bluuebunny/crossref_metadata_2025_split huggingface dataset:
Numerical investigation of the impact of cavitation on the pressure fluctuations in a Francis turbine at deep part load conditions
Authors: Wack J , Grübel M , Conrad P , Von Locquenghien F , Jester-Zürker R , Riedelbauch S 
Published: October 2022

Abstract
Hydropower plants can play an important role to stabilize the electrical grid when the share of volatile renewable energies increases. This goes hand in hand with a more flexible operation of the turbines and longer operation in off-design operating points. The increased pressure fluctuations that occur at off-design conditions may reduce the lifetime of the turbines. Consequently, this should be considered within the design process to develop efficient but also robust turbines. To derive suitable simulation setups that can be used in the design process, it is crucial to have a deep understanding of specific flow phenomena. As pressure fluctuations are of main importance in terms of fatigue, it is important that simulations can capture them well. Within this study, a deep part load operating point of a Francis turbine at model scale is investigated. By using three different meshes the influence of mesh refinement on the predicted pressure fluctuations is analyzed. Furthermore, by means of single- and two-phase simulations it is investigated how the pressure fluctuations are affected by the occurrence of cavitation. The results show only minor differences between the different meshes, whereas significant differences are observed between a single- and two-phase treatment.
How angular-momentum flux controls Francis turbine full-load surge
Authors: Dörfler P K , Wack J , Riedelbauch S 
Published: October 2022

Abstract
Hydropower plants equipped with Francis turbines may enter unstable conditions at high turbine discharge [13], with strong pulsations of power output and internal pressure. The cause are oscillations of a vapor-filled cavity in the vortex flow downstream of the turbine runner, interacting with the draft tube and penstock pressure. The paper describes the structure and simulation results of a 1D distributed-parameter model for full-load surge, parametrized from results of a high-resolution unsteady two-phase CFD simulation study (Wack [14]) of high-load pulsations in the reduced-scale model of a medium-specific speed Francis turbine. In earlier research on the same model turbine, Müller [12] had suggested that runner blade cavitation could cause instability. To clarify this, variation of turbine intake flow was sup-pressed, and the influence of pressure variation via runner blade cavitation on the relative runner exit flow angle and angular-momentum flux was simulated. Due to the upstream boundary condition, the influence of mass-flow gain χ is replaced by a cavitation gain factor ψ, likewise delayed by the limited speed of swirl propagation. It is found that gain factors obtained from steady-state simulation cannot be directly used with a lumped-parameter model because the swirl effect is much reduced due to its phase changes along the vortex; using the steady-state gain factors would thus exaggerate the influence of swirl on cavity volume. Simulation in frequency domain with realistic swirl transport delay confirms that instability can occur because in some parts of the cavitation zone the swirl provides oscillation power.
Identification of the precessing vortex core in a hydro turbine model using local stability analysis and stochastic modeling
Authors: Litvinov Ivan , Sieber Moritz , Oberleithner Kilian 
Published: October 2022

Abstract
Stochastic modeling and local linear stability analysis (LSA) is employed to predict the onset of the precessing vortex core (PVC) in the hydro turbine model. The method of the stochastic modeling based on the pressure fluctuation signals correctly predicts the instability of the azimuthal mode m = 1 at flow rates below 0.7Q

c
. This is in line with local LSA that shows that the azimuthal modes m = 1 and m = 2 are absolutely unstable below the flow rate of 0.7Q

c
. The absolute instability of mode m = 2 is a new observation in the part load regimes of hydro turbines and plays a significant role in the dynamics of the PVC. As demonstrated in this paper, local LSA and stochastic modelling are both methods to uncover the driver of the PVC using sparse experimental data stemming from either spatially resolved but non-timeresolved PIV snapshots or single-point time-resolved wall pressure recordings, respectively. This makes these methods suitable to be applied to configurations of industrial relevance.
On the existence of adverse pressure gradient in decelerated swirling flow and its suppression by water jet injection
Authors: Urban Ondřej , Rudolf Pavel 
Published: October 2022

Abstract
The paper revisits the theory originally published by Hall that explains the presence of adverse pressure gradient at the axis of a diffuser with swirling flow. It presents the fields of the axial pressure derivative obtained by CFD simulations of flow in a swirl generator, which support the theory. The last part brings cautionary results regarding the suppression of the backflow caused by this adverse pressure gradient by water jet injection. On a case with a strong swirl intensity leading to a strong vortex rope, it is shown that the injected jet may be deflected to the outer walls and rotate with the outer swirling flow, eventually leading to stronger pressure pulsations than in the original state. Further investigation shows that high enough diameter of the jet is needed to ensure its stability.
Measurement of sediment particle size distribution and shape using dynamic imaging analysis
Authors: Arora N , Kumar A , Singal S K 
Published: October 2022

AbstractHydropower plants operating in fragile mountainous regions face severe sediment erosion of hydraulic turbines, resulting in decreased efficiency, frequent power interruptions, and downtime for maintenance. The sediment size and shape are detrimental to the hydraulic turbines and influence sediment erosion. Sediment management is a crucial aspect of tackling sediment erosion in hydraulic turbines. The traditional method of measuring particle size and shape is time-consuming, labour-intensive, and usually requires statistical analysis. The present study applies the recent technique of dynamic imaging analysis to determine the particle size distribution and shape of sediment. Five different silt-sand mixtures have been used to check the ability of dynamic imaging analysis. Further, the repeatability and reliability of the results were checked by carrying out five repeated measurements of each sample under the same condition in a short period of time. This study will help the researchers and sedimentologists to adopt the dynamic imaging analysis technique for measuring particle size and shape and help the instrument developer in designing a better future version of the instrument.
Problem Identification and Condition Monitoring status of Nepalese Power Plants
Authors: Sapkota P , Chitrakar S , Neopane H P , Thapa B 
Published: October 2022

Abstract
Condition monitoring is the process of detecting faults in machine components with the help of measurement & analysis of pre identified parameters in a hydro power plant. This paper presents the different problems identified in the selected hydropower plants owned by Nepal Electricity Authority. The power plants have been selected on the basis of type of turbines and total power output. They include all types of turbines that has been in use in Nepal i.e., Pelton, Francis and Bulb turbines. Similarly, they have been selected in accordance to power output i.e., from 1 MW to 144 MW. The questionnaire has been developed and surveyed for the identification of problems in these hydropower plants as well as status of measurement devices. The paper presents the status of different measurement processes, instruments and sensors being used in the selected hydropower plants. The paper further presents possibly of condition monitoring and fault detection process that can be implied in these power plants.
Lattice Boltzmann simulation of the ERCOFTAC pump impeller
Authors: Roos Launchbury D , Casartelli E , Mangani L 
Published: October 2022

Abstract
The analysis of unsteady flows is becoming more and more important in the field of hydraulic turbomachinery applications, both for stable and especially for unstable operating conditions and other transient phenomena. Unsteady Reynolds-averaged Navier-Stokes (RANS) methods for incompressible flows, implemented via finite volume or finite element approaches, reach their limit in both accuracy and acceptable computational time. The accuracy mainly suffers due to the strong dependence of the solution on turbulence model variants. Furthermore, incompressibility of the flow and the linked implicit pressure equation makes large eddy simulations (LES) of Navier-Stokes-based methods for such flows prohibitively expensive. An alternative to Navier-Stokes fluid solvers are methods based on the solution of the discrete Boltzmann equation, so-called Lattice Boltzmann Methods (LBM). The purpose of this work is to present first results of an ongoing project involving the simulation of unsteady hydraulic turbomachines at high Reynolds numbers. For this purpose, the ERCOFTAC pump impeller was selected as a test case. Results obtained with the current implementation of the in-house code suggest, that the present methodology could provide a viable alternative for the analysis of unsteady flow phenomena in hydromachines.
Multidisciplinary optimization of a Francis turbine runner
Authors: Tengs E , Charrassier F , Jordal M , Iliev I 
Published: October 2022

Abstract
A fully automated multi-disciplinary design optimization procedure for a Francis turbine runner has been developed in a previous task of the Horizon2020-HydroFlex project. The design optimization was limited to blade design, with the goal of improving the hydraulic efficiency and torque, and reducing the harmonic structural stresses. This is to ensure that the turbine is less prone to fatigue, but still performs well hydraulically. Results from the numerical optimization are presented in this paper. From the design optimization, two runner designs are highlighted. One that performs significantly better than a reference design, and one that performs significantly worse. It is observed that small, but significant improvements can be obtained in both torque and efficiency, while at the same time reducing the structural stresses drastically. This shows that there might be previously unknown areas in the design space that can be explored, especially on the structural side.
Numerical simulation of transient processes for pumped-storage power station with fixed and variable speed units
Authors: Cao R , Guo W C , Liu Y , Wu F L , Bai Q J 
Published: October 2022

Abstract
The pumped-storage power stations (PSPSs) with variable speed units (VSUs) have been emerging in recent years, and the research on the transient processes of those PSPSs is of great significance. In this paper, the transient processes of a PSPS with four units, i.e. three fixed speed units (FSUs) and one VSU, sharing one tunnel are numerically simulated under the conditions of large fluctuation and small fluctuation. Firstly, combining the calculation method of hydraulic transient processes and the basic equations of pump turbine, doubly-fed induction motor (DFIM) and governor, a hydraulic-mechanical-electrical coupling model of PSPS is established. Secondly, the data of an actual PSPS is substituted into the model and the corresponding parameters are set. Then, the transient processes of typical conditions of large fluctuation and small fluctuation are simulated respectively. Finally, according to the numerical simulation results, the characteristics of the transient processes of PSPS with three FSUs and one VSU are analyzed. The results show that, under large fluctuation conditions, whether the initial operating states of FSUs and VSU are consistent or not, the transient processes are similar. Under small fluctuation conditions, the VSU can adjust the active power to meet the grid demand more quickly than the FSUs.
Optimization of guide vanes for variable speed Francis turbines – Human controlled vs. automated approach
Authors: Stojkovski Filip , Markov Zoran 
Published: October 2022

Abstract
Guide vanes of hydraulic turbines can maximise turbine efficiency and directly influence on turbine operating characteristics. A hydraulic analysis is carried out on geometrically parameterised guide vane foils, to maximise efficiency and potentially observe which configuration gives operating range expansion. Variable-speed operated turbines in hydropower plants tend to run with improved performance at off-design operating points due to the speed variations. A combination of variable speed turbine operation and guide vane shape optimization gives a win-win situation in water to energy usage ratio. Human controlled developed designs and CFD calculations were previously done. The obtained results are compared with automated approach in ANSYS Workbench using Design of Experiments. The numerical tests were made on parametrically developed guide vane cascades, where the turbine efficiency is the evaluated parameter and it is observed in the range of several rotational speeds and calculated flow rates. The cascade design which produces less turbine efficiency deviations for the tested rotational speed range is later exported and hydraulically and geometrically compared with the developed design from the manual approach. The tested configurations of the guide vanes are developed on the basis of Francis 99 turbine model from the Waterpower Laboratory at the Norwegian University of Science and Technology in the existing geometric space for the guide vanes and blades axis positioning circumference.
An engineering approach for the prediction of fatigue damage in the part load regime
Authors: Von Locquenghien F , Leibing B 
Published: October 2022

Abstract
Due to the volatile electricity received from solar and wind power plants, the energy market requires highly variable hydropower machines which are able to cope with fast changes of their operating regimes. With the need for flexible operation, Francis runners are exposed to various operating conditions outside the traditional operating range. Hence, the runners must be specifically designed for long time part load operation to meet hydraulic and structural requirements. Therefore, it is inevitable to have a reliable but also practical engineering approach in the design phase of the projects in order to ensure sufficient fatigue life of the Francis runners. Deep part load operation mainly consists of stochastic loads acting on the runner blades. Previous publications show that it is reasonable to normalize the Rainflow matrix of stress amplitudes to represent the characteristic behaviour at deep part load operation. Hence, it is feasible to utilize the normalized Rainflow matrix to predict the fatigue damage of Francis runner designs without prototype measurements. Within this paper, a scaling method to derive project specific fatigue results for deep part load is introduced and subsequently validated through an intercomparison of available strain gauge measurement data and prediction. Based on the results, the accuracy of the proposed engineering approach and possible impacts of different boundary conditions are discussed.
Corrosion fatigue investigations on cast, forged and welded nickel-martensitic steels for hydro turbines
Authors: Leibing B , Von Locquenghien F , Rückle D , Schellenberg G 
Published: October 2022

Abstract
Facing an increased demand for flexible operation of hydro power plants (HPPs), Francis runners are substantially subjected to operating conditions beyond the traditional operating ranges. In order to ensure a reliable service of major HPP components covering their complete service life, a fundamental understanding of both load and material fatigue behavior is essential. Due to the cyclic loading in water, reliable corrosion fatigue data is required for the runner fatigue assessments. In previous publications, a state-of-the-art corrosion fatigue testing campaign within a collaborative German research project was described and results for a common cast steel used for the manufacturing of hydro runners were presented. The material testing program for the 13 % Chromium and 4 % Nickel steel has advanced since then and important aspects beyond varying water qualities and different stress ratios have been investigated. Within this conference contribution, corrosion fatigue investigations that were carried out on cast, forged and welded nickel-martensitic steels are described and recently gained results are presented.
PZT actuators as on-board instruments to reduce vibrations and strains in submerged structures
Authors: Sánchez-Botello X , De La Torre O , Roig R , Costa-Castelló R , Escaler X 
Published: October 2022

Abstract
This paper presents the design and evaluation of an active control system to reduce vibrations and strains of submerged structures. For that, a metallic disk mounted in a test rig has been equipped with on-board waterproof strain gauges and accelerometers, to measure its structural response, and with an on-board PZT actuator patch, to provide the required control force. This control force is computed in real-time by an optimal algorithm that determines the exact voltage to be supplied to the PZT patch in order to mitigate any increase of vibrations and/or strains detected by the sensors. In order to capture the dynamics of the structure when it is excited both in air and submerged in water, a plant model has been identified from the Frequency Response Functions (FRFs) obtained experimentally from the measured vibrations and numerically from simulated strains. With these plant models, two different control techniques based on a Linear Quadratic Gaussian (LQG) controller and on an H

∞
controller have been implemented and simulated under different types of excitations. The obtained results are satisfactory and support the need to continue the research to validate them experimentally.
Solving the challenging puzzle of the Grimsel II runner vibration
Authors: Titzschkau M , Dörfler P 
Published: October 2022

Abstract
After 30 years of operation, the runners of the four 100 MW high-head Francis turbines successively developed fatigue cracks starting from the hub-side trailing-edge fillets. Assessment of alternating stress in normal turbine operation revealed that stress levels were not nearly large enough to explain the damages. Consequently, a test campaign comprising start and stop manoeuvres in addition to normal turbine operation was conducted in 2017. Strain gauge signals obtained on-board of the runner blades were recorded together with casing vibration and acoustic signals. Intense high-frequency vibration centred around 611 Hz was detected in three transient conditions of operation: at speed-no-load (SNL), during coast-down and during back-to-back pump start. Common properties of these conditions were very low to no discharge, strong secondary flow in the runner and speed between 50 and 100 %. Due to the extremely narrow bandwidth, three possible ways of excitation were considered and checked: rotor-stator interaction (RSI), rotor-dynamic instability and Von Kármán excitation. RSI was excluded due to speed-independent frequency. Strong sidebands in the stationary-frame frequency spectra - obviously caused by runner shroud deformation shapes ND1 through ND5 - excluded any feedback from runner side chamber pressure; thus leaving Von Kármán excitation as the only possible diagnosis. Accordingly, a very safe profile was selected and implemented for the runner blade trailing edges. Subsequent field-testing at two modified units confirmed that the vibration problem at the harmful operation points had hereby been eliminated.
Numerical Simulation and Experimental Verification of Downstream Fish migration in a Bulb turbine
Authors: Benigni H , Schneider J , Reckendorfer W , Schiffer J , Tuhtan J , Leithner S , Zenz G , Meusburger P 
Published: October 2022

Abstract
Downward migrating fish can be damaged when passing through turbines in the direction upstream to downstream. The overall influence on the fish population is not known yet. In recent decades, many publications with steady-state CFD calculations for the passage of fish have been published. However, transient CFD calculations show that the range in which fish pass the runner itself is not easily predictable but rather random, influenced by the current position of the runner, thus affecting the probability of survival. The pressures, which are also well represented by steady-state CFD calculations, are compared by applying “Live Fish” (typical types for the middle of Europe) as well as “Barotrauma Detections Sensors (BDS)” which were exposed to the area of the turbine intake at different heights and monitored during downstream migration.
The Semi-Air-Cushion Surge Tank: Design Principles and Case Study
Authors: Richter W , Zenz G , Vereide K 
Published: October 2022

Abstract
Surge tanks are required to enable power and frequency governing of hydraulic machines in hydropower plants with pressure tunnels. To improve the hydraulic performance and reduce the construction costs, a new concept of semi-air-cushion surge tank (semi-ACST) is proposed. The semi-ACST can be particularly effective for upgrading and retrofitting of existing hydropower plants, when additional hydraulic machine capacity is added to an existing waterway. The key design element is a crown throttle, constructed as an inverted weir placed in the crown of the chamber. The purpose is to intentionally trap an air pocket during filling of the lower chamber. The air is released slowly through defined air pipes so that the air in practice contributes equivalent to added volume in the upper chamber. Thus, the semi-ACST improves the dampening of mass oscillation, without increasing the surge tank volume. The semi-ACST has been investigated and developed with multiphase 3D CFD simulations with RANS turbulence modelling. The design principles have been tested on the case-study Tonstad hydropower plant (960 MW). In the presented case study, the semi-ACST is proposed as an extension of one of the three exiting hydraulically coupled surge tanks.
1D numerical study of a multistage centrifugal pump during transient operation
Authors: Alerci Aldo , Vagnoni Elena , Nicolet Christophe , Hugo Nicolas , Paolone Mario 
Published: October 2022

Abstract
Hydropower plants (HPPs) are acknowledged to be fundamental assets to provide generation flexibility to power systems hosting substantial amount of stochastic renewable energy sources. Within this context, providing this flexibility enforces HPP units to cross-over transient operating conditions or to operate in off-design conditions. The evaluation of the impact that such transients have on the hydraulic machines is a fundamental step to assess the operating costs in terms of lifetime reduction of the machine structural integrity.
When these assessments are performed on experimental test-rigs featuring a machine’s reduced scale models, testing transient regimes may potentially trigger harmful pressure profiles along the piping system as well as resonant phenomena on the test-rig. Simulating how the several state variables evolve during the experimental tests allows for the prediction of damaging circumstances and, eventually, prevent performing experimental tests in detrimental conditions.
In this regard, this paper illustrates a numerical method able to reliably reproduce the experimental test-rig behavior while preserving the consistency of the scaled similarities with respect to a specific case study HPP: the Veytaux I pumped-storage hydropower plant located in Switzerland.
Moreover, in this paper, the developed 1D numerical model of the EPFL Technological Platform for Hydraulic Machines experimental test-rig is presented since some of the characteristic transient sequences of the studied HPP are simulated on this model and compared to the prototype scale 1D numerical results.