Figure 4 - uploaded by Arturo Rivetti
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Representation of pressure fields and water vapour cavity volumes (2-Phase) for constant speed factor n ED , varying GVO [°].
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Francis turbines operating at high load conditions produce a typical flow pattern in the draft tube cone characterized by the presence of an axisymmetric central vortex. This central cavity could become unstable, generating synchronic pressure pulsations, usually called self-excited oscillations, which propagate into the whole machine.
The on-set a...
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... E-26° OP# B -30° Another CFD sequence was simulated for constant n ED corresponding to peak efficiency and varying GVO, as could be seen on figure 4.OP# F shows an incipient vortex. In conjunction with OP# D, it permits to draw the beginning of the high load vortex curve, which limits the free vortex zone in the efficiency hill diagram. ...
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Hydropower plants are indispensable to stabilize the grid by reacting quickly to changes of the energy demand. However, an extension of the operating range towards high and deep part load conditions without fatigue of the hydraulic components is desirable to increase their flexibility. In this paper a model sized Francis turbine at low discharge op...
Citations
... 3 The adverse pressure gradient arising as a consequence of the draft tube's diverging geometry is also a trigger mechanism for the breakdown of the core flow. At part-load, the co-rotating flow often leads to a precessing spiral vortex core and large pressure fluctuations (see, for example, Favrel et al. 4,5 and Pasche et al. 6,7 ) Breakdowns also occur at high-load where the vorticity distribution may contribute to the onset of either spiral 8,9 or axisymmetric [10][11][12] structures. At the BEP, the limited amount of swirl generally allows for lower velocity and pressure fluctuations, and the flow remains mostly axial. ...
Hydraulic turbines sometimes exhibit a sharp efficiency drop around the best efficiency point. The drop is known to originate from large flow separations in their draft tubes, limiting their ability to recover part of the residual kinetic energy exiting the runner. While the conditions leading to the onset of these separations are not yet understood, the potentially unstable vorticity distribution at the runner exit led to the hypothesis that those separations are the result of an interaction between the flow at the center of the draft tube and the boundary layer at the walls. To study this hypothesis, the turbulent flow inside the draft tube of a bulb turbine was measured with time-resolved particle-image velocimetry (TR-PIV). In this work, coherent structures are identified from spectral proper orthogonal decomposition (SPOD) of the velocity fields to correlate changes in their topology with the efficiency drop. Special attention is given to the periodic vortical motions in the runner's wake, whose shape and energy content are found to be linked to the flow rate. Three-dimensional reconstructions of the underlying structure reveal a shift in its topology that correlates with the efficiency drop and separations at the wall. In addition, comparisons of the SPOD coefficients with the runner position show that the phase angle between the structure and the runner remains the same for each operating condition, suggesting a link with a rotating flow imbalance in the runner blade channels.
... Besides partial load, Francis turbines have been operated at excess flow rate regimes. Francis turbines operating at excess load conditions can generate high-pressure oscillations [12][13][14][15][16]. At an excessive flow rate, an axisymmetric central vortex presence in the draft tube center causes synchronic pressure pulsations. ...
... RANS simulations were carried out to model the vortex precession and accompanying cavitational effects [14] for different pressure distributions at the inlet of the draft tube. A cavitation model was implemented, and the air volume fraction, as well as the pressure and head, were monitored as a function of time. ...
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.
... At excessive load, the vortex phenomenon experiences strong bubble vortex cavity behavior, leading to an axisymmetric central vortex and synchronic pressure pulsations. The velocity, vorticity, and pressure fields in the draft tube are immensely influenced by the axisymmetric central vortex, which leads to instabilities in the hydraulic system and reduces the turbine performance [7][8][9][10][11]. Another significant pattern is a precession of the vortex rope occurs under partial load (at a flow rate of 85% to 65% of the design point rate). ...
The performance of a pump-turbine under partial flow rates, 85%, 75%, and 65%, is studied using the LES model. The power signal, velocity, vorticity, and pressure field is presented over the blades and throughout the draft tube. Pressure fluctuations are probed at various locations over the wall of the draft tube. Examining the flow field in the blade region can provide further insights into the system performance. Flow-induced pressure fluctuations can disrupt system stability.
For this turbine, a strong swirling region is observed around the draft tube walls, causing pressure fluctuations. The size and intensity of this region decrease with the flow rate. A vortex rope is present in all cases. At the design point, the strength is constant throughout the draft tube. However, at partial load, the rope is weakened along the draft tube. Between the region dominated by the vortex rope and the wall, there is a swirling shear layer, which moves closer to the wall as the flow rate decreases.
Both the magnitude of pressure fluctuations at the wall and the pressure difference over the blade decrease with the flow rate. The decreased pressure differences over the blade represent less power produced, and the decline in fluctuation magnitude at the wall represents more system stability. For this turbine, there appears to be a trade-off between power and strength of pressure fluctuations.
... The mismatch in flow between the runner blades and the guide vanes during part load (PL) and high load (HL) conditions results in a high swirling outflow from the runner exit, which generates a vortex core in the draft tube. [3][4][5][6][7] The resulting velocity diagrams at the runner outlet for the BEP, PL, and HL conditions are shown in Fig. 1. The flow exiting from the runner at the BEP condition is almost axial when entering into the draft tube, as shown in Fig. 1(a). ...
... In this case, a counter-rotating vortex core develops in the draft tube. 5,6 In the present scenario, the designers' approach is to develop a turbine with maximum efficiency at low cost. This approach has already reduced the weight of the turbine significantly. ...
... 2,11 The vortex core phenomenon of the draft tube during offdesign conditions is associated with the vortex breakdown. 13 Several theories of vortex breakdown exist: (1) growth in hydrodynamic instability with time, 14 (2) flow transition from supercritical to subcritical, 15 (3) boundary layer separation, 16 (4) transition of vortex states from one helical symmetry to another, 17 (5) stagnant and critical state of flow, 18,19 and (6) axial velocity change from a jet-like to wake-like profile before and after breakdown. 20,21 An abrupt change of the flow structure in the fluid domain may also be defined as the vortex breakdown, as presented by Hall. ...
This study presents the experiments performed on a model Francis turbine during load acceptance from best efficiency point to high load conditions. The Reynolds number varies from 7.3 × 10⁵ to 9.0 × 10⁵ during the measurement. A vortex core is generally observed in the draft tube of the Francis turbine at high load operation. However, the mechanism of formation of the core is not yet highlighted using an experimental flow-field study. This paper illustrates the mechanism involved in the formation of the vortex core using synchronized velocity and pressure measurements. The measurements are performed on a model Francis turbine. A fully developed vortex core is observed in the draft tube at high load operation, and the formation of the core originates with the formation of the stagnant, reverse, and recirculating flow regions during load acceptance. The vortex core rotates in the direction opposite to the runner rotation. The axial velocity profiles are observed to change from jet-like to wake-like during the formation of a vortex core. The large velocity gradients represent the sharp transition in the flow around the center axis of the draft tube. The severe pressure fluctuations corresponding to rotor–stator interaction and pressure waves are observed in the draft tube and vaneless space.
... Interestingly, the flow is more concentrated towards the left wall of the draft tube at BEP and HL conditions as compared to that of the SL and PL conditions. The torch-like vortex core at HL rotates in a direction opposite to the runner direction (Dorfler et al., 2013;Magnoli, 2014;Rodriguez, Rivetti, & Lucino, 2016) and that pushes the flow towards the opposite wall of the draft tube as compared to PL and BEP condition. Amiri, Mulu, Raisee, and Cervantes (2016) showed that the flow symmetry in an elbow draft tube is a function of turbine operating conditions and flow conditions at the draft tube inlet. ...
This paper presents the experiments performed on a high head model Francis turbine during start-up. Synchronized time dependent pressure and velocity measurements were performed to investigate the instabilities in the turbine. A total of four steady state operating points, namely synchronous load, part load, best efficiency point, and high load are considered to perform the turbine start-up. The runner angular speed was observed to increase almost exponentially during the guide vane positions from completely closed to no load condition. The frequency of wave propagation due to the interaction between runner blades and guide vanes was observed to follow the trend of increase of runner angular speed. A vortex rope frequency was captured in the draft tube during synchronous load to part load of the start-up. Two different mechanisms, namely, the development of stagnation point and the available recirculation regions were observed to cause the formation of vortex rope in the draft tube.
