Vortex Rope Formation in a High Head Model Francis Turbine

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Rahul Goyal ◽  
Michel J. Cervantes ◽  
B. K. Gandhi
Keyword(s):  
Physical Mechanism ◽  
Swirling Flow ◽  
Draft Tube ◽  
Dynamic Pressure ◽  
Angular Position ◽  
Francis Turbine ◽  
Head Model ◽  
Stagnation Region ◽  
Vortex Rope ◽  
High Head

Francis turbine working at off-design operating condition experiences high swirling flow at the runner outlet. In the present study, a high head model Francis turbine was experimentally investigated during load rejection, i.e., best efficiency point (BEP) to part load (PL), to detect the physical mechanism that lies in the formation of vortex rope. For that, a complete measurement system of dynamic pressure, head, flow, guide vanes (GVs) angular position, and runner shaft torque was setup with corresponding sensors at selected locations of the turbine. The measurements were synchronized with the two-dimensional (2D) particle image velocimetry (PIV) measurements of the draft tube. The study comprised an efficiency measurement and maximum hydraulic efficiency of 92.4 ± 0.15% was observed at BEP condition of turbine. The severe pressure fluctuations corresponding to rotor–stator interaction (RSI), standing waves, and rotating vortex rope (RVR) have been observed in the draft tube and vaneless space of the turbine. Moreover, RVR in the draft tube has been decomposed into two different modes; rotating and plunging modes. The time of occurrence of both modes was investigated in pressure and velocity data and results showed that the plunging mode appears 0.8 s before the rotating mode. In the vaneless space, the plunging mode was captured before it appears in the draft tube. The physical mechanism behind the vortex rope formation was analyzed from the instantaneous PIV velocity vector field. The development of stagnation region at the draft tube center and high axial velocity gradients along the draft tube centerline could possibly cause the formation of vortex rope.

scholarly journals Manipulation of the swirling flow instability in hydraulic turbine diffuser by different methods of water injection

2018 ◽  
Vol 180 ◽  
pp. 02090 ◽  
Author(s):  
Pavel Rudolf ◽  
Jiří Litera ◽  
Germán Alejandro Ibarra Bolanos ◽  
David Štefan
Keyword(s):  
Swirling Flow ◽  
Flow Instability ◽  
Draft Tube ◽  
Water Injection ◽  
Hydraulic Turbine ◽  
Operating Conditions ◽  
Francis Turbine ◽  
Necessary Condition ◽  
Vortex Rope ◽  
Wide Range

Vortex rope, which induces substantial pressure pulsations, arises in the draft tube (diffuser) of Francis turbine for off-design operating conditions. Present paper focuses on mitigation of those pulsations using active water jet injection control. Several modifications of the original Susan-Resiga’s idea were proposed. All modifications are driven by manipulation of the shear layer region, which is believed to play important role in swirling flow instability. While some of the methods provide results close to the original one, none of them works in such a wide range. Series of numerical experiments support the idea that the necessary condition for vortex rope pulsation mitigation is increasing the fluid momentum along the draft tube axis.

Experimental Evidence of Hydroacoustic Pressure Waves in a Francis Turbine Elbow Draft Tube for Low Discharge Conditions

2009 ◽  
Vol 131 (8) ◽  
Author(s):  
Jorge Arpe ◽  
Christophe Nicolet ◽  
François Avellan
Keyword(s):  
Pressure Fluctuation ◽  
Swirling Flow ◽  
Draft Tube ◽  
Flow Direction ◽  
Pressure Fluctuations ◽  
Operating Conditions ◽  
Hydropower Plant ◽  
Francis Turbine ◽  
Scale Model ◽  
Vortex Rope

The complex three-dimensional unsteady flow developing in the draft tube of a Francis turbine is responsible for pressure fluctuations, which could prevent the whole hydropower plant from operating safely. Indeed, the Francis draft tube is subjected to inlet swirling flow, divergent cross section, and the change of flow direction. As a result, in low discharge off-design operating conditions, a cavitation helical vortex, so-called the vortex rope develops in the draft tube and induces pressure fluctuations in the range of 0.2–0.4 times the runner frequency. This paper presents the extensive unsteady wall pressure measurements performed in the elbow draft tube of a high specific speed Francis turbine scale model at low discharge and at usual plant value of the Thoma cavitation number. The investigation is undertaken for operating conditions corresponding to low discharge, i.e., 0.65–0.85 times the design discharge, which exhibits pressure fluctuations at surprisingly high frequency value, between 2 and 4 times the runner rotation frequency. The pressure fluctuation measurements performed with 104 pressure transducers distributed on the draft tube wall, make apparent in the whole draft tube a fundamental frequency value at 2.5 times the runner frequency. Moreover, the modulations between this frequency with the vortex rope precession frequency are pointed out. The phase shift analysis performed for 2.5 times the runner frequency enables the identification of a pressure wave propagation phenomenon and indicates the location of the corresponding pressure fluctuation excitation source in the elbow; hydroacoustic waves propagate from this source both upstream and downstream the draft tube.

Unsteady vortical flow simulation in a Francis turbine with special emphasis on vortex rope behavior and pressure fluctuation alleviation

2017 ◽  
Vol 231 (3) ◽  
pp. 215-226 ◽  
Author(s):  
Xianwu Luo ◽  
An Yu ◽  
Bin Ji ◽  
Yulin Wu ◽  
Yoshinobu Tsujimoto
Keyword(s):  
Pressure Fluctuation ◽  
Swirling Flow ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Flow Simulation ◽  
Vortical Flow ◽  
Francis Turbine ◽  
Vortex Rope ◽  
Spiral Vortex ◽  
Baroclinic Torque

Hydro turbines operating at partial flow conditions usually have vortex ropes in the draft tube that generate large pressure fluctuations. This unsteady flow phenomenon is harmful to the safe operation of hydropower stations. This paper presents numerical simulations of the internal flow in the draft tube of a Francis turbine with particular emphasis on understanding the unsteady characteristics of the vortex rope structure and the underlying mechanisms for the interactions between the air and the vortices. The pressure fluctuations induced by the vortex rope are alleviated by air admission from the main shaft center, with the water-air two phase flow in the entire flow passage of a model turbine simulated based on the homogeneous flow assumption. The results show that aeration with suitable air flow rate can alleviate the pressure fluctuations in the draft tube, and the mechanism improving the flow stability in the draft tube is due to the change of vortex rope structure and distribution by aeration, i.e. a helical vortex rope at a small aeration volume while a cylindrical vortex rope with a large amount of aeration. The preferable vortex rope distribution can suppress the swirl at the smaller flow rates, and is helpful to alleviate the pressure fluctuation in the draft tube. The analysis based on the vorticity transport equation indicates that the vortex has strong stretching and dilation in the vortex rope evolution. The baroclinic torque term does not play a major role in the vortex evolution most of the time, but will much increase for some specific aeration volumes. The present study also depicts that vortex rope is mainly associated with a pair of spiral vortex stretching and dilation sources, and its swirling flow is alleviated little by the baroclinic torque term, whose effect region is only near the draft tube inlet.

scholarly journals Numerical and experimental investigation of the effect of baffles on flow instabilities in a Francis turbine draft tube under partial load conditions

2019 ◽  
Vol 11 (1) ◽  
pp. 168781401882446 ◽  
Author(s):  
Xing Zhou ◽  
He-gao Wu ◽  
Chang-zheng Shi
Keyword(s):  
Swirling Flow ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Tangential Velocity ◽  
Operating Conditions ◽  
Dynamics Simulation ◽  
Flow Instabilities ◽  
Francis Turbine ◽  
Vortex Rope ◽  
Partial Load

An improved method for preventing vortex rope formation and alleviating the associated pressure fluctuations in turbine draft tubes is investigated using baffles in the draft tube to hinder the swirling flow emerging from a Francis turbine runner. A strong swirl produces flow instabilities and pressure fluctuations. Partial load operating conditions at the rated water head and three flow rates are taken into consideration. It is demonstrated using a computational fluid dynamics simulation that this method effectively eliminates the vortex rope, particularly when using four baffles. The amplitude of the pressure pulsation in the draft tube modified with four baffles was 0.42 times that in a traditional draft tube. The baffles were found to reduce the tangential velocity of the flow in the draft tube and consequently hinder the development of the fierce swirling flow. This type of decrease is more significant compared to the gradual decay due to viscous effects of the solid wall in a traditional draft tube. The conclusion was verified by the results of experiments conducted using a novel device. The measured increase in turbine efficiency exceeded 3% at the evaluated partial loading point, indicating improved economic performance of the turbine.

scholarly journals Effect of Air Injection on the Internal Flow Characteristics in the Draft Tube of a Francis Turbine Model

Processes ◽  
2021 ◽  
Vol 9 (7) ◽  
pp. 1182
Author(s):  
Seung-Jun Kim ◽  
Yong Cho ◽  
Jin-Hyuk Kim
Keyword(s):  
Flow Rate ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Flow Characteristics ◽  
Internal Flow ◽  
Air Injection ◽  
Low Flow ◽  
Francis Turbine ◽  
Navier Stokes ◽  
Vortex Rope

Under low flow-rate conditions, a Francis turbine exhibits precession of a vortex rope with pressure fluctuations in the draft tube. These undesirable flow phenomena can lead to deterioration of the turbine performance as manifested by torque and power output fluctuations. In order to suppress the rope with precession and a swirl component in the tube, the use of anti-swirl fins was investigated in a previous study. However, vortex rope generation still occurred near the cone of the tube. In this study, unsteady-state Reynolds-averaged Navier–Stokes analyses were conducted with a scale-adaptive simulation shear stress transport turbulence model. This model was used to observe the effects of the injection in the draft tube on the unsteady internal flow and pressure phenomena considering both active and passive suppression methods. The air injection affected the generation and suppression of the vortex rope and swirl component depending on the flow rate of the air. In addition, an injection level of 0.5%Q led to a reduction in the maximum unsteady pressure characteristics.

Hydraulic Instability Onset Detection in Kaplan Turbines by Monitoring Shaft Vibrations

2012 ◽  
Author(s):  
P. Pennacchi ◽  
P. Borghesani ◽  
S. Chatterton ◽  
A. Vania
Keyword(s):  
Experimental Test ◽  
Turbine Unit ◽  
Swirling Flow ◽  
Draft Tube ◽  
Operating Conditions ◽  
Low Flow ◽  
Operating Life ◽  
Vortex Rope ◽  
Power Load ◽  
Helical Vortex

Design of hydraulic turbines has often to deal with hydraulic instability. It is well-known that Francis and Kaplan types present hydraulic instability in their design power range. Even if modern CFD tools may help to define these dangerous operating conditions and optimize runner design, hydraulic instabilities may fortuitously arise during the turbine life and should be timely detected in order to assure a long-lasting operating life. In a previous paper, the authors have considered the phenomenon of helical vortex rope, which happens at low flow rates when a swirling flow, in the draft tube conical inlet, occupies a large portion of the inlet. In this condition, a strong helical vortex rope appears. The vortex rope causes mechanical effects on the runner, on the whole turbine and on the draft tube, which may eventually produce severe damages on the turbine unit and whose most evident symptoms are vibrations. The authors have already shown that vibration analysis is suitable for detecting vortex rope onset, thanks to an experimental test campaign performed during the commissioning of a 23 MW Kaplan hydraulic turbine unit. In this paper, the authors propose a sophisticated data driven approach to detect vortex rope onset at different power load, based on the analysis of the vibration signals in the order domain and introducing the so-called “residual order spectrogram”, i.e. an order-rotation representation of the vibration signal. Some experimental test runs are presented and the possibility to detect instability onset, especially in real-time, is discussed.

scholarly journals Influence of Upstream Disturbances on the Vortex Structure of Francis Turbine Based on the Criteria of Identification of Various Vortexes

Energies ◽  
2021 ◽  
Vol 14 (22) ◽  
pp. 7626
Author(s):  
Tao Guo ◽  
Lihui Xu ◽  
Wenquan Wang
Keyword(s):  
Flow Field ◽  
Vortex Structure ◽  
Draft Tube ◽  
Francis Turbine ◽  
Vortex Identification ◽  
Vortex Rope ◽  
Blade Passage ◽  
Small Opening ◽  
Turbulent Characteristics ◽  
Passage Vortex

The inter-blade passage vortex, the vortex rope of the draft tube, and the vortex in the guide apparatus are the characteristics of flow instability of the Francis turbine, which may lead to fatigue failure in serious cases. In the current study, in order to accurately capture the transient turbulent characteristics of flow under different conditions and fully understand the flow field and vortex structure, we conduct a simulation that adopts sliding grid technology and the large-eddy simulation (LES) method based on the wall-adapting local eddy viscosity (WALE) model. Using the pressure iso-surface method, the Q criterion, and the latest third-generation Liutex vortex identification method, this study analyzes and compares the inter-blade passage vortex, the vortex rope of the draft tube, and the outflow and vortex in the guide apparatus, focusing on the capture ability of flow field information by various vortex identification methods and the unique vortex structure under the condition of a small opening. The results indicate that the dependence of Liutex on the threshold is small, and the scale range of the flow direction vortex captured by Liutex is wider, but the ability of the spanwise vortex is relatively weak. The smaller the opening, the more disorderly the vortexes generated in each component and the more unstable the flow field. In the draft tube, the original shape of the vortex rope is destroyed due to the interaction between vortexes. Under the condition of a small opening, an inter-blade passage vortex is generated, affecting the efficient and stable operation of the turbine.

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

2020 ◽  
Vol 152 ◽  
pp. 770-780 ◽  
Author(s):  
Huan Cheng ◽  
Lingjiu Zhou ◽  
Quanwei Liang ◽  
Ziwu Guan ◽  
Demin Liu ◽  
...  
Keyword(s):  
Draft Tube ◽  
Francis Turbine ◽  
Vortex Rope
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