Large Eddy Simulations of Francis Turbine Operating at Ultra-Low Loads

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. The unit operated at 50% and 40% of the best efficiency design flowrate. Contours of vorticity, velocity, pressure, and iso-surfaces of Q-Criterion were presented to characterize the effects on the draft tube. Probes placed alongside the draft tube measure the pressure signal to investigate the flow-induced pressure fluctuations inside the turbine unit. The maximum intensity of pressure fluctuations at 50% partial load was 22.66% of the turbine head, while the strength of the pressure fluctuations was 26.36% at 40% partial load. A large number of unorganized smaller vortices observed in the draft tube contribute to the creation of pressure fluctuations. Two pressure modes can be easily recognized (1) high frequency with low amplitude pressure fluctuations and (2) low frequency with high amplitude fluctuations. These pressure fluctuations could be harmful to the structural integrity of the unit and also have undesirable influences on the operational stability of the hydro-turbines.

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

Volume 10: Fluids Engineering ◽

10.1115/imece2020-23378 ◽

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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Performance of Francis Turbine Operating at Excess Load Condition

Volume 3: Computational Fluid Dynamics; Micro and Nano Fluid Dynamics ◽

10.1115/fedsm2020-20053 ◽

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.

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

Energies ◽

10.3390/en13071734 ◽

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.

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

Journal of Applied Mechanics ◽

10.1115/1.4026818 ◽

2014 ◽

Vol 81 (6) ◽

Cited By ~ 15

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.

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

Volume 3B: Fluid Applications and Systems ◽

10.1115/ajkfluids2019-4867 ◽

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.

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Large-eddy simulations of a swirling flow in a model Francis turbine

Journal of Physics Conference Series ◽

10.1088/1742-6596/2119/1/012026 ◽

2021 ◽

Vol 2119 (1) ◽

pp. 012026

Author(s):

E V Palkin ◽

M Yu Hrebtov ◽

R I Mullyadzhanov

Keyword(s):

Swirling Flow ◽

Draft Tube ◽

Large Eddy Simulations ◽

Francis Turbine ◽

Mean Flow ◽

Fixed Frequency ◽

Precessing Vortex Core ◽

Large Eddy ◽

Low Load ◽

Helical Vortex

Abstract We performed Large-eddy simulations of the flow in a model air Francis turbine in a range of low-load regimes with a swirler rotating at fixed frequency. All investigated regimes revealed the presence of coherent helical vortex structure in the draft tube: the precessing vortex core. We identified the frequency of this instability and obtained mean flow velocity fields to be utilized in further works.

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

Advances in Mechanical Engineering ◽

10.1177/1687814018824468 ◽

2019 ◽

Vol 11 (1) ◽

pp. 168781401882446 ◽

Cited By ~ 4

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.

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

Processes ◽

10.3390/pr9071182 ◽

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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