteristics Analysis of a High-Head Prototype Pump-Turbine During Turbine Start-Up: Effects of the Clearance
Authors: Zhang Shaozheng , Huang Xingxing , Yin Xilong , Bi Huili , Wang Zhengwei 
Published: October 2022

Abstract
The dramatic changes in the internal flow and the corresponding structural behavior during the turbine start-up transient process of pump-turbines are extremely complex. The clearances in the upper crown chamber and bottom ring chamber affect the results of the flow field and structural field of the pump-turbine runner. Most of the previous studies ignored the effects of the clearance flow field to simplify the numerical simulations. In this study, numerical calculations were performed on the entire flow passage of a high-head prototype pump-turbine during the start-up in turbine mode, and the model with and without the clearance are analysed respectively. The causes of the flow field characteristic difference and external characteristic difference caused by the existence of intermediate clearance in the model are studied in detail. The results show that the clearance flow field has a great influence on the axial forces on the runner, which is mainly due to the higher pressure of the clearance flow field compared with the flow field in the runner; At the same time, because the existence of clearance flow field only has a small effect on the distribution of internal flow field, the hydraulic torque of runner considering clearance effects is basically the same as the one without clearance.
Fluid-Structure Coupling Analysis of a Pump-turbine unit during the Pump Shutdown Transient Process
Authors: Yang Mengqi , Huang Xingxing , He Qilian , Bi Huili , Hu Dongyang , Hu Dejiang , Wang Zhengwei 
Published: October 2022

Abstract
Hydropower is a major renewable clean energy and is widely used worldwide. The reversible pump-turbine unit of the pumped-storage power station is able to work in two main operating modes as required by the power grid: turbine-mode for power generation and pump-mode for power storage. In order to absorb unstable energies such as wind and solar energy and improve the quality of the electricity, reversible pump-turbines need to frequently change operating conditions, and experience more start-stops under different operating modes in a short period. The unstable flow during these transient processes will lead to high-level stresses on the structural components of the pump-turbine units. Therefore, it is of great engineering and academic significance to study the flow characteristics and structural characteristics of the unit during the transient processes. This paper has established a numerical calculation model for a prototype reversible pump-turbine unit, has carried out the CFD calculations of the pump-turbine fluid domains during the pump shutdown transient process, and has analysed the corresponding structural dynamic characteristics of the stationary components of the unit with the fluid-structure coupling method. The pressure variation trend of the spiral case outlet during pump shutdown has the same trend as that of the spiral case domain, and the guide vane flow domain. The maximum flow-induced deformation and stress of the stationary structures have a strong correlation with the axial thrust values of the head cover. The maximum deformation occurs at the inner edge of the head cover, and the maximum stress appears in the fillet of the stay vane leading edge. An increase in the number of shutdowns will result in a higher real risk of fatigue damage to the stay vanes. The conclusions obtained are of great value for safe operation, field condition monitoring, fault diagnosis, and predictive maintenance of the pump-turbine units.
Spatiotemporal analysis of sheet and cloud cavitation and its damage potential
Authors: Hatzissawidis G , Kerres L , Ludwig G J , Pelz P F 
Published: October 2022

Abstract
The cavitation regime has a substantial influence on the damage potential, thus it has to be considered in any specific investigation. For this purpose, we set up a test rig at the Technische Universität Darmstadt using a Circular Leading Edge hydrofoil (CLE) to analyse the damage potential of sheet and cloud cavitation. Exceeding a critical Reynolds number Re
c, the cavitation regime transitions from harmless sheet cavitation to aggressive cloud cavitation. High-speed recordings of the cavitation regime are correlated with high frequency pressure data from a wall-mounted piezoelectric pressure transducer. Spatial and temporal content of the cavitating flow are captured applying proper orthogonal decomposition (POD) to the high-speed recordings. In order to determine the damage potential of the cavitation regime we apply a copper foil on the hydrofoil surface, on which plastic, crater-shaped deformations due to bubble collapses occur. Images of the surface are recorded before and after each run via two-dimensional Pit-Count microscopy. We correlate spatial modes from the cavitating flow field with the eroded surface rate from pitting tests leading to the result that cloud cavitation associated with increasing cloud size is more aggressive. A power law is identified where pitting rate increases with fourteenth power of the Reynolds number.
Research on Sediment Laden Flow Characteristics in Water Conveyance System
Authors: Qi Dunzhe , Bi Huili , Zeng Haiyan , Shen Yubin , Wang Zhengwei 
Published: October 2022

Abstract
In the long-distance water conveyance channels or pipelines of sediment laden flow, due to the change of the flow discharge, there may be different levels of siltation condition, and regular dredging work needs to be carried out. If the water conveyance channels are an open system, it is easier to dredge, but if it is a pressurized pipeline system, dredging is more difficult. Some water delivery irrigation projects of sediment laden flow could not be promoted due to the unresolved problem of siltation. This paper takes a long-distance gravity flow pipeline system as the research object. The distribution branch pipe on the main pipeline is connected to the downstream reservoir for field irrigation. According to the pipe data, the one-dimensional full pipeline numerical model and the three-dimensional pipe calculation model were established respectively. The flow velocity calculation of main pipeline under the condition of simultaneous irrigation of different numbers of distribution branch pipes, and the distribution of different velocities in the pipe was obtained. At the same time, the three-dimensional unsteady flow analysis was carried out to analyse the flow characteristics and siltation patterns in the main pipeline under different flow discharge. It is helpful to predict the silt phenomenon that may exist in the water pipeline in advance by calculation, and give technical suggestions for the pipeline design.
Artificial neural networks for predicting the onset of overload instability in Francis turbines
Authors: Zhao W Q , Presas A , Egusquiza M , Valero C , Moraga G 
Published: October 2022

Abstract
Overload instability is a self-excited phenomenon that occurs in Francis turbines working over the Best Efficiency Point. It provokes huge power swings and pressure fluctuations in the hydraulic circuit. One particular issue is that this phenomenon appears suddenly and just before its onset the machine can operate in a very stable manner. In this study, we show that artificial intelligence techniques such as Neural Networks can be used to evaluate the risk of overload instability several seconds before its appearance. Experimental data, acquired during several overload instability tests in a huge prototype, has been used. The techniques proposed in this paper could be used in advanced condition monitoring systems and could permit a safer operation of the turbine working at high loads.
Dynamic response analysis of a reduced scale Kaplan turbine model operating in propeller mode
Authors: Escaler X , Sánchez-Botello X , Roig R , De La Torre O , Mulu B , Högström C-M 
Published: October 2022

Abstract
In the frame of the AFC4Hydro H2020 research project, an extensive measurement campaign has been carried out on a reduced scale Kaplan turbine model at the Vattenfall Research and Development facility in Älvkarleby, Sweden. The objective of the tests has been to monitor and characterize the dynamic response of the machine when it operates in propeller mode with a fixed blade angle corresponding to the best efficiency point. A series of measurements have been taken at steady state operating conditions, such as speed no load, part load and best efficiency point, as well as during transient conditions. The turbine has been instrumented with sensors to measure vibrations, displacements, strains and pressures both off-board and on-board. The excitation due to a rotating vortex rope at part load and the structural response induced by it have been clearly measured by the system. A sub-synchronous frequency with a maximum amplitude has been identified for a given part load condition. The evolution of this frequency with the discharge level and the presence of cavitation has been evaluated. Moreover, the response induced by the speed no load condition has been compared against the best efficiency point. Finally, a turbine start-up sequence has been analyzed in the time-frequency domain.
Flow rate evaluation using water hammer equations – Reformulation of the method
Authors: Dunca G , Cervantes M J 
Published: October 2022

Abstract
The present paper aims to improve the applicability of the pressure-time method for flow rate evaluation by eliminating the necessity to cut off the machine discharge completely and taking into account the compressibility of the fluid. As a result, the flow rate determination is possible for any load changes, increase or decrease. The method uses the water hammer equations with the differential pressure measured between two cross-sections. Furthermore, the instantaneous flow rate is analytically linked to the pressure variation and the losses. The method is presented and validated against numerical results.
CFD and FEM analysis to migrate from fixed- to variable-speed on the Alto Lindoso HPP
Authors: Pacot Olivier , Delannoy Joachim , André François , Segoufin Claire , Delgado João , Roque Miguel , Münch-Alligné Cécile 
Published: October 2022

Abstract
The need to decarbonise our energy system push us toward the use of different clean renewable energy sources, such as solar, wind and hydroelectric energy. It is well known that some of these renewable energy sources suffer of intermittence, which requires the conventional dispatchable power plants to become more flexible to respond to any power change to ensure a stable electrical grid. For this specific purpose, the European project XFLEX HYDRO is focusing on the hydropower sector, with seven demonstrators in three different countries to demonstrate the possibility to enhance the flexibility, one option being to use the variable-speed technology combined with smart digital control. Among these seven demonstrators, one is the Alto Lindoso hydropower plant owned by EDP and located in North-West of Portugal. It consists of two fixed-speed vertical Francis turbines with a high head of approximately 275 m and a rated power of 317 MW. The objective of this demonstrator is to assess the potential of a variable speed using the Doubly Fed Induction Machine (DFIM) technology to increase the flexibility of the power and the lifetime of the turbine. Therefore, this paper explores numerically the hydraulic behaviour of the model runner when operated at a selected part load conditions (60% BEP) with the rotational speed changed by ±10%. Second, a FEM analysis is carried out to evaluate the runner blades damage rate. This preliminary study showed that in term of wear and tear, the variable speed technology would bring in an advantage.
Numerical design methodology for reversible Deriaz turbine with high energy performance and reduced fish impacts
Authors: Kassanos I , Alexopoulos V , Anagnostopoulos J 
Published: October 2022

AbstractThe increased interest for further exploitation of hydropower and pumped hydro storage sites of lower head and capacity, and the tightening on the other hand of the environmental terms and restrictions, would require the design of corresponding hydro turbines and reversible machines with improved environmental performance. This work presents a numerical methodology to optimize the design of a Deriaz turbine in order to achieve high energy efficiency and improved fish friendly behaviour in both pump and turbine operation modes. Various numerical simulation and optimization software and tools are used, while the geometry of the machine is fully parameterized to allow for wide design modifications and corresponding numerical tests. At first, the most important design parameters and their variation range limits are identified by a sensitivity study. Next, several multi-objective optimization procedures are carried out, using general quantitative targets for a comparative evaluation of the various machine runner designs. The results show a complex interdependence or competitiveness of the machine efficiency and the various fish impact performance indices that requires careful analysis. The developed and applied methodology can produce one or more candidate designs that satisfactorily meet all the desired objectives.
Sensitivity study of numerical and geometrical parameters for structural mechanical analyses in the automatic design process of hydraulic machines
Authors: Fraas S , Tismer A , Riedelbauch S 
Published: October 2022

Abstract
In the present paper a sensitivity study of numerical and geometrical parameters related to structural mechanical analyses of a hydraulic machine is presented. The sensitivity study is carried out for the blades of two different hydraulic machines, a propeller blade of a submersible vehicle and a runner blade of a kinetic river turbine. The focus of this work is on static structural mechanical analyses, which are performed with the open source finite element program CalculiX. Parameters for the sensitivity study are numerical parameters like the global mesh size as well as the local mesh refinement at certain locations of the blade. Furthermore, the sensitivity of geometrical parameters such as the blade thickness at different locations or the shape of the fillet is investigated. The results of the sensitivity study show that the numerical parameters have a small influence compared to the geometrical parameters. The most dominant geometrical parameter is the blade thickness at the hub. In case of a high load due to the centrifugal force, the influence of the blade thickness at the tip of the blade increases.