bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimization of Francis Turbine Runner for Variable Speed Operations with minimization of sediment erosion
Authors: Ghimire Amul , Gautam Saroj , Chitrakar Sailesh , Neopane Hari Prasad
Published: October 2022

Sediment erosion is one of the most notorious phenomenon damaging the turbines, in the Himalayan region. Francis turbine, which is one of the most used type of hydro turbine in the world as well as in Nepal, is affected quite severely, more so when operated in off design conditions. This study has taken a reference design of a Francis turbine from a hydropower in Nepal to optimize it for operation in variable speeds. Minimization of sediment erosion and maximization of efficiency are taken as the objective functions of the optimization, for which blade angles at trailing edge and blade angle distribution are taken as the design variables. The design space of the runner are constrained such that the optimized design could replace the existing runner in the turbine. Latin Hypercube Sampling technique is used to populate the design space such that the design variables are divided randomly to create required number of designs in the design space. Computational Fluid Dynamic analysis are performed on simplified numerical models of the samples to predict their performance and Sediment Erosion Rate Density (SERD), under various operating conditions. The results of output parameters, obtained from CFD, along with the design variables, are used to develop an approximation model relating the objective functions with the design parameters. NSGA-II optimization technique is used to search for the optimum design. The paper presents the comparison of the sediment erosion and efficiency of the reference runner and optimized runners under various operating conditions. It also presents an outline of the process used to optimize the runner for variable speed operation with minimum sediment erosion.
A Simulation Study of Hydraulic Vibration caused by Clearance Flow in a Pump Turbine
Authors: Hu H P , Xia M , Qiao M , Wang W , Wang J Z , Wang Z W
Published: October 2022

The pumped-storage power station needs to adjust the load to meet the needs of the power grid at any time and cannot always operate under the best working conditions. In practical operation, vibration phenomenon with complex inducements occasionally occurs. In the stable operation of the Baishan energy storage power station, the headcover vibrates obviously. To find the cause of vibration, this paper modeled the unit flow area, used the numerical simulation method for steady and unsteady calculation, extracted the pressure fluctuation information in the flow field, and then analyzed the possibility of strong vibration caused by the instability of the flow field. The results show that there is a low frequency component of pressure fluctuation caused by the spiral motion of the fluid in the sealing groove in the upper canopy clearance flow, which may be the inducer of the vibration of the headcover.
Evaluation of startup time for a model contra-rotating pump-turbine in pump-mode
Authors: Fahlbeck J , Nilsson H , Salehi S
Published: October 2022

A larger part of the electricity is today from intermittent renewable sources of energy. However, the energy production from such sources varies in time. Energy storage is one solution to compensate for this variation. Today pumped hydro storage (PHS) is the most common form of energy storage. Usually, it requires a large head, which limits where it can be built. In the EU project ALPHEUS, PHS technologies for low- to ultra-low heads are explored. One of the concepts is a contra-rotating pump-turbine (CRPT). The behaviour of this design at time-varying load conditions is today scarce. In the present work, the impact of the startup time for a CRPT is analysed through computational fluid dynamics (CFD) simulations. The analysis includes a comparison between a coarse and a fine CFD model. The coarse model produces acceptable results and is 50 times cheaper, this model is thus used to assess the startup time. It is found that longer startup times generate lesser loads and peak values. A startup time of 10 s may be a sufficient alternative as the peak loads are heavily reduced compared to faster startups. Furthermore, there is not much difference between a startup time of 20–30 s.
Experimental Analysis of a Centrifugal pump in pump mode and turbine mode
Authors: Pokharel N , Sapkota P , Ghimire A , Thapa B S , Thapa B
Published: October 2022

The national grid of Nepal faces several hurdles during its operation, especially in remote areas of hilly and mountain region. Citizens face blackouts of days to weeks, due disturbances caused in the grid in those regions. Micro hydro can be a very strategic backup for the national grid in those regions. However, due to lack of subsidy, which used to be provided in the past, for development of micro hydro, micro developers are unable to afford the construction cost of the micro hydro. Thus using Pumps that are readily available in the market at a much cheaper price, compared to custom built hydro turbines, as turbines can be a very economical alternative for such cases. But the concept of pump as turbine has not been utilized in Nepalese market. In this study, a centrifugal pump abundantly available in the Nepalese market has been used to evaluate its performance in turbine mode as well as in pump mode. A characteristic performance curve of the pump operated in turbine mode is obtained from the experimental data. The Best efficiency point and the operating regime is also determined based on the results obtained from experiments. The head conversion factor and discharge conversion factor calculated based on the data measured from the experiment, at various operating conditions, are compared to previous works done by other researchers. The performance obtained from the experimental analysis is comparable to the performance of the turbines installed in the micro hydro power projects. PAT can be a game changer in the micro hydro sector, by providing a very economical and technically viable option of expensive hydro turbines.
Parametric design and multi-objective optimization on blade high-pressure side of a pump-turbine
Authors: Qin Y L , Li D Y , Wang H J , Liu Z S , Wei X Z , Wang X H
Published: October 2022

High pressure side, as the runner outlet at pump mode and runner inlet at turbine mode, playing an important role in controlling hydraulic loss and flow characteristics in vaneless region, affecting both steady characteristics and unsteady characteristics. While when it comes to three-dimensional inverse design of a pump-turbine blade, scholars are forces on the design method in runner flow channel, ignoring the geometry profile of high-pressure side. Hence, in present paper, concepts “swept”, “bowed (lean)” and “twisted” are innovatively introduced, eight new parameters are proposed to control the profile of blade high pressure side. And then a multi-objective optimization design system consisting of geometry generation, computational fluid dynamics, design of experiment, approximation model, multi-objective genetic algorithm, and self-organization map is built. Efficiency at both pump mode and turbine mode and S margin at turbine mode are selected as optimization objectives in the first optimization step. Then, the hump margin is regard as the objective in the second optimization step. Based on the two-step optimization, a runner with optimized high-pressure side is obtained and CFD results show that compared with the original runner, the efficiency at rated points and the margin of unsteady characteristics can be increased. The parametric design method on high-pressure side presented in our paper can be regard as an essential supplement to the traditional design method in hydraulic machinery.
Pressure pulsation of a single-stage storage pump with 1300 m delivery head
Authors: Benigni H , Bodner C , Leithner S , Kirilowitsch C , Giefing D , Penninger G , Giese M , Jaberg H , Meusburger P
Published: October 2022

A new storage pump replaced 3 multistage pumps to transport water between the Kreuzeck and the Reisseck power plant sides. Located in the valley of the two reservoirs, it provides impressive performance data for single-stage operation at n = 3000 rpm. Hydraulics already utilised for significantly lower heads are the basis of this pump. The pump was adapted to the new location, and CFD-based development was applied. Within the scope of a model test, the performance data (Q-H-Eta and Q-H-Sigma) were to be confirmed and measured, including the inflow and outflow situation. In addition, the pressure pulsations had to be verified. Finally, installing air vessels should reduce pressure pulsations on the suction and pressure sides. Furthermore, modifications in the bladed areas of the impeller and the guide vane section should enhance this reduction.
Mesh dependence analysis for simulating unsteady cavitation around a plane convex hydrofoil
Authors: Puga D , Escaler X , Hidalgo V , Luo X
Published: October 2022

The mesh significantly influences the quality of the numerical results in computational fluid dynamics (CFD) problems due to its correlation with turbulence models. In the present paper a structured mesh was generated using Gmsh and two semi-structured meshes were generated using snappyHexMesh to determine the suitable mesh distribution for simulating unsteady cavitation around a plane convex hydrofoil. The numerical simulation was conducted by using the software OpenFOAM with the k-ω SST SAS turbulence model and the Zwart-Gerber-Belamri (ZGB) cavitation model. Besides, the experimental results obtained by the Laboratory for Hydraulic Machines of École Polytechnique Fédérale de Lausanne were used to validate the numerical results. The results showed that both structured and semi-structured meshes predicted the cavitation pattern and the maximum cavity length. The semi-structured mesh with suitable refinement reproduced in detail the dynamic behavior of unsteady cavitation, while the structured mesh efficiently reproduced the phenomenon.
Draft tube guide vane system to mitigate pressure pulsations
Authors: Joy J , Raisee M , Cervantes M J
Published: October 2022

The present study introduces the concept of mitigating pressure pulsations in a hydro-turbine draft tube. The concept refers to using an adjustable guide vane system in the draft tube. The adjustability relates to its ability to rotate around an axis. The test rig for the experimental study is a high-head Francis model turbine. Three sets of guide vanes are distributed evenly circumferentially in the draft tube. Each guide vanes consists of two hydrofoils. The upper hydrofoil can move around an axis. The lower hydrofoil is fixed. The turbine operating head for the experiments was 12 m. The operating condition considered is at part load, for Q/Q

BEP
= 0.71. The results indicate that using the guide vanes in the draft tube, the plunging mode of the rotating vortex rope becomes insignificant for nearly all upper hydrofoil configurations considered. The reduction in the rotating mode of the vortex rope is between 50% and 80%. The vortex rope frequency shifts from 0.307·f

0
and varies between 0.33·f

0
to 0.617·f

0
, which is a function of upper hydrofoil angles
Global linear stability analysis of the flow inside a conical draft tube
Authors: Mitruţ R , Bucur D M , Dunca G , Cervantes M J
Published: October 2022

The paper presents the numerical simulations of the flow inside the draft tube of Francis-99 turbine at the part load (PL) operating condition. The rotating vortex rope (RVR) is a phenomenon that occurs during the PL operating regime inside the draft tube of hydraulic turbines. To reduce the computational cost, the numerical simulations are carried out in two steps. Firstly, steady state numerical simulations are performed in a reduced geometry of the runner which is made of a runner passage and part of the draft tube. The velocity profiles from the steady state simulation are used as a boundary condition for unsteady numerical simulation on the inlet of the full draft tube geometry. The velocities from the numerical simulations are time-averaged over a period of 5 RVR rotations and validated with the experimental velocities averaged over the same period. Further, a two-dimensional (2D) linear global stability analysis is performed on a plane extracted from the cone of the draft tube using the time-averaged flow. The frequency of three-dimensional (3D) flow simulation and of the 2D stability analysis are found to be in good agreement with the experimental frequency.
Impact of runner crown shape modifications on the onset of the precessing vortex core
Authors: Lückoff F , Naster M , Müller J S , Sieber M , Litvinov I , Oberleithner K
Published: October 2022

In this experimental study, a passive flow control concept is provided to extend the stable part load operation regime by shifting the rise of the precessing vortex core (PVC), also known as vortex rope, toward lower mass flows respectively deeper part loads. A parametrized runner crown design working as a passive flow control device is derived. This control device aims for shifting the bifurcation point of the PVC to lower flow rates. To determine the most influential design parameters and derive an optimized runner crown design, a design of experiments (DoE) approach is used. This DoE approach is based on data obtained from differential pressure sensors inside the draft tube wall of a generic hydro turbine test rig using air as working fluid. By means of stochastic modeling, the growth rate of the PVC mode is derived from the statistics of the measured pressure signals. The growth rate is used to estimate the bifurcation point of the PVC characterized by a certain normalized flow rate. It is shown that the stable part load operation regime is extended by up to 25% due to the passive impact of the modified runner crown. Moreover, the operational range featuring considerable PVC-induced pressure oscillations is diminished and the pressure recovery of the draft tube is improved compared to the baseline case.
On the relevance of the runner crown for flow instabilities in a Francis turbine
Authors: Müller J S , Lückoff F , Kaiser T L , Oberleithner K
Published: October 2022

The precessing vortex core (PVC), also known as vortex rope, in a draft tube of a Francis-99 hydro turbine is investigated. The goal is to increase our comprehension of the root of the PVC in order to attenuate or suppress the PVC, thus extending the stable operational range below the best efficiency point at part load conditions. Unsteady Reynolds-averaged Navier– Stokes simulations are conducted and used as a basis for all the analyses performed in this work. The discrete Fourier transform (DFT) and the spectral proper orthogonal decomposition (SPOD) as data-driven methods and the linear stability analysis (LSA) as a physics-based, operator-driven method are used to examine the PVC in detail. With the DFT and SPOD, two dominant modes are found inside the draft tube. Likewise, the LSA reveals two distinct linear instabilities of single-helical and double-helical shape, which agree with the findings of the SPOD in terms of spatial shape and temporal frequency. A particular focus is laid upon the region upstream of the draft tube. An adjoint-based sensitivity analysis reveals that both instability modes are highly sensitive to mean flow modifications inside the transitional segment between runner and draft tube, such as induced by passive control devices. The knowledge of these sensitivities will guide to an optimized runner and draft tube design for controlling the PVC and the double-helical mode.
Vortex rope interaction with radially protruded solid bodies in an axial turbine: a numerical study
Authors: Holmström H , Sundström J , Cervantes M J
Published: October 2022

Radially protruded solid rods and their interaction with the rotating vortex rope at part load condition are investigated numerically on an axial model turbine.

The commercially available software ANSYS CFX was used to perform the simulation, and the test case was the Porjus U9 Kaplan turbine model operating at a fixed runner blade angle at part load condition. Four rods, with a rod diameter equal to 15% of the runner diameter were evenly distributed in a horizontal plane in the draft tube cone and protruded to a length set to intercept the RVR in its unperturbed trajectory. It is shown that the RVR plunging (synchronous) mode is completely mitigated upstream and downstream of the protruded rods. The RVR rotating (asynchronous) mode is reduced by 47% and 63% at the two monitor positions located upstream of the protruding rods, while only a minor reduction occurs to the first RVR harmonic at the monitor positions located downstream of the protruded rods. The perturbed RVR experiences an increased angular velocity due to the flow area decrease caused by the protruding rods, thus increasing the RVR frequency by approximately 53% compared to the unperturbed value. Investigation of the swirling flow indicates a locally increased swirl in the center of the draft tube downstream of the protruded rods which could explain the reduction of the RVR pressure amplitude. The overall turbine efficiency with solid rods protruded causes a marginally efficiency reduction of 0.85%. However, as the RVR pressure pulsations are reduced significantly, a more comprehensive investigation of the rods impact on the turbine performance and life time should be performed to elucidate the suitability of using solid rod protrusion for RVR mitigation.
Application of wireless measurement technology in hydraulic turbine test
Authors: Deng Xiangping , Wang Xiuling , Liu Zhiyuan
Published: October 2022

At present, the measurement of internal characteristics of hydraulic machinery runner (blade pressure pulsation, pressure field and dynamic stress) and blade torque is difficult to be realized by the existing wired measurement methods due to the measurement object is a high-speed rotating part. This paper will introduce the system composition and application in hydraulic machinery of the wireless measurement technology one by one. In the measurement of model hydraulic machinery, it is mainly used to measure the rotating part. The sensors work in water, including measuring the internal characteristics of the runner (blade pressure pulsation, pressure field and dynamic stress) and blade torque. The wireless measurement technology that is by laying the corresponding measurement unit at the measured part, the signal sensed by the sensor is collected and stored by the node, and then wirelessly transmitted through the node, and the data is received through the wireless gateway and enters the computer acquisition system.
Comparison of fatigue loading in Francis turbine runners using extreme values interpolation
Authors: Pham Q H , Gagnon M , Page M
Published: October 2022

Extreme values of strain signals are important for hydroelectric turbine runners fatigue assessment. However, due to measurements limitations such as short data length and limited subset of measured operation conditions, the extreme values are rarely fully captured. Our study aims to estimate the extreme values of runner strain at non-measured operating conditions by interpolating the extreme components from the measured ones. The method is based on the peaks over threshold technique and the kriging interpolation. A case study with two similar Francis turbines (same design and power plant) is presented. The comparison is made between the use of independent interpolation models and a combined model for the two turbines. This helps assess the assumption that similar turbines share similar fatigue loading. If that is the case, a common model for the whole fleet of similar turbines design in a given facility could be considered, and thus contribute to reduce the uncertainties related to unmeasured loadings without the need for in-situ measurements on every runner.
Measurement and analysis on natural frequency of runner blade of hydraulic turbine by hammering method
Authors: Cao Dengfeng , Ding Jiaren , Zhou Ye , Jiang Cuiwei
Published: October 2022

Hammering method is a common solution to the measurement on natural frequency of solid components. Due to the difficulty to directly measure the natural frequency of a rotating runner in the water, numerical simulation is widely applied to predict the runner behaviours. To verify the influence coefficient of the natural frequency in the water than that in the air, specific measurements are taken on a model runner both in the water and air and a prototype runner by hammering method. Test results show that the ratio of natural frequency in the water to in the air lies in 0.692 ∼ 0.753 on the model runner, and 0.746 ∼ 0.812 on the prototype runner. Practice in this paper shows that hammering method is a helpful way to predict the natural frequency of runner blade in the water, then the operation risk of resonance may be avoided.
Approach to sediment monitoring in Hydro Power Plant
Authors: Almeida Vinícius , D’Agostini Neto Alexandre , Jung Alexander , Kratzsch Axel , Sozio Felipe , Gissoni Humberto , Papp Guilherme , Asencio Jaime Danus , Nieto Diaz Jose Manuel , Gonçalves Manuel , Garcés Octavio , Rohne Wolfgang
Published: October 2022

The presence of mineral sediments in water led to significant erosion damage to a power plant in the Chilean Andes. The plant is operated by Enel and is equipped with three vertical Francis power units, with approximately 25 MW each operating under a net head of around 120 m. The overall machine condition with reported significant damage on the turbine caused by erosion led to a modernization project with runner replacement and output and efficiency increase. In order to ensure a proper sediment measurement and quantification during the turbine operation, Voith developed, supplied and installed a sediment monitoring system in power unit #01. The monitoring system, developed using the OnCare.Health Hydro platform, applies optical sensors installed directly in the spiral case inlet section to measure the water turbidity. Other operational parameters are recorded, processed and stored together in the same database. Two optical sensors were installed in the spiral case inlet section and provide turbidity readings. The first one is positioned directly flush to the flow. The second one is located in a sampling pipe that collects water from four different points of the same cross section. An automatic water sampler installed in the sampling pipe collects water periodically, which is sent to a laboratory for particle analysis. The laboratory results allow an identification of important parameters such as sediments concentration, particle sizes and materials that are used for correlation with the indicated turbidities. The main data analysis shows that the power unit operated most of the time close to the rated condition. A comparison between turbidities indicated by both sensors shows an offset, probably associated to the particles distribution along the cross section. A system calibration process successfully correlated suspended sediment concentration and turbidity values to allow online indications of the sediment concentration. The system has been operating for almost two years in a close cooperation with the engineering teams of the plant owner and system provider. Main results and system findings for this application will be presented and discussed in this paper.
PIV measurements in the draft tube of a down-scale propeller turbine: uncertainty analysis
Authors: Sotoudeh Nahale , Shiraghaee Shahab , Andersson Robin , Sundstrom Joel , Raisee Mehrdad , Cervantes Michel
Published: October 2022

In this study, the flow in the conical section of the draft tube of a propeller turbine has been investigated at the best efficiency point and part-load operating conditions using 2D and stereoscopic 3D particle image velocimetry. Since the flow in the turbine is periodic, it is necessary to study the mean flow field rather than the instantaneous one to identify the flow characteristics from a statistical standpoint. However, the statistical convergence of the obtained mean velocity is questionable. Thus, the current work proposes a methodology for investigating the convergence of mean velocity profiles based on the central limit theorem. The methodology is applied to the best efficiency point and part-load results. The results show that 3D PIV results have lower uncertainty than 2D PIV results because measuring the tangential velocity component affects uncertainty, only measured in 3D PIV. The uncertainty difference is more significant, especially in part-load operation, due to the presence of the rotating vortex rope, and therefore a more accurate measurement is necessary to produce a reliable mean flow field. Furthermore, the convergence of the mean velocity profile is faster, with lower uncertainty for best efficiency point results since, at the part-load condition, the tangential velocity component of the flow is higher. In addition, the converged mean velocity profiles show a backflow region with minor rotation in the center, surrounded by a high rotational axial flow during the part-load operation of the turbine.
Research of Water Consumption Rate Measurement Based on Thermodynamic Method
Authors: Zhou Ye , Duan Lian , Cao Dengfeng , Li Ligang
Published: October 2022

As an important index which can be used to evaluate the economic operation result for hydropower station, the water consumption rate of hydropower unit can be acquired with hill chart curve of hydraulic turbine, the relationship curve between flow discharge of hydropower station and water level of reservoir. However, considering that the hill chart curve of turbine is calculated and converted by the model test, there are certain errors when used on the prototype machine, which also brings limitations to the applicability. According to the definition of water consumption rate, it can be determined by the unit’s net head and efficiency. The static water head of the unit can be obtained by measuring pressure for spiral case inlet and draft tube outlet. Therefore, the key issue for the measurement of unit water consumption rate is the measurement of turbine flow discharge. Therefore, according to the operating characteristics of high-head turbines, this article adopts thermodynamics to measure the flow discharge of turbine, studies the installation and implementation of high- and low-pressure section test instruments with the site condition of a hydropower project, and analyses of the water consumption rate of units under different conditions such as the multiple-unit’s operation, various water heads, etc. The analysis results show the relationship and trend for the water consumption rate and unit operation condition, which can provide references for the optimal dispatch operation of multi-units’ hydropower stations, achieving the purpose of reducing the water consumption rate of power generation and improving the efficiency of power generation.
Virtual sensors to generate turbine runner blade strains from indirect measurements
Authors: Gagnon Martin , Vouligny Luc , Cauchon Luc , Giroux Anne-Marie
Published: October 2022

Strain measurements on turbine blades are difficult and costly tasks. Such measurements, when carried out, generally only happen during the runner commissioning. This gives rise to two problems. The first is that some of the sensors often stop functioning properly during the measurement campaign, which leads to distorted data, and the second is that runner blade strains are not available for long-term monitoring after the measurement campaign. To alleviate the consequences of distorted or missing values, we propose the use of neural networks to automate the imputations of missing values in measurement campaign data using virtual sensors. Three types of network architecture are proposed: Long Short-Term Memory (LSTM) in different multi-stage/multi-layer configurations in Nonlinear Auto-Regressive Neural Networks with exogenous input (NARXNN), injector multi-scale attention network (Injector MA-Net), and a combined architecture using both. The performance of these architectures will be compared in four situations: the loss of strain gauge rosette branches; the loss of a complete strain gauge rosette; the loss of data on a complete blade; and the absence of strain data, which is related to the problem of identifying which sensors could be used for long-term monitoring. The performance of the proposed algorithms will be evaluated on real case scenarios from a measurement campaign during a recent unit commissioning.
Application of a Coupled Reynolds Stress Model to a Swirl-Driven Diffuser Flow
Authors: Del Rio A , Casartelli E , Mangani L , Roos Launchbury D
Published: October 2022

Swirling flow is a dominant feature in a significant number of technical applications. Hydraulic turbines at part-load are strongly affected by the related vortex rope phenomenon. Its dynamic behavior has a negative impact on the operating performance and durability of the machine.

CFD can be used to get additional insight in this complex phenomenon but requires a valid simulation model able to capture the relevant flow physics, which is driven by highly anisotropic turbulent structures. The simulation results are therefore strongly affected by the turbulence modeling. A swirl apparatus (AC6-14), for which extensive experimental data is available, is used in this work for the assessment and validation of different turbulence models. The state-of-the-art SST k-ω model, with and without curvature correction, is compared to a coupled full Reynolds stress model. All models are integrated into a pressure-based coupled flow solver.

The investigation revealed that both, SST k-ω with curvature correction and the full Reynolds stress model better predict the time-averaged velocity profiles in the diffuser compared to standard SST k-ω. The swirl component is thereby best captured with the Reynolds stress model. All models deliver a reasonable frequency spectrum for the dynamic behavior of the vortex rope. However, flow visualization shows that standard SST k-ω is not capable of predicting the shape and size of the vortex rope accordingly. Both, SST k-ω with curvature correction and the full Reynolds stress model, can be used in the future for more detailed flow investigations, which include also the assessment of flow control measures.
Comparison of mechanical stress in Pelton Buckets between simulation and measurement
Authors: Rentschler M , Weibel P , Isler P , Lais S , Gervais N
Published: October 2022

Thinner structures, lead to higher efficiencies, but also to higher mechanical stresses. The increase of the hydraulic performances of Pelton runners is hence limited by mechanical stress constraints. The design process of Pelton runner therefore includes stress and hydraulic assessment during profile development phase. This assessment is based on the combination of Finite Element Method (FEM) for the mechanical stress and computational fluid dynamics (CFD) for the hydraulic behavior of the machine. This combined approach enables the generation of hydraulic profiles with high efficiency while respecting defined limits for the stress. The validation of the combined simulation is achieved by comparison with prototype measurements. For several projects the runners have been equipped with strain gauges in the root and close to the cut-out to record the deformation for different operating points. The comparison with numerical simulation results shows some limitations of the approach but concludes with a satisfying agreement between numerical simulation and measurement.
Effect of blade layout on performance of low specific speed pump operating in turbine mode
Authors: Štefan David , Chabannes Lilian , Hudec Martin , Bezdíček Jiří , Rudolf Pavel
Published: October 2022

The very low specific speed pump (nq = 8.9) operated in turbine mode was analyzed. The experimental and numerical studies were carried out in order to show effect of different blade layout on pump-as-turbine (PaT) performance. In total three different impellers were analyzed. One impeller consisting of four main blades and two impellers consisting of four main blades and different arrangement of splitter blades. Either single splitter blade or two splitter blades are placed between each of main blades. While measurements pointed out the main PaT performance, the simulations enable to analyze internal flow fields and point out the mechanisms of performance variation using different impellers. The main aim of this study is to clarify usability of very low specific speed pump for energy recovery in terms of pump as turbine operation or for storage capability in terms of pump-turbine.
Eigenfrequencies of rotating discs in dense fluid: imposed modal motion approach
Authors: Nennemann B , Monette C , Braun O , Biner Daniel , Münch-Alligné Cécile
Published: October 2022

The mode split on disc like structures rotating in a dense fluid leads to a deviation of eigenfrequencies at high rotational speeds compared to their values in still water. Predicting eigenfrequencies correctly is essential to avoid fatigue cracks on prototype turbine runners. Analytical models for simple geometric configurations and complex numerical models using fully coupled fluid structure interaction to predict the mode split on arbitrary geometries exist. We are presenting a complementary approach of intermediate complexity applicable to arbitrary geometries. Mode shapes and modal parameters are computed by finite element analysis in still water. These mode shapes are imposed with a harmonic variation in time during an unsteady computational fluid dynamics computation. From the interaction between the flow and the modal motion, the modal force and the modal work can be computed. These can be converted into added modal mass and hydrodynamic damping and further into the shift of the eigenfrequency under rotation due to the fluid for a given mode. The tendencies of the frequencies with rotation compare reasonably well with experimental data. The numerical method can be applied to disc rotation speeds far beyond the range of experimental data revealing interesting tendencies and a phenomenological interpretation of the cause of the mode split.
Numerical investigation of the structural behaviour of a headwater bifurcation in hydraulic short circuit
Authors: Khalfaoui K , Tismer A , Riedelbauch S
Published: October 2022

Within the XFLEX HYDRO project, hydraulic short circuit operation (HSC) is being investigated as one of the possibilities to enhance the flexibility of pumped storage powerplants. Therefore, the structural behaviour of a headwater piping bifurcation of a pumped-storage power plant is investigated. Based on unsteady CFD simulations, time history mechanical simulations are carried out to assess the dynamic integrity of the structure in pump (PP), turbine (TT) and hydraulic short circuit operation. Since the plant at hand was initially designed for PP and TT operation, simulating these load conditions delivers a base of assessment for the admissibility of HSC loading. Herein, the focus of the assessment lays on the welded joints. First, a modal analysis of the rock-supported structure is conducted to identify the required numerical analysis method to be employed for the time-history structural simulations. The CFD pressure fluctuations are then applied to the structure within time history FE analyses. The dynamic structural simulations deliver low stress fluctuations which go hand in hand with the low pressure fluctuations previously determined using CFD. Besides, the structure exhibits comparable stress oscillations for all three loading conditions. The latter stress oscillations lie below the fatigue limit of the welds. The study herewith presents promising findings towards safe HSC operation ensuring more flexibility of the power plant at hand.
On the auto-oscillation effect of Kaplan rotors during load rejection
Authors: Weber W , Hübner B , Koutnik J
Published: October 2022

High vibrations due to auto-oscillation effects can occur at power units with Kaplan type runner during load rejection. These vibrations may be present in the turbine braking mode when the wicket gates are almost closed, and the rotating runner pushes the water towards the draft tube resulting in higher pressure below the runner blades. The upward leakage flow between runner blades and discharge ring is suspected to cause the vibration phenomenon.

Within this contribution, a fluid-structure-interaction (FSI) technique is applied to identify the mechanism of self-excitation. The investigation is carried out on a disk that represents the simplified geometry of a Kaplan runner. Computational fluid dynamics (CFD) simulations are used in order to obtain parameters in terms of stiffness, damping and inertia of the fluid to be applied in the following shaft line analysis. Damped modal analyses of the shaft line allow for evaluation of the stability of natural frequencies and corresponding mode shapes of the shaft line. A resulting negative damping indicates the occurrence of auto-oscillation behavior of the shaft line. For the operating condition under investigation, mode shapes with negative damping are obtained with this approach and a dynamic bending moment on the runner due to lateral movement in combination with a tilting at the runner in these mode shapes is identified as source of instability.

Based on these findings, an analytical approach for modeling the relevant effect is derived and compared to CFD result as well as results from acoustic finite element modelling. Finally, conclusions for avoidance of self-excited vibrations at Kaplan runners are proposed.
On the numerical assessment of flow losses and secondary flows in Pelton turbine manifolds
Authors: Hahn F J J , Semlitsch B , Bauer C
Published: October 2022

Different methods combining analytical guidelines, numerical simulations, and the manufacturer’s experience are employed to design and optimise the distributor manifold of a Pelton turbine. All these methods have in common to assume undisturbed straight inflow to the manifold, thus neglecting the upstream flow conditions. Our numerical simulations executed with the open-source code OpenFOAM v2012 show that the insufficient consideration of the upstream flow situation may lead to inaccurate predictions of the manifold flow. Five turbulence models were tested in our simulations, and the inflow turbulence intensity was varied from 1% to 10%. The flow quality was assessed by evaluating the head loss coefficient from total pressure drop, the head loss coefficient from the entropy production, the secondary velocity ratio upstream the injectors and the mass flow imbalances in the injectors. The study of turbulence models revealed that the k-ω Shear Stress Transport model predicted the head loss and secondary flows with reasonable accuracy. Also, the computationally less expensive model of Spalart-Allmaras leads to similar results and therefore emerges as an appropriate model for optimisation. The simulations with varying inflow turbulence intensity levels indicate a flow pattern change, if the specified inflow turbulence intensity is less than 4%. Below this value of inflow turbulence intensity, a significant increase of the secondary flow upstream of the injectors was observed.
Scale-resolving simulations of the flow in the Francis-99 turbine at part-load condition
Authors: Arabnejad Mohammad Hossein , Nilsson Håkan , Bensow Rickard E.
Published: October 2022

In this paper, we investigate the formation of the Rotating Vortex Rope (RVR) using scale-resolving methods, SAS and Wall-Modeled LES (WMLES). We compare the results from these simulation methods with the experimental data of the Francis-99 workshop. This comparison shows that the general features of the RVR can be captured with both methods. However, using WMLES methods would lead to a better quantitative agreement between the velocity profiles in the draft tube in the simulation and the experiment. The reasons for this better agreement are discussed in detail. A comparison of the pressure fluctuations in the draft tube captured in the simulations and the experiment is also presented. This comparison shows that all simulations under-predict the Root Mean Square (RMS) of these pressure fluctuations, although the RMS values predicted by the WMLES simulation are closer to the experimental values.
Towards accurate vibration prediction of pump-turbine runners in deep partial load operation
Authors: Khalfaoui K , Zorn M , Riedelbauch S
Published: October 2022

Within the XFLEX HYDRO project, operating power plants in low load conditions is being investigated as one of the possibilities to enhance the flexibility of hydropower. Therefore, the structural dynamic behaviour of a pump-turbine runner is numerically analysed in deep partial load (DPL) operating points. Based on highly resolved unsteady CFD simulations performed prior to this work, the pressure fluctuations are extracted and mapped onto the structure for subsequent Finite Element (FE) analyses. Herein, an efficient hybrid methodology combining purely structural and coupled Fluid-Structure-Interaction (FSI) modelling is applied. The obtained simulation results are compared to experimental measurements. Herein, the influence of several factors such as the CFD mesh size, the turbulence model, the simulated time window width, and the structural simulation type is studied. As a result, the numerical outcome is validated with good accuracy. The importance of highly resolved CFD simulations with appropriate turbulence modelling is highlighted. Besides, the necessity of coupled acoustic-structural analyses to accurately resolve the dynamic response to excitation by rotor stator interaction (RSI) pressure shapes is demonstrated despite the chaotically dominated nature of the structural vibrations. The results of this work can be used to extend the operating range of the pump-turbine runner and, most importantly, to calibrate numerical models to predict the structural vibrations accurately and efficiently in DPL and finally ensure safe operation of these components.
Decomposition of the structural response of the Francis-99 runner during resonance.
Authors: Solemslie Bjørn W. , Østby Petter T. , Iliev Igor
Published: October 2022

In this work, the complex structural response of the Francis-99 turbine runner is further investigated by decomposition of the output signal from previous Laser Doppler Vibrometry (LDV) measurements into the motion of each nodal diameter. During the structural measurements, the non-rotating runner was installed in the turbine pit, submerged in a non-flowing water, and excited with piezoelectric patches mounted on the hub. The patches were excited with phase shifted sinusoidal voltage to create overall excitation of the runner with a desired number of nodal diameters. The deflection of selected locations on the trailing edges were scanned with LDV, one point at a time, and the global movement was reconstructed by combining the data for all points.

The Francis-99 runner has its blades bolted to an over-dimensioned hub and shroud, where the hub is not fully axi-symmetrical and has several hollows in it. This, together with the fact that one patch was found to be non-functional, is believed to have excited other ND patterns in addition to the one that was intentionally excited, therefore contaminating the movement of the trailing edges with movements that does not belongs to the excited ND. To mitigate this and create a better representation of the movement of the trailing edge, which is not affected by the bleed from other ND, the LDV signal for each excited frequency of a particular ND is post-processed using discreet Fourier transformation to decompose the motion of each nodal diameter in the range ND0 to ND7. This unveils the contribution of each nodal diameter within the output signal where a spike is seen for the excited ND in all measurements. Influence from other nodal diameters were found, where the failed patched is believed to cause a ND1 like movement. In addition the clustering of multiple eigenmodes with differing nodal diameters previously found in narrow frequency bands were also found as interfering contribution when exciting at the relevant frequencies.
Effect of unbalancing mass placement in hydropower generators with floating rotor rim
Authors: Rondon D , Aidanpää J O , Gustavsson R
Published: October 2022

Dynamic models for hydropower generators treat the rotor part as a rigid body; however, many studies have shown the opposite. The electromagnetic force distribution of deformed rotors is uneven, creating Unbalance Magnetic Pull, causing high forces on generator components leading to a risk of fatigue, therefore shortening the life of machines. Unbalancing masses can worsen the asymmetries of the rotor, which would further increase the effect of the electromagnetic interactions. This paper evaluates the rotor response using different unbalancing masses at the rotor and at the poles to quantify their impact in displacements and exciting frequencies. The model employed in this paper is based on the equation of motion derived using Lagrange equations in both co-rotating and stationary frames of reference, considering the effects of Centrifugal loads, Coriolis, and magnetization of poles. Different unbalancing mass placements affect different variables; extra weights in the poles contribute predominantly to the deformation of the rim, while the unbalance in the shaft affects the position of the shaft; a combination of placements was also studied. The simulations were performed and compared with and without radial electromagnetic forces, showing how the presence of magnetized poles further deforms the shapes of the rotor.
On the topic of added mass and acoustics in High Head Francis Runners
Authors: Østby P. T. K. , Knutsen S. , Billdal J. T.
Published: October 2022

Methods used to calculate the eigenmodes and eigenfrequencies of a Francis runner with the added mass from water are well established and have been verified by several experiments. The common explanation of the water’s role is the added mass it brings to the runners structural eigenmodes and thereby reducing the eigenfrequencies. In this paper we calculate the first eigenfrequency of the runner from the acoustic properties of the runner channel and compare the result with full FEA calculations. Using this method we show that the local speed of sound for the water in the channel can be as low as 200m/s and that the density of the runner is not necessarily an important parameter for the first eigenfrequency with some Nodal Diameters of High Head Runners.
CFD simulations of hydraulic short-circuits in junctions, application to the Grand’Maison power plant
Authors: Decaix J , Drommi J-L , Avellan F , Münch-Alligné C
Published: October 2022

In the framework of the XFLEX HYDRO H2020 European Project, the pumped-storage power plant of Grand’Maison (France), owned by Electricité De France, focuses on the implementation of the hydraulic short-circuit (HSC) operating mode. This mode increases the flexibility in pumping mode, which helps the integration of intermittent energies. Grand’Maison is divided into two power houses: the first features four Pelton turbine units and the second eight reversible pump-turbines units. A trifurcation splits the flow into three penstocks, each is then split into two branches that feed each power house. The HSC operating mode, which consists in operating the pumps and the Pelton turbines simultaneously, changes the flow paths in the junctions compared to the pump mode. The power plant was not designed to operate in HSC mode over a long duration, therefore an assessment of its feasibility is necessary. 151 computational fluid dynamic simulations are carried out for two bifurcations and one trifurcation. The numerical simulation results show that the local head losses in HSC mode represent less than 1% of the gross head. No flow instabilities are observed at the bifurcations contrary to the trifurcation. Additional analyses are required to better understand the flow in the trifurcation.