Pressure Fluctuation Mitigation in a Francis Turbine With Water Injection: Computational Study

2018 ◽  
Author(s):  
Muhannad Altimemy ◽  
Cosan Daskiran ◽  
Bashar Attiya ◽  
I-Han Liu ◽  
Alparslan Oztekin
Keyword(s):  
Pressure Fluctuation ◽  
Draft Tube ◽  
Computational Study ◽  
Water Injection ◽  
Francis Turbine ◽  
Navier Stokes ◽  
Tube Surface ◽  
Vortex Rope ◽  
Central Vortex ◽  
Dynamics Simulations

Computational fluid dynamics simulations were performed on Francis turbine using Reynolds-averaged Navier-Stokes (RANS) with k-ω SST turbulence model. Simulations were conducted at the turbine’s best efficiency point with a Reynolds number of 2.01 × 107. Water injection was admitted from the runner cone in the stream-wise direction. The aim of this process was to investigate the influence of water injection on the turbine performance and the pressure pulsation. The water injection did not affect the nominal value of the turbine’s power generation. Straight vortex rope was observed at the centerline of the draft tube. Moreover, helix-shaped vortex ropes were obtained near the draft tube surface. The water injection expands the central vortex rope, but it did not suppress or disrupt the helix-shaped peripheral vortex rope near the draft tube surface. The pressure fluctuation became less regular after the water injection, but the fluctuation level remained similar.

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.

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.

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.

Mitigation of Flow-Induced Pressure Fluctuations in a Francis Turbine Using Water Injection

2019 ◽  
Author(s):  
Muhannad Altimemy ◽  
Bashar Attiya ◽  
Cosan Daskiran ◽  
I-Han Liu ◽  
Alparslan Oztekin
Keyword(s):  
Power Generation ◽  
Turbulent Flow ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Water Injection ◽  
Operating Conditions ◽  
Francis Turbine ◽  
Partial Load ◽  
Flow Induced Vibrations ◽  
Dynamics Simulations

Abstract Computational fluid dynamics simulations are conducted to characterize the spatial and temporal characteristics of the turbulent flow fields inside Francis turbine operating at the design and partial load regimes. High-fidelity large eddy simulations turbulence model is applied to investigate the flow-induced vibrations in the draft tube of the unit. The water injection at 4% rate from the runner cone is implemented to control the flow-induced pressure fluctuations. The simulations are conducted at the turbine design point and two partial load operations with and without water injection. It has been demonstrated that the water injection has a profound influence in the turbulent flow structure and the pressure field inside the draft tube at the partial load operating conditions. To evaluate the effectiveness of the water injection techniques in mitigating flow-induced fluctuations, the probes at various locations along the wall of the draft tube are used to monitor the pressure signals. It appears to be a reduction in the level of pressure fluctuations by the water injection at both partial load operating regimes. However, we could not draw a firm conclusion about the level of mitigation of flow-induced vibrations. Simulations should be carried out for much longer flow time. Water injection hardly influenced the unit power generation. Hence water injection can be employed effectively without a major liability on the power generation.

Unsteady Pressure Analysis of a Swirling Flow With Vortex Rope and Axial Water Injection in a Discharge Cone

2012 ◽  
Vol 134 (8) ◽  
Author(s):  
Alin Ilie Bosioc ◽  
Romeo Susan-Resiga ◽  
Sebastian Muntean ◽  
Constantin Tanasa
Keyword(s):  
Draft Tube ◽  
Partial Discharge ◽  
Pressure Fluctuations ◽  
Water Injection ◽  
Water Jet ◽  
Francis Turbine ◽  
Experimental Investigations ◽  
Vortex Rope ◽  
Conical Diffuser ◽  
Jet Control

The variable demand of the energy market requires that hydraulic turbines operate at variable conditions, which includes regimes far from the best efficiency point. The vortex rope developed at partial discharges in the conical diffuser is responsible for large pressure pulsations, runner blades breakdowns and may lead to power swing phenomena. A novel method introduced by Resiga et al. (2006, “Jet Control of the Draft Tube in Francis Turbines at Partial Discharge,” Proceedings of the 23rd IAHR Symposium on Hydraulic Machinery and Systems, Yokohama, Japan, Paper No. F192) injects an axial water jet from the runner crown downstream in the draft tube cone to mitigate the vortex rope and its consequences. A special test rig was developed at “Politehnica” University of Timisoara in order to investigate different flow control techniques. Consequently, a vortex rope similar to the one developed in a Francis turbine cone at 70% partial discharge is generated in the rig’s test section. In order to investigate the new jet control method an auxiliary hydraulic circuit was designed in order to supply the jet. The experimental investigations presented in this paper are concerned with pressure measurements at the wall of the conical diffuser. The pressure fluctuations’ Fourier spectra are analyzed in order to assess how the amplitude and dominating frequency are modified by the water injection. It is shown that the water jet injection significantly reduces both the amplitude and the frequency of pressure fluctuations, while improving the pressure recovery in the conical diffuser.

scholarly journals Computational and Theoretical Analyses of the Precessing Vortex Rope in a Simplified Draft Tube of a Scaled Model of a Francis Turbine

2016 ◽  
Vol 139 (2) ◽  
Author(s):  
Girish K. Rajan ◽  
John M. Cimbala
Keyword(s):  
Discharge Coefficient ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Three Dimensional ◽  
Francis Turbine ◽  
Navier Stokes ◽  
Detached Eddy Simulation ◽  
Mean Flow ◽  
Scaled Model ◽  
Vortex Rope

Results on flows in a draft tube of a constant-head, constant-specific speed, model Francis turbine are presented based on computational fluid dynamics (CFD) simulations and theoretical analysis. A three-dimensional, unsteady, Navier–Stokes solver with the detached-eddy simulation (DES) model and the realizable k–ϵ (RKE) model is used to analyze the vortex rope formed at different discharge coefficients. The dominant amplitude of the pressure fluctuations at a fixed point in the draft tube increases by 13 times, and the length of the rope increases by 3.4 times when the operating point of the turbine shifts from a discharge coefficient of 0.37 to 0.34. A perturbation analysis based on a steady, axisymmetric, inviscid, incompressible model for the mean flow is performed to obtain a Sturm–Liouville (SL) system, the solutions of which are oscillatory if the discharge coefficient is greater than 0.3635, and nonoscillatory otherwise.

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.

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