bluuebunny/crossref_metadata_2025 huggingface dataset:
Modal testing of the Francis-99 runner
Authors: Solemslie Bjørn W , Dahlhaug Ole Gunnar , Sigurdsson Bjarki , Tengs Erik O
Published: June 2021

The Francis-99 low specific speed Francis runner has undergone much study in the years since it was first made available to the public. The High Head Francis(HiFrancis) project, hosted by the Waterpower Laboratory at NTNU, have conducted multiple experimental campaigns to supply validation data for numerical simulations of the RSI phenomenon for the runner. Due to the amount of instrumentation on board the runner an asymmetry exists in the runner structure that naturally affects the structural response of the runner. From numerical analysis the runner has been found to exhibit multiple natural modes of vibration, within narrow frequency bands. In order to validate the identified natural frequencies, with their corresponding mode of vibration and the amplitude, an experiment has been conducted. The experiment utilises piezo electric patches mounted on the runner hub to excite the runner in a defined mode of vibration and the response at the trailing edge of all blades is measured with a 1D Laser Doppler Vibrometry scanner. Preliminary results are shown that demonstrate the possibilities with the presented experimental method.
Experimental study and parametric analysis for primary frequency regulation of hydropower unit
Authors: Wang Cong , Wang De-kuan , Zhang Jian-ming
Published: June 2021

With the integration of new large-scale energy sources into grids, power system stability is facing huge challenges. The primary frequency regulation of hydropower generators is an important regulation method for grid frequency stability and power balance. It can effectively suppress the grid frequency fluctuations caused by the high proportion of new energy sources, and it plays a very important role in maintaining the stability of the power system. The regulation parameters of the primary frequency regulation of a hydropower unit have a significant impact on its regulation capabilities and performance indices. The experimental analysis of the effects of the regulation parameters is crucial for the safe and stable operation of a hydropower unit. A frequency regulation parameter test on a hydropower station was conducted in this study to determine the effects of the regulation parameters on the performance of the primary frequency regulation. This provides a reference for the selection of the regulation parameters of a hydropower unit.
Experimental influence of a splitter vane on the flow fields in a wide-angled diffuser
Authors: Guo Miao , Meng Xv , Zuo Zhigang , Liu Shuhong
Published: June 2021

Based on PIV and high-speed photography technology, the lengths and area ratios of flow separation zone under different Reynolds numbers Re in a diffuser with a fixed angle (28º), with and without a splitter vane were obtained. The flow field, vorticity and the flow distribution (hydrogen bubble tracing) at the downstream vicinity of the outlet of the diffuser under three representative conditions (Re = 1000, 6000 and 12000) were given. The experimental results show that with increasing Re, the area proportion of the separation zone decreases, but the length proportion of the separation zone increases, because the thickness of the separation zone along the vertical flow direction is reduced. Due to the existence of vane, the area of the corresponding separation zone is always smaller than that without the vane.
Numerical prediction of slurry erosion and its influence on prevailing flow conditions using a dynamic mesh method
Authors: Hankeln F , Kirschner O , Riedelbauch S
Published: June 2021

With the help of Computational Fluid Dynamics (CFD), the amount and location of slurry erosion can be predicted using renowned models. However, all common models do not change the hydraulic contour of the system and can therefore be interpreted as a CFD-post-processing step. Thus, the influence of slurry erosion on the fluid flow is currently not considered and the effects of changing flow conditions on the system performance or perhaps energy efficiency are not predictable. Building upon this state of the art, a numerical method based on the idea of a dynamic mesh procedure is developed. The method uses the predicted erosion to locally deform the mesh during simulation. The method contains the generation of an array-based mesh structure, the precise movement of affected boundary nodes including the tracking of inner nodes as well as a reconstructing procedure to improve mesh quality after deformation. The whole method is implemented in a commercial CFD-code. The functionality of the method is examined using the example of a standardized water pump. The comparison to corresponding experimental results revealed an accurate prediction of the erosion distribution in the pump’s front casing. Furthermore, the evaluation of the pump’s performance curves indicated an accurate prediction of the changing flow field caused by erosive wear.
Numerical assessment of parameters influencing the modal response of a Kaplan turbine model
Authors: Roig Rafel , De La Torre Oscar , Jou Esteve , Escaler Xavier
Published: June 2021

The present study intends to investigate numerically the impact of: i) the fluid added mass, ii) the turbine rotational speed and iii) the variation of the runner blade bearing stiffness due to the turbine water head on the modal response of a reduced scale Kaplan turbine. The impact of each variable has been quantified by means of a series of numerical modal analyses of the turbine in vacuum and in water, and at rest and rotating. It has been found that the natural frequencies of the Kaplan turbine model are sensitive to all the variables investigated in the present paper, the added mass being the factor which has the greatest impact.
Development of a novel numerical framework in OpenFOAM to simulate Kaplan turbine transients
Authors: Salehi Saeed , Nilsson Håkan , Lillberg Eric , Edh Nicolas
Published: June 2021

A novel numerical framework in OpenFOAM is proposed in this work, to simulate transient operation of Kaplan hydraulic turbines. Such transient operations involve a variation of both runner blade and guide vane angles, which also gives rise to a flow rate variation. A numerical simulation of such a process is very challenging, since it requires a deformation of both guide vane and runner meshes, with mesh slip conditions at arbitrarily shaped surfaces, at the same time that the runner mesh is rotating around the turbine axis. The currently available mesh morphing methodologies in OpenFOAM are not able to properly accomplish this. Thus a novel framework for OpenFOAM, including dynamic mesh solvers and boundary conditions, is developed to tackle this problem.

The new framework is utilized to simulate the flow during transient operation of the U9-400 Kaplan turbine model. The guide vanes and runner blades are rotated individually around their own axes with a constant rotational speed, while the runner is rotating, and the flow rate is linearly changed with the guide vane angle. It is shown that the novel numerical framework can successfully be utilized to simulate the load change of Kaplan turbines.
Numerical Simulation of Hydraulic Turbine During Transient Operation Using OpenFOAM
Authors: Salehi Saeed , Nilsson Håkan , Lillberg Eric , Edh Nicolas
Published: June 2021

Power generation from intermittent renewable energy resources (e.g. wind, solar) requires regulation of the electric grid. Although most hydraulic turbines are designed to work in their best efficiency points, nowadays they are being used more often under varying operating conditions to stabilize the electric grid. Unstable and varying conditions of fluid flow in hydraulic turbines during transient operation cause significant pressure fluctuations and load variations that could negatively affect the turbine lifetime. Therefore, the development of high-fidelity numerical tools for hydraulic turbine flow during transient operation, i.e. changing from one condition to another or during start-up and shut-down, is of great importance for the lifetime prediction of the machines.

In the present work, we are investigating the capabilities of the OpenFOAM open-source CFD tool to predict such phenomena. The transient operation of hydraulic turbines most of the time involves changing the guide vane angles while the runner is rotating, which must thus also be allowed by the employed numerical techniques. The high-head Francis-99 turbine is used as a test case, due to the availability of the geometry and rich experimental data. The turbulence resolving computations are performed using the SAS turbulence model. The numerical results are validated against the experimental data and compared with each other in terms of accuracy and usability. The results are also used for describing the flow behaviors during the shutdown.
Numerical and experimental study of pump as turbine for sediment affected micro hydropower project in Nepal
Authors: Pokharel Nischal , Ghimire Amul , Thapa Bhola , Thapa Biraj Singh , Qian Zhongdong , Guo Zhiwei
Published: June 2021

Designing and manufacturing site-specific turbines for small hydropower is not economical. Using abundantly available Pump, from the market, as Turbine (PAT) instead of designing a completely different turbine can be much more economical for small-scale hydropower. Lot of research have been going throughout the world on this and has already proven its effectiveness. In Nepalese hydropower, including the ones already developed and the ones that will be developed in the future, Francis turbines are supposedly the suitable turbine of choice. However, designing and manufacturing Francis turbine is a tedious task and the local manufacturers, who are expert in manufacturing Cross-flow turbines; do not have the technology and competence to manufacture the modern Francis turbines. Sediment in Himalayan rivers are the major hurdles of operation as they reduce the lifetime of the turbine by a very large factor, shooting up the maintenance cost of the hydropower. The operational region of Francis turbine and PAT overlaps quite a lot, thus indicating that PAT can be used in many of the hydropower in Nepal. The Chinese and Indian pump manufacturers are already renowned in developing a wide range of pumps and supplying them in Nepal. Despite having such great opportunity, pumps have never been used in turbine mode in Nepalese hydropower. In addition to that, no research has been done, in Nepal, regarding the effects of sediment on pumps being operated as pump or turbine. This paper describes performance of pump in erosive environment using Computational Fluid Dynamics (CFD). It also compares the results obtained from the CFD analysis of sediment erosion in pump operated in pump mode as well as turbine mode with other researchers work. Finally, it compares the results obtained from the CFD with the results obtained from the experiment.
A marine propeller as a hydrokinetic turbine – CFD analysis of energy characteristics
Authors: Ristić Bogdan , Božić Ivan , Simić Aleksandar
Published: June 2021

It is sometimes economically unreasonable to develop specific turbines, but rather to use a well-known concept of a pump in the turbine mode – PAT (Pump As Turbine). Inspired by PAT, the scope of this paper covers the assessments of the possibility to use marine propellers as runners for the hydrokinetic turbines. Numerical analyses are conducted for selected propeller geometry with the aim of evaluating its energy characteristics in the turbine regime. The chosen propeller belongs to the conventional, thoroughly experimentally tested, Wageningen B-screw series. The experimental data were used for validation of the numerical results in propeller regimes. Based on the obtained results, the researched non-optimized turbine in its optimum regime achieves a satisfactory power coefficient which is close to the value of other types of contemporary hydrokinetic turbines efficiencies.
Pump-Turbines in Conventional Hydropower Plants
Authors: Dagsvik Helene Njølstad , Storli Pål-Tore Selbo
Published: June 2021

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.
Effect of the leakage flow in runner on flow characteristics of a Francis turbine model
Authors: Kim S J , Choi Y S , Cho Y , Choi J W , Hyun J J , Joo W G , Kim J H
Published: June 2021

In the Francis turbine, there are gaps between the runner and the stator structure as the labyrinth seals. The flows inside gaps have the complicated flow phenomena; these complicated flows can affect the hydraulic performance and the downstream of the Francis turbine as the leakage flow. Therefore, it is important to investigate the flow characteristics induced by the runner gaps. However, normally the runner gap that has very complicated flow structure is often disregarded by considering the time and cost of the numerical analysis. But, it is necessary to apply all flow structures correctly to confirm the more accurate and reliable flow phenomena. In this study, the flow characteristics of gaps at the hub and shroud of the Francis turbine model runner were investigated with adding the leakage flow in the runner cone. In addition, the internal flow characteristics were compared by considering with and without the runner gaps application. For observing the influence of the leakage flow on the hydraulic performance and flow characteristics, the three-dimensional steady Reynolds-averaged Navier-Stokes analyses were performed using the shear stress transport turbulence model. The efficiency was decreased by applying the runner gaps; and the complicated flow phenomena were captured in the runner gaps.
Numerical and experimental analysis of pressure fluctuations in the draft-tube of a Francis turbine using the swirl number
Authors: Khozaei M H , Favrel A , Masuko T , Yamaguchi N , Watanabe R , Miyagawa K
Published: June 2021

Pressure fluctuations in the draft-tubes of hydraulic turbines are the most dominant issues that may cause damages to a hydraulic power plant. When the frequency of the fluctuations coincides with one natural frequencies of the plant, resonance with high amplitude vibrations and pressure pulsations occurs, dramatically impacting the integrity of the plant. This research focuses on the part-load behaviour of a medium specific-speed Francis turbine (with specific speed 184 m.kW), designed by the authors in the framework of an industrial project. This paper combines the results of model tests with CFD simulation to investigate the part-load behaviour of the turbine. To do so, performance of the turbine is numerically simulated at one operating condition by solving the RANS equations via the commercial code ANSYS CFX 19.0. The k-ω SST turbulence model is used to predict the turbine performance. In addition, model tests are conducted in the range of 20% to 110% guide vane opening. To capture pressure fluctuations in the draft-tube, 12 static pressure sensors are installed in 4 different sections of the draft-tube. The no-swirl and other iso-swirl lines of the draft tube flow are first determined by using CFD results. Furthermore, pressure signals from model tests are analysed by performing cross spectral density analysis. Two transitions in the behaviour of the precessing vortex under part-load conditions occurring at given values of swirl number are observed. Finally, it is observed that linear correlations between the Strouhal number of the vortex rope frequency and the swirl number can be established within the 2nd and 3rd part-load regimes, independently of the values of the speed coefficient.
Numerical simulation of the unsteady cavitating flow in a Francis turbine draft tube at Upper-Part-Load (UPL) conditions
Authors: Liu Z. , Favrel A. , Takahashi W. , Miyagawa K.
Published: June 2021

At part-load conditions, Francis turbines experience the development of a precessing vortex rope in their draft tube (DT), rotating with a frequency between 0.2 and 0.5 times the runner frequency. It induces pressure pulsations at the precession frequency in the whole hydraulic system, undesirable vibrations and noise putting at risk the stability of the system and the lifetime of the machine components. In specific machines, a dramatic amplification of the noise and vibrations can be observed between 70% and 85% of the design conditions. In these cases, synchronous pressure pulsations with a high frequency, typically between 2 and 4 times the runner frequency, are observed, referred to as the so-called Upper-Part-Load (UPL) pulsations. These pulsations are induced by a self-excitation of the entire hydraulic system including the cavitation vortex rope at one of its high order eigenmodes. Besides, the cavitation vortex rope features an elliptical cross-section rotating around the vortex axis at a high frequency. In this manuscript, unsteady two-phase flow simulations using a Scaled-Adaptive-Simulation (SAS) turbulence model and ZGB cavitation model of a Francis turbine draft tube at 80% of the design condition are performed to clarify the mechanisms responsible for the formation of the elliptical shape of the vortex and the UPL pulsations. However, the numerical simulation with a constant inlet boundary condition cannot reproduce the UPL pulsation phenomenon since it is a self-excited oscillation at one eigenmode of the complete system, which is not considered. Therefore, the UPL pulsations component is extracted from the measured pressure fluctuations data by applying a band-pass filter and is then set as the inlet boundary condition in the numerical simulation. The flow field is therefore artificially excited which aims to confirm whether the same phenomenon can be observed and to compare with the flow field obtained with constant inlet boundary condition. The numerical simulations are validated by experimental results, and the wave propagation in the DT is clarified.
Efficiency measurement on horizontal Pelton turbine by thermodynamic method
Authors: Cao Dengfeng , Zhou Ye , Pan Luoping , Wang Junjie
Published: June 2021

Thermodynamic method is a practical solution to determine the turbine efficiency of heads in excess of 100m without need for discharge measurement directly. Efficiency measurement is performed on a horizontal Pelton turbine with 650m rated head and two jets by thermodynamic method in this paper. Determination of specific hydraulic energy, specific mechanical energy, discharge and efficiency of Pelton turbine are discussed accordingly. To avoid the uncertain impact from unstable flow and velocity profile at low pressure section, an improved method to determine the turbine discharge is introduced, in which discharge may be worked out without need for flow velocity measurement at low pressure section.
Transient TEHD Analysis of a Thrust Bearing Based on Two-Way Fluid-Solid-Thermal Interaction
Authors: Cao Jingwei , Zhai Liming , Luo Yongyao , Wang Zhengwei
Published: June 2021

Thrust bearing is a key component of hydroelectric generator and bears the axial load of the unit. During transient processes, such as start and stop of the unit, thrust bearing usually occurs wear phenomenon at pad surface, which affects the stable operation of the unit and reduces service life of the bearing. Based on two-way fluid-solid-thermal interaction, this paper did transient thermal-elastic-hydrodynamic (TEHD) analysis of a tilting pad thrust bearing under the pump mode. For the fluid region, the Reynolds equation and the viscosity-temperature equation are solved. For the solid region, transient heat conduction equation and basic thermal-elasticity equation are considered. The data transferred in the fluid-solid interface satisfy the fluid-solid-thermal interaction equation. The variation of the lubricant oil temperature, the oil film force, the pad surface inclination angle and axial velocity of pad and mirror plate are analysed, which provide theoretical basis for the design of bearing and the safe operation of the unit.
Numerical study of hydraulic axial force of prototype pump-turbine pump mode’s stop with power down
Authors: Mao Zhongyu , Tao Ran , Bi Huili , Luo Yongyao , Wang Zhengwei
Published: June 2021

Pumped storage power station is regarded as a key component in modern social power system. It usually requires pump-turbine start and stop frequently. The hydraulic axial thrust is one of the most essential effects on the stable operation during start and stop. The hydraulic axial thrusts of pump-turbine pump mode’s stop with power down based on 1D transient flow simulation and 3D CFD simulation were carried out in this paper. The hydraulic axial thrusts of two operation conditions show different trends in early stage. In later stage, results show similar characteristics with load-rejection process of turbine mode. The components of Fz
including the crown, band and blade reach an obvious peak value with unit operation status change. This study provides the reference for the safety and stable operation of pump storage power stations.
Electricité de France Hydro’s overview of the First Israeli Hydro-Electric Pumped Storage Power Plant: GILBOA PSPP
Authors: Maruzewski P. , Sautereau T. , Fauriel D. , Drommi J.-L. , Ciocan G. , Laffly E. , Sapir Y. , Barak H.
Published: June 2021

The Israeli Public Utilities Authority (PUA) has decided to increase the instantaneous power available on the grid by adding Pumped Storage Power Plants (PSPP) to the existing generation capacity. PSPP Investments Ltd. is a private investor that has decided to develop the GILBOA PSPP. The capacity of the GILBOA PSPP is 300MW (2x 150 MWe). The project performance has to comply with PUA regulation for PSPP, and with all relevant Israeli laws and standards. The GILBOA PSPP is located at the Gilboa Mountain, in the north of Israel, near the town of Beit She’an (about 120 km North of Tel Aviv). As during IAHR 2016, EDF Hydro presented different items from feasibility studies to installations on site including model tests and manufacturing process. This current paper will give an overview of the GILBOA PSPP through short descriptions of EDF Hydro feedback during the end of the installation, during commissioning and during meetings for preparation of acceptance tests with Israeli Electric Company. The main equipment from the 2 units (reversible Francis pumps-turbines, motor-generators and associated auxiliaries) to the hydraulic gates as well as units ‘control and protection systems are provided by GE Renewable Energy which will also ensure Operating and Maintenance of the PSPP. GE Renewable Energy has introduced several technical innovations that will be presented in this paper as for instance, a dedicated trolley for the installation and the maintenance of the main inlet valves, or the thrust bearing with composite material PolyPadTM. The GILBOA PSPP is capable of connecting on IECo grid in approximately one minute.
Low-frequency oscillations in hydroturbines caused by cavitation together with the phase transitions effects
Authors: Kuibin P A
Published: June 2021

Among the unsteady phenomena taking place in hydroturbines one can separate oscillations caused by a cavitation bubble behind the runner. Usually a concept of cavitation compliance is used to model the cavity pulsations (Chen et al.). There was demonstrated the destabilizing effect of the diffuser and swirl. Kuibin et al. had found that the effect of swirl on stability is much smaller than that described by Chen et al. In the present work, the same analysis is repeated, but taking into account influence of the evaporation and condensation on dynamics of the cavity appearing in the draft tube. During analysis a simplified spherical cavity form was considered.
Modal Decomposition of Large- and Small-Scale Cloud Cavitation
Authors: Hatzissawidis G , Ludwig G J , Pelz P F
Published: June 2021

Cavitation clouds cover a frequency ω and wavenumber k spectrum limited by an upper bound due to the span of a blade or the diameter of a nozzle. The lower bound is given by the size of the eddies of a turbulent flow. Interpreting cavitation clouds as ring vortices and the formation as a challenge between re-entrant jet and asymptotic sheet growth, this becomes clear. To investigate the temporal and spatial contain of cavitation clouds, experiments were conducted on an unmodified and a modified hydrofoil with an artificial roughness i.e. an obstacle. From the high-speed measurements, we observe two cavitation regimes, (i) periodic large-scale cloud cavitation for the modified and (ii) small-scale cloud cavitation for the unmodified hydrofoil. For the latter, a coherent pattern is not visible to the naked eye. Sparsity-Promoting Dynamic Mode Decomposition is applied to the high-speed measurements to capture the dominant coherent structures from the complex flow field. This allows us to identify the underlying physical mechanism leading to cloud detachments. Within the paper we give the temporal frequencies of cloud detachments in both observed cavitation regimes.