Rotating Vortex Rope Research Articles

A Novel Guide Vane System Design to Mitigate Rotating Vortex Rope in High Head Francis Model Turbine

Guide vanes are a mechanical system used to direct flow in the desired direction. At lower operating conditions, implementing a guide vane system in the draft tube of a hydro-turbine can decrease the excess swirl in the flow and, thus, reduce pressure fluctuations. The present study discusses a numerical methodology to design an effective guide vane system in the draft tube of a high head Francis model turbine. The numerical method is computationally efficient and thus, saves excess computational time and data storage required for parametric analysis of the guide vane system. The aim is to mitigate the ‘rotating’ vortex rope with minimum additional losses in the turbine. The factors considered for the guide vane system design are a) number of guide vanes, b) chord length, c) span, d) inlet-outlet angles of the guide vanes, and e) their position in the draft tube. The parametric study comprises a) ideal guide vane design and b) realistic guide vane design study. The ideal guide vane design study was with the standalone draft tube domain. The realistic guide vane design study used the passage domains of the distributor, runner, and complete draft tube. From the ideal guide vane design study, a guide vane system with two or three guide vanes of chord 86% of runner radius and leading-edge span of 30% of runner radius effectively mitigates the rotating vortex rope. The system with three guide vanes is the most efficient when placed at some distance below the runner exit with mitigation above 95%

Numerical Simulation and Experimental Validation of a Kaplan Prototype Turbine Operating on a Cam Curve

The role of hydropower has become increasingly essential following the introduction of intermittent renewable energies. Quickly regulating power is needed, and the transient operations of hydropower plants have consequently become more frequent. Large pressure fluctuations occur during transient operations, leading to the premature fatigue and wear of hydraulic turbines. Investigations of the transient flow phenomena developed in small-scale turbine models are useful and accessible but limited. On the other hand, experimental and numerical studies of full-scale large turbines are challenging due to production losses, large scales, high Reynolds numbers, and computational demands. In the present work, the operation of a 10 MW Kaplan prototype turbine was modelled for two operating points on a propeller curve corresponding to the best efficiency point and part-load conditions. First, an analysis of the possible means of reducing the model complexity is presented. The influence of the boundary conditions, runner blade clearance, blade geometry and mesh size on the numerical results is discussed. Secondly, the results of the numerical simulations are presented and compared to experimental measurements performed on the prototype in order to validate the numerical model. The mean torque and pressure values were reasonably predicted at both operating points with the simplified model. An analysis of the pressure fluctuations at part load demonstrated that the numerical simulation captured the rotating vortex rope developed in the draft tube. The frequencies of the rotating and plunging components of the rotating vortex were accurately captured, but the amplitudes were underestimated compared to the experimental data.

Flow-induced pulsations in Francis turbines during startup - A consequence of an intermittent energy system

Hydraulic turbines are increasingly responsible for regulating the electric grid, due to the rapid growth of the intermittent renewable energy resources. This involves a large increase in the number of starts and stops, which cause severe flow-induced pulsations and fluctuating forces that deteriorate the machines. Better knowledge of the evolution of the flow in the machines during transients makes it possible to avoid hazardous conditions, plan maintenance intervals, and estimate the costs of this new kind of operation. The present work provides an in-depth and comprehensive numerical study on the flow-induced pulsations and evolution of the flow field in a high-head model Francis turbine during a startup sequence. The flow simulation is carried out using the OpenFOAM open-source CFD code. A thorough frequency analysis is conducted on the fluctuating part of different pressure probes and force components, utilizing Short-Time Fourier Transform (STFT) to extract the evolution of the frequency and amplitude of pulsations. Low-frequency oscillations are detected during the startup, which are induced by the complex flow structure in the draft tube. A decomposition is performed on the draft tube pressure signals, and the variations of the synchronous (plunging) and asynchronous (rotating) modes are studied. The plunging mode is stronger at minimum and deep part load conditions, whereas the rotating mode is dominant during the presence of the Rotating Vortex Rope (RVR) at part load. The velocity field in the draft tube is validated against experimental data, and the complex flow structures formed during the startup procedure are explained using the λ2 vortex identification method.

Influence of runner cone profile and axial water jet injection in a low head Francis turbine at part load

Development of rotating vortex rope (RVR) at part load (PL) operation is a source of pressure fluctuations in draft tube and power swings, which may lead to runner failure under extreme conditions. Fluid injection methodologies like air and water injection may be employed to mitigate the RVR and the pressure pulsations associated with it. A small turbine test rig is being developed at IIT Roorkee (India) to study the effect of the fluid injection measures (air/ water) in the Francis turbine. The present work summarizes a preliminary numerical investigation of the runner cone design and water jet injection to be used in the test rig. The runner cone is modified to incorporate provisions for axial water jet injection. Two different runner cone designs have been compared based on power output, efficiency, pressure recovery, and pressure pulsations in the draft tube for four different jet discharges. The water jet affects both the magnitude and the frequency of the pressure pulsations associated with RVR, and improves the overall efficiency. The results also indicate that the water jet injection may not always be effective and may increase pressure fluctuations in some cases.

An in-depth numerical analysis of transient flow field in a Francis turbine during shutdown

Power production from intermittent renewable energy resources, such as solar and wind, has increased in the past few decades, leading researchers and engineers to establish techniques to preserve a stable electrical grid. Consequently, hydraulic turbines are being used more frequently in transient operating modes to regulate the grid. The present work provides a comprehensive numerical study on the transient flow field of a high-head Francis turbine model throughout the shutdown sequence. The computations were performed using OpenFOAM, utilizing the SST-SAS turbulence model. A Laplacian smoothing scheme is employed to conduct the mesh deformation of the guide vane domain. The time-averaged draft tube velocity field at the steady Best Efficiency Point (BEP) is validated against experimental data. Then different aspects of the transient flow field in the shutdown sequence are carefully assessed and explained for the first time. Short-Time Fourier Transform (STFT) analysis is carried out on the fluctuating part of the static pressure and force signals. High-amplitude low-frequency oscillations, due to the formation of a Rotating Vortex Rope (RVR) were observed during a specific period of the shutdown sequence. Thereafter, at deep part load conditions, the RVR vanishes and, a wide range of stochastic frequencies are identified at minimum load. A signal coherence analysis was accomplished to distinguish the deterministic and stochastic frequencies. The variation of the velocity field in the draft tube is described in detail with the help of velocity triangles. An in-depth explanation of the formation and variation of vortical structures during the whole sequence is presented. The physical mechanism of formation and destruction of the RVR is thoroughly explained.

Turbine Test Rig to Investigate Flow Instabilities in Draft Tube

Time-dependent phenomena in a reaction turbine such as rotor-stator interactions (RSI) and rotating vortex rope (RVR) contribute to pressure fluctuations, vibrations, and even failure of the machine. A small scale test rig is being developed to investigate RSI and RVR and to study RVR mitigation methods. The test rig is designed for a maximum available head and discharge of 8 meters and 0.05 m3/s, respectively. Provisions are made to operate the test rig in open and closed loops. The test rig is flexible to operate for the turbines of high specific speed (high head Kaplan turbine or very low head Francis) to low specific speed (high head Francis). The runner of the test rig is 200 mm in diameter at draft tube inlet. The turbine is connected with a variable speed generator to run up to 1000 rpm. This paper presents the design and arrangements of the test rig components according to the head and discharge conditions. Scale down design calculations for a model are performed using IEC 60193.

Proper orthogonal decomposition of turbulent swirling flow of a draft tube at part load

A large scale vortical structure, i.e., rotating vortex rope (RVR), generally forms at part load (PL) operation of a reaction turbine due to the combined effect of swirling flow at the runner outlet and the adverse pressure gradient in the draft tube. Even with the data analysis through advanced measurement techniques like Laser Doppler velocimetry (LDV) and Particle Image Velocimetry (PIV), the information available about the flow instabilities developed due to RVR is still incomplete. This paper presents an application of the proper orthogonal decomposition (POD) method to analyze the flow field inside the draft tube cone at PL operation of a high-head model Francis turbine. The POD analysis is performed on 250 PIV snapshots containing the axial and radial velocities. The results show that the first eight modes contain more than 95% of the total kinetic energy (KE) of the flow field and are associated with the organized motion of the flow. The first mode contains more than 50% of the total energy, and the axial velocity profile reconstructed with the first mode is identical to the mean axial velocity flow field. The maximum dissipation of the turbulent kinetic energy (TKE) occurs through unorganized motion.

Mitigation of Draft Tube Pressure Pulsations by Radial Protrusion of Solid Bodies into the Flow Field: An Experimental Investigation

An experimental investigation of frequential protrusion of four solid rods into the draft tube of a propeller turbine operating under partial discharge has been undertaken. The effectiveness of mitigating the pressure fluctuations associated with the rotating vortex rope (RVR) has been quantified using pressure measurements on the wall of the draft tube cone. Three azimuthal configurations of the phase difference between the rods, and four protrusion lengths were investigated. It is shown that the rotating component of the RVR decreases, irrespective of the azimuthal configuration and protrusion length, with imposed phase differences in the same direction as the runner rotation being the most effective, reducing the amplitude of the rotating component by a maximum of 62%. However, for each azimuthal configuration, the plunging mode of the RVR is amplified for large protrusion lengths, with the smallest amplification occurring for the case of 180 degrees phase difference between protrusions. Therefore, to quantify the most efficient configuration in mitigating the harmful effects of the RVR, an overall assessment of its effects on the entire turbine must be made before a conclusion can be drawn.

Influence of the runner cone design on the pressure fluctuations in the draft tube of a low head Francis turbine

This paper presents a preliminary numerical investigation on the design of the runner cone for a small turbine test rig under development at IIT Roorkee, India. The test rig is designed to study the formation and mitigation of rotating vortex rope (RVR) in the draft tube of a low head Francis turbine. The original runner cone design needs to be modified to include the provision for axial jet injection. Three different configurations, namely, a truncated cone and two original length cones with different nozzle positions, have been compared with the original cone to choose a suitable one for further investigations. The entire three-dimensional flow passage of the turbine unit has been simulated using the shear stress transport turbulence model after carrying out a mesh independence test on the original design. The proposed designs are compared at three different operating points. The amplitude and frequency of the pressure pulsations in the draft tube have been analyzed using the fast Fourier transform (FFT) method.

Study of Flow Characteristics inside Francis Turbine Draft Tube with Adjustable Guide Vanes

Numerical investigation was performed on a semi-model (one stay vane, two guide vanes, one runner passage inclusive of one main and one splitter blade, and the draft tube) of a high-head Francis turbine model with adjustable guide vanes in the draft tube. The motive of the present study is to investigate the possibility to mitigate the rotating vortex rope (RVR) formed at part load operating condition. Each guide vane in the draft tube consists of two hydrofoils. The upper hydrofoil is adjustable according to the flow angles leaving the runner. The lower hydrofoil is stationary and corresponds to the flow angle at best efficiency point (BEP).The factors considered while designing these guide vanes were a) number of guide vanes, b) chord length, c) span and d) position in the draft tube. The preliminary results indicate that the RVR pressure amplitude was suppressed by 97% compared to the reference model with no guide vanes at part load (PL) operating condition. An 8.7% increment in the draft tube pressure recovery was obtained which indicates that implementation of the guide vanes in the draft tube could positively impact the turbine efficiency beside the mitigation of the pressure pulsations at PL operation.

Vortex rope mitigation with azimuthal perturbations: A numerical study

A novel method to mitigate the rotating vortex rope is investigated numerically on a propeller turbine using ANSYS CFX. Pulsating momentum is injected in a horizontal plane in the diffuser cone from four evenly spaced jets. Three mitigation strategies are tested; M1 in which the momentum is injected perpendicular to the axial flow direction, M2, which exhibit a 12 degree angle against the tangential velocity in the diffuser cone, and finally M3, which exhibit the same horizontal angle as M2 but at a 15 % higher flow rate. It is shown that mitigation attempts M1, M2 and M3 decrease the amplitude of the rotating mode by 51%,96% and 97%, respectively. The amplitude of the plunging mode, on the other hand, increase for all mitigation attempts. However, the amplitude of the plunging mode of the unperturbed RVR is an order of magnitude smaller than the rotating mode, and thus, the overall amplitude of the pressure fluctuations in the diffuser decreases significantly. The more efficient mitigation using attempt M2 and M3 are explained using velocity contour in the diffuser cone, which show that the RVR is significantly reduced downstream of the injection plane in between injections, which is not the case for attempt M1.

Optimization of axial water injection to mitigate the Rotating Vortex Rope in a Francis turbine

Rotating Vortex Rope (RVR) resulting from flow instabilities inside draft tube affects the hydraulic turbine's efficiency and wear. Among the passive and active RVR mitigation methods, the water injection method has been widely investigated lately. The present research focuses on the dynamics of RVR mitigation using the water injection method and the optimization of exergy loss during the water injection. The numerical simulations are performed using a reduced turbine geometry consisting of one stay vane, two guide vanes, one runner passage by applying periodic boundary conditions at side boundaries, and a complete draft tube. The pressure fluctuations, velocity field, and the RVR structure are well captured using the Shear Stress Transport - Scale Adaptive Simulation (SST-SAS) turbulence model. The results of the local swirl number during the load variation show that the onset of flow instabilities and RVR formation are created due to a decrease of the axial momentum below the runner cone. The required axial momentum flux of water injection is calculated for a stable swirl number and the local minimum ratio of dimensionless loss to the pressure recovery factor. The numerical results show that the rotating and plunging components of the pressure fluctuations inside the draft tube reduce during the water injection. The pressure recovery of the draft tube improves. The dynamics of RVR mitigation are visualized using λ2 criterion and the spectrogram of pressure probes on the draft tube wall. An optimum water flow rate and jet velocity for a fixed axial momentum are proposed. The results show that a water jet with a large radius and low velocity is more effective to mitigate the RVR and minimizing the draft tube losses.

Reduced Numerical Modeling of a High Head Francis Turbine Draft Tube at Part Load

In the present study, a reduced model of the Francis-99 model turbine was investigated numerically at part load operating condition. The reduced model consists of a standalone draft tube domain of the Francis-99 model turbine. Numerical studies performed in the past on nearly complete hydro-turbine models (inclusive of the spiral casing, distributor domains, runner, and draft tube) reportedly consist of a large number of computational grids. This may increase the computational costs and data storage required to perform numerical analysis, which could be a setback for future research on new design concepts and optimization study of the draft tube domain. The reduced model was developed by mapping the phase averaged axial, radial, and tangential velocity profiles from the runner exit to the inlet of the standalone draft tube domain. Additionally, turbulent kinetic energy (k) and turbulent eddy dissipation (ε) variables were also considered for better flow prediction inside the draft tube domain. Two methods for mapping inlet boundary conditions were considered in the present study. In the first method, the entire planar profile of the runner-draft tube interface was considered. In the second method, the variables along a radial profile at the runner exit were considered with an axis-symmetric flow assumption over the entire draft tube inlet plane. The numerical results obtained from the Francis-99 reduced model turbine were validated against the numerical model of the NVKS Francis-99 model turbine (with available structured mesh) that was also analysed using the passage flow numerical technique and available experimental results. The results were found to be in reasonable agreement, with each other. The present study could be useful for the future mitigation study of rotating vortex rope by modifying the draft tube domain.

An Indirect Measurement Methodology to Identify Load Fluctuations on Axial Turbine Runner Blades.

Smooth integration of intermittent energy sources, such as solar and wind power, into the electrical grid induces new operating conditions of the hydraulic turbine by increasing the off-design operations, start/stops, and load variations. Therefore, hydraulic turbines are subject to unstable flow conditions and unfavorable load fluctuations. Predicting load fluctuations on the runner using indirect measurements can allow for optimized operations of the turbine units, increase turbine refurbishment time intervals, and avoid structural failures in extreme cases. This paper investigates an experimental methodology to assess and predict the flow condition and load fluctuations on a Kaplan turbine runner at several steady-state operations by performing measurements on the shaft in the rotating and stationary frame of references. This unit is instrumented with several transducers such as miniature pressure transducers, strain gages, and proximity probes. The results show that for any propeller curve of a Kaplan turbine, the guide vane opening corresponding to the minimum pressure and strain fluctuations on the runner blade can be obtained by axial, torsion, and bending measurements on the shaft. Torsion measurements on the shaft could support index-testing in Kaplan turbines particularly for updating the cam-curve during the unit operation. Furthermore, a signature of every phenomenon observed on the runner blade signals, e.g., runner frequency, rotating vortex rope components, and rotor-stator interaction, is found in the data obtained from the shaft.

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.

Investigation of Rotating Vortex Rope formation during load variation in a Francis turbine draft tube

Rotating Vortex Rope (RVR) has been a matter of focus for years due to the major effects on hydraulic turbine’s efficiency. The exact procedure of RVR formation is still vague. The present research focuses on the dynamics of the RVR formation during the load variation employing transient numerical simulations. Two different geometries including the full geometry and the reduced one, which consists of one stay vane, two guide vanes, one runner blade, one splitter blade and full draft tube, are considered. In order to capture the transient swirling flow features inside the draft tube, the Shear Stress Transport-Scale Adaptive Simulation (SST-SAS) model is utilized to approximate the turbulent stresses. The pressure results inside the draft tube agree well with the experimental measurements. Moreover, the velocity results show the central low-axial-velocity and high-tangential-velocity region in the draft tube properly. The flow structure is visualized using λ2 criterion. The dynamic of RVR and the physics behind the RVR formation are investigated during the load variation. The results indicate four flow regimes with different characteristics during RVR formation. The first flow regime is a stable swirling structure occurring at Best Efficiency Point (BEP). The second flow regime occurs at the beginning of the load variation where signs of flow instabilities appear. These instabilities are temporary and washed down by the upstream flow. Expanding the instabilities and creating the vortical structures in the draft tube are the important flow features in the third flow regime. The fourth flow regime is the presence of a developed rotating rope occurring at the Part Load (PL) condition. The flow regimes differ according to the size and shape of the stalled region during load rejection inside the draft tube cone. They also reveal that despite some shortcomings, the reduced model is reliable to simulate the RVR transient formation. The full geometry simulations could be also applicable for practical problems provided that the modified time step is slightly greater than the main blade rotational angle is used.

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.

Experimental and numerical analysis of turbulent velocity fluctuations in a Francis turbine draft tube in part load operation

The part load operation of turbines with non-regulated runner blades is often associated with the occurrence of a rotating corkscrew-shaped vortex rope in the draft tube. Due to its negative influence on power plants, this phenomenon is an important factor in the design process of a water turbine. Investigations using CFD aim to predict the draft tube flow, especially regarding the efficiency and the losses of the turbine. As there are still discrepancies between CFD simulations and experimentally gained data, a research project on this subject was initiated, including extensive measurements with 2D-Laser-Doppler-Anemometry. CFD simulations were carried out using high-resolution hybrid RANS-LES turbulence models. In this paper, the interrelation of the turbulent velocity fluctuations and the periodic vortex rope movement is being examined. Therefore, the measured data is provided time averaged and phase resolved. The specialities of the swirling flow influenced by the periodic vortex concerning the turbulence are illustrated on two planes, one downstream the runner in the cone and one at the lowest position of the draft tube. The results show good conformity with the CFD data including periodic part of the velocity fluctuations. The dependence of the velocity field on the rotating vortex rope is significantly higher in the cone than at the end of the draft tube bend.

Formation of Rotating Vortex Rope in the Francis-99 Draft Tube

The aim of this research is to understand the mechanism(s) of RVR formation during the changes in operating condition from the Best Efficiency Point (BEP) to Part Load (PL). A Computational Fluid Dynamic (CFD) methodology by the means of ANSYS-CFX is applied on a reduced high head Francis turbine model. The reduced model consists of one stay vane, two guide vanes, one runner blade, one splitter and a full draft tube. Numerical simulation is first performed at BEP as well as PL to ensure the appropriate employment of turbulence models and boundary conditions. In the second step, the inlet boundary conditions are changed linearly from BEP to PL in order to achieve the transient conditions inside the draft tube. The initial condition of the second step is the converged BEP result. The transient simulation is continued until the RVR is fully developed in the draft tube at part load condition. The numerical results for BEP, PL and BEP to PL are in a good agreement with the experimental data. The effect of the RVR is considered from two aspects. The first one is the frequency, and the amplitude of the pressure pulsations induced by the RVR in the draft tube. The second one is the velocity field in the draft tube which is investigated over time during load rejection. Moreover, the flow structure is visualized using the λ2 criterion. The mechanism(s) of RVR formation and damping is accurately investigated by the presented approach. Furthermore, the results provide a better understanding of the physics behind the RVR formation. The obtained results aim to design an effective RVR controlling approach.

Experimental investigation of flows inside draft tube of a high-head pump-turbine

Abstract Due to fast and frequently switch between pumping and generating modes several times daily and continuously increase to operate at off-design conditions, understanding the detail of flows is a prerequisite to the design and safety operation of a pump-turbine. Because complex flow phenomena occur inside the draft tube at different operating conditions, its flow patterns are highly demanding to be accurately investigated with reliable approach. Laser Doppler Velocimetry measurements and vortex ropes visualization inside the draft tube of a high-head pump-turbine with the specific speed nq = 24 have been performed at various operating conditions both in turbine and pump modes. Measurements and observations were taken for the speed factor nED = 0.21 and 0.19 at loads ranging from 40% to 120% of the rated power at turbine mode, and the possible operating range at pump mode. The distribution of measured axial and tangential velocities, the size of backflow zones and the rotation of vortex rope are all in line with the hydraulic design and model test results. The LDV measurement has been applied to accurately analyze the swirl flow pattern and quantify swirling intensity, the region of backflow and vortex rope, as well as to precisely determine the location of the rope-free zone combining with vortex ropes visualization. Measured instantaneous tangential velocities have been applied to accurately distinguish the processional frequency of a rotating vortex rope inside the draft tube from other fluctuations at part loads. Influence of the splitter blades can be also observed in both time and frequency domain of measured instantaneous velocities. This study can provide rather valuable information for analysis and validation in different operating regimes. The presented methodology and techniques have been demonstrated very helpful to successfully developed high-head pump-turbines for two pumped storage power plants in China.