Influence of the velocity field at the inlet of a Francis turbine draft tube on performance over an operating range

In this paper, the effect of water, air, and their combined injection from two different injection points is studied in order to reduce vorticity effects in a draft tube of prototype turbine working at three operating points. The flow from spiral case to the end of draft tube is simulated using the shear stress transport k–ω turbulence and two-phase models. Using an appropriate validation method, acceptable results were obtained under the noninjection condition. To determine suitable number of points and inlet flow rate for air injection as well as the appropriate nozzle diameter for air and water injection, a new method which considers the ratio of total loss to the pressure recovery factor is used, in addition to using the traditional method which calculates the total loss in the draft tube. Comparing results of the three types of injections shows air injection in the operating range greater than 70% of turbine design flow rate, is much more effective than water injection or the combination of air and water injection. However, in the operating range below 70%, either water or air injections are not suitable, but combination of these two fluids can improve system performance.

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Modelling and optimization of the velocity profiles at the draft tube inlet of a Francis turbine within an operating range

Journal of Hydraulic Research ◽

10.1080/00221686.2015.1119763 ◽

2016 ◽

Vol 54 (1) ◽

pp. 74-89 ◽

Cited By ~ 12

Author(s):

Tiberiu Ciocan ◽

Romeo F. Susan-Resiga ◽

Sebastian Muntean

Keyword(s):

Draft Tube ◽

Velocity Profiles ◽

Francis Turbine ◽

Operating Range ◽

Modelling And Optimization

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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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Numerical investigation of the cavitation dynamic parameters in a Francis turbine draft tube with columnar vortex rope

Journal of Hydrodynamics ◽

10.1007/s42241-019-0035-z ◽

2019 ◽

Vol 31 (5) ◽

pp. 931-939 ◽

Cited By ~ 3

Author(s):

Jing Yang ◽

Ling-jiu Zhou ◽

Zheng-wei Wang

Keyword(s):

Numerical Investigation ◽

Draft Tube ◽

Francis Turbine ◽

Dynamic Parameters ◽

Vortex Rope ◽

Columnar Vortex

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Influence of Upstream Disturbances on the Vortex Structure of Francis Turbine Based on the Criteria of Identification of Various Vortexes

Energies ◽

10.3390/en14227626 ◽

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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Comparison of Axial Water and Air Injections in the Draft Tube of a Francis Turbine for RVR Mitigation

10.1115/fedsm2021-65503 ◽

2021 ◽

Author(s):

Subodh Khullar ◽

Krishna M. Singh ◽

Michel J. Cervantes ◽

Bhupendra K. Gandhi

Keyword(s):

Pressure Pulsation ◽

Draft Tube ◽

Water Injection ◽

Air Injection ◽

Flow Instabilities ◽

Pressure Recovery ◽

Francis Turbine ◽

Tube Flow ◽

Pressure Pulsations ◽

Jet Injection

Abstract The presence of excessive swirl at the runner outlet in Francis turbines operating at part load leads to the development of flow instabilities such as the rotating vortex rope (RVR). The presence of RVR causes severe pressure pulsations, power swings, and fatigue damage in the turbine unit. Air and water injection in the draft tube have been reported to reduce the detrimental effects of RVR formation in the Francis turbines. Air injection is one of the oldest and most widely used methods. In contrast, water jet injection is a relatively new methodology. The present work reports the numerical simulations performed to compare the respective effectiveness of these methods to mitigate the RVR and the related flow instabilities. The efficacy of the two methods has been compared based on the pressure pulsations and pressure recovery in the draft tube cone. The results show that the air and water injection influence the draft tube flow field in different ways. Both air and water injection led to a reduction in pressure pulsation magnitudes in the draft tube cone. However, the air injection led to a negative pressure recovery while the water injection improved the draft tube action.

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