Numerical Simulation of Pressure Vibrations in a Francis Turbine Draft Tube With Air Admission

2014 ◽  
Author(s):  
Renfang Huang ◽  
An Yu ◽  
Xianwu Luo ◽  
Bin Ji ◽  
Hongyuan Xu
Keyword(s):  
Draft Tube ◽  
Load Condition ◽  
Flow Simulation ◽  
Rotation Frequency ◽  
Francis Turbine ◽  
Stable Operation ◽  
Hydraulic Efficiency ◽  
Operation Condition ◽  
Vortex Rope ◽  
Part Load

The pressure vibrations in a draft tube are harmful for the stable operation for a Francis turbine at part load conditions. In this paper, air admission is proposed to depress those pressure vibrations. The unsteady flow in a Francis turbine, whose hydraulic performance has been tested experimentally, is simulated at a part load operation condition. The flow simulation is conducted using RANS methods coupling with SST k-ω turbulence model. The results indicate the pressure vibrations in the turbine are reasonably predicted by the present numerical method. Based on the calculations, the dominant pressure vibration component for a hydro turbine operated at part-load condition is caused by the vortex rope in draft tube, and its frequency is near 0.2 times of the runner rotation frequency. The frequencies of pressure vibration do not change by air admission, and the pressure vibration amplitude decreases with the air admission. Further, the depression effect for pressure vibration would be improved if air admission is from the crown holes instead of the spindle hole. The results also indicate that the turbine hydraulic efficiency changes periodically with the pressure vibration induced by vortex rope in turbine draft tube, would be degraded with air admission from the spindle hole, and improved with air admission from the crown holes. With the increase of air admission, the turbine hydraulic efficiency would improve. The present research will be helpful for the safe operation of Francis turbines.

Generation of Twin Vortex Rope in the Draft-Tube Elbow of a Francis Turbine During Deep Part-Load Operation

2021 ◽  
Author(s):  
Mohammad Hossein Khozaei ◽  
Arthur Favrel ◽  
Toshitake Masuko ◽  
Naoki Yamaguchi ◽  
Kazuyoshi Miyagawa
Keyword(s):  
Linear Correlation ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Model Tests ◽  
Francis Turbine ◽  
Deep Part ◽  
Swirl Number ◽  
Vortex Rope ◽  
Part Load

Abstract This paper focuses on the generation of twin vortex rope in the draft-tube elbow of a Francis turbine at deep part-load operation through analyzing the results of model tests along with numerical simulations. Model tests, including pressure fluctuations measurements, are conducted over 10 speed factors. By considering the frequency of the pressure fluctuations with respect to the swirl intensity at the runner outlet, the part-load operating range is divided into three regimes, with two clear transitions between each occurring at swirl numbers 0.4 and 1.7. For operating conditions with a swirl number S>0.4, a linear correlation between the frequency of the precessing vortex core and the swirl number is established. During deep part-load regime (S>1.7), low-frequency pressure fluctuations appear. Their frequency feature another linear correlation with the swirl number. Unsteady CFD simulation of the full domain is performed to elucidate the generation mechanisms of the low-frequency fluctuations. By tracking the center of the vortical structures along the draft-tube, generation of three vortices in the elbow responsible for the pressure fluctuations at the lowest frequency is highlighted: the main PVC hits the draft-tube wall in the elbow resulting in its break down into three vortices rotating with half the rotational speed of the PVC. Two of the vortices rotate with opposite angular position, constituting a structure of twin vortices. The periodic rotation of these three vortices in the elbow induces the low-frequency pressure fluctuations.

Characteristics and Control of the Draft-Tube Flow in Part-Load Francis Turbine

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Ri-kui Zhang ◽  
Feng Mao ◽  
Jie-Zhi Wu ◽  
Shi-Yi Chen ◽  
Yu-Lin Wu ◽  
...  
Keyword(s):  
Swirling Flow ◽  
Draft Tube ◽  
Flow Behavior ◽  
Load Condition ◽  
Francis Turbine ◽  
Tube Flow ◽  
Reversed Flow ◽  
Part Load ◽  
Axial Pressure Gradient ◽  
Flow Phenomena

Under part-load conditions, a Francis turbine often suffers from very severe low-frequency and large-amplitude pressure fluctuation, which is caused by the unsteady motion of vortices (known as “vortex ropes”) in the draft tube. This paper first reports our numerical investigation of relevant complex flow phenomena in the entire draft tube, based on the Reynolds-averaged Navier–Stokes (RANS) equations. We then focus on the physical mechanisms underlying these complex and somewhat chaotic flow phenomena of the draft-tube flow under a part-load condition. The flow stability and robustness are our special concern, since they determine what kind of control methodology will be effective for eliminating or alleviating those adverse phenomena. Our main findings about the flow behavior in the three segments of the draft tube, i.e., the cone inlet, the elbow segment, and the outlet segment with three exits, are as follows. (1) In the cone segment, we reconfirmed a previous finding of our research group based on the turbine’s whole-flow RANS computation that the harmful vortex rope is an inevitable consequence of the global instability of the swirling flow. We further identified that this instability is caused crucially by the reversed axial flow at the inlet of the draft tube. (2) In the elbow segment, we found a reversed flow continued from the inlet cone, which evolves to slow and chaotic motion. There is also a fast forward stream driven by a localized favorable axial pressure gradient, which carries the whole mass flux downstream. The forward stream and reversed flow coexist side-by-side in the elbow, with a complex and unstable shear layer in between. (3) In the outlet segment with three exits, the forward stream always goes through a fixed exit, leaving the other two exits with a chaotic and low-speed fluid motion. Based on these findings, we propose a few control principles to suppress the reversed flow and to eliminate the harmful helical vortex ropes. Of the methods we tested numerically, a simple jet injection in the inlet is proven successful.

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.

Prediction of the Pressure Pulsation in a Draft Tube for a Part Load Condition Using the LES Approach

2015 ◽  
Author(s):  
Olivier Pacot ◽  
Chisachi Kato ◽  
Yang Guo ◽  
Yoshinobu Yamade
Keyword(s):  
Load Condition ◽  
Francis Turbine ◽  
Sharp Change ◽  
Back Side ◽  
Vortex Rope ◽  
Part Load ◽  
Eddy Simulation ◽  
Computational Mesh ◽  
Large Eddy ◽  
Lift And Drag

The present paper focuses on the vortex rope that arises when operating a model Francis turbine at a part load condition: 65% of the Best Efficiency Point (BEP). The investigation is performed numerically using the Large Eddy Simulation (LES) approach with the Dynamic Smagorinsky Model (DSM). Such approach and turbulence model are implemented in the overset finite element open source code, FrontFlow/blue (FFB). Furthermore, a cavitation model is implemented allowing computations for non-cavitating and cavitating conditions. Thanks to the use of the K supercomputer, located at Kobe in Japan, and to the use of large computational mesh (123 million elements), it is shown that the frequency of the precession of the vortex rope as well as the head can be accurately computed. However, the predicted amplitude of the fluctuation did not fully agree with the experiment. Differences in a particular region near the back side of the elbow are about 35%. A comparison between the variation of the size of the vortex rope and the swirl number has been investigated and showed a clear relation. The location of the vortex rope and the minimum of the pressure were also investigated and showed that they do not fully share the same location. Furthermore, in a preliminary study to the computation of the cavitating vortex rope, computations of the flow around a Clark-11.7% hydrofoil under cavitation condition and for angles of attack of 2° and 8° are carried out. The results showed the common issue for this computation, i.e. the sharp change of the lift and drag coefficients could not be accurately predicted. Currently underway are the computation of the cavitating vortex rope. The effect of the cavitation on the vortex rope will be studied and reported at a later stage.

Cavitating Turbulent Flow Simulation in a Francis Turbine Based on Mixture Model

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
Shuhong Liu ◽  
Liang Zhang ◽  
Michihiro Nishi ◽  
Yulin Wu
Keyword(s):  
Turbulent Flow ◽  
Mixture Model ◽  
Draft Tube ◽  
Flow Simulation ◽  
Cavitating Flow ◽  
Francis Turbine ◽  
Phase Flow ◽  
Two Phase ◽  
Vortex Rope ◽  
Unsteady Cavitating Flow

As a numerical method to study the cavitation performance of a Francis turbine, the mixture model for the cavity/liquid two-phase flow is adopted in the cavitating turbulent flow analysis together with the re-normalization group (RNG) k-ε turbulence model in the present paper. The direct coupling numerical technique is used to solve the governing equations of the mixture model for the two-phase flow. Unsteady cavitating flow simulation around a hydrofoil of ALE15 is conducted as preliminary evaluation. Then, the cavitating flow in a Francis turbine is treated from the steady flow simulation since the feasibility of the cavitation model to the performance prediction of the turbine is the present major concern. Comparisons of the computational results with the model test data, i.e., the cavitation characteristics of hydraulic efficiency and the overload vortex rope at the draft tube inlet being reproduced reasonably, indicate that the present method has sufficient potential to simulate the cavitating flow in hydraulic turbines. Further, the unsteady cavitating flow simulation through the Francis turbine is conducted as well to study the pressure fluctuation characters caused by the vortex rope in the draft tube at partial load operation.

Hydraulic Disturbance Method to Reduce the Pressure Fluctuation in Francis Turbine Draft Tube

2011 ◽  
Author(s):  
Zhangchao Li ◽  
Jinshi Chang ◽  
Xingying Ji ◽  
Wanjiang Liu ◽  
Zhe Xin
Keyword(s):  
Flow Rate ◽  
Pressure Fluctuation ◽  
Draft Tube ◽  
Water Injection ◽  
Three Dimensional ◽  
Flow Simulation ◽  
Francis Turbine ◽  
Vortex Rope ◽  
Helical Vortex ◽  
Velocity Circulation

For a Francis turbine, when operating at partial flow rate the fixed-pitch runner shows a strong swirl at the runner outlet which induces a helical vortex (so-called vortex rope) in the draft tube. The precessing vortex rope causes severe pressure fluctuation which effects the steady and secure operating of the turbine. Three-dimensional unsteady turbulent flow simulation with RNG k-ε turbulence model of complete flow passage of a model Francis turbine at partial discharge is performed. To verify the simulation, the model turbine is tested on the test rig at the Harbin Electric Machinery Co., Ltd. (HEC), China. An ideal result of the simulation is obtained. The simulation predicts the shape of the helical vortex successfully in the draft tube cone, and the predicted values of the pressure fluctuation frequencies and amplitudes agree well with the test data. The hydraulic disturbance method is introduced, i.e., injecting water with velocity circulation from the runner cone to reduce the pressure fluctuation in Francis turbine draft tube. The injected water with velocity circulation destroys the forming mechanism of vortex rope and eliminates the pressure fluctuation accordingly. The flow in the turbine with water injection is simulated, and it is indicated that with appropriate flow rate and velocity circulation water injection the pressure fluctuation in the draft tube is reduced effectively.

Analysis and Control of Part-Load Unsteady Flow in Francis Turbine’s Draft Tube

2007 ◽  
Author(s):  
Ri-Kui Zhang ◽  
Feng Mao ◽  
Jie-Zhi Wu ◽  
Shi-Yi Chen ◽  
Yu-Lin Wu ◽  
...  
Keyword(s):  
Mass Flux ◽  
Pressure Fluctuation ◽  
Draft Tube ◽  
Load Condition ◽  
Vortical Flow ◽  
Francis Turbine ◽  
Effective Control ◽  
Complex Flow ◽  
Part Load ◽  
Axial Pressure Gradient

By using the Reynolds-averaged Navier-Stokes (RANS) equations, the complex unsteady vortical flow in the entire draft tube of a Francis turbine under a part-load condition, with severe low-frequency pressure fluctuation, is investigated numerically to gain an in-depth understanding of the physical characters of the flow including its stability and robustness, and thereby to seek effective control means to alleviate or even eliminate the strong pressure fluctuation. Our main findings are as follows: In the cone segment of the draft tube, the vortex rope is due to the global instability of the flow caused crucially by the reversed axial flow at the inlet. In the elbow segment of the draft tube, the reversed flow coexists side by side with a fluid channel that carries the mass flux downstream due to favorable axial pressure gradient. In the outlet segment of the draft tube, the mass-flux channel always goes through a fixed outlet, leaving the other two with nearly zero flux. The entire draft-tube flow, although undesired under part-load condition, forms a globally robust system. The principles for effectively controlling this complex flow are proposed. A simple water jet injection at the inlet is numerically proven successful.

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.

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