Kaplan Turbine Model Research Articles

Experimental Fitting of Efficiency Hill Chart for Kaplan Hydraulic Turbine

The development of hydroelectric technology and much of the “knowledge” on hydraulic phenomena derive from scale modeling and “bench” tests to improve machinery efficiency. The result of these experimental tests is mapping the so-called “hill chart”, representing the “DNA” of a turbine model. Identifying the efficiency values as a function of the specific parameters of the flow and energy coefficient (which both identify the operating point) allows us to represent the complete behavior of a turbine in hydraulic similarity with the original model developed in the laboratory. The present work carries out a “reverse engineering” operation that leads to the definition of “an innovative research model” that is relatively simple to use in every field. Thus, from the experimental survey of the degree of efficiency of several prototypes of machines deriving from the same starting model, the hill chart of the hydraulic profile used is reconstructed. The “mapping” of all the characteristic quantities of the machine, together with the physical parameters of the regulating organs of a four-blade Kaplan turbine model, also made it possible to complete the process, allowing to identify not only the iso-efficiency regions but also the curves relating to the trend of the angle of the impeller blades, the specific opening of the distributor, and the identification of critical areas of cavitation. The development of the hill chart was made possible by investigating the behavior of 33 actual prototypes and 46 characteristic curves derived from the same reference model based on practical experiments for finding the optimal blade distributor “setup curve”. To complete this, theoretical characteristic curves of “not physically realized” prototypes were also mapped, allowing us to complete the regions comprising the diagram. The study of the unified hill charts found in previous documentation of the most famous manufacturers was of great help. Finally, the validation of the “proposed procedure” was obtained through the experimental survey of the actual efficiency of the new prototype based on the theoretical values defined in the design phase on the chart obtained with the method described.

Assessment of Fiber Bragg Grating sensors for monitoring induced strains on the draft tube cone of hydraulic turbines

Abstract In hydraulic turbines, several flow instabilities can take place inside the draft tube cone during off-design and transient operating conditions such as the rotating vortex rope which can severely damage the structure if sustained in time. In the frame of the AFC4Hydro H2020 research project, an extensive measurement campaign has been carried out to monitor and predict this rotating vortex rope phenomenon in a reduced scale Kaplan turbine model at the Vattenfall Research and Development facility in Älvkarleby, Sweden. The hydraulic turbine model has been operated in propeller mode with a fixed blade angle corresponding to its best efficiency point. Several sensors have been placed along the test stand to monitor vibrations, strains and pressures. Concretely, the present paper assesses the performance of using Fiber Bragg Grating sensors to measure the strains induced on the draft tube cone walls with high-spatial resolution in three different zones of influence: the upper and lower flanges and the vertical cone wall between the runner outlet and the elbow. To do so, a total of 3 arrays embedding a total of 48 Fiber Bragg Grating sensors were glued inside three grooves previously machined on these particular areas of the draft tube cone. Analysing the frequency response of the different Fiber Bragg Grating sensors, the strain patterns induced by the rotating and plunging components of the rotating vortex rope have been precisely determined. Moreover, their impacts at the different part load conditions tested have also been quantified.

On the Vaneless Space Vortex Structures in a Kaplan Turbine Model Operating at Speed No Load

The growing installation of intermittent renewable energy sources is forcing hydraulic turbines to work more frequently at speed no load when dangerous vaneless space vortex structures and stochastic flow phenomena can occur. An experimental campaign has been performed in a reduced-scale Kaplan turbine model at speed no load for different combinations of guide vane and runner blade angles under non-cavitation and cavitation conditions. Several simultaneous vaneless space vortex structures, all of them inducing torsional rotor vibrations, have been observed. Nonetheless, only one of them has been found to dominate over the rest depending on the blade and guide vane angles. Off-board pressures, torques and vibrations as well as on-board strains have been measured to characterize their nature, intensity, dynamic behavior and induced structural response. Their precession frequencies have been found to depend on the discharge factor, the number of vortices and their location inside the vaneless space. Under cavitation conditions, their dynamic behavior has not been significantly altered but the induced structural response has increased at the low-pressure side of the turbine.

Fatigue damage analysis of a Kaplan turbine model operating at off-design and transient conditions

The current renewable energy market forces hydraulic turbines to operate for longer periods of time at off-design and transient conditions. Their life expectancy is then decreased due to the wear provoked by flow instabilities and stochastic flow excitations. This study presents an experimental investigation into the fatigue damage induced on the runner blades of a Kaplan turbine model when working at speed-no-load (SNL), part load (PL) and during ramps of load. The unit was equipped with on-board sensors on the blades and the shaft as well as with off-board sensors installed on the supporting structure and the draft tube cone. The results reveal that operation at SNL induces more fatigue damage on the runner blades than at PL. The damage is then mainly induced by stochastic flow excitations at SNL and by the rotating mode of the rotating vortex rope (RVR) at PL. The ramps of load, when crossing each operating condition, cause levels of damage similar to those found in stationary operation. Finally, it was proved that the blade fatigue damage can be estimated from on-board shaft measurements at any condition. However, the blade fatigue damage can only be estimated using off-board measurements when the RVR is fully developed at PL.

A semi-implicit slip algorithm for mesh deformation in complex geometries, implemented in OpenFOAM

Many engineering applications of computational fluid dynamics (CFD) comprise extensive movement of objects that necessitate complex dynamic mesh treatments. In particular, the mesh motion process frequently requires a proper slipping of mesh points on highly curved surfaces. The currently available implementation of explicit slip boundary conditions in OpenFOAM fails to allow large deformations of the mesh without severely degrading the mesh quality and inverting some of the cells. Thus, a robust semi-implicit slip algorithm, based on the Laplacian smoothing methodology, is developed in the present work to tackle this issue. The algorithm is in fact performed in two steps, one explicit and one implicit. The OpenFOAM implementation of the algorithm includes different mesh motion solvers and boundary conditions, based on the displacement or velocity of points. The method is first verified using simple, yet relevant, test cases, and it is shown that the developed algorithm significantly outperforms some of the well-known proprietary CFD codes. Then, it is applied to a complex practical CFD case study. An engineering application that requires the features of the developed mesh motion algorithm is the transient operation of Kaplan turbines. These double-regulated machines simultaneously adjust the guide vane and runner blade angles while changing the operating condition. CFD simulations of such transient operations are highly complex, as they involve mesh deformation of the guide vane passage and simultaneous mesh deformation and rigid-body rotation of the runner blade passage. The mesh deformation requires points to slip on the curved hub and shroud surfaces while preserving the cell quality in tiny blade clearances. Therefore, the feasibility of the developed algorithm is evaluated for a load rejection sequence of a Kaplan turbine model. Program summaryProgram Title: Semi-implicit slip mesh motionCPC Library link to program files:https://doi.org/10.17632/wztc26vh7b.1Developer's repository link:https://github.com/salehisaeed/semiImplicitSlipLicensing provisions: GPLv3Programming language: C++Nature of problem: CFD simulations of numerous engineering fluid flows, such as transient operation of hydraulic turbines, involve an immensely complicated mesh motion process consisting of simultaneous mesh deformation and mesh slipping on highly curved surfaces. The available standard mesh motion methodology in OpenFOAM lacks some features to simulate this elaborate mesh motion. The introduced program addresses this problem by developing a new dynamic mesh algorithm.Solution method: The program implements a robust semi-implicit algorithm for slipping the mesh points on curved surfaces. The algorithm includes two steps, namely, an explicit step based on the general slip condition and an implicit step based on the Dirichlet condition. It employs the Laplacian smoothing equations to spread the mesh deformation into the domain. Additionally, a solid-body rotation may be added on top of the deformed mesh, which could be useful for modeling the runner region in transient operation of Kaplan turbines which contains simultaneous mesh deformation and solid-body rotation of the mesh.

Vortex rope interaction with radially protruded solid bodies in an axial turbine: a numerical study

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.

Evaluation of fish-related properties of Kaplan turbines at the design phase: simulation-based outcomes vs. experimental data

The development of turbine technology faces growing demands to maximize the survival of migratory fish entrained into turbine flows. In this context, two strategies have emerged for quantifying hazardous hydraulic conditions: computer-based evaluations at design stage and recordings with autonomous sensors deployed in prototypes. The former is a desktop evaluation with many modelling assumptions (idealization) and the latter is a field technique that introduces various unknown and uncontrollable factors (uncertainty.) The present work introduces and implements a third method based on test rig measurements and the corresponding computer-based predictions of conditions that negatively affect fish survivability. The experimental work was conducted in a five-bladed Kaplan turbine model in which miniaturized autonomous sensors (SF Mini, developed at the Pacific Northwest National Laboratory, U.S.) measured fish-relevant hydraulic features. The modelling work involved flow simulations according to industry practices and the representation of fish trajectories through the simulated flow conditions. We compared both the experimental measurements and CFD outcomes and discussed the challenges and advantages of the modelling strategies, as well as the benefits for turbine engineers in need of incorporating effective design concepts to mitigate fish mortality through turbines.

Dynamic response analysis of a reduced scale Kaplan turbine model operating in propeller mode

In the frame of the AFC4Hydro H2020 research project, an extensive measurement campaign has been carried out on a reduced scale Kaplan turbine model at the Vattenfall Research and Development facility in Älvkarleby, Sweden. The objective of the tests has been to monitor and characterize the dynamic response of the machine when it operates in propeller mode with a fixed blade angle corresponding to the best efficiency point. A series of measurements have been taken at steady state operating conditions, such as speed no load, part load and best efficiency point, as well as during transient conditions. The turbine has been instrumented with sensors to measure vibrations, displacements, strains and pressures both off-board and on-board. The excitation due to a rotating vortex rope at part load and the structural response induced by it have been clearly measured by the system. A sub-synchronous frequency with a maximum amplitude has been identified for a given part load condition. The evolution of this frequency with the discharge level and the presence of cavitation has been evaluated. Moreover, the response induced by the speed no load condition has been compared against the best efficiency point. Finally, a turbine start-up sequence has been analyzed in the time-frequency domain.

On the Rotating Vortex Rope and Its Induced Structural Response in a Kaplan Turbine Model

The rotating vortex rope, which can be decomposed in the rotating and the plunging modes, is the origin of pressure fluctuations in the draft tube cone when hydraulic turbines operate at part load, compromising the structural integrity and limiting the output load. A measurement campaign was carried out in a Kaplan turbine model which is a replica of the experimental 10 MW Porjus U9 prototype machine along a propeller curve to study the rotating vortex rope’s excitation levels and the induced structural responses. A complete set of sensors mounted on-board and off-board was used to measure pressures, forces, torques, accelerations, displacements, and strains. The characteristic frequencies and amplitudes of the pressure fluctuations and of the corresponding induced loads and vibrations associated with the two modes were quantified in a wide range of operating conditions at part load. The two modes are detected at different frequencies depending on the sensor position. Moreover, their frequencies change depending on the discharge and present different amplitudes depending on the mode. Particularly, the rotating mode shows higher amplitudes than the plunging mode in the majority of positions and directions measured.

Experimental and numerical modal analysis of a reduced scale Kaplan turbine model

This paper presents an experimental modal test of a Kaplan turbine model and provides the corresponding analysis of the results. The modal test of the rotor including the runner, the shaft and the generator was performed using, as exciters, an impact hammer and a shaker and, as sensors, several accelerometers. Additionally, numerical models of the rotor with the runner surrounded by air (dry condition) or submerged in water (wet condition) were also built. By comparing the numerical and experimental results, the main modes of vibration of a runner blade in dry conditions and of the rotor shaft both in dry and wet conditions have been identified and discussed. The most significant deviations between experimental and numerical natural frequencies were found around 10% for the rotor shaft and around 6% for the runner blade. Moreover, it was observed that the fourth mode of vibration presents the highest added mass effect with a Frequency Reduction Ratio of about 6.7% and the second mode of vibration shows the lowest damping ratios in both dry and wet conditions with values of about 1.5 and 2.1%, respectively.

Development of a novel numerical framework in OpenFOAM to simulate Kaplan turbine transients

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 assessment of parameters influencing the modal response of a Kaplan turbine model

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.

Kaplan Turbine Model Validation for Large Grid Frequency Disturbances

The grid frequency stability is decreasing in many AC grids due to less inertia in these systems, and their stability is commonly evaluated through simulation. Hydropower often delivers frequency control, yet research on hydropower model validation for large grid frequency disturbances is scarce. We performed frequency control tests on a Kaplan unit and evaluated three models (called the standard grey-box, Kaplan grey-box, and white-box models) against validation data. The grey-box model parameters were identified with grey-box system identification, whereas the white-box model used index test data to relate guide vane opening and runner blade angle to net head, flow, and efficiency. All models accounted for steering mechanism backlash. The results showed that the white-box model was overall more reliable than the two grey-box models. However, all models had difficulties in simulating the output power the first few seconds after a large change in frequency. Moreover, backlash sizes had considerable performance impacts on all models. The uncertainties in hydropower model response during the first few seconds after a disturbance emphasize the importance of sufficient stability margins in grid frequency stability simulations.

Numerical Simulation of the Flow in a Kaplan Turbine Model during Transient Operation from the Best Efficiency Point to Part Load

The aim of this study is to develop a reliable numerical model that provides additional information to experimental measurements and contributes to a better exploitation of hydraulic turbines during transient operation. The paper presents a numerical analysis of the flow inside a Kaplan turbine model operated at a fixed runner blade angle during load variation from the best efficiency point (BEP) to part load (PL) operation. A mesh displacement is defined in order to model the closure of the guide vanes. Two different types of inlet boundary conditions are tested for the transient numerical simulations: linear flow rate variation (InletFlow) and constant total pressure (InletTotalPressure). A time step analysis is performed and the influence of the time discretization over the fluctuating quantities is discussed. Velocity measurements at the corresponding operating points are available to validate the simulation. Spectrogram plots of the pressure signals show the times of appearance of the plunging and rotating modes of the rotating vortex rope (RVR) and the stagnation region developed around the centerline of the draft tube is captured.

Dynamic measurements on a Kaplan turbine: model – prototype comparison

The purpose of this paper is to validate in prototype, the results related to the description of dynamic behaviour obtained in a Kaplan turbine model. A comparative analysis between model and prototype measurement at homologous operation conditions is presented. The phenomena of interest are tip vortex cavitation development and rotor stator interaction (rsi), associated to operation at high power outputs. The selection of suitable signal processing tools allows a clear identification of the role played by the modulation of the main rsi frequencies on cavitation phenomena. The comparison between pressure acceleration measurements on model and prototype at draft tube wall are consistent, both in time and frequency domain analysis. The acceleration measurement at the draft tube wall (manhole) is suggested as a trustworthy dynamic indicator for prediction (model) and monitoring (prototype) purposes.

Numerical analysis of a Kaplan turbine model during transient operation

Hydropower plants are currently being intensively employed for electrical grid regulation. As a consequence, the frequency of start/stops and load variations is considerably increasing, leading to the operation of hydraulic turbines under improper conditions. During the last years, studies have focused on Francis turbines. The present paper aims to investigate a Kaplan turbine model. The flow through the turbine is modelled during transient operation, from the best efficiency point to a part load operating point, using a moving mesh for the guide vane displacement. The simulations are validated against experimental velocity profiles. A time step sensitivity analysis is performed in order to determine the optimum discretization time. The possibility of using large time steps is explored. The numerically simulated unsteady pressure pulsations on the runner blades are analysed. The influence of the inlet boundary conditions on the accuracy of numerical simulations is studied. The results show that a linear flow rate variation defined during the guide vane closure leads to an overestimation of the turbine head compared to the experimental value due to an overestimation of losses. The second type of boundary conditions, a constant total pressure, results in an underestimation of the flow rate compared to the experimental value due again to an overestimation of the losses.

Fluid added polar inertia and damping for the torsional vibration of a Kaplan turbine model runner considering multiple perturbations

A water turbine runner is exposed to several perturbation sources with different frequencies, phases, and amplitudes both at the design and off-design operations. Rotor-stator interaction, cavitation, rotating vortex rope, and blade trailing edge vortices are examples of such perturbations which can disturb the runner. The rotor dynamic coefficients require being determined to perform a reliable dynamic analysis. Fluid added inertia, damping, and stiffness have previously been investigated for individual perturbation frequencies for the torsional vibration of a Kaplan turbine model runner. However, a number of perturbation sources mostly take place simultaneously and alter the dynamics of the runner. Soltani et al. [1] have evaluated the torsional added parameters for a Kaplan turbine runner using numerical simulations considering single perturbation frequency. In the present work, the fluid added parameters are assessed in the presence of multiple perturbation sources. A similar methodology is used. A single-degree-of-freedom (SDOF) model for the dynamic model and unsteady Reynolds-averaged Navier–Stokes approach for the flow simulations are assumed. Perturbations with different frequencies are applied to the rotational speed of the runner to determine the fluid added parameters for the torsional vibration. A number of previously investigated frequencies are chosen and their combinations are investigated. In addition, two different phase shifts are considered between the applied perturbations to study the effect of phase. Two more test cases with higher perturbation amplitude are also conducted to investigate its influence on the fluid added inertia and damping. The results are compared with the previous study and the interaction of multiple perturbations on the added parameters is investigated.

Study on the Accuracy of RANS Modelling of the Turbulent Flow Developed in a Kaplan Turbine Operated at BEP. Part 2 - Pressure Fluctuations

The aim of the paper is to investigate the limitations of unsteady Reynolds-averaged Navier-Stokes (RANS) simulations of the flow in an axial turbine. The study is focused on modelling the pressure pulsations monitored on the runner blades. The scanned blade geometry renders the meshing process more difficult. As the pressure monitor points are defined on the blade surface the simulation relies on the wall functions to capture the flow and the pressure oscillations. In addition to the classical turbulence models, a curvature correction model is evaluated aiming to better capture the rotating flow near curved, concave wall boundaries. Given the limitations of Reynolds-averaged Navier-Stokes models to predict pressure fluctuations, the Scale Adaptive Simulation-Shear Stress Transport (SAS-SST) turbulence model is employed as well. The considered test case is the Porjus U9, a Kaplan turbine model, for which pressure measurements are available in the rotating and stationary frames of reference. The simulations are validated against time-dependent experimental data. Despite the frequencies of the pressure fluctuations recorded on the runner blades being accurately captured, the amplitudes are considerably underestimated. All turbulence models estimate the correct mean wall pressure recovery coefficient in the upper part of the draft tube.

Load Rejection Transient Process Simulation of a Kaplan Turbine Model by Co-Adjusting Guide Vanes and Runner Blades

To obtain the flow mechanism of the transient characteristics of a Kaplan turbine, a three-dimensional (3-D) unsteady, incompressible flow simulation during load rejection was conducted using a computational fluid dynamics (CFD) method in this paper. The dynamic mesh and re-meshing methods were performed to simulate the closing process of the guide vanes and runner blades. The evolution of inner flow patterns and varying regularities of some parameters, such as the runner rotation speed, unit flow rate, unit torque, axial force, and static pressure of the monitored points were revealed, and the results were consistent with the experimental data. During the load rejection process, the guide vane closing behavior played a decisive role in changing the external characteristics and inner flow configurations. In this paper, the runner blades underwent a linear needle closure law and guide vanes operated according to a stage-closing law of “first fast, then slow,” where the inflection point was t = 2.3 s. At the segment point of the guide vane closing curve, a water hammer occurs between guide vanes and a large quantity of vortices emerged in the runner and the draft tube. The pressure at the measurement points changes dramatically and the axial thrust rises sharply, marking a unique time in the transient process. Thus, the quality of a transient process could be effectively improved by properly setting the location of segmented point. This study conducted a dynamic simulation of co-adjustment of the guide vanes and the blades, and the results could be used in fault diagnosis of transient operations at hydropower plants.

Numerical analysis of fluid-added parameters for the torsional vibration of a Kaplan turbine model runner

The impact of fluid on the runner of a hydraulic turbine is a recurrent problem. Fully coupled fluid–structure simulations are extremely time-consuming. Thus, an alternative method is required to e...