Vortex Rope Research Articles

Characterization of The Rotating Vortex Rope Pressure Oscillations in a Kaplan Model Turbine Draft Tube

Characterization of The Rotating Vortex Rope Pressure Oscillations in a Kaplan Model Turbine Draft Tube

Experimental investigation on the characteristics and alleviation of the upper part load pressure fluctuation in a Francis turbine

The Francis turbine is a renowned reaction hydraulic turbine known for its remarkable ability to convert the energy of flowing water into mechanical energy. Operating near the best efficiency point, the Francis turbine experiences the upper part load pressure fluctuation, which is characterized by an abrupt high amplitude increment and decay and can put the stability of a power plant at risk. This study experimentally investigated the amplitude-frequency characteristics of this special pressure fluctuation. The findings indicate that the most powerful amplitude reaches up to 15.35% of the tested water head in the vaneless space, and the characteristic frequency ranges from 1.6 to 2.3 times the rotating frequency. Notably, the study has found that the vortex rope in the draft tube exhibits a slender and spring-like within the upper part load region, and it looks like a spring stretching and compressing during the evolution of vortex rope, implying energy accumulation and release. Furthermore, a technical approach to alleviate high-amplitude pressure fluctuation in upper part load region is proposed that controls the velocity distributions at the runner outlet. A comparison of experimental test evidence shows that an impressive result is obtained with a reduction of the maximum amplitude from 10.75% to 3.52% in spiral casing, and from 15.35% to 3.03% in the vaneless space. In addition, the first-order frequencies at the spiral casing are primarily reduced, while the draft tube shows local increment, and the runner blade passage frequency becomes dominant in the vaneless space.

Axial water jet injection in a low head Francis turbine at part load

The part load operation of a Francis turbine generally develops a rotating vortex rope (RVR) in the draft tube, generating flow instabilities and pressure pulsations. The present work reports the axial water injection method to mitigate the RVR and its associated pulsations in the draft tube. The runner cone is modified to install a nozzle of three different designs for water jet injection. Water is injected axially at the rate of 0.5% to 2% of the main flow. Spectral analysis is performed to assess the pressure amplitude and dominating frequency affected by the water injection. Spectrogram and spatial harmonic decomposition show a reduction in synchronous and asynchronous pulsation amplitudes. The pressure recovery in the draft tube cone is also improved. The spectral analysis shows that the water injection at 1% and 1.25% is the most effective. The water injection above 1.25% to 2% causes pulsations near the draft tube bend as the water jet bounces back after striking the elbow. A sharp decrease in the Strouhal frequency is observed above 0.5% jet injection. The efficiency of the turbine is also improved by water jet injection.

Upper part-load instability in a reduced-scale Francis turbine: an experimental study

Francis turbines with medium or high specific speeds may experience a particular type of instability in the upper part load in which the precessing vortex has an elliptical shape. The occurrence of the upper part-load instability (UPLI) is accompanied by large-amplitude pressure fluctuations at a distinct frequency between 2 and 4 times the runner rotational speed. This paper experimentally investigates UPLI for a reduced-scale Francis turbine. To investigate the causal factors of this instability, draft tube pressure measurements, particle image velocimetry, and high-speed flow visualizations have been performed at several operating points under cavitation and cavitation-free conditions. It is shown for the first time that for an operating point within the UPLI range, the vortex always features a circular section in cavitation-free conditions, which is preserved even after the initial appearance of cavitation. It is only below a certain Thoma number that the vortex section turns into an ellipse and shows an abrupt increase in pressure fluctuations. Analysis of the phase-averaged velocity fields reveals that a concentrated vortex with a large precession radius is a prerequisite for UPLI, while the instantaneous velocity fields clearly illustrate the asymmetric velocity distribution around the elliptical vortex. The existence of a breathing mode and the intermittent formation of two side vortices along the elliptical vortex rope are also evidenced by high-speed flow visualizations. These results provide a much deeper insight into the flow structures that favor the development of UPLI and help delimit its thresholds to higher precision, and thus, prevent its occurrence during turbine operations.

Runner cone drilling and slotting to reduce vortex rope - induced pressure fluctuations in a Francis turbine

Pressure fluctuations caused by helical vortex rope in the draft tube is one of the main reasons for the instability of Francis turbines. Previous studies showed that drilling holes on runner cone can reduce the amplitude of pressure fluctuation. In this study, based on the runner cone which has been drilled one hole, two kinds of new runner cones are designed by means of drilling two more holes (2 holes runner cone) and drilling a slot (2 holes and 1 slot runner cone). Pressure fluctuation and flow pattern observation experiments as well as numerical simulations at partial load operation condition in a Francis turbine were carried out for the three types of runner cones. Analyses were performed to reveal the generation mechanism of the low frequency pressure fluctuations. The results showed that low frequency fluctuations are related to the cyclical rotation of helical vortex rope. Holes and slots on the runner cone have little impact on the efficiency of the Francis turbine. Slotting on the runner cone can reduce the amplitude of pressure fluctuation induced by vortex rope to a maximum of 29.41%. Hence, proper slotting on the runner cone can effectively reduce the low-frequency fluctuation caused by vortex rope in Francis turbines. These findings provide a new idea to improve the hydraulic instability of Francis turbines induced by vortex rope.

Numerical Analysis of the Flow by Using a Free Runner Downstream the Francis Turbine

The current requirements of industrialized countries require the use of as much renewable energy as possible. One significant problem with renewable energy is that the produced power fluctuates. Currently, the only method available for energy compensation in the shortest time is given by hydroelectric power plants. Instead, hydroelectric power plants (especially the plants equipped with hydraulic turbines with fixed blades) are designed to operate in the vicinity of the optimal operating point with a maximum ±10% deviation. The energy market requires that hydraulic turbines operate in an increasingly wide area between −35% to 20% from the optimum operating point. Operation of hydraulic turbines far from the optimum operating point involves the appearance downstream of the turbine of a decelerated swirling flow with hydraulic instabilities (known in the literature as the vortex rope). The main purpose of this paper is to investigate numerically a new concept by using a free runner downstream on the main hydraulic runner turbine more precisely in the draft tube cone. The free runner concept requires rotations at the runaway speed with vanishing mechanical torque. The main purpose is to redistribute the total pressure and the moment between the shaft and the periphery. In addition, the free runner does not modify the operating point of the main hydraulic turbine runner.

Experimental and numerical investigation of vortex flows and pressure fluctuations in a high-head pump-turbine

Vortex flows and pressure fluctuations, especially in the draft tube and vaneless space, have crucial impact on stable and safety operation for a high-head pump-turbine due to be fast adjusted and frequently switched between pump and turbine modes and often down to the deep part load. To more accurately investigate the vortex flows and pressure fluctuations in a high-head pump-turbine with splitter blades runner, a reliable approach by combined experiments with numerical simulations is presented in this paper. Laser Doppler Velocimetry measurements and high-speed camera observations inside the draft tube have been conducted at all the investigated points. Velocity profiles, vortex ropes visualization, the instantaneous tangential velocity and pressure fluctuations in both measured and simulated are presented to validate the numerical simulations and classify the vortex flow patterns. Meanwhile, the relationship between formation, development and breakdown of the vortex rope, the processional frequency and swirl intensity has been linked up and experimentally validated. Variation regularity of pressure fluctuations along the flow passage at different operating conditions has been quantitatively substantiated with measurements and predictions. Validation of predicted results versus experimental data shows that the presented numerical simulations can give very good results of vortex structures and the predicted fluctuation's frequencies. The pressure fluctuations amplitude of both the measured and the predicted in turbine mode agree well in the vortex rope-free zone, but has relatively larger deviations out of the vortex rope-free zone. It also demonstrated that the runner with splitter blades can restrain development of vortex flow and decrease the amplitude of pressure fluctuations for a high-head pump-turbine. The presented approach has been applied to successfully developed the high-head pump-turbines for several Pumped Storage Power Plants in China and demonstrated very useful in hydraulic optimizations.

Experimental investigation of an adjustable guide vane system in a Francis turbine draft tube at part load operation

Hydraulic turbines are increasingly used to regulate the electrical grid. They thus have to operate more often at off-design conditions where instabilities may develop leading to large pressure pulsations decreasing the life of the machines. In the present study, an adjustable guide vane system in the draft tube of a single regulated hydraulic turbine is developed. The aim is to mitigate the pressure pulsations when the turbine operates at part load operational regimes without significantly compromising the hydraulic efficiency of the turbine. The draft tube guide vane system, named DTGV, is composed of three guide vanes. Each guide vanes comprise two individual hydrofoils. The upper hydrofoil can rotate around an axis up to ±45°, and the lower is fixed. Therefore, the guide vanes are adjustable. The rotating vortex rope mitigation was investigated at two part-load operational regimes at a 12 m turbine head, a) Q/QBEP = 0.59, and b) Q/QBEP = 0.71. The results indicate that the DTGV system significantly mitigates the pressure pulsations, thereby improving the flexible operation of the turbine. The best mitigation outcome was for 0 GV configuration at both part-load operating conditions. The mitigation effects are better when the operating condition approaches BEP, and the rotating mode of the pressure pulsations tends to go towards the runner frequency (f/f0 = 1). The hydraulic efficiency is found to be marginally affected by the guide vane system at part load operations and is improved at the best efficiency point (Q/QBEP = 1) and high load Q/QBEP = 1.21.

Evolution of unsteady vortex structures in the tip region of an axial compressor rotor

The evolution of unsteady vortex structures in the tip region of an axial compressor rotor is investigated based on delayed detached eddy simulation. The vortex structures are identified by the LTcri method, and the velocity fields are visualized by the particle tracing method. The results show that the evolution of the tip leakage vortex (TLV) can be divided into three phases: the generation phase, the development phase, and the dissipation phase. The unsteadiness of the flow field mainly appears in the dissipation phase as a consequence of the unsteady secondary tip leakage. There are three primary unsteady vortex structures caused by the tip leakage flow (TLF), and these vortex structures are related to each other as a feedback loop. The intermittent formation of the vortex ropes leads to the breakdown of the TLV and thus results in the roll-up of the backflow vortex (BFV) due to the radial velocity gradient. The secondary leakage of the BFV locally enhances the TLF jet and affects the formation of the vortex ropes in turn. This feedback loop causes the unsteady behavior of the TLF and has great impacts on the performance and stability of the compressors.

Investigation of cavitating vortex rope instabilities and its suppression inside a Francis turbine model with Thoma number variation

Cavitating vortex rope at part load (PL) condition at lower values of the Thoma number (σ) induces severe pressure fluctuation and efficiency reduction in a Francis turbine, which ultimately hinders continuous energy production. Installation of fins at draft tube (DT) can mitigate these instabilities and can safeguard the turbine operation with lower maintenance costs. The effect of fins on hydraulic performance and internal flow physics at PL condition with the variation of σ is examined in the present numerical investigation. For the two extreme opposite values of σ, the flow characteristics are predicted accurately for the turbine with and without fins by conducting transient simulations using ANSYS-CFX. The numerical findings on the structured and unstructured grid points are validated with the experimental results. The turbine's performance remains constant for higher values of Thoma numbers, and as the value decreases, the performance declines. The cavitation vortex rope formation inside the DT with fins is mitigated significantly at the minimum σ, while at the maximum value, the vortex rope with bubble generation is restricted. Compared to the without fin case, the swirl intensity is minimized remarkably (68%) with the presence of fins at the lowest σ. The maximum cavitation rate is manifested by the DT without fins, which is about 60% higher than the DT with fins. At minimum σ, extreme pressure pulsations are induced inside the DT without fins, which are reduced by 43% in the finned draft tube. Therefore, stable energy production is maximized with the installation of fins at both Thoma numbers.

Study of vortex rope for the flow field pulsation law

When a hydro turbine deviates from the best efficiency conditions and carries the partial load, the helical vortex rope appears in the center of the draft tube. It will cause the unstable operation. In order to study the characteristics of the vortex rope more clearly, this study monitored the pressure pulsation of the draft tube through large eddy simulation. The temporal-spatial features of spiral vortex rope are well analyzed and visualized by pulsation tracking network (PTN). The results show that the draft tube vortices can be divided into spiral vortex, fragmented vortex and reunited vortex. The frequency of 1.9 Hz is the dominant frequency, which is located in the center of the spiral vortex rope. The area proportion of 1.9 Hz is reduced to 3.63% at the outlet. There are three types of phase difference characteristics. Type-1 is through the center and the whole road section. Compared with linearity, S-shape represents the radial hysteresis of circumferential pressure wave propagation. Type-2 is similar to a circle. Type-3 is similar to the combination of Type-1 and Type-2. This study is of great significance for the design and operation stability of hydro turbine.

Investigation of Flow-Induced Instabilities in a Francis Turbine Operating in Non-Cavitating and Cavitating Part-Load Conditions

The integration of intermittent renewable energy resources to the grid system requires that hydro turbines regularly operate at part-load conditions. Reliable operation of hydro turbines at these conditions is typically limited by the formation of a Rotating Vortex Rope (RVR) in the draft tube. In this paper, we investigate the formation of this vortex using the scale-resolving methods SST-SAS, wall-modeled LES (WMLES), and zonal WMLES. The numerical results are first validated against the available experimental data, and then analyzed to explain the effect of using different scale-resolving methods in detail. It is revealed that although all methods can capture the main features of the RVRs, the WMLES method provides the best quantitative agreement between the simulation results and experiment. Furthermore, cavitating simulations are performed using WMLES method to study the effect of cavitation on the flow in the turbine. These effects of cavitation are shown to be highly dependent on the amount of vapor in the RVR. If the amount of vapor is small, cavitation induces broadband high-frequency fluctuations in the pressure and forces exerted on the turbine. As the amount of cavitation increases, these fluctuations tend to have a distinct dominant frequency which is different from the frequency of the RVR.

Flow Evolution and Energy Loss Mechanism in Accidental Shutdown Process of a Large Submersible Mixed-Flow Pump System

To investigate the accidental shutdown process for a large vertical and submersible mixed-flow pump unit in a drainage pumping station, a three-dimensional numerical method for this transition process was proposed according to the angular momentum balance theorem and rigid body rotation technology. We obtained the transient performance curves of the unit and the internal flow characteristics of the full flow channel. In addition, we also analyzed the energy loss distribution of the through-flow components during this progress based on the entropy production theory. The results show that the whole runaway process needs to go through four stages: the pump mode, the pump braking mode, the turbine mode, and the stable runaway mode. The pressure amplitude changes greatly in impeller and guide vanes, and the main fluctuation frequency is the blade frequency. There are higher harmonic frequencies in the dynamic rotor-stator interface. As the rotating speed increases in turbine mode, the negative pressure area near the impeller blade’s trailing edge gradually increases. The entropy production method can be used to determine the location, intensity of energy loss during the transient process. If the rotating speed exceeds the allowed value in the runaway turbine mode, the interaction between blade tip vortex and hub vortex rope may cause loss of stability.

Numerical investigation of air admission influence on the precessing vortex rope in a Francis turbine

Precessing vortex rope (PVR) plays a key role in inducing hydraulic resonance in Francis turbines operating at partial load, possibly degrading power plant stability and availability. Air injection into the runner cone is a suitable mitigating alternative; however, the influence mechanism of air injection on PVR remains unclear. The principal objective of this study was to establish response relationships between the characteristic parameters and air injection by the method of computational fluid dynamics (CFD) considering the fluid components of water, water vapour and air. The findings show that cavitation flow can be completely suppressed by slight air injection; however, the helical vortex structures are persisted at 1.0% and 2.0% air volume fractions, and the static pressure recovery is improved together with a slight increase in the hydraulic loss. At 3.0% air volume fraction, the vortex structure completely disappears, leaving an umbrella-shaped structure, with no pressure vibration arising in the turbine. Moreover, the physical mechanism of reducing the pressure amplitudes is clarified. This results clarify the influence mechanism of air injection on PVR, and contribute to steadily extending the flexibility of the operating range of the turbine during the engineering application.

S-shaped characteristics of pump turbine with large guide vane opening by experimental and numerical analysis

Pumped storage hydropower is perhaps the most reliable solution to the energy crisis, but its development and application are limited by hydraulic instability, such as S-shaped characteristics. In this paper, based on the S-shaped region pressure fluctuation test of a model pump turbine, the connection between the guide vane opening and S-shaped pressure fluctuation is established, and a guide vane opening of 28 mm is selected for numerical analysis to expand the discussion of the S-shaped pressure fluctuation characteristics to large guide vane openings. The frequency components of the vaneless space are extracted by fast Fourier transform analysis, and the rotational stall frequency is identified. Among them, the blade passage frequency and its multiplier are induced by the rotor–stator interaction, and the low-frequency component of pressure fluctuations is induced by the rotational stall and vortex rope. Based on this, the internal flow and flow rate along the S-shaped characteristic curve and their correlation with the rotational stall are analyzed. In addition, the percentage of and variation in the entropy production rate in the S-shaped region are also analyzed.

A new draft tube shape optimisation methodology of introducing inclined conical diffuser in hydraulic turbine

A grid-connected integrated system incorporating cascade hydropower, solar/photovoltaic, and wind power requires a hydraulic turbine to operate frequently under off-design conditions, resulting in a series of instability issues, such as pressure fluctuations. In this study, a new draft tube shape optimisation methodology of introducing an inclined conical diffuser for a Francis turbine was proposed to eliminate vortex rope and mitigate the associated pressure fluctuations. Four different inclination angles in the range of 0°–16.8° were considered to investigate the effectiveness of the modified draft tube. The results demonstrated that the maximum pressure fluctuation amplitude within the draft tube was 1.64% times the equivalent pressure under the rated water head for the modified draft tube, showing an 18.4% reduction compared with that of the traditional case. In addition, an overall index of Pdegree was proposed to quantitatively measure the mitigating impact of this type of modified draft tube on the pressure fluctuations. When the inclination angle is 16.8°, the Pdegree on the downstream and upstream sides decreases by 34.1% and 23.1%, respectively, when compared to the traditional draft tube. Based on a comprehensive comparison of the vortex rope, instantaneous axial velocity, and pressure distribution, the mitigating mechanism of the modified draft tube was further explored. Finally, further optimisation direction was pointed out by means of entropy production theory.

Research on the Vortex Rope Control Techniques in Draft Tube of Francis Turbines

Francis turbines are most widely used in hydropower due to their characteristics which include a fast response and wide time-scale operation. The vortex rope inside Francis turbines is a common flow phenomenon, which always causes strong vibration, pressure pulsations, fatigue load, and even serious failure of the components. Vortex suppression methods can effectively change the velocity and pressure distribution of the flow field in the draft tube, reduce the volume of vortex rope and the amplitude of pressure pulsation, inhibit the development of cavitation erosion, and improve the operation stability of the hydro turbine. However, the vortex suppression method is not suitable for all working conditions, and the vortex suppression effect is also different. There are still many problems with how to analyze the vortex suppression effect and practicability of the turbine from multi-dimensions. It is of great significance to analyze the vortex suppression techniques and their practicability in hydraulic turbines from various aspects. The primary focus of the present study is to analyze the hazards of vortex rope in draft tubes and summarize the methods of suppressing vortex rope and pressure pulsation. This review article provides a basis for controlling the vortex rope in the draft tube, which can help the designers choose the suitable control method to mitigate it. Future research directions are also briefly discussed.

Mitigation of swirling flow with a vortex rope by passive installations—Theory, simulations, and experiments

This work is focused on reduction of swirling in cases from hydraulic machinery and, thus, mitigation of instabilities associated with swirling flow. We study this problem analytically using the torque integral equation, numerically using computational fluid dynamics (CFD) simulations, and experimentally using a swirl generator that generates swirling flow approximating that in a draft tube of a hydro turbine operating at a part load, featuring an unsteady spiral vortex—the vortex rope. The analytical description elucidates the effect of different installations on the circumferential velocity. Unconventional conical perforated installations targeting at increasing the dissipation were proposed and tested. The rather unsatisfactory results led to proposing fins placed unconventionally away from the wall, closer to the diffuser axis, which were subsequently shown to be able to reliably suppress the unsteady vortex rope. Their effect is documented by an analysis of CFD results including the proper orthogonal decomposition as well as experimental observations and measurements.

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.

Vortex-Pressure Fluctuation Interaction in the Outlet Duct of Centrifugal Pump as Turbines (PATs)

The outlet duct is the only outlet flow component of the pump as turbines (PATs). The flow state within it not only affects its operation stability but also influences the safe operation of downstream process equipment. The undesirable flow phenomenon of the vortex is the main reason for pressure pulsations and vibrations; therefore, it is important to adopt simulation and experimental methods to reveal the vortex-pressure fluctuation interaction characteristics in the outlet duct of PATs. Firstly, the spatial and temporal evolution of the vortex in the outlet duct under different operation conditions were compared based on the Q-vortex identification criterion. Subsequently, the frequency components corresponding to local high amplitude vorticity and pressure pulsations were clarified. Finally, the relationship between vortex evolution and the pressure pulsations in the outlet duct was established. The results showed that the flow rates of the turbine significantly affected the spatial and temporal evolution of the vortex rope in the outlet duct. The front chamber leakage flow and vortex shedding from the blade trailing edge also influenced the vortex distribution characteristics in the outlet duct. The dominant frequency of the pressure pulsation in the outlet duct was 6 fn under different operating conditions, and the amplitude of the pressure pulsation increased with the flow rates. The effect of vortex evolution on the local pressure pulsation characteristics decreased with increasing flow rates. The results can be used to improve and stabilize the operation and further optimization of PATs.