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.

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.

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.

Simulations and Measurements of Pressure Oscillations Caused by Vortex Ropes

2006 ◽  
Vol 128 (4) ◽  
pp. 649-655 ◽  
Author(s):  
Zhengwei Wang ◽  
Lingjiu Zhou
Keyword(s):  
Stokes Equations ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Three Dimensional ◽  
Suction Side ◽  
Rotational Frequency ◽  
Francis Turbine ◽  
Pressure Oscillations ◽  
Hydraulic Unit ◽  
Load Conditions

Pressure oscillations caused by vortex rope were measured in the draft tube of a prototype Francis turbine. The three-dimensional, unsteady Reynolds-averaged Navier-Stokes equations with the RNG κ−ϵ turbulence model were solved to model the flow within the entire flow path of the prototype hydraulic unit including the guide vanes, the runner, and the draft tube. The model was able to predict the pressure fluctuations that occur when operating at 67–83% of the optimum opening. The calculated frequencies and amplitudes of the oscillation show reasonable agreement with the experiment data. However, the results at 50% opening were not satisfactory. Pressure oscillations on the runner blades were found to be related to the precession of vortex ropes which caused pressure on the blades to fluctuate with frequencies of −fn+fd (fn is the rotational frequency and fd is vortex procession frequency). The peak-to-peak amplitudes of the pressure oscillations on the blades at the lower load conditions (67% opening) were higher than at higher load conditions (83% opening). Fluctuations on the suction side tended to be stronger than on the pressure side.

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.

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

2020 ◽  
Author(s):  
Muhannad Altimemy ◽  
Justin Caspar ◽  
Saif Watheq ◽  
Alparslan Oztekin
Keyword(s):  
Turbulent Flows ◽  
Structural Integrity ◽  
Draft Tube ◽  
Temporal Structure ◽  
Pressure Fluctuations ◽  
High Amplitude ◽  
Large Eddy Simulations ◽  
Francis Turbine ◽  
Partial Load ◽  
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. 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.

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 Hydraulic Vibration and Possible Exciting Sources Analysis in a Hydropower System

2021 ◽  
Vol 11 (12) ◽  
pp. 5529
Author(s):  
Aili Shen ◽  
Yimin Chen ◽  
Jianxu Zhou ◽  
Fei Yang ◽  
Hongliang Sun ◽  
...  
Keyword(s):  
Vibration Mode ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Experimental Tests ◽  
Dominant Frequency ◽  
Rotational Frequency ◽  
Operating Conditions ◽  
Mode Shapes ◽  
Vortex Rope ◽  
Hydropower System

To understand the hydraulic vibration characteristics in a traditional hydropower system and identify possible exciting sources that may induce serious hydraulic vibrations in the flow passage, experimental tests and numerical calculations were conducted for different operating conditions. The experimental results show that the pressure fluctuations are mainly related to the vortex rope phenomena in the draft tube, and the dominant frequency of pressure fluctuation is 0.2~0.4 times the runner rotational frequency (fn). The numerical results show all the attenuating factors are negative, which indicates the system itself is stable on the condition that all the hydraulic elements have steady operating performance. The free vibration analyses confirm that the frequency range of the vortex rope in the draft tube partly overlaps the natural frequencies of the hydropower system. Apart from the vortex rope, the runner rotational frequency is another common frequency that is approximately equal to the frequency of the 10th vibration mode. From the vibration mode shapes, it is inferred that a small disturbance in its frequency close or equal to a specific natural frequency of the vibration mode could induce large pressure oscillations in the tail tunnel. In light of the system’s response to different forcing frequencies, the vortex rope formed under off-design conditions and runner rotational frequency is verified to be the potential exciting source of a traditional hydropower system, and the frequency 0.2 fn is much more dangerous than other disturbances to the system.

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 Flow-related mechanisms of draft tube surges observed at partial load of a francis turbine.

1985 ◽  
Vol 51 (467) ◽  
pp. 2406-2413 ◽  
Author(s):  
Michihiro NISHI ◽  
Shigenori MATSUNAGA ◽  
Katsuhiko NISHITANI ◽  
Takashi KUBOTA ◽  
Yasutoshi SENOO
Keyword(s):  
Draft Tube ◽  
Francis Turbine ◽  
Partial Load

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.

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