Vortex Rope Research Articles

Prediction of the precessing vortex core in the Francis-99 draft tube under off-design conditions by using Liutex/Rortex method

The turbulent flow in the draft tube of a Francis turbine is very complicated while working under off-design conditions. Although the off-design conditions were widely studied, the vortex core line in the draft tube of a Francis turbine with splitter blades is not well understood, especially the vortex rope property. This letter presents a prediction of the behavior of the vortex rope in the draft tube of the Francis-99 turbine obtained by the computational fluid dynamics (CFD), where the Liutex/Rortex method, as the most recent vortex definition, is applied to analyze the periodical precession of the vortex rope in the draft tube cone. The advantage of this Liutex/Rortex method is shown by its enhanced ability to represent the vortex rope structurewith the vortex-core lines. Furthermore, since it seems to be very hard to define a sharp boundary surface for the whole vortex structure, it is advantageousfocusing only on the vortex core line,by which different vortex structures can be clearly differentiated. The evolution of the vortex core and the process of the vortex breakdown in the draft tube are revealed, which might help to comprehend the development of the turbulent flow in the draft tube.

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.

Computational analysis of vortex rope in a hydroturbine of Tucuruí dam

There is a relevant hydraulic potential in Amazon rivers, from which it is generated most of the energy available for the largest part of Brazil. One of the most important hydropower plants in the Amazon is Tucurui dam, which can produce 8370 MW through Francis turbines. This kind of turbine often presents hydrodynamic issues due to the vortex rope formation, which can induce vibrations and reduce assembly operating life. The present work concentrates in a computational study to determine the frequency pressure fluctuations in a hydraulic generation unit of the Tucurui hydroelectric power plant. A numerical technique is applied for modeling the turbulent flow throughout the generation unit in a transient regime using finite volume method. For the analysis of velocity and pressure fields on the hydrogenerator, a 3D fully turbulent transient modeling for characterizing the pressure fluctuations at the runner outlet is performed. The structure of vortex rope formed is investigated in order to evaluate the hydrodynamic phenomena resulting from the turbine operating condition. Low-frequency pressure pulsations are caused by the movement of the vortex rope at the runner outlet. Additionally, vortex shedding dynamics is heavily influenced by the turbine operating conditions, demonstrating the importance of such investigation. For 63% distributor opening, the structure of the vortex rope reaches a big size in the suction duct, increasing the possibility of vibration. For 89% distributor opening, the turbine operates under overload condition, and the vortex rope becomes unstable. Comparisons with field data show the efficiency and validity of the numerical modeling, demonstrating good agreement.

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.

Investigation of Pressure Fluctuation and Pulsating Hydraulic Axial Thrust in Francis Turbines

Under the circumstances of rapid expansion of diverse forms of volatile and intermittent renewable energy sources, hydropower stations have become increasingly indispensable for improving the quality of energy conversion processes. As a consequence, Francis turbines, one of the most popular options, need to operate under off-design conditions, particularly for partial load operation. In this paper, a prototype Francis turbine was used to investigate the pressure fluctuations and hydraulic axial thrust pulsation under four partial load conditions. The analyses of pressure fluctuations in the vaneless space, runner, and draft tube are discussed in detail. The observed precession frequency of the vortex rope is 0.24 times that of the runner rotational frequency, which is able to travel upstream (from the draft tube to the vaneless space). Frequencies of both 24.0 and 15.0 times that of the runner rotational frequency are detected in the recording points of the runner surface, while the main dominant frequency recorded in the vaneless zone is 15.0 times that of the runner rotational frequency. Apart from unsteady pressure fluctuations, the pulsating property of hydraulic axial thrust is discussed in depth. In conclusion, the pulsation of hydraulic axial thrust is derived from the pressure fluctuations of the runner surface and is more complicated than the pressure fluctuations.

Pressure fluctuation and flow instability in S-shaped region of a reversible pump-turbine

To understand the pressure fluctuations occurring in S-shaped region of the pump-turbine and their relationships with rotor-stator interaction, rotating stall and vortex rope, the numerical and experimental studies of a reversible pump-turbine were conducted under turbine, runaway, turbine brake, small flow rate and reverse pump operating modes. The experimental results show that pressure fluctuations under turbine mode were most affected by the guide vane opening. This effect was weaker under turbine brake mode and weaker still under reverse pump mode. At a guide vane opening of 19 mm, the maximum fluctuation peak in the vaneless space is 5.7 times, 6.9 times and 2.0 times greater than that in the spiral casing inlet, draft tube cone and draft tube elbow, respectively. Unsteady simulation results for the whole flow passage of the pump-turbine show that asymmetric flow occurred in the runner under turbine, runaway and turbine brake operating modes. Meanwhile, an obvious rotating stall vortex with a frequency of 0.65fn was generated in turbine brake mode, resulting in relatively large pressure fluctuations in the vaneless space. At the small flow rate, flow separation in the vaneless space became very obvious, while that in the runner weakened, resulting in decreased pressure fluctuations in the vaneless space. Under reverse pump mode, pressure fluctuations in the vaneless space were relatively small because of obvious symmetrical flow separation in the runner. Pressure fluctuations in the draft tube were mainly attributed to the combined actions of rotating stall and vortex rope phenomena. What’s more, the fluctuation amplitudes at the rotating stall frequency of 0.65fn and the vortex rope frequencies of 0.30fn and 0.17fn were relatively large.

Swirl number based transposition of flow-induced mechanical stresses from reduced scale to full-size Francis turbine runners

At part load conditions, Francis turbines are subject to the emergence of a hydrodynamic instability in their draft tube, referred to as precessing vortex rope. It induces pressure pulsations in the water passages at the precession frequency of the vortex, leading to additional vibrations and dynamic loads on the runner blades. The prediction of both the dynamic behaviour of the vortex rope and the resulting dynamic loads over a wide operating range is of importance to improve the runner design and robustness on the one hand and to assess additional fatigue and related maintenance costs on the other hand. Such a prediction, either with numerical simulation or reduced scale physical model tests, remains however challenging. The present paper aims at introducing a methodology to assess the vortex behaviour, the related pressure fluctuations and the resulting dynamic strains on the runner over the complete part load operating range. It is based on reduced scale physical model tests of a Francis turbine, including the measurement of the pressure and the load on the runner with instrumented blades. It is shown that the influence of both the discharge factor and speed factor on the vortex dynamics behaviour and related pressure fluctuations can be represented by a single parameter; the swirl number. The correlation with the swirl number is further extended to the dynamic strains induced by the vortex rope on the runner blades. Similar mechanical load and pressure measurements are finally performed on the full-scale machine during a power ramp-up and the results are compared to the empirical correlations established on the reduced scale physical model.

Experimental study of swirling flow from conical diffusers using the water jet control method

The actual requirements of the energy market enforce the hydraulic turbine to operate far from the best efficiency point. When hydraulic turbines operate at partial discharge, downstream of the runner (in the conical diffuser), the decelerated swirling flow becomes highly unstable. In these conditions a spiral vortex breakdown occurs, also known in engineering literature as the precessing vortex rope. The flow unsteadiness produced by the vortex rope results in severe pressure fluctuations that hinder the turbine operation or may cause accidents. We propose the water jet method for decelerated swirling flow with vortex rope from conical diffuser to mitigate the unsteadiness. The method involves injecting water at the inlet of the conical diffuser. Initial experimental investigations of the unsteady pressure field at conical diffuser wall reveal that the water injection method mitigates the pressure pulsations associated to the precessing vortex rope. In this paper, we investigate experimental measurements of the unsteady velocity field in a conical diffuser using LDA (Laser Doppler Anemometry). The main goal of the paper is to show how different water-jet discharge rates change the velocity field and the characteristics of the vortex rope in the wake.

A method of evaluating the vortex rope strength in draft tube of Francis turbine

A precessing vortex core in the draft tube of Francis turbine is considered as unstable flow conditions and the cause of many undesirable system instabilities for hydraulic power stations. The feature of vortex rope, pressure fluctuations and runner outlet velocity at part-load conditions are investigated experimentally and numerically in this paper. An obvious vortex rope could be observed at 0.88Q11BEP point and produces the most intensive periodic pressure fluctuation at the vortex rotation frequency fv along the whole passage of Francis turbine. At other conditions away from this point, the vortex rope becomes unobvious or even invisible, and the associated pressure fluctuation amplitude also reduces. This trend could be well represented by the new parameters G and Vs, which could be used to determine the strength of helical vortex field based on the gradient of time-averaged velocity on horizontal section of the draft tube cone. Transient simulations using VLES turbulence model could obtain satisfying results by comparing with experimental data, especially in terms of the time-averaged axial velocity profile resolution. Therefore, this new method enables the designers to evaluate vortex strength in engineering applications.

Inter-Blade Vortex and Vortex Rope Characteristics of a Pump-Turbine in Turbine Mode under Low Flow Rate Conditions

Pump-turbines are often used to provide a stable power supply with a constant frequency in response to intermittent renewable energy resources. However, existing pumped-storage power stations often operate under off-design conditions because of the increasing amounts of inconsistent renewable resources that have been added to the grid. Under off-design low flow rate conditions, inter-blade vortex and vortex rope phenomena usually develop in the runner and draft tube passages, respectively, in turbine mode. These vortices cause complicated flow patterns and pressure fluctuations that destabilize the operation of the pump-turbine system. Therefore, this study investigates the influence of correlation between the inter-blade vortex and vortex rope phenomena under low flow rate conditions. Three-dimensional steady- and unsteady-state Reynolds-averaged Navier–Stokes equations were calculated with a two-phase flow analysis using a shear stress transport as the turbulence model. The inter-blade vortices in the runner passages were captured well at the low flow rate conditions, and the vortex rope was found to develop within a specific range of low flow rates. These vortex regions showed a blockage effect and complicated flow characteristics with backflow in the passages. Moreover, higher unsteady pressure characteristics occurred at locations where the vortices were especially pronounced.

Investigation into the outlying swirl instability in the hydro-turbine draft tube under part-load operation

The swirling trajectory of the draft tube flow improves the pressure recovery. Resultant flow instabilities are still problematic for an optimized hydropower generation. Engineering solutions are getting increasing importance. However; the depth of wall-mounted countermeasures is a challenging parameter. Performance assessment of countermeasures would rate dampened pressure pulsations in terms of their sources so as to place them at a proper location. A method for evaluating the instability in the outlying domain of the draft tube cone is herein suggested. Two part-load operating points with opposite polarity in pressure pulsations have been investigated. The numerical approach of Shear Stress Transport has been used, and experimental agreement of measurements for the pressure spectra is verified at the wall. Inlet flow conditions and the growth of the core instability are described. The outlying domain of the draft tube has been found to be composed of three distinct zones of influence: the shortest upper cone with upstream travelling influences, the longest middle cone with core excitation, and the lowermost cone dominated by the backward elbow influence. The wall response lies within a definite high-frequency range proper to the operating point. During admission, the frequency range becomes even wider due to high kinetic energy and the highest pressure amplitude of the draft tube is developed by the impact of blade passing frequency. The excitation from the vortex rope precession in the outlying domain strongly depends on the operating condition. Thus, further works should extend this study over a wider operation range, and mount pressure sensors on countermeasures in order to identify their own instability contribution.

Visualization of the elliptical form of a cavitation vortex rope and its collapse by two cameras

This article presents preliminary results of an experimental study of the upper-part load instability and the associated elliptical form of the cavitation vortex rope in a Francis turbine draft tube. The influence of the operating parameters on the onset and the development of the instability is first briefly studied by pressure measurements in the draft tube. Visualizations of the cavitation vortex rope and its associated elliptical form are performed by using two synchronized high-speed cameras spaced by an angle of 90°. This allows to reconstruct the instantaneous position of the vortex center along the draft tube. It is confirmed that using a single camera leads to biased estimations of the cavitation volume fluctuations. The breathing behaviour of the vortex rope, responsible for the pressure pulsations according to several authors, cannot be definitely demonstrated without a proper reconstruction of the vortex shape based on high-speed videos from several positions. Finally, unique intermittent collapses of cavitation in the vortex center, giving rise to two distinct cavities connecting on both sides the vortex rope, are highlighted.

Experimental investigation of vortex ring formation as a consequence of spiral vortex re-connection

The unsteady shape variation of a cavitating spiral vortex is experimentally studied pointing out the repetitive creation of vortex ring (or vortex loop). To generate the spiral vortex structure, a newly designed vortex generator is employed. This generator is used to study the decelerated swirling flow leading to the spiral vortex formation as a consequence of spiral vortex breakdown, which is considered to be the main triggering mechanism for the occurrence of the coherent vortex rope structure in the Francis turbine draft tube operated at part load conditions. Thanks to its design, the vortex generator enables to change the ratio between fluxes of axial momentum and tangential moment of momentum of generated swirl. Using this set-up, the behavior of the vortex structure changes in a similar way as the flow rate variation in the draft tube of Francis turbine. At certain flow conditions, the spiral vortex movement is characterized by sudden spiral entanglement leading to disconnection of a vortex ring and followed by spiral reconnection. Thanks to the transparent diffuser of the swirl generator apparatus, both high-speed camera recording of the cavitating vortex and PIV measurements of velocity fields at cavitationfree conditions are employed. The main aim of this paper is to link the visual observation of the above described vortex dynamics with the velocity fields measured in one longitudinal and one cross-sectional planes.

Analysis of a propeller turbine operated in a full load operating point

A full load operating point of a propeller turbine is investigated both numerically and experimentally. Steady state and unsteady flow simulations, using four different turbulence models with grid sizes up to 100 million elements, are validated with experimental results which are performed according to IEC 60193 standard. Due to the high swirl number of S = -0.175 at the runner outlet, steady state and unsteady RANS simulations are no longer capable to predict the correct flow field in the draft tube of the turbine. Hence, unsteady simulations with scale-resolving turbulence models, are performed. As a result of the high swirl a full load vortex rope is generated starting at the runner hub developing in the center of the draft tube. The integral quantities head and torque as well as the axial and tangential velocity components at five evaluation lines in the draft tube are validated with experimental results. Velocity components are both time- and phase-averaged, using the runner as trigger signal. Additionally, a comparison of local quantities, head loss in the draft tube and the pressure recovery factor is carried out. The turbulence quantities of the different approaches are evaluated at the evaluation lines applying the viscosity ratio or energy spectra at discrete points in the draft tube of the turbine.

POD analysis of a vortex rope with transient boundary conditions

The paper presents the results of a POD analysis of the flow in a swirl generator. The generator has two separate inlets – one straight axial inlet and one tangential inlet ended by a spiral case. This allows to set different operating points, which are in turn connected with different shapes of the vortex rope present in the flow field. In this case, the investigation is aimed at linearly varying flow rates at the boundaries, namely, the ratio of the tangential flow rate is increased from 40 % up to more than 80 % of the total flow rate, which is maintained constant. Substituting the POD modes into the energy equation provides additional insight into the energy fluxes between the modes and consequently into a major part of the turbulent flow energy cascade as the input data were gained from a CFD simulation using scale-resolving Stress-Blended Eddy Simulation model. The POD modes can be used to construct a simplified model of the vortex rope dynamics covering a wider operating range rather than just one point.

A Novel Passive Method to Control the Swirling Flow with Vortex Rope from the Conical Diffuser of Hydraulic Turbines with Fixed Blades

In this paper, we introduce a novel passive control method to mitigate the unsteadiness effects associated to the swirling flows with self-induced instabilities. The control method involves a progressive throttling cross-section flow at the outlet of the conical diffuser. It adjusts the cross-section area with a diaphragm while maintaining all positions of the circular shape centered on the axis. It improves the pressure recovery on the cone wall while the pressure fluctuations associated with the self-induced instability are mitigated as it adjusts the cross-section area. It can adjust the diaphragm in correlation with the operating conditions of the turbine. We investigated the passive control method on a swirl generator, which provides a similar flow as a hydraulic turbine operated at a partial discharge. The plunging and rotating components are discriminated using the pressure fluctuation on the cone wall to provide a clear view of the effects induced by this passive control method. As a result, the novel proof of concept examined in this paper offers valuable benefits as it fulfils a good balance between the dynamical behavior and the hydraulic losses.

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.

Influences of Rotational Speed Variations on the Flow-Induced Vibrational Performance of a Prototype Reversible Pump Turbine in Spin-No-Load Mode

Abstract During the spin-no-load mode, vibrational performance of the reversible pump turbine is an important criterion for the evaluation of the operational performances of the power station. In the present paper, the influences of rotational speed variations on the vibrational performances of the whole unit (including the top cover, the upper, and the lower brackets) are experimentally investigated with discussions of their sources and propagation characteristics. According to the whole vibrational levels and the dominant frequencies of the vibration signals obtained at the top cover, the investigated cases with different rotational speeds could be divided into three partitions with their main characteristics given as follows. In the first partition (with low rotational speeds), the vibrational level is quite limited, and its source is the pressure fluctuation generated by the swirling vortex rope in the draft tube. In the second partition (with medium rotational speeds), the vibrational level gradually increases and its source is the mechanical aspects of the impeller rotation. In the third partition (with high rotational speeds), the vibrational level is prominent with a prominent swirling vortex rope in the draft tube and intensive rotor–stator interactions in the vaneless space (VS). For the vibrations of the upper and the lower brackets, the vibrations mainly originate from the mechanical aspects of the impeller rotation and the amplitudes of the dominant frequency also increase with the increment of the rotational speed. Finally, differences between the vibrational performances of the spin-no-load mode and the generating mode are discussed.

Numerical Simulation of the Transient Flow in a Pump-Turbine During the Load Rejection Process With Special Emphasis on the Cavitation Effect

Abstract At present, pumped-storage power technology is the only available and effective way for the load balancing and energy storage in the grid network scale. During the frequent switch back and forth conditions, there are severe pressure pulsation and cavitation in pump-turbines. However, their generation mechanism has not been determined yet. This work contributes to the numerical simulation of the transient behaviors in a prototype pump-turbine during the load rejection process with special emphasis on cavitation effect. In this study, the two-dimensional dynamic remesh and variable speed slide mesh methodologies were employed to perform the simulation of the transient single-phase flow and cavitation flow in a pump-turbine. The simulation results of single-phase flow and cavitation flow were both consistent with the experimental data except in local regions based on the experimental validation of prototype tests. However, the numerical results considering cavitation effects have a better behavior than those of single-phase flow in the predictions of pressure pulsation and rotational speed. Then, the cavitation flow simulation results were analyzed deeply, especially in pressure pulsation and cavitation flow field. Analysis revealed that three typical complex frequency components of pressure were captured in the cavitation flow, which significantly affect the axial hydraulic thrust on the runner. And it is validated that they are primarily induced by the cavity collapse near the trailing edges of the runner blades in reverse pump mode and the interaction between cavitation and vortex rope in draft-tube in turbine mode.

On the High-partial-load Pulsation in Francis Turbines

Francis turbines with high specific speed may produce intense pulsations of system pressure when operating at approximately 70-85% of best-efficiency discharge. Frequencies between 1 and 4 times runner rotation are typical. The paper recalls and explains the particular set of properties. The pulsation is due to instability of the second-lowest eigenmode of the hydraulic system containing a cavitating draft tube vortex. A 1D model in frequency domain is used - with emphasis on the crucial role of the finite velocity of swirl propagation - to show how this mode can become unstable and give rise to a strong ‘breathing’ pulsation. A parameter study shows the well-known influence of draft tube pressure and why the behavior of prototype turbines is often different from the reduced-scale model. Some of the phase properties are explained by a 2D acoustic model. The phase reversal between upstream and downstream pressures is a consequence of the mode shape, with roughly two quarter waves of pressure along the cavitating vortex rope. The stability is quite sensitive to 2D effects; therefore the present 1D model, while useful for understanding the phenomena, is not likely to permit reliable predictions.