Vortex Rope In Draft Tube Research Articles

Intensity recognition of vortex ropes in draft tube of a prototype pump turbine using an optimized CNN-BiLSTM framework with multi-head self-attention mechanism

Swirling vortex rope in draft tube (DT) is a typical hydraulic instability of a pump turbine (PT) in the pumped storage plant (PSP). In view of the potential hazards of the vortex rope, accurate recognition of its intensity is of great significance to maintain the stable operation of the PT. Due to the limitations of shallow learning algorithms during intelligent recognition, an adaptive deep learning framework is innovatively proposed in this study. Firstly, the measured high-precision pressure fluctuation signals based on the prototype PT in a Chinese PSP that can reflect different intensities of vortex ropes in the DT are utilized as the input data. Secondly, a preliminary deep learning framework that integrates convolutional neural network (CNN), bidirectional long short-term memory (BiLSTM) and multi-head self-attention mechanism (MHSAM) is constructed. Then, the Bayesian optimization algorithm (BOA) is utilized to adaptively determine several hyperparameters of the framework. And an adaptive BOA-CNN-BiLSTM-MHSAM framework is established to recognize different intensities of vortex ropes in the DT. Finally, the recognition performance of the proposed framework is demonstrated through comparing with other deep learning frameworks. And the recognition results illustrate that the proposed BOA-CNN-BiLSTM-MHSAM framework can be utilized to effectively recognize different intensities of vortex ropes in the DT. It will be a good technical reserve to improve the intelligent level of the monitoring system of the PSP.

Investigation on formation mechanism of cavitation channel vortex in the runner of Francis turbine

Abstract Cavitation channel vortices generally cause flow state more complex in the runner, leading to the work efficiency is reduced and the cavitation erosion on flow components is enhanced. In this paper, simulation of cavitation flow is carried out for a Francis turbine model, and the evolution of vortices in the runner during cavitation development is analyzed under partial load conditions. The results show that cavitation has a great influence on the morphology and distribution of attached vortices between blades, trailing vortices at the outlet edge of the blades, and vortex near the cone in the runner. With the gradual development of cavitation, the invasion effect of vortex rope in draft tube on the runner is enhanced, and the vortex near runner cone is gradually connected as a whole. Under the effect of the downstream reverse pressure gradient in local low-pressure area, the flow separation area gradually expands to the blade and upstream, and more singular points evolve on flow separation lines, which leads to more trailing vortices at the outlet of the blade and cavitation channel vortices are formed in serious cavitation stage.

A Comparative Study of the Mode-Decomposed Characteristics of the Asymmetricity of a Vortex Rope with Flow Rate Variation

In hydro turbines, the draft tube vortex rope is one of the most crucial impact factors causing pressure pulsation and vibration. It is affected by operating conditions due to differences in the flow rate and state and can be symmetric or asymmetric along the rotational direction. It may influence the stability of draft tube flow. To achieve a better understanding, in this work, dynamic mode decomposition is used in a draft tube case study of a simplification of a vortex rope. As the flow rate increases, the shape of the vortex rope becomes clear, and the flow rotation becomes more significant as the inlet flow rate increases. Dynamic mode decomposition was used to determine the relative frequencies, which were 0 (averaged), 0.7 times, and 1.4 times the features of the reference frequency. As the inlet flow rate increases, the order of high-energy modes and their influence on the vortex rope gradually increase, and this characteristic is exhibited further downstream of the draft tube. When the inlet flow rate is low, the impact of mode noise is greater. As the flow velocity increases, the noise weakens and the rotation mode becomes more apparent. Identifying the mode of the vortex flow helps extract characteristics of the vortex rope flow under different operating conditions, providing a richer data-driven basis for an in-depth analysis of the impact of operating conditions on the flow stability of a draft tube.

Experimental study of pressure pulsation in a large-scale hydropower plant with Francis turbine units and a common penstock

The effects of partial load operation on pressure pulsation and pressure pulsation propagation to off-operation units are significant • Pressure pulsation measurements are provided for an actual hydropower plant. • Rotor-stator interactions and draft tube vortex rope were studied simultaneously. • Pressure pulsation propagates from operating units to off-operation units. • In partial loads, the most significant wave in waterway is draft tube vortex rope. • The turbine rotor-stator interaction waves are localized and flow rate dependent. As a limiting factor in power generation, especially in part-loads, turbulence is considered an important phenomenon in hydropower plants. In this study, the pressure pulsation measurements are done for Darian Hydropower Plant in Iran, which is composed of three 70 MW units with a common penstock. The frequency spectrum for different features such as rotor-stator interactions, draft tube vortex rope, and surge tank fluctuations is obtained and discussed in detail. Pressure pulsation measurements were obtained while one of the units (i.e., Unit I) was in operation, and the other units (Units II, III) were out of operation. In addition to the in-operation units, the effect of pressure pulsation on the out of operation units is also studied comprehensively. The results show that to avoid the destructive effect of pressure pulsation, the output power of the in-operation unit should not be lower than 55 MW (i.e., a part load lower than 78% of the full load). Results demonstrate that at lower power levels pressure pulsation is significant likely because the flow characteristics are rather significantly different than the conditions considered for the design of the plant. Moreover, it is found that most of the pressure waves generated by the turbulent flow of the in-operation unit waterway have considerable impacts on the neighboring units. A very good agreement between the expected theoretical frequencies and measured frequencies are observed for various pressure pulsation phenomena, as well.

Draft Tube Vortex Rope Analysis of a Pump-turbine Unit under Different Operating Conditions

Pumped-storage power stations which can be flexibly adjusted for power generation and energy storage, are widely constructed worldwide. Pumped-storage units work under different operating conditions according to the demand of the power grid, and vortex ropes in the draft tube will appear and show different shapes under the partial load conditions of the unit in generation mode. The unstable pressure pulsation caused by the eccentric vortex ropes inside the draft tube may cause the vibration of the unit during operation. So it is important to study the characteristics of draft tube vortex ropes for flow and vibration analysis. In this paper, a three-dimensional model of the unit is fully established, and the unsteady calculations of 60 %, 86.7%, and 100 % load conditions in turbine mode are carried out by numerical calculations to obtain the flow characteristics of the full flow field. The pressure and velocity distributions, shapes of vortex rope in the draft tube under different load conditions are extracted from the CFD analyses, respectively. The pressure pulsations of different monitoring points set up inside the draft tube are evaluated in the time domain and frequency domain. The results show that the vortex rope in the draft tube is more obvious under small flow discharge, and the vortex rope itself is not sensitive to the selection of Q criterion threshold, but the vortex on the draft tube wall is greatly affected by the selection of the threshold. The influence of draft tube vortex ropes on the vibration of the pumped-storage unit is also discussed.

Study on the method of reducing the pressure fluctuation of hydraulic turbine by optimizing the draft tube pressure distribution

Francis turbines within hydropower plants are required to operate under off-design conditions to meet the grid demand and ensure stability. In line with this, part-load conditions are generally accompanied with draft tube vortex rope appearance and associated pressure pulsations, which may cause structural vibrations and possible plant disruption for severe cases. Therefore, this study makes an attempt weaken the draft tube vortex rope and the associated pressure pulsations for a Francis turbine under part-load conditions (0.6Q opt. ), through the modification of runner hub extension (RHE) design. Four RHE designs were numerically tested, namely Type A, B, C, and D. Experimental tests were carried out to explore real time vortex rope dynamics, which were replicated by numerical simulation method. The RHE design was found to considerably influence the draft tube pressure field characteristics through its filling effect to low pressure zones. The draft tube pressure pulsation analysis showed that all RHE designs exhibited the same pulsation frequency equivalent to 28% of the runner rotational frequency, with different amplitudes. With Type C design, the draft tube vortex rope energy was considerably weakened, leading to lowest pressure pulsation amplitude. Note that this design is a cylindrical RHE with a thickened tail. • Experiments are conducted on Francis turbine part-load conditions. • The draft tube flow is simulated with DES model reproducing the vortex rope presence. • Pressure distribution characteristics were analyzed for different runner hub shapes. • Fast Fourier Transform-based pressure pulsations spectra were extracted for every shape.

Experimental research on pressure fluctuation characteristics of high-head Francis turbine under part load conditions

Pressure fluctuation at partial load operation is an obvious and harmful phenomena in Francis turbines. It could be induced by rotor stator interaction and cavitation vortex rope in draft tube and must be minimized during the design of Francis turbine. To reduce the pressure fluctuation at partial load operation, accurate assessment of the cause of this phenomena is crucial. In this paper, a reduced-scale physical model of a high head Francis turbine is used as the research object, and the test technique is applied to investigate the excitation source of the pressure fluctuation at two partial load operation conditions. Based on experimental data, it can be concluded that the amplitude of pressure fluctuation in the draft tube significant is increased with the decrease of the power of turbine, which is indicated that the stability of turbine deteriorated sharply with the power reduced. But the dominant frequency of the pressure fluctuation is almost the same. And the strong pressure fluctuation in the draft tube can be transmitted to the upstream components, which are affected the pressure fluctuation caused by rotor stator interaction.

Effect of Fins on the Internal Flow Characteristics in the Draft Tube of a Francis Turbine Model

Undesirable flow phenomena in Francis turbines are caused by pressure fluctuations induced under conditions of low flow rate; the resulting vortex ropes with precession in the draft tube (DT) can degrade performance and increase the instability of turbine operations. To suppress these DT flow instabilities, flow deflectors, grooves, or other structures are often added to the DT into which air or water is injected. This preliminary study investigates the effects of anti-cavity fins on the suppression of vortex ropes in DTs without air injection. Unsteady-state Reynolds-averaged Navier–Stokes analyses were conducted using a scale-adaptive simulation shear stress transport turbulence model to observe the unsteady internal flow and pressure characteristics by applying anti-cavity fins in the DT of a Francis turbine model. A vortex rope with precession was observed in the DT under conditions of low flow rate, and the anti-cavity fins were confirmed to affect the mitigation of the vortex rope. Moreover, at the low flow rate conditions under which the vortex rope developed, the application of anti-cavity fins was confirmed to reduce the maximum unsteady pressure.

On the use of artificial neural networks for condition monitoring of pump-turbines with extended operation

Abstract Because of the addition of stochastic supply to the power grid introduced by new renewable energies, hydropower is required to respond rapidly to the fluctuation between power demand and supply. Consequently, hydropower units must work under extreme off-design conditions, where the machines are more prone to suffer from damages and shorter useful lives. Novel supervision and monitoring techniques which are able to compare the revenues with the remaining useful life of the turbine unit are required to cope with these new scenarios. In this paper, the upgrading of an existing monitoring system to deal with the extended operating range of a pump-turbine is discussed. Previously, the machine operating range was from 50% to 100% power and now it is from 20% to 100% power. For that purpose, the vibration signals collected from the current monitoring system have been used. First, the autoregressive mapping of the overall levels measured in the machine for all the extended operating conditions has been carried out. Back-propagation neural network was applied for the mapping. Second, the complex hydraulic phenomena that may occur in the extended operating range that can produce accelerated wear and tear have been studied. Typical phenomena are excessive turbulence, draft tube vortex rope, cavitation erosion, excessive vibration and excessive stresses in the runner. For each of these abnormal operating conditions, several features (condition indicators) were selected and mapped on the operation hill-chart using neural networks. The consequences of each abnormal operation have been analyzed with physics-based models. With the mapping, the zones where operation is not recommended can be identified and the effects estimated.

Very large eddy simulation of the vortex rope in the draft tube of Francis turbine

There is usually a strong swirling flow between runner and draft tube of Francis turbine at part load, which would lead to a vortex rope in draft tube. RANS and LES models are used by lots of authors to simulate this phenomenon, and the calculated results is almost similar with test observation. However, these turbulent models can’t predict the length of helical vortex rope exactly, especially the low pressure and high vorticity in the core of vortex. Very large eddy simulation, as a hybrid RANS-LES methodology, could combine the advantages of RANS and LES, which has been proved by many authors. This paper presents a VLES simulation of draft tube flow field compared with other three turbulence models. The results show that VLES model is able to effectively reduce the eddy viscosity and velocity dissipation in vortex rope field, leading to lower pressure and longer rope than other models Moreover, it is demonstrated that proper mesh is also significant for VLES to predict the cavitation vortex rope observed in model test.

Behavior of Francis turbines at part load. Field assessment in prototype: Effects on power swing

Francis turbines are increasingly demanded to work at part load in order to satisfy the electricity generation demand and to compensate the non-constant generation of other renewable sources such as wind or solar. At part load, Francis turbines present complex flow patterns dominated by cavitation and especially by the draft tube vortex rope. The vortex rope phenomenon affects the structural behavior of the machine as well as the hydraulic circuit response and, for certain conditions, it could become unstable. At this situation, the output power can significantly swing endangering the electrical grid stability. This condition has to be well-known and avoided as far as possible. In this paper, the phenomenon of the power swing at part load in prototypes is studied. For this purpose, different Francis turbine prototypes have been selected. The units have different specific speeds and therefore different power, head and geometry. All of them present the power swing phenomenon at part load. Different sensors, such as accelerometers, proximity probes, pressure sensors or strain gauges, have been placed in different positions of all the machines in order to detect and quantify the phenomenon. The relationship between the hydraulic phenomenon and the output power swing is presented for all the turbines studied. Results obtained permit to asses on the on-site detection of power swing as well as the possibility to avoid it in real-time.

Behavior of Francis turbines at part load. Field assessment in prototype: Effects on the hydraulic system

At present, hydropower plants play an important role because of their flexibility in the generation of electricity. Today, the required operating range of turbine units is expected to extend to deep part load. At part load the draft tube vortex rope is generated and, in some cases, can produce large pressure pulsations limiting the operation of the turbine. The draft tube vortex rope has been much studied.In this paper, the effects of the part load behavior on the hydraulic system in existing power plants has been studied. Different hydropower units with Francis turbines has been selected for this purpose. All of them present strong vibrations in penstocks and other machine components at part load. Vibrations, pressure fluctuations and other parameters were measured in different positions of the turbine, the draft tube and the penstock. Signals were acquired during transients and for several operating loads. The signals in time and frequency domain and other signal treatments for these cases are shown.

Influence of draft tube water injection system on cavitation behaviour in a full-scale Francis turbine with visual access

This paper describes the results of a comprehensive and ongoing project to control unsteady fluid-dynamic behaviour and associated effects at Svorka hydro power plant (25 MW, 260 m, 600 rpm), operated by Statkraft in Norway. This Francis unit had an original air admission system installed on the draft tube wall and turbine shaft. However, these systems did not perform satisfactorily to prevent high intensity pressure fluctuations due to vortex rope at part load. To mitigate them, a complementary draft tube water injection system, designed by Flow Design Bureau AS (FDB), was installed in 2010. Moreover, the runner suffered from cavitation pitting on the blades and a measuring campaign with several accelerometers and an acoustic emission sensor concluded that the unit was susceptible to high load cavitation erosion. Given the complexity of the turbine flow, it was decided to install four transparent acrylic glass windows on the draft tube, allowing for visual access to the runner blades and outlet flow. To evaluate the influence of the draft tube injection system on the cavitation behaviour, a series of measurements of cavitation intensity were carried out. High frequency data from the sensors were processed with demodulation techniques at various expected modulation frequencies, particularly the blade passing frequency and the frequency of the draft tube vortex rope. Furthermore, the pressure pulsations were measured and quantified with pressure sensors on the draft tube wall. The results indicate that draft tube water injection has little or no effect on the measured cavitation intensity in the runner. However, other aspects of the machine, including the seasonal and daily variation of the submergence level, must be taken into consideration. In conclusion, it appears that additional long-term observations are needed to clarify the global dynamic behaviour along the entire operating range.

Analysis on the internal flow field in vaneless space and draft tube of one reversible pump turbine during load rejection under turbine mode

The development of pumped storage plants presents quicken tendency since the deregulation of energy market, for balancing the unstable grid caused by electricity generated by renewable energy. Whilst pumped storage plants are increasingly operating at off-design conditions in order to meet the requirement of electrical grid, therefore performance of unit at off-design conditions has become more important. In this work, three dimensional unsteady calculations were carried out for simulating pump turbine load rejection period based on the dynamic sliding mesh method and turbulent model of detached eddy simulation, also weakly compressible fluid was considered. The results was analyzed in both time domain and frequency domain to study the internal flow field characteristics, while flow field evolution in vaneless space between guide vane and runner was also investigated, reasons of vortex rope formation and characteristics of vortex rope in draft tube were studied.

The influence of runner cone perforation on the draft tube vortex in Francis hydro-turbine

In Francis hydro-turbine, the pressure oscillation in the draft tube is the main source of flow-induced vibration and noise. The structural design of runner cone exerts significant influence on the tangential flow velocity within the draft tube and the pressure fluctuations downstream. In this work, we propose a simple and effective method by perforating through-holes on the runner cone to weaken or eliminate pressure fluctuations induced by the vortex ropes in draft tube. An 2 and 4 holes are perforated on the lower half of runner cone, and numerical simulations and experiments are conducted to understand the changing of phenomenology and features with regard to vortex ropes and pressure levels, comparing with the original unperforated case. The analyses are done for two different guide vane openings, ?, and the results were validated by experiments with high-speed photography. It is found that at the two openings, the runner cone with perforations can reduce the volume of vortex ropes, and decrease the pressure fluctuation level. The motion of vortex ropes tends to be less violent with more perforated holes. Therefore, the runner cone perforation has the potential of reducing the vortex rope strength.

Numerical investigation on radial impeller induced vortex rope in draft tube under partial load conditions

The vortex rope phenomena appearing in a draft tube under partial load operations were investigated. The efforts were carried out by using a homogeneous equilibrium vapor-liquid two-phase model and a blending SST turbulence model, and the former takes account of the weak compressibility of mixed media. The vortex rope phenomena were induced by the radial impeller and three kinds of flow regimes were illustrated. Vortex rope was visible at some conditions and it displayed various patterns, while no obvious vortex rope was found under some conditions. It was found that the plant cavitation number determines that whether the vortex rope was visible or invisible. An interesting profile was that a secondary vortex filament ring entwining the vortex rope appeared clearly near the design condition. Both flow structures and pressure characteristics were studied to depict the vortex mechanism and the characteristics of the η-σ curves. Further analysis found that the shedding frequency of the vortex ring is similar with that of the precession movement.

Experimental Study of the Francis Turbine Pressure Fluctuations and the Pressure Fluctuations Superposition Phenomenon Inside the Runner

The pressure fluctuations in both the rotating runner and the other fixed components in a model Francis turbine under various loads were experimentally measured by means of onboard measuring equipment in the runner and data storage device on the shaft in this study. Large pressure fluctuations were observed under both small guide vane opening and large guide vane opening conditions. Flow separation at the blade suction surface led to large pressure fluctuations for small guide vane openings, the unsteady flow around the inlet on the blade pressure side led to large pressure fluctuations for large openings. The pressure fluctuations correlation between the runner and other components of the turbine, mainly the draft tube, was analyzed in detail for both small guide vane opening (12 deg) and large guide vane opening (30 deg). The results show that the pressure fluctuations in the runner space increased by the superposition of draft tube vortex rope pressure fluctuations and runner inter blade vortices pressure fluctuations, resulting in much larger pressure fluctuations in the runner space than in other components.

Vortex ropes in draft tube of a laboratory Kaplan hydroturbine at low load: an experimental and LES scrutiny of RANS and DES computational models

ABSTRACTWe report on the examination of several approaches to simulate computationally the unstable regime of a model Kaplan turbine operating at off-design load. Numerical simulations complemented by laboratory experiments have been performed for a 60:1 scaled-down laboratory turbine model using two Reynolds-averaged Navier–Stokes (RANS) models (linear eddy viscosity model (LEVM), and a Reynolds stress model (RSM), including realizable , k-ω SST, and LRR), detached eddy simulation model (DES), and large eddy simulation model (LES). Unlike the LEVM, the RSM, DES, and LES reproduced the mean velocity components and the intensities of their fluctuations and pressure pulsations well. The underperformance of the LEVM is attributed to the high eddy viscosity as a consequence of an excessive production of the turbulent kinetic energy due to the models’ inability to account for the turbulent stress anisotropy and the stress-stain phase lag, both naturally accounted for by the RSM. This led to a much larger modelled and a smaller resolved turbulent kinetic energy compared to those in the RSM.

Numerical simulation of a cavitating draft tube vortex rope in a Francis turbine at part load conditions for different σ-levels

In the present study, numerical investigations of a Francis turbine at model scale are performed. A part load operating point is selected for the analysis with a focus on the cavitating draft tube vortex rope. For three σ-levels, different aspects are investigated. First, the extension of the vapor volume is analyzed and compared to high-speed visualization recordings from the measurements. Furthermore, the pressure fluctuations for different σ-levels are analyzed and compared to experimental data at a measuring point in the draft tube cone. Finally, the impact of the different cavitation volumes on the axial and circumferential velocity distribution is investigated. In addition, the SST and SAS turbulence models are compared for two pressure levels.

The numerical simulation of draft tube cavitation in Francis turbine at off-design conditions

Purpose – The vortex ropes in draft tube of Francis turbine always cause fluctuation and vibration, which consequently threaten the safety and stability of hydro turbines. The purpose of this paper is to use a cavitation flow computational method to simulate spiral vortex ropes under part load conditions and columnar vortex ropes under high-load conditions in draft tube. The unsteady cavitating flow characteristics in draft tube and its interaction with runner cavitation were analyzed. Design/methodology/approach – The calculation method was verified by cavitation simulation around a 3D hydrofoil. The results show that the Large Eddy Simulation (LES) turbulence model with the Zwart-Gerber-Blemari cavitation model have comparative advantage in cavitation simulations whether from capture of cavity shape or prediction of pressure changes. So it was chosen to simulate the two-phase cavitation flow in Francis turbine. The boundary conditions for inlet and outlet were set to inlet total pressure and outlet static pressure. The finite volume method with the central difference was adopted to discretize the equations. Findings – The calculated Thoma number agreed well with the experimental data. The vortex rope diameter and length increased with the cavitation development for both of the two types of vortex ropes conditions. The maximum peak-to-peak values of pressure pulsations located in the draft tube elbow part under all of the Thoma numbers conditions. Under spiral vortex rope conditions, the pressure pulsation in the same section of draft tube cone show obviously phase shift. The vortex rope affects the development of runner cavitation, which induces the symmetric and axisymmetric cavitation region in the suction side of blades for spiral and columnar vortex rope condition, respectively. Research limitations/implications – The mesh independence had been checked only in non-cavitation flow; in addition, the mesh density did not well satisfy the requirements of LES due to the limitations of computing power. The higher mesh density on a simplified model with one blade flow path and the entire draft tube may be helpful for obtaining more precise results. Originality/value – The spiral and columnar vortex ropes in a Francis turbine were compared and analyzed. The annular hydraulic jump appeared in the columnar vortex rope conditions has little effects on the pressure pulsations. The uneven flow field caused by spiral vortex led to the asymmetric cavitation development.