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A detailed understanding of turbulent fluid particle breakup mechanisms is essential for the accurate modeling of gas/liquid and liquid/liquid dispersions. The design of a fully automated setup for the three-dimensional serial exa...
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A detailed understanding of turbulent fluid particle breakup mechanisms is essential for the accurate modeling of gas/liquid and liquid/liquid dispersions. The design of a fully automated setup for the three-dimensional serial examination of the single bubble breakup process in a stirred tank, ensuring high repetition rates necessary for the additionally automated statistical analysis, is described. The implementation of a three-dimensional automatic bubble breakup tracking tool is illustrated. At last, exemplary bubble breakup trajectories that show the benefits and limitations of the developed system and method are discussed.
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The bubble breakups in a jet bubbling reactor are captured using a high-speed camera and the velocity field is measured by particle image velocimetry. Two typical breakup patterns, jet breakup and jet-vortex breakup are observed. ...
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The bubble breakups in a jet bubbling reactor are captured using a high-speed camera and the velocity field is measured by particle image velocimetry. Two typical breakup patterns, jet breakup and jet-vortex breakup are observed. The breakup time interval of the jet-vortex breakup is two orders of magnitude higher than the jet breakup. The probability of the jet-vortex breakup and the jet breakup accounting in the total breakup events increases and decreases with the jet velocity and the mother bubble size, respectively. The bubble breakup region increases with the jet velocity. The bubble breakup frequency increases with the turbulent dissipation rate and the mother bubble size. The average number of daughter bubbles increases with the Weber number. An L-shaped daughter bubble size distribution is observed. Empirical correlations are established for the bubble breakup frequency, the average number of daughter bubbles and daughter bubble size distribution, and fitted well with the experimental results.
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A fully automated three-dimensional observation of single bubble breakup trajectories in a stirred tank is demonstrated to gain unbiased and statistically relevant information about the breakup process. The mother bubble size is k...
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A fully automated three-dimensional observation of single bubble breakup trajectories in a stirred tank is demonstrated to gain unbiased and statistically relevant information about the breakup process. The mother bubble size is kept constant, independently of the stirring rate. The investigated parameter in this work is the power input. Three-dimensional bubble breakup trajectories and heat maps for the initial breakup location probability for the bottom and side views are provided. The influence of the stirrer blade angle position, at the moment of bubble detachment from the capillary, on the breakup probability is analyzed. The breakup positions are linked to the current flow field, related to the stirrer blade angle, within the tank.
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Full scale bubbly flow experiments were performed on a 6 m flat bottom survey boat, measuring the void fraction, bubble velocity and size distributions as the bubbles naturally entrained at the bow of the boat interact with the bo...
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Full scale bubbly flow experiments were performed on a 6 m flat bottom survey boat, measuring the void fraction, bubble velocity and size distributions as the bubbles naturally entrained at the bow of the boat interact with the boat's boundary layer. Double-tip sapphire optical probes capable of measuring bubbles down to 50 mu m in diameter were specifically designed and built for this experiment. The probes were positioned under the hull at the bow near the bubble entrainment region and at the stern at the exit of the bottom flat plate. Motorized positioners were used to vary the probe distance to the wall from 0 to 50 mm. The experiments were performed in fresh water (Coralville Lake, IA) and salt water (Panama City Beach, FL), at varying velocities with most data analysis performed at 10, 14 and 18 knots. The results indicate that the bubbles interact significantly with the boundary layer. At low velocity in fresh water, bubble accumulation under the hull and coalescence are evident by the presence of large bubbles at the stern. At high speeds bubble breakup dominates and very small bubbles are produced near the wall. It is also observed that salt water inhibits coalescence, even at low boat speeds. The void fraction increases with speed beyond 10 knots and peaks near the wall. Bubble velocities show slip with the wall at all speeds and exhibit large RMS fluctuations, increasing near the wall. (C) 2015 Elsevier Ltd. All rights reserved.
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We present a new method that allows to control the bubble size and formation frequency in a planar air-water co-flow configuration by modulating the Water velocity at the nozzle exit. The forcing process has been experimentally ch...
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We present a new method that allows to control the bubble size and formation frequency in a planar air-water co-flow configuration by modulating the Water velocity at the nozzle exit. The forcing process has been experimentally characterized determining the amplitude of the water velocity fluctuations from measurements of the pressure variations in the water stream. The effect of the forcing on the bubbling process has been described by analyzing the pressute signals in the air stream in combinatiOn with visualizations performed with a high-speed camera. We show that, when the forcing amplitude is sufficiently large, the bubbles can be generated at a rate different from the natural bubbling frequency, f(n), which depends on the water-to-air velocity ratio, Lambda = u(n)/u(q), and the Weber number, We = rho(w)u(n)(2)H(0)/sigma, where 110 is the half-thickness of the air stream at the exit slit, rho(w), the water density and a the surface tension coefficient. Consequently, when the forcing is effective, monodisperse bubbles, of sizes smaller than those generated without stimulation, are produced at the prescribed frequency, f(f) > f(n). The effect of the forcing process on the bubble size is also characterized by measuring the resulting intact length, 1, i.e. the length of the air stem that remains attached to the injector when a bubble is released. In addition, the physics behind the forcing procedure is explained as a purely kinematic mechanism that is added to the effect of the pressure evolution inside the air stream that would take place in the unforced case. Finally, the downstream position of the maximum perturbation amplitude has been determined by a one-dimensional model, exhibiting a good agreement with both experiments and numerical simulations performed with OpenFOAM. (C) 2016 Elsevier Ltd. All rights reserved.
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An experimental study investigated how freely rising ellipsoidal bubbles approach each other, make contact and coalesce or breakup. Pulsed planar swarms of 10–20 bubbles with E?tv?s numbers from 6.0 to 27.5 were released simultan...
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An experimental study investigated how freely rising ellipsoidal bubbles approach each other, make contact and coalesce or breakup. Pulsed planar swarms of 10–20 bubbles with E?tv?s numbers from 6.0 to 27.5 were released simultaneously in aqueous solutions of 0–48 wt% sugar with Morton numbers from 3.2 × 10?11 to 3.7 × 10?6. Bubble interaction was recorded by a video camera following the rising bubbles. Essentially, all coalescence and breakup events occurred after, not during, wake-induced collisions by a complex process related to the bubble vortex shedding cycle. This same process was also found in multi-bubble clusters and may account for excess turbulent kinetic energy generation in bubbly flow.
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We study experimentally the behaviour of a bubble injected into a horizontal liquid solid-body rotating flow, in a range of rotational velocities where the bubble is close to the axis of rotation. We first study the stretching of ...
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We study experimentally the behaviour of a bubble injected into a horizontal liquid solid-body rotating flow, in a range of rotational velocities where the bubble is close to the axis of rotation. We first study the stretching of the bubble as a function of its size and of the rotation of the cell. We show that the bubble aspect ratio can be predicted as a function of the bubble Weber number by the model of Rosenthal (J. Fluid Mech., vol. 12, 1962, 358–366) provided an appropriate correction due to the impact of buoyancy is included. We next deduce the drag and lift coefficients from the mean bubble position. For large bubbles straddling the axis of rotation, we show that the drag coefficient $C_D$ is solely dependent on the Rossby number $Ro$, with $C_D \approx 1.5/Ro$. In the same limit of large bubbles, we show that the lift coefficient $C_L$ is controlled by the shear Reynolds number $Re_{shear}$ at the scale of the bubble. For $Re_{shear}$ larger than 3000 we observe a sharp transition, wherein large fluctuations in the bubble aspect ratio and mean position occur, and can lead to the break-up of the bubble. We interpret this regime as a resonance between the periodic forcing of the rotating cell and the eigenmodes of the stretched bubble.
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Drops impacting at low velocities onto a pool surface can stretch out thin hemispherical sheets of air between the drop and the pool. These air sheets can remain intact until they reach submicron thicknesses, at which point they r...
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Drops impacting at low velocities onto a pool surface can stretch out thin hemispherical sheets of air between the drop and the pool. These air sheets can remain intact until they reach submicron thicknesses, at which point they rupture to form a myriad of microbubbles. By impacting a higher-viscosity drop onto a lower-viscosity pool, we have explored new geometries of such air films. In this way we are able to maintain stable air layers which can wrap around the entire drop to form repeatable antibubbles, i.e. spherical air layers bounded by inner and outer liquid masses. Furthermore, for the most viscous drops they enter the pool trailing a viscous thread reaching all the way to the pinch-off nozzle. The air sheet can also wrap around this thread and remain stable over an extended period of time to form a cylindrical air sheet. We study the parameter regime where these structures appear and their subsequent breakup. The stability of these thin cylindrical air sheets is inconsistent with inviscid stability theory, suggesting stabilization by lubrication forces within the submicron air layer. We use interferometry to measure the air-layer thickness versus depth along the cylindrical air sheet and around the drop. The air film is thickest above the equator of the drop, but thinner below the drop and up along the air cylinder. Based on microbubble volumes, the thickness of the cylindrical air layer becomes less than 100 nm before it ruptures.
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We consider the evolution of the bulk bubble-size distribution N(a, t) of large bubbles (Weber number We >> 1) under free-surface entrainment described generally by an entrainment size distribution I(a) with power-law slope gamma ...
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We consider the evolution of the bulk bubble-size distribution N(a, t) of large bubbles (Weber number We >> 1) under free-surface entrainment described generally by an entrainment size distribution I(a) with power-law slope gamma and large-radius cutoff a(max). Our main focus is the interaction between turbulence-driven fragmentation and free-surface entrainment, and, for simplicity, we ignore other mechanisms such as degassing, coalescence and dissolution. Of special interest are the equilibrium bulk distribution N-eq(a), with local power-law slope (beta) over tilde (eq)(a), and the time scale tau(c) to reach this equilibrium after initiation of entrainment. For bubble radii a << a(max), we find two regimes for the dependence of N-eq(a) on the entrainment distribution. There is a weak injection regime for gamma >= -4, where (beta) over tilde (eq)(a) = -10/3 independent of the entrainment distribution; and a strong injection regime for gamma < -4, where the power-law slope depends on gamma and is given by <(beta)over tilde>(eq) = gamma + 2/3. The weak regime provides a general explanation for the commonly observed -10/3 power law originally proposed by Garrett et al. (J. Phys. Oceanogr., vol. 30 (9), 2000, pp. 2163-2171), and suggests that different weak entrainment mechanisms can all lead to this result. For a similar to a(max), we find that N-eq(a) exhibits a steepening deviation from a power law due to fragmentation and entrainment, similar to what has been observed, but here absent other mechanisms such as degassing. The evolution of N(a, t) to N-eq(a) is characterised by the critical time tau(c) proportional to C-f epsilon(-1/3) a(max)(2/3), where epsilon is the turbulence dissipation rate and C-f is a new constant that quantifies the dependence on the daughter size distribution in a fragmentation event. For typical breaking waves, tc can be quite small, limiting the time t less than or similar to tau(c) when direct measurement of N(a, t) might provide information about the underlying entrainment size distribution.
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Capillary instability of a liquid cylinder immersed in another liquid is analyzed using viscous potential flow. An effect of viscosity on the irrotational motion may be introduced by evaluating the viscous normal stress at the liq...
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Capillary instability of a liquid cylinder immersed in another liquid is analyzed using viscous potential flow. An effect of viscosity on the irrotational motion may be introduced by evaluating the viscous normal stress at the liquid-liquid interface on the irrotational motions. In a second approximation, the explicit effects of the discontinuity of the shear stress and tangential component of velocity which cannot be resolved pointwise in irrotational flows, can be removed in the mean from the power of traction integrals in the energy equation by the selection of two viscous corrections of the irrotational pressure. The actual resolution of these discontinuities presumably takes place in a boundary layer which is not computed or needed. We include the irrotational stress and pressure correction in the normal stress balance and compare the computed growth rates to the growth rates of the exact viscous flow solution. The agreement is excellent when one of the liquids is a gas; for two viscous liquids, the agreement is good to reasonable for the maximum growth rates but poor for long waves. Calculations show that good agreement is obtained when the vorticity is relatively small or the irrotational part is dominant in the exact viscous solution. We show that the irrotational viscous flow with pressure corrections gives rise to exactly the same dispersion relation as the dissipation method in which no pressure at all is required and the viscous effect is accounted for by evaluating the viscous dissipation using the irrotational flow. (c) 2005 American Institute of Physics.
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