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.

Performance of Francis Turbine Operating at Excess Load Condition

2020 ◽  
Author(s):  
Muhannad Altimemy ◽  
Justin Caspar ◽  
Alparslan Oztekin
Keyword(s):  
Flow Field ◽  
Draft Tube ◽  
Turbine Blades ◽  
Pressure Fluctuations ◽  
Load Condition ◽  
Operating Conditions ◽  
Francis Turbine ◽  
Partial Load ◽  
Dynamics Simulations ◽  
Excess Load

Abstract Computational fluid dynamics simulations are conducted to characterize the spatial and temporal characteristics of the flow field inside a Francis turbine operating in the excess load regime. A high-fidelity Large Eddy Simulation (LES) turbulence model is applied to investigate the flow-induced pressure fluctuations in the draft tube of a Francis Turbine. Probes placed alongside the wall and in the center of the draft tube measure the pressure signal in the draft tube, the pressure over the turbine blades, and the power generated to compare against previous studies featuring design point and partial load operating conditions. The excess load is seen during Francis turbines in order to satisfy a spike in the electrical demand. By characterizing the flow field during these conditions, we can find potential problems with running the turbine at excess load and inspire future studies regarding mitigation methods. Our studies found a robust low-pressure region on the edges of turbine blades, which could cause cavitation in the runner region, which would extend through the draft tube, and high magnitude of pressure fluctuations were observed in the center of the draft tube.

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 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.

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

Energies ◽  
2020 ◽  
Vol 13 (7) ◽  
pp. 1734
Author(s):  
Xing Zhou ◽  
Changzheng Shi ◽  
Kazuyoshi Miyagawa ◽  
Hegao Wu ◽  
Jinhong Yu ◽  
...  
Keyword(s):  
Draft Tube ◽  
Renewable Energy Sources ◽  
Pressure Fluctuations ◽  
Dominant Frequency ◽  
Rotational Frequency ◽  
Rapid Expansion ◽  
Francis Turbine ◽  
Axial Thrust ◽  
Partial Load ◽  
Hydropower Stations

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.

Flow in the Simplified Draft Tube of a Francis Turbine Operating at Partial Load—Part I: Simulation of the Vortex Rope

2014 ◽  
Vol 81 (6) ◽  
Author(s):  
Hosein Foroutan ◽  
Savas Yavuzkurt
Keyword(s):  
Experimental Data ◽  
Axial Velocity ◽  
Draft Tube ◽  
Three Dimensional ◽  
Operating Conditions ◽  
Turbulence Closure ◽  
Francis Turbine ◽  
Outer Flow ◽  
Vortex Rope ◽  
Partial Load

Numerical simulations and analysis of the vortex rope formation in a simplified draft tube of a model Francis turbine are carried out in this paper, which is the first part of a two-paper series. The emphasis of this part is on the simulation and investigation of flow using different turbulence closure models. Two part-load operating conditions with same head and different flow rates (91% and 70% of the best efficiency point (BEP) flow rate) are considered. Steady and unsteady simulations are carried out for axisymmetric and three-dimensional grid in a simplified axisymmetric geometry, and results are compared with experimental data. It is seen that steady simulations with Reynolds-averaged Navier–Stokes (RANS) models cannot resolve the vortex rope and give identical symmetric results for both the axisymmetric and three-dimensional flow geometries. These RANS simulations underpredict the axial velocity (by at least 14%) and turbulent kinetic energy (by at least 40%) near the center of the draft tube, even quite close to the design condition. Moving farther from the design point, models fail in predicting the correct levels of the axial velocity in the draft tube. Unsteady simulations are performed using unsteady RANS (URANS) and detached eddy simulation (DES) turbulence closure approaches. URANS models cannot capture the self-induced unsteadiness of the vortex rope and give steady solutions while DES model gives sufficient unsteady results. Using the proper unsteady model, i.e., DES, the overall shape of the vortex rope is correctly predicted and the calculated vortex rope frequency differs only 6% from experimental data. It is confirmed that the vortex rope is formed due to the roll-up of the shear layer at the interface between the low-velocity inner region created by the wake of the crown cone and highly swirling outer flow.

Flow in the Simplified Draft Tube of a Francis Turbine Operating at Partial Load—Part II: Control of the Vortex Rope

2014 ◽  
Vol 81 (6) ◽  
Author(s):  
Hosein Foroutan ◽  
Savas Yavuzkurt
Keyword(s):  
Flow Rate ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Companion Paper ◽  
Francis Turbine ◽  
Control Technique ◽  
Outer Flow ◽  
Vortex Rope ◽  
Stagnant Region ◽  
Partial Load

Numerical simulations and investigation of a method for controlling the vortex rope formation in draft tubes are carried out in this paper, which is the second part of a two-paper series. As shown in the companion paper, formation of the vortex rope is associated with a large stagnant region at the center of the draft tube. Therefore, it is concluded that a successful control technique should focus on the elimination of this region. In practice, this can be performed by axially injecting a small fraction (a few percent of the total flow rate) of water into the draft tube. Water jet is supplied from the high-pressure flow upstream of the turbine spiral case by a bypass line; thus, no extra pump is needed in this method. It is shown that this method is very effective in elimination of the stagnant region in a simplified draft tube operating at two part-load conditions, i.e., at 91% and 70% of the best efficiency point (BEP) flow rate. This results in improvement of the draft tube performance and reduction of hydraulic losses. The loss coefficient is reduced by as much as 50% for the case with 91% of BEP flow rate and 14% for the case with 70% of BEP flow rate. Unsteady, three-dimensional simulations show that the jet increases the axial momentum of flow at the center of the draft tube and decreases the wake of the crown cone and thereby decreases the shear at the interface of the stagnant region and high velocity outer flow, which ultimately results in elimination of the vortex rope. Furthermore, reduction (by about 1/3 in the case with 70% of BEP flow rate) of strong pressure fluctuations leads to reliable operation of the turbine.

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.

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.

Numerical Simulation of Cavitating Turbulent Flow in a Francis Turbine with Draft Tube Natural Air Admission

2013 ◽  
Vol 291-294 ◽  
pp. 1963-1968
Author(s):  
Hong Ming Zhang ◽  
Li Xiang Zhang
Keyword(s):  
Turbulent Flow ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Francis Turbine ◽  
Transfer Model ◽  
Mass Transfer Model ◽  
Governing Equations ◽  
Operation Analysis ◽  
High Head ◽  
Volume Method

The paper presents numerical analysis of cavitating turbulent flow in a high head Francis turbine with draft tube natural air admission at part load operation. Analysis was performed by OpenFOAM code. A mixture assumption and a finite rate mass transfer model were introduced. The finite volume method is used to solve the governing equations of the mixture model and the pressure-velocity coupling is handled via a Pressure Implicit with Splitting of Operators (PISO) procedure. The pressure distribution and the flow of air in the draft tube are analyzed in detail. Simulation results show that the pressure fluctuations on the draft tube wall can reduce with natural air admission.

Francis Turbine Operation at Excess Flow Rate Using Large Eddy Simulation: The Effect of Water Injection

2020 ◽  
Author(s):  
Muhannad Altimemy ◽  
Justin Caspar ◽  
Saif Watheq ◽  
Alparslan Oztekin
Keyword(s):  
Flow Rate ◽  
Draft Tube ◽  
Tube Wall ◽  
Temporal Structure ◽  
Pressure Fluctuations ◽  
Water Injection ◽  
Design Flow ◽  
Francis Turbine ◽  
Reduced Pressure ◽  
Large Eddy

Abstract High-fidelity Large Eddy Simulations (LES) were conducted to characterize the spatial and temporal structure of turbulent flows in an industrial-sized Francis turbine running at 120% of the design flow rate. Injection at a 4% and 8% flow rate is applied and investigated as a mitigation method for pressure-induced fluctuations along the draft tube. Contours of velocity and vorticity in the draft tube are presented to examine the effects of water injection. Probes placed alongside the draft tube measure the pressure signal and compare both operational regimes to characterize the pressure fluctuations. The intensity of pressure fluctuations along the draft tube wall is an order of magnitude smaller compared to that at the center. As the injection is applied, the intensity of the pressure fluctuations along the draft tube wall is increased while the intensity of pressure fluctuations in the center of the draft tube is reduced. Pressure probes in the center of the draft tube measure an 86% to 57% reduction in amplitude for 4% to 8% flow rate injection, respectively. There is a 30% to 40% increase in fluctuations along the wall for 4% to 8% flow rate injection, respectively. These changes in flow structure are due to the dissipation of the vortex rope as the injection is applied.

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