bluuebunny/crossref_metadata_2025 huggingface dataset:
Observational constraints on anisotropies for bouncing alternatives to inflation
Authors: Agullo Ivan , Olmedo Javier , Wilson-Ewing Edward
Published: October 2022

We calculate how primordial anisotropies in the background space-time affect the evolution of cosmological perturbations for bouncing alternatives to inflation, like ekpyrosis and the matter bounce scenario. We find that the leading order effect of anisotropies in the contracting phase of the universe is to induce anisotropies in the cosmic microwave background with a very concrete form: a scale-invariant quadrupolar angular distribution. Sub-leading effects are the generation of higher-order moments in the angular distribution, as well as cross-correlations between scalar and tensor modes. We also find that observational constraints from the cosmic microwave background on the quadrupole moment provide strong bounds on allowed anisotropies for bouncing alternatives to inflation that are significantly more constraining than the bounds previously obtained using scaling arguments based on the conjectured Belinski-Khalatnikov-Lifshitz instability.
Design of an optically-transparent ultra-broadband microwave absorber
Authors: Gao Mian , Chen Qiang , Zheng Yue-Jun , Yuan Fang , Sun Zhan-Shan , Fu Yun-Qi
Published: October 2022

The optical window of low-observable platform needs to be compatible with ultra-broadband absorption, hence an optically-transparent absorber with ultra-broadband absorption is designed and analyzed in this paper. The transparent materials indium-tin-oxide (ITO) film and polymethylmethacrylate (PMMA) are selected as the lossy layer and the supporting dielectric layer, respectively. The optically-transparent ultra-broadband absorber (OT-UBA) is composed of three layers of ITO square patterns, three layers of PMMA dielectric and a uniform ITO plane. The ITO square patterns can realize arbitrary equivalent series of RC (resistor and capacitor) circuit, so that three layers of ITO square patterns together with the ITO plane can achieve ultra-broadband absorption based on the equivalent circuit optimization. Measured results shows that the 90%-absorption bandwidth covers 2–17 GHz while the light transmittance achieves 59.6% with a total thickness of only 12.9 mm.
Analysis of the interaction between the ϕ meson and nucleus
*
Authors: Wang 王 Xiao-Yun 晓云 , Dong 董 Chen 晨 , Wang 王 Quanjin 全进
Published: October 2022

In this study, we systematically investigate the ϕ meson and nucleus interaction by analyzing and fitting the cross sections of
\begin{document}$ \gamma N $\end{document}

\begin{document}$ \rightarrow \phi $\end{document}

N reactions near the threshold, where N represents the nucleus. Using the vector meson dominant model, the distribution of the ϕ-N scattering length is presented as a function of energy, and the results show that there is a slight increase in scattering length with increasing energy. Based on this, the average scattering length of a ϕ-proton is obtained as
\begin{document}$ 0.10\pm0.01 $\end{document}
fm by combining experimental data and theoretical models. Moreover, the average scattering length of the ϕ-deuteron interaction is derived to be
\begin{document}$ 0.014\pm0.002 $\end{document}
fm for the first time. Furthermore, the effect of the momentum transfer
\begin{document}$|t_{{\rm min}}|$\end{document}
on the ϕ-N scattering length at the threshold is discussed. The obtained results not only provide important theoretical information for a more comprehensive and accurate study of the ϕ-N scattering length, but also a basis for future experimental measurements of ϕ meson production.
An Analytical quasi 2D steady-state Francis turbine model using first principles
Authors: Storli P-T , Trivedi C
Published: October 2022

Accurate models are required for obtaining accurate simulation results. High fidelity numerical models exist, but for the simulations of large hydropower conduit systems one-dimensional (1D) models are still required. For the operation and characteristics of hydraulic machinery the state variables in 1D pipe computation are not necessarily providing sufficient level of detail on what happens inside the machine to accurately predict component and system behaviour. This work is looking into the physics of the spatial distribution of the flow at the outlet of a Francis turbine runner in order to include 2D effects into a 1D analysis. The key finding is a differential equation describing the distribution of the relative velocity W between the flow and the runner outlet, which enables numerical integrating for finding the distribution of W itself, and subsequently the circumferential and meridional components needed to execute the integrals turning the 2D results into functions of the 1D state variables. The presented work is reflecting condition at the best efficiency point, but ongoing work is expanding this to map the entire region of a turbine runner. The approach should be very relevant for future implementations into digitalization schemes such as digital twins
An experimental investigation on the effects of cylindrical rods in a draft tube at part load operation in down-scale turbine
Authors: Shiraghaee S , Sundström J , Raisee M , Cervantes M J
Published: October 2022

The present work examines the effects of the radial protrusion of four cylindrical rods at different lengths within the flow field of a down-scaled turbine draft tube under part-load operating conditions. Four rods were placed on the same plane 90 degrees apart. The protrusion length was varied from zero to approximately 90 % of the draft tube radius. Time-resolved pressure measurements were performed to quantify the effect of the rod protrusion, using two pressure sensors at the same vertical level 180 degrees apart. Such sensor configuration enabled the decomposition of the signals into rotating and plunging components of the rotating vortex rope (RVR). The results show that different levels of mitigation are achieved for the rotating and plunging components depending on the protrusion length. The effects on the plunging component differ from the ones on the rotating component. The RVR plunging pressure pulsations slightly increase with the initial rod protrusion and then significantly drop after a certain length. On the contrary, the rotating component of the pressure pulsation amplitudes immediately decreases with the onset of rod protrusion. However, an optimum length is obtained in both cases where the highest mitigation occurs before reaching the maximum protrusion. This observation falls in line with the previous investigations conducted for oscillatory rod protrusions, further approving the point that a closed-loop controller should accompany the mitigation technique to achieve optimum mitigation.
Characterization of no-load conditions for a high head Francis turbine based on the swirl level
Authors: Fortin M , Nennemann B , Houde S
Published: October 2022

This paper compares the average flow topology in the draft tube cone of a high head Francis turbine operated at full-gate opening no-load (runaway) and speed-no-load (SNL). The comparison is based on the swirl level in the turbine quantified with the angular momentum parameter (RCu11) and the Swirl number. This study shows that RCu11 only depends on the flow angle at the guide vane outlet, the distributor height and the runner outlet diameter. The Swirl number has strong limitations in characterizing the flow at runaway and SNL and is unsuitable for no-load conditions.
Development of a test rig for investigating the performance of a model Francis turbine
Authors: Lama R , Gautam S , Lama L , Neopane H P , Chitrakar S
Published: October 2022

Sediments flowing with water in the Himalayan Rivers of Nepal erodes the components of the turbine heavily. The design of hydraulic turbines prioritizes the hydraulic performance of the turbine, neglecting erosion challenges. The necessity of turbine design and its model testing has been felt in the past two decades in Nepal. An attempt is made to fill such conditions by developing a test rig for testing the 92 kW model Francis turbine at Turbine Testing Lab, Kathmandu University, Nepal. The model turbine is a scaled-down model of a 4.1 MW Francis turbine of a Jhimruk Hydropower Plant in Nepal severely affected by sediment erosion. The design of the prototype turbine was carried out considering the hydraulic performance as well as erosion resistivity. The prototype turbine was scaled-down utilising model similarity conditions to meet the lab’s specifications. Each component of the rig has been optimized using CFD to match the lab’s specifications. Comparing the results from CFD and experiment on velocity, pressure, torque, and RPM measurements were comparable.
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

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

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

This 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

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

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

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

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

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

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

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

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

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

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

The 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

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.
URANS numerical simulations of pulsating flows considering streamwise pressure gradient on asymmetric diffuser
Authors: Grecu I S , Dunca G , Bucur D M , Cervantes M J
Published: October 2022

The paper focuses on implementing the wall model developed by Manhart, in Reynolds Averaged Navier - Stokes (RANS) turbulence models used in the field of Computational Fluid Dynamics (CFD). This wall model considers the influence of the streamwise pressure gradient in addition to the existing wall models used in the usual CFD codes. In the present work, two RANS numerical simulations are carried out using the k-ω Shear Stress Transport (SST) turbulence model on an asymmetric diffuser geometry. One numerical simulation is carried out using the implementation of the Manhart wall model in the k-ω SST turbulence model, and the other numerical simulation is performed using the standard formulation of the k-ω SST turbulence model. The numerical simulations carried out using the Manhart wall model and the standard formulation of the k-ω SST are compared with experimental measurements made on the asymmetric diffuser experimental installation. The numerical simulations are carried out using a free, open-source CFD tool, Code_Saturne. The comparisons between numerical simulations and the experimental data are in good agreement in the boundary layer of the flow inside the diffuser. The Manhart wall model had a faster convergence resulting in a shorter simulation time.
Towards numerical simulation of high part load pulsations in Francis turbines
Authors: Chirkov D , Shcherbakov P , Skorospelov V , Turuk P
Published: October 2022

One of the most interesting operating points of Francis turbines is the high part load point, which is characterized by synchronous pressure pulsations of large amplitude. Physical mechanisms of these pulsations are not well understood. Experimental observations show that the frequency of the pulsations is 1.5 to 4 times higher than the runner rotation frequency and there exists a 180 degree phase shift of pressure signal measured in the spiral casing and in the draft tube. Numerical modeling this operating point is challenging as it has to account several complex phenomena: cavitation, rotating vortex rope and interaction of the turbine flow with the whole water system of the test rig. In the present paper we carried out a series of 1D-3D calculations of a model turbine in an attempt to simulate the high part load pulsations. 1D hydro-acoustic equations were used for the upstream waterways, while 3D Reynolds averaged Navier-Stokes model of two-phase mixture was used for the turbine. Although the main factors were taken into account, these computations were not able to capture the high part load pulsations. Namely, the frequency and amplitude of the pressure pulsations were significantly lower than those in the experiment, and there was no phase shift in turbine domain. In order to investigate the shortcomings of the present model, we considered a problem of propagation of disturbances in two-phase “liquid-gas” flows. Several 2D test cases were numerically investigated using both incompressible and compressible gaseous phase model. It was shown that in the absence of phase transfer the compressible model correctly represents the speed of sound in both homogeneous and stratified flow regimes. From the other hand, in case of phase transfer the model damps out propagation of pressure disturbances in upstream direction. This behavior explains the failure of the present 1D-3D model to correctly describe the acoustic properties of the two-phase flow in the draft tube, playing a crucial role in the development of high part load pulsations.
Characterization of the Fluid Damping in Simplified Models of Pump-Turbines and High Head Francis Runners
Authors: Moraga G , Valero C , Valentín D , Egusquiza M , Xia X , Zhou L , Presas A
Published: October 2022

In order to satisfy the power demand in the electrical grid, hydraulic turbine units frequently work under off-design operation conditions and pass through transient events. These operation conditions can lead to high vibration amplitudes in the turbine runners, decreasing their useful life, and in some cases to premature failures. To determine and to understand the behaviour of the fluid damping is a relevant topic, because this parameter limits the maximum amplitude in resonance conditions. The runner of some types of turbines, such as reversible pump-turbine and high head Francis turbine, can be modelled as a disk-like structure, due to their similar mode shapes. Because of this, in this work, the fluid damping of a vibrating disk was studied. The disk was submerged in water and was put in a resonant state at different vibration amplitudes. Moreover, this structure was excited at different distances to a rigid surface, in order to analyse the effects of the distance between the runner and the casing. The main effects on the fluid damping were determined and characterized, showing a dependency of the fluid damping ratio on the different parameters.
On-site measurements of the dynamic behaviour of Pelton turbines in the context of predictive maintenance
Authors: Chiarelli M , Hasmatuchi V C , Amini A , Decaix J , Vetsch D F , Boes R M , Münch-Alligné C
Published: October 2022

The operating modes of pumped storage power plants, such as that of the Forces Motrices Hongrin-Léman (FMHL) in Switzerland, have drastically changed over the past decades due to the emergence of new renewable energies. The number of starts and stops experienced by machines increased significantly leading to broader fatigue cycles on the different parts of the machine. Therefore, the development of predictive maintenance tools is paramount to increase the availability and reinforce operational safety of the power plant. In this context, vibration measurements were performed on a 60 MW Pelton turbine of one of the ternary units of the FMHL power plant during dynamic and steady state operations. Non intrusive instrumentation has been deployed including accelerometers on the bearing. Comparing the ramp-up and ramp-down of the Pelton turbine, a hysteresis of the vibration level for a similar power has been observed. A reduction of the vibration levels when an additional injector is engaged during the ramp-up is observed. This reduction correlates strongly with the load reduction on the buckets when the total flow rate is distributed over more injectors, leading to a smaller flow rate per injector.
Validation of deep part load dynamic stresses for axial runners
Authors: Monette C , Chamberland-Lauzon J , Nennemann B
Published: October 2022

To accommodate renewable energy production and load demand variability, hydropower plant owners need to increase their operating range regardless of their units’ original design load envelop. When this increased operating range is an issue for the fatigue life of the old runners, solutions need to be found with the design of a new runner to sustain those new challenging loads of the increased operating range. In recent years, many papers have been published to show the challenging loads on Francis runners at speed-no-load and deep part load conditions. Andritz demonstrated a good numerical prediction capability for stress levels at deep part load conditions for Francis runners. However, for axial units, very little has been published. Very recently, some papers showed good predictability by CFD of the flow behavior at deep part load including the vortices present at those conditions. This paper demonstrates the prediction capability of the numerical tools by comparing strain gage measurements on an axial runner to CFD-FEA stress predictions. The measurement campaign was conducted conjointly by the unit owner and the manufacturer for research purposes. In the deep part load operating zone under the effect of columnar vortices, frequency analysis of the measured vibrations and strain gage signals confirmed the flow behavior predicted by CFD, and the measured dynamic strain amplitudes were well predicted. Numerical prediction of dynamic stress in the complete range from 0-100% power of the measured unit as well as detection of high vibration zones was successful.
Anti-interference characteristics of variable speed pumped storage units for load cases of hydraulic disturbances
Authors: Huang Yifan , Yang Weijia , Li Yulan , Zhao Zhigao , Yang Jiandong
Published: October 2022

Variable speed pumped storage unit (VSPSU) is an innovative technology, and investigations regarding operating characteristics of VSPSU are significant. However, there are relatively few studies towards hydraulic disturbances of VSPSUs. The hydraulic disturbance means that pumped-storage units, which were originally in steady operation, are inevitably affected by suddenly operating changes of other units in the same waterway system. Normally, it will cause the power output of fixed-speed units to oscillate, and adversely affect the safety of generators and stability of power systems. Hence, investigations of VSPSUs’ characteristics under hydraulic disturbances are necessary. In this paper, VSPSU using doubly fed induction machine (DFIM) is adopted as a study case, and fast power control strategy is used in the controlling of VSPSU. The simulation result shows that VSPSU has good anti-interference performance, and normally operating VSPSU can still maintain stable power output and stator current even under disturbance from other units. However, despite the good anti-interference performance of VSPSU, the mechanical power of VSPSU lags far behind the electromagnetic power, and the hydraulic-mechanical transient processes of VSPSU are similar to that of a fixed-speed unit. The findings in this paper could provide understanding and guidance for practical operation of VSPSUs.
Contribution for the roadmap of hydraulic short circuit implementation: Case of Grand-Maison pumped storage power plant
Authors: Landry C , Nicolet C , Badina C , Pichon H , Drommi J-L
Published: October 2022

The objectives of the 2050 energy policy based on the decarbonization of the electric power networks generate drastic changes for grid balancing with a massive integration of non-dispatchable Renewable Energy Sources. Hydroelectric power plants already significantly support electricity power system flexibility with innovative solutions such as variable speed units, fast frequency control, fast generating to pumping modes transition, high ramping rate, inertia emulation, etc.

For pumped storage power plants (PSP), a quick solution to increase the flexibility without large investment is to operate the power plant in hydraulic short circuit (HSC) mode. This technological solution is simple to implement, but requires an in-depth study of various technical aspects among which the hydraulic transients of the new operating modes is of high importance from the installation’s safety perspective. In the framework of XFLEX HYDRO H2020 European research project, the exploitation of this solution is under implementation at Grand-Maison PSP. Located in the French Alps, Grand-Maison PSP is equipped with 8 reversible multi-stage Francis pump-turbines and 4 Pelton turbines, for a total installed capacity of 1800MW, thus being the largest PSP in Europe and one of the major PSP in the world. The waterway includes a headrace tunnel, a headrace surge tank, 3 parallel penstocks feeding the 12 units operated under a maximum gross head of 955mWC.

In this paper, after a description of the general HSC considerations, the 1D model of the Grand Maison PSP and the related validation are presented. Finally, the most critical load cases in HSC operation are described to identify the potential hydraulic transient issues, such as extreme water levels in the upstream surge tank, maximum static pressure along the pressure shaft and minimum static pressure along the tunnels. The analysis performed for Grand Maison PSP is a contribution to the roadmap for the implementation of HSC operation in pumped storage power plant and will be made available as a public deliverable of the XFLEX HYDRO H2020 European research project.
Optimization of turbine start-up sequence of a full size frequency converter variable speed pump-turbine
Authors: Schmid J , Alligné S , Biner D , Münch-Alligné C , Hugo N , Nicolet C
Published: October 2022

The stability of the electricity grid will be disrupted by the massive integration of new renewable energies. Hydropower plants have a major role to play in this transformation of the electricity market by increasing their operational flexibility and their ability to provide ancillary services. However, this flexibility may lead to an accelerated degradation of mechanical components. By changing the turbine operating point far from the best efficiency point or by increasing the number of transient manoeuvres such as start and stop sequences, unsteady flow phenomena, cavitation development and additional wear and tear stress the unit’s components and impact its lifetime. The present work aims to provide preliminary insight on the optimization of the start-up sequence of a 5 MW reversible Francis pump-turbine equipped with a Full Size Frequency Converter (FSFC). The goal of the optimization approaches are to determine a start-up sequence which minimizes the runner damage, the penstock fatigue and the water losses. The objective functions are evaluated by 1D hydraulic transient simulations with the SIMSEN software and which it allow to compare the relative mitigation between the conventional fixed-speed start-up and the linear variable-speed start-up equipped with a Full Size Frequency Converter.
Heat treatment optimization of 13Cr4Ni stainless alloy used as hydro turbine materials
Authors: Dahal D R , Khatri K.C R H , Tandukar S , Kapali A , Ghimire A , Panthee A , Chitrakar S , Thapa B
Published: October 2022

In Nepal and the regions along with the Andes, sediment erosion is one of the major causes of the failure of hydro turbines. Every year lots of the components of turbines exposed to sediment-laden water got eroded resulting unexpected shutdown of the power plants. The cheapest method of repairing these eroded runners and other components of turbines are welding which may induce residual stress, embrittlement, and increase carbide content. Therefore, post welding heat treatment especially stress relieving annealing is always carried out to reduce the induced residual stresses, enhance strength and other mechanical properties as well as reduce the risk of cracking. As heating time is relative of low importance, great care should be taken during heat treatment to maintain the soaking time and cooling rate. Holding time for heat treatment will vary with the thickness of casting sections involved, but should be sufficiently long enough to heat all sections to a uniform temperature throughout. The cooling rate determines the material properties at room temperature and rapid cooling avoids the formation of chromium carbide. In this paper, the post-weld heat treatment used for repair and maintenance of turbines in Nepal is studied along with the investigation of heat treatment conditions on mechanical properties and microstructures of 13Cr4Ni stainless steel used as turbine materials to optimize the heat treatment itself. First of all relative analysis of five different heat treatment conditions with respect to ‘no heat treatment’ regarding mechanical properties i.e. toughness, hardness, and fatigue life was carried out. Heat treatments of the specimen were done in a muffle furnace. Hardness, toughness, and fatigue strength of the specimen using the Rockwell Hardness Test, Charpy Impact Test, and Fatigue Life Test were conducted respectively. Subsequently, microstructures of all the heat treatment conditions are closely examined using the metallurgical microscope. It was observed that holding time plays a great impact on improving mechanical properties.
Energy scale of dielectric coupling in antiferromagnetic insulators
Authors: Zhang Jiarui , Chen Cai , Xiang Shuqing , Zhang Junchao , Qi Qi , Huang Rui , Yu Yang , Chen Kai , Han Zhida , Yuan Guoliang , Liu Junming , Zhu Jinsong
Published: October 2022

To probe the energy of dielectric coupling between the dipole chains and the quasipolarons based on extension of the polaron concept, dielectric and magnetic properties of antiferromagnetic insulators, namely Ca4-xCuxTi4O12 (x = 3, 2 and 1), are investigated at different temperatures. The energy is estimated larger than 56 meV of the dipole interaction energy, but smaller than 88 meV of the dipole-chain thermal activation energy which is for the Dissado-Hill type dielectric relaxation. In particular, the quasipolaron ingredient is at least 28 meV of the total superexchange interaction which bond one singe titanium ion of the dipole chain with four quasipolarons. Then, the energy of the coupling is in the range from 84 meV to 88 meV. The experimental observation, suggests the dielectric coupling may provide an approach to obtain large permittivity in strongly-correlating systems of antiferromagnetic insulators.