bluuebunny/crossref_metadata_2025_split huggingface dataset:
Pump-Turbines in Conventional Hydropower Plants
Authors: Dagsvik Helene Njølstad , Storli Pål-Tore Selbo 
Published: June 2021

Abstract
This paper proposes an innovative approach to retrofit existing hydropower plants with suitable upper and lower reservoirs into pumped storage. The conventional methods are both comprehensive and expensive, mainly due to the increased risk of cavitation during pumping operation. In an existing facility, the installed runner is already sufficiently submerged to avoid unfortunate cavitation, but replacing the runner with a reversible pump-turbine demands an even further increase of the static pressure at the low-pressure side of the turbine. An alternative to the traditional method is discussed in this research by introducing the concept of a booster pump installed in the draft tube. Increasing the pressure at the inlet of the pump-turbine during pumping could eliminate the risk of cavitation and enable operation of a pumped storage plant without submerging the runner further. This conceptual paper discusses the motive for developing pumped storage and presents two uses for the proposed booster pump; eliminating cavitation and correcting for some of the necessary lifting head of the pump-turbine. The project is a part of the Norwegian Research Centre for Hydropower Technology, HydroCen.
Numerical flow simulation of pump stability for pumped storage units with analysis and visualization of the dynamic flow patterns
Authors: Magnoli M V , Giese M 
Published: June 2021

Abstract
With the increasing participation of alternative energy conversion sources in the energy market, the demand for energy storage and grid regulation services keep steadily growing. Pumped storage stations stand out currently as the most suitable and effective solution for these purposes. This leads to tougher requirements regarding the operation flexibility, regulation band and operating range of pumped storage units with reversible pump-turbines or ternary sets (with single or multi-stage pumps), operating at fixed or variable speed. Through the extension of the pump stable region, the pump regulation band can be extended (assuming the proper cavitation safety is provided) and in the case of reversible units the design of the hydraulic machine for turbine mode operation can be accordingly adjusted. The objective here is to model and examine details of the flow phenomena occurring as the pump goes through the unstable region at flows below minimum operating flow. A detailed understanding of the flow phenomena in this region can provide insights to improve the hydraulic design and extend the stable region of operation. Traditional model tests only provide data about the pump characteristic curve. The focus here will be transient computational fluid dynamic (CFD) simulations of a pump-turbine as it goes through the unstable low flow pumping region. The finite volume model includes the complete hydraulic machine from draft tube to spiral case and hybrid turbulence models were used, in this case scale adaptive simulation (SAS). For the analysis and assessment of the transient fluid flow at the different pump operating conditions, integral quantities such as the lift head were evaluated, the velocity field was visualized with flow streamlines, the pressure field with contour plots at the hydraulic surfaces and the vortical structures were identified, quantified and plotted using the criterion. The transient fluid flow calculation was started at an operating point in the pump stable operating range. The simulated pump flow was progressively reduced during the given numerical simulation until the set-in of the pump instability with the break-down of the lift head and pump flow continued to be reduced until the lift head started rising again. The numerical simulations were carried out for a low specific-speed reversible pump-turbine. Detailed model test results were available for the selected design and have been used to validate the numerical simulations.
Experimental investigation of a free surface oscillation in a model pump-turbine
Authors: Maly Anton , Bauer Christian 
Published: June 2021

Abstract
Hydro power plants provide balancing services to the power grid and the amount is expected to increase in the near future. One of these services is the compensation of reactive power and the regulation of the power factor. Therefore, hydro power units equipped with synchronous machines are operated in so called synchronous condenser mode. During this operation the runner is dewatered and rotates in air above a free surface at the draft tube cone. As a result, a free surface oscillation is excited under some conditions. This paper deals with the experimental investigation of this flow phenomenon. Therefore, model tests are conducted at the hydraulic laboratory of the Institute of Energy Systems and Thermodynamics at TU Wien. The investigated model pump-turbine is equipped with a draft tube cone made of acrylic glass for optical investigations. In addition to the standard instrumentation, fast response pressure transducers are installed at different planes and positions. The machine setup and measurement conditions aim at the investigation of the free surface oscillation and shall prevent other influences. The experiments are performed for both runner rotation directions, different initial water levels at the draft tube and different densimetric Froude numbers. High speed camera recordings are conducted and the amplitude of the water formation is determined by post processing the captured images. In periodic state the frequency of the free surface oscillation is extracted out of the pressure signals. Finally general trends and dependencies are deduced from the acquired data.
A Turbulence Model Assessment for Deep Part Load Conditions of a Francis Turbine
Authors: Wack J , Beck J , Conrad P , von Locquenghien F , Jester-Zürker R , Riedelbauch S 
Published: June 2021

Abstract
Today, hydropower plants received increased attention as they have the ability to stabilize the electrical grid. However, this is accompanied by more start-stop sequences as well as operation in off-design conditions. One big task is to develop improved runner designs that can resist the increased pressure fluctuations. In the scope of this study, a Francis turbine is simulated for a deep part load operating point with the use of the SAS model and the more recently developed SBES model with ANSYS CFX. Both turbulence models are hybrid RANS-LES models. The simulation results are compared against model test data from a measurement at high cavitation number. For the evaluation of the pressure fluctuations comparison is furthermore made against experimental data at low cavitation number. All in all, SAS and SBES model predict similar flow characteristics. Both, the velocity profiles in the PIV measurement plane and the predicted pressure fluctuations, show only minor differences. The comparison against experimental data shows that the amplitude of the broadband frequency spectrum, which results from the occurrence of vortices, is overestimated at the investigated locations on the runner blades.
Reconstruction of the Francis 99 main runner blade using a hybrid parametric approach.
Authors: Cerriteño A , Delgado G , Galván S , Dominguez F , Ramírez R 
Published: June 2021

Abstract
The runner blade shape has been described as a set of discrete sections joining hub and shroud, resulting in such 3D free geometrical forms of considerable complexity that the search for an appropriate parametric approach for its reconstruction and optimization is still a matter of research field. For this reason, the main purpose of this study has been to reconstruct a real and twisted runner blade with a required accuracy through a methodology that uses several algorithms to combine sections and surfaces parametric approaches. Using the surface approach, the free-form shape of the 3D camber surface was defined by a multi-directional interpolation procedure. Using the section approach a Direct Profiling method was tested to fitting the hydrofoil section to the real one. For the blade reconstruction, ten profiles were stacked along the spanwise direction of the real blade to obtain by interpolation the pressure and suction sides using a lofting procedure, thus achieving an acceptable continuous and smooth surface definition. We assume that this methodology could be efficient to reconstruct a damaged and worn free-form blade, especially when the original surface geometry implies sections with different Thickness Distribution along the spanwise direction.
Simplified simulation of a small Pelton turbine using OpenFOAM
Authors: Hidalgo V , Díaz C , Erazo J , Simbaña S , Márquez D , Puga D , Velasco R , Mafla C , Barragán G , Parra C , Valencia E , Luo X 
Published: June 2021

Abstract
In Ecuador, the implementation of hydroelectric plants has had remarkable growth in the energy sector due to its high efficiency, low environmental impact and opportunities to generate employment. One of the sectors with the greatest benefits due to this type of energy has been the rural sector, where several small power plants with a generating capacity between 500KW and 1MW have been installed, which are usually Pelton turbines. In the present research it is necessary to use reverse engineering followed by a numerical study to performance of small Pelton turbines due to the fact that geometry dimensions have not been provided. For this purpose, the geometry has been obtained by applying reverse engineering using a technique based on the solidification of polymers. A silicon elastomer is spread over the buckets to obtain a rubber-type shell and it is then cover with plaster to maintain the shape of the rubber. A 3D scanner is used to obtain a cloud of points which is then processed to generate a faceted surface model. The three-dimensional model is edited and improved using CAD software. A basic simulation is presented to show that this reverse engineering process can be used to perform numerical analysis of Pelton turbines.
Emergency gates - model scale tests at turbine runaway condition
Authors: Angulo M. , Rivetti A. , Díaz L. , Lucino C. , Liscia S. 
Published: June 2021

Abstract
Emergency gates are the last link in the chain of safety of turbo-groups in case of distributor failure, safeguarding the power station from severe damage. These gates can be located at the turbine intake or at the outlet of the draft tube and can be controlled by gantry cranes or hoist hydraulic cylinders. Gates must descend with high flow for a short time to prevent the turbine from spinning at runaway velocity for periods longer than admissible, as that would entail the rise of uplift and downpull forces that may jeopardize their stability. Indeed, at the prototype scale, the closing maneuver entails a certain risk, because of which it is usually tested avoiding extreme conditions.
In this work, the operation of emergency gates was tested against more severe conditions on a reduced-scale physical model. The case study involves three emergency gates controlled by gantry cranes and located at the intake of a large Kaplan turbine which underwent high levels of vibration when operated at prototype scale.
Model tests were aimed at detecting and quantifying hydraulic phenomena that might emerge during operation with an eye on the proposal of alternative designs. Unlike most tests of this sort, the experimental setup includes the runner of the turbine assembled on a test rig, which allows for a more realistic flow distribution along the vanes during the gate closure under runaway conditions.
Steady state tests were carried out under runaway conditions, while stems of servomotors enabled the regulation of the position of the gate. Downpull forces were found to start at 12 % of the gate opening. Flow asymmetry was observed, gate on the left of the semi-spiral casing being the most affected by higher flow velocities. The runner vortex rope frequency was measured also at gate lip for some particular conditions.
Numerical investigation of the effects of hydrofoil vibrations on the unsteady behavior of cavitation
Authors: Geng Linlin , Chen Jian , De La Torre Oscar , Escaler Xavier 
Published: June 2021

Abstract
In order to better understand the fluid-structure interaction (FSI) in hydraulic machines, this study aims to numerically investigate the flow around an oscillating hydrofoil with special attention on the effect of unsteadiness on leading edge cavitation. The cavitating flow has been modelled as incompressible with a Shear Stress Transport (SST) turbulence model coupled with a transition model, and with a mass transport equation between vapor and water. The sinusoidal motion of the hydrofoil has been implemented by using a moving mesh technique. The numerical results, which have been validated with experimental data, show that a critical frequency exists showing two different behaviors. For pitching frequencies above this critical frequency, the cavitation shedding frequency is linearly dependent on the pitching frequency. Moreover, this change of cavitation dynamics is explained by a pressure phase shift and an amplitude change observed as the pitching frequency increases.
Scale adaptive simulation of unsteady cavitation flow around a plane convex hydrofoil with a semi-cylindrical obstacle
Authors: Hidalgo V , Escaler X , Díaz A , Luo X , Simbaña S , Márquez D , Hernandez P , Valencia E 
Published: June 2021

Abstract
The present study focuses on the numerical simulation of unsteady cavitating flow around a plane-convex hydrofoil with a semi-cylindrical obstacle, which is based on the cavitation-erosion experiment perform at LMH-EPFL using the vortex cavitation generator tunnel. The turbulence model k-ω SST SAS method, which presents advantages in terms of computational consumption and reproduction of the phenomenon, has been applied in OpenFOAM version 4 to reproduce the unsteady behavior of cavitating flow. Additionally, the Zwart-Gerber-Belamri (ZGB) cavitation model has been applied, based on a previous work where this model was implemented in OpenFOAM. The model is based on Rayleigh Plesset equation, which considers small cavities with changes of void fraction for condensation and vaporization and using empirical calibration numbers based on previous research. Regarding the mesh development, the present work explores two configurations of grid mesh containing hexahedra (hex) and split-hexahedra (split-hex) automatically generated from triangulated surface geometries based on previous numerical studies. The aforementioned method aims to optimize computational demand and phenomenon reproducibility. Results show that the unsteady cavitating flows behavior has been reproduced with good accuracy and shows special details which are important for erosion studies in futures works.
Cavitation modelling using real gas state equation: A conceptual study
Authors: Hanimann L , Mangani L , Casartelli E , Darwish M , Claramunt K 
Published: June 2021

Abstract
The prediction of the onset of cavitation to allow investigations of the flow in off-design conditions during design is unavoidable for a variety of applications. Numerical modeling of the cavitation phenomena is however computationally challenging due to the nature of the phenomena. High density ratios, steep pressure gradients, the presence of multiple phase including phase change and hence to occurrence of real gas thermophysical behavior would cause a computational effort unmanageable during design. Today’s models are therefore greatly simplified representations of the reality. Conservation equations are usually solved using a mixture approach with flow properties being a volume fraction weighted average of two incompressible fluids thus neglecting real gas effects. The source term from the phase exchange however usually poses significant stability problems. In this article we will therefore present an approach for cavitation modeling using high-order polynomial real-gas state equations to accurately predict the properties of water at any state. Computational performance is improved by the use of structured tabulation using high order interpolation methods. The numerical results are then compared against measurement data and conventional cavitation model results.
Francis turbine dynamic stresses measurements at model and prototype scales
Authors: J. Chamberland-Lauzon , J. DiSciullo , C. Monette , B. Nennemann , S. Afara 
Published: June 2021

Abstract
A strain gauge and pressure measurement campaign on a model runner with a modern design was performed. The reference prototype runner was commissioned in 2013 and strain gauge measurements on the prototype are available for comparison. The model runner was fabricated by stereolithography in a plastic composite selected to obtain mechanical homology. The instrumented model runner was tested from 14% to 100% of the maximum guide vane opening at various speed factors (nED
) by combining different head levels and rotating speeds as well as varying the cavitation coefficient (sigma) so that the effect of each studied parameter can be identified and quantified. By adjusting the speed and the head of the model test, it was also possible to induce rotor-stator resonance of the runner and to confirm expected hydrodynamic damping behaviour. Finally, the model scale pressure and stress measurements are compared to the ones obtained by both prototype measurements and CFD/FSI simulations.
Operational Modal Analysis of Francis Turbine Runner Blades Using Transient Measurements
Authors: Gagnon M , Dollon Q , Nicolle J , Morissette J-F 
Published: June 2021

Abstract
To prevent dynamic amplification or resonance during normal operation, which could result in large blade deformations and premature fatigue failure, structural mode frequencies are predicted using numerical models during design. However, such numerical models could benefit from validation using data obtained during commissioning. Here, we propose the use of asynchronous transient experimental measurements to identify regions where interaction between structural modes and Rotor-Stator Interaction (RSI) harmonics can be detected. From these regions, modal parameters can be estimated and compared with numerical predictions. This paper presents the methodology used to extract these modal parameters and estimate their uncertainties using data from a Francis runner recently commissioned by Hydro-Québec.
A proposal for the dynamic strain interpolation on hydroelectric turbine runner
Authors: Pham Quang Hung , Gagnon Martin , Antoni Jérôme , Tahan Antoine , Monette Christine 
Published: June 2021

Abstract
The large operation range of hydroelectric turbine, caused by the increasing changes in the electrical network usage, leads to higher dynamic stress fluctuation on runner. However, the experimental data cannot cover all the possible operating conditions. The missing data in some turbine operation zones generates a challenge for the recovery of the highest dynamic strain. This paper proposes a methodology for the interpolation of such strains between operating conditions by using the kriging method. The case study focuses on the interpolation of the part load rope which is an important component in the dynamic strain of a Francis turbine runner. Two interpolation approaches, inspired by spatio-temporal kriging and cokriging, are applied and compared. Finally, some suggestions are proposed to improve the recovery of operating conditions using interpolation process.
Experimental investigation of swirl number influence on spiral vortex structure dynamics
Authors: Štefan D , Hudec M , Uruba V , Procházka P , Urban O , Rudolf P 
Published: June 2021

Abstract
The hydraulic turbines are recently forced to operate far away from the optimal conditions in order to balance fluctuations in electricity generation. In case of Francis, pump and propeller turbines, using only single control component of guide vanes, it means that in regions where the high residual swirl enters the draft tube, the flow is decelerated and convenient conditions for the vortex rope development are created. Such flow conditions are considered to be the triggering mechanism for occurrence of different forms of vortex structures in the Francis turbine draft tube, e.g. spiral or straight vortex rope at part load or full load respectively. Independently on the vortex rope shape the unsteady pressure fields develop producing periodic stress on turbine components and possibly resulting in noise, blade cracks, runner lift, power swing, etc. To study and mimic such flow conditions, a simplified device of vortex generator apparatus is employed. Thanks to its design, the vortex generator enables to change the ratio between fluxes of axial momentum and tangential moment of momentum of generated swirl. Then, the behavior of vortex structure changes in a similar way as the flow rate variation in the draft tube of Francis turbine. For above mentioned reasons the unsteady cavitating spiral vortex is experimentally studied using both high speed video record and particle image velocimetry (PIV). The main focus is on change of vortex dynamics regarding to the swirl number variation. The proper orthogonal decomposition (POD) together with the classical fast Fourier transformation (FFT) are employed to extract dominant modes and frequencies from experimental data.
Dynamic behaviour of a full-load cavitation vortex in a Francis turbine draft tube excited at its eigenfrequencies
Authors: Favrel A , Vagnoni E , Gomes Pereira J , Sakamoto M , Yamaishi K , Müller A , Avellan F , Miyagawa K 
Published: June 2021

Abstract
The operating range of Francis turbines is limited at full-load conditions by the formation of a cavitation vortex rope that may enter self-oscillations under certain conditions. This induces severe pressure pulsations in the entire system, as well as output power swings putting at risk the integrity of the electro-mechanical components. The understanding of the underlying physical mechanisms and the prediction of the stability of hydropower units at full-load conditions are therefore crucial to ensure a safe extension of their operating range. In the present paper, the dynamic behaviour of a stable cavitation vortex rope at full load is investigated by high-speed visualizations while the test rig is excited at its first and second hydroacoustic eigenfrequencies. It is first demonstrated that the cavitation volume and the pressure in the draft tube are more likely to oscillate at the first eigenfrequency, in agreement with the observations of self-excited oscillations at the first eigenfrequency of the cavitation vortex rope during unstable full-load conditions. In addition, it is observed that the amplitude of both the cavitation volume and pressure fluctuations in the draft tube reach a limit value when the amplitude of the excitation is further increased. Further investigations will determine if this behaviour can be generalized to any full-load conditions and will focus on the determination of the hydro-acoustic parameters of the draft tube cavitation flow based on the behaviour of the vortex rope during forced oscillations.