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

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.

Investigation on Combined Air and Water Injection in Francis Turbine Draft Tube to Reduce Vortex Rope Effects

2018 ◽  
Vol 141 (5) ◽  
Author(s):  
Moona Mohammadi ◽  
Ebrahim Hajidavalloo ◽  
Morteza Behbahani-Nejad
Keyword(s):  
Flow Rate ◽  
Draft Tube ◽  
Water Injection ◽  
Total Loss ◽  
Design Flow ◽  
Air Injection ◽  
Francis Turbine ◽  
Two Phase ◽  
Operating Range ◽  
Two Fluids

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.

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.

Characteristics and Control of the Draft-Tube Flow in Part-Load Francis Turbine

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Ri-kui Zhang ◽  
Feng Mao ◽  
Jie-Zhi Wu ◽  
Shi-Yi Chen ◽  
Yu-Lin Wu ◽  
...  
Keyword(s):  
Swirling Flow ◽  
Draft Tube ◽  
Flow Behavior ◽  
Load Condition ◽  
Francis Turbine ◽  
Tube Flow ◽  
Reversed Flow ◽  
Part Load ◽  
Axial Pressure Gradient ◽  
Flow Phenomena

Under part-load conditions, a Francis turbine often suffers from very severe low-frequency and large-amplitude pressure fluctuation, which is caused by the unsteady motion of vortices (known as “vortex ropes”) in the draft tube. This paper first reports our numerical investigation of relevant complex flow phenomena in the entire draft tube, based on the Reynolds-averaged Navier–Stokes (RANS) equations. We then focus on the physical mechanisms underlying these complex and somewhat chaotic flow phenomena of the draft-tube flow under a part-load condition. The flow stability and robustness are our special concern, since they determine what kind of control methodology will be effective for eliminating or alleviating those adverse phenomena. Our main findings about the flow behavior in the three segments of the draft tube, i.e., the cone inlet, the elbow segment, and the outlet segment with three exits, are as follows. (1) In the cone segment, we reconfirmed a previous finding of our research group based on the turbine’s whole-flow RANS computation that the harmful vortex rope is an inevitable consequence of the global instability of the swirling flow. We further identified that this instability is caused crucially by the reversed axial flow at the inlet of the draft tube. (2) In the elbow segment, we found a reversed flow continued from the inlet cone, which evolves to slow and chaotic motion. There is also a fast forward stream driven by a localized favorable axial pressure gradient, which carries the whole mass flux downstream. The forward stream and reversed flow coexist side-by-side in the elbow, with a complex and unstable shear layer in between. (3) In the outlet segment with three exits, the forward stream always goes through a fixed exit, leaving the other two exits with a chaotic and low-speed fluid motion. Based on these findings, we propose a few control principles to suppress the reversed flow and to eliminate the harmful helical vortex ropes. Of the methods we tested numerically, a simple jet injection in the inlet is proven successful.

Influence of upstream disturbance on the draft-tube flow of Francis turbine under part-load conditions

2018 ◽  
Vol 30 (1) ◽  
pp. 131-139 ◽  
Author(s):  
Ting Chen ◽  
Xianghao Zheng ◽  
Yu-ning Zhang ◽  
Shengcai Li
Keyword(s):  
Draft Tube ◽  
Francis Turbine ◽  
Tube Flow ◽  
Part Load ◽  
Load Conditions

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.

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.

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