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This study experimentally investigates the effects of bubble coalescence on the heat fluxes during nucleate pool boiling. A microheater array was used to generate vapor bubbles in FC-72 liquid with constant surface temperature bou...
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This study experimentally investigates the effects of bubble coalescence on the heat fluxes during nucleate pool boiling. A microheater array was used to generate vapor bubbles in FC-72 liquid with constant surface temperature boundary conditions while the heat flux at selected locations was measured for various superheats using a high speed data acquisition system. The heat flux for boiling with coalescence was found to fluctuate much more than when only a single bubble formed on the surface due to the vaporization of the liquid layer trapped between the bubbles and oscillations of the bubbles after coalescence which resulted in relatively long periods of fairly high heat fluxes. The observations also showed that some coalescence events were not accompanied by an increase in the heat transfer rate as the liquid layer between the bubbles was physically pushed away by the rapid bubble growth during the inertial bubble growth stage instead of evaporating. A mechanistic model was developed to predict the conditions for which the heat fluxes do not increase during coalescence. In many other instances, the coalescence dynamics were such that the only bubble remaining on the surface was swept off by the liquid motion from the previous coalescence event with no nucleation of another bubble. This was followed by a long period without bubble nucleation followed by sudden nucleation and explosive coalescence of two bubbles giving rise to a large bubble which departed violently with a large increase in the heat transfer rate.
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Microbubbles, small gas bubbles of diameter less than 1 mm, are desirable in many important chemical and process engineering applications. Their behaviour differs from that of large bubbles due to very high surface energy and slow...
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Microbubbles, small gas bubbles of diameter less than 1 mm, are desirable in many important chemical and process engineering applications. Their behaviour differs from that of large bubbles due to very high surface energy and slow motion velocity. Unfortunately, producing them efficiently and economically has been until recently a problem. Paper discusses one of the reasons, apparently not yet recognised in the available literature. Because of the slow ascent motion associated with the small size, the microbubble, after leaving the aerator exit, inevitably gets into close proximity with the microbubble that was formed at the same exit earlier. Quite often, they merge to form a single substantially larger bubble. The conjunctions are so fast and take place at a scale so small that they have apparently escaped attention. Author discovered this limiting effect to bubble smallness by using a long-distance macro-lens and a high-speed camera.
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The motion of nearly spherical air bubbles rising in a column in stagnant water is measured at Reynolds numbers ranging from ReD = 0.2 to 35. The relative velocity is found to be dependent on the distance between bubbles and on th...
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The motion of nearly spherical air bubbles rising in a column in stagnant water is measured at Reynolds numbers ranging from ReD = 0.2 to 35. The relative velocity is found to be dependent on the distance between bubbles and on their diameter. For the larger bubbles, the relative velocities increased with decreasing distance, reaching maximum values just prior to contact. For the smaller bubbles, the relative velocity decreased prior to coalescence. For the entire range of Reynolds number considered, the wake-induced relative motion results in collisions between bubbles. These collisions culminate in coalescence at the present levels of water purity and surface tension. In order to understand the basic features of the measured relative motion, a simple model is developed. It is based on the known flow field and viscous-wake structure around a single bubble, and examines how other bubbles move within this field. Oseen flow for Re << 1 and potential flow with a thin wake for Re >> 1 are assumed. The approximations involved limit the validity of the model to distances larger than a few bubble diameters. The general agreement between the predictions and experimental results suggest that the model contains the most relevant mechanisms that govern the interaction, within its range of validity. Prior analyses for non-deformable bubbles that predicted an equilibrium due to balance between pressure gradients and wake-induced motion, are contradicted by the observed coalescence. A possible cause for the discrepancy is bubble deformation.
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Critical coalescence concentration (CCC) was determined in a laboratory-scale mechanical flotation cell for a series of coalescence inhibiting inorganic salts (KCl, NaCl, Na_2SO_4, CaCl_2 and MgSO_4) compared to two commercial fro...
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Critical coalescence concentration (CCC) was determined in a laboratory-scale mechanical flotation cell for a series of coalescence inhibiting inorganic salts (KCl, NaCl, Na_2SO_4, CaCl_2 and MgSO_4) compared to two commercial frothers (methyl isobutyl carbinol, Dowfroth 250C). The salt CCC values ranged from 0.07 M (MgSO_4) to 0.31 M (KC1 and NaCl) and correlated with ionic strength. The CCC values are compared to transition concentrations in the literature. The effect of salts on gas dispersion in flotation systems is discussed.
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The dynamic process or structural studies of adsorption and desorption phenomena on the fluid interface has not been clearly established because of its difficulties in experimental instrumentation.In this work,we focused on the bu...
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The dynamic process or structural studies of adsorption and desorption phenomena on the fluid interface has not been clearly established because of its difficulties in experimental instrumentation.In this work,we focused on the bubble coalescence phenomenon,which is known to be strongly affected by trace impurities such as 1 ppm or smaller.We measured the time course of the scattered He-Ne laser intensity irradiated at the contact surface of two bubbles,and the bubble coalescence time was determined in an aqueous solution of n-butanol with concentrations of 5.0,7.5,and 10.0 mol/m3.The butanol concentration was so dilute that surface tension was little affected.At each butanol concentration,the bubble coalescence time varied with bubble age,which is the elapsed time from the formation of a bubble.The coalescence time quickly increased in a few seconds since its formation.After that,it gradually increased till the age of a few hundreds seconds,and then suddenly increased again.The initial value of the bubble coalescence time at the bubble age of zero could not be determined because of the difficulty in experiment,however,the value just after the first increase clearly depended on the butanol concentration.The bubble age at the onset of the second sudden increase also clearly depended on the butanol concentration.The slope between the first and the second sudden increase was larger for the higher butanol concentration.The dependence of the bubble coalescence time on the bubble age obtained in this work can be a help to elucidate the dynamic process of adsorption and desorption at the interface.
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We propose a new bubble dynamics model to study the evolution of a suspension of bubbles over a wide range of vesicularity, and that accounts for hydrodynamical interactions between bubbles while they grow, deform under shear flow...
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We propose a new bubble dynamics model to study the evolution of a suspension of bubbles over a wide range of vesicularity, and that accounts for hydrodynamical interactions between bubbles while they grow, deform under shear flow conditions, and exchange mass by diffusion coarsening. The model is based on a lattice Boltzmann method for free surface flows. As such, it assumes an infinite viscosity contrast between the exsolved volatiles and the melt. Our model allows for coalescence when two bubbles approach each other because of growth or deformation. The parameter (disjoining pressure) that controls the coalescence efficiency, i.e., drainage time for the fluid film between the bubbles, can be set arbitrarily in our calculations. We calibrated this parameter by matching the measured time for the drainage of the melt film across a range of Bond numbers (ratio of buoyancy to surface tension stresses) with laboratory experiments of a bubble rising to a free surface. The model is then used successfully to model Ostwald ripening and bubble deformation under simple shear flow conditions. The results we obtain for the deformation of a single bubble are in excellent agreement with previous experimental and theoretical studies. For a suspension, we observe that the collective effect of bubbles is different depending on the relative magnitude of viscous and interfacial stresses (capillary number). At low capillary number, we find that bubbles deform more readily in a suspension than for the case of a single bubble, whereas the opposite is observed at high capillary number.
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A simulation model for 3D polydisperse bubble column flows in an Eulerian/Eulerian framework is presented. A computationally efficient and numerically stable algorithm is created by making use of quadrature method of moments (QMOM...
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A simulation model for 3D polydisperse bubble column flows in an Eulerian/Eulerian framework is presented. A computationally efficient and numerically stable algorithm is created by making use of quadrature method of moments (QMOM) functionalities, in conjunction with appropriate breakup and coalescence models. To account for size dependent bubble motion, the constituent moments of the bubble size distribution function are transported with individual velocities. Validation of the simulation results against experimental and numerical data of Hansen [1] show the capability of the present model to accurately predict complex gas-liquid flows.
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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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Growth of an isolated bubble and horizontal coalescence events between bubbles of dissimilar size were examined during pool nucleate boiling of water on a horizontal, electrically-heated titanium foil 25 urn thick. Wall temperatur...
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Growth of an isolated bubble and horizontal coalescence events between bubbles of dissimilar size were examined during pool nucleate boiling of water on a horizontal, electrically-heated titanium foil 25 urn thick. Wall temperature measurements on the back of the foil by high-speed IR camera, synchronized with high-speed video camera recordings of the bubble motion, improved the temporal and spatial resolution of previous observations by high-speed liquid crystal thermography to 1 ms and 40 um, respectively, leading to better detailed maps of the transient distributions of wall heat flux. The observations revealed complex behaviour that disagreed with some other observations and current modelling assumptions for the mechanisms of heat transfer over the wall contact areas of bubbles and interactions between bubbles. Heat transfer occurred from the entire contact area and was not confined to a narrow peripheral triple-contact zone. There was evidence of an asymmetrical interaction between bubbles before coalescence. It was hypothesised that a fast-growing bubble pushed superheated liquid under a slow-growing bubble. Contact of this liquid with regions of the wall that had been pre-cooled during bubble growth caused local reductions in the wall heat flux. During coalescence, movement of liquid under both bubbles caused further changes in the wall heat flux that also depended on pre-cooling. Contraction of the contact area caused a peripheral reduction in the heat flux and there was no evidence of a large increase in heat flux during detachment. Boiling on very thin foils imposes special conditions. Sensitivity to the thermal history of the wall must be taken into account when applying the observations and hypotheses to other conditions.
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Understanding the dynamics of oscillating bubbles beneath a free surface is crucial to many practical applications including airgun-bubble clusters, underwater explosions, etc. In this paper, an experimental and numerical study of...
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Understanding the dynamics of oscillating bubbles beneath a free surface is crucial to many practical applications including airgun-bubble clusters, underwater explosions, etc. In this paper, an experimental and numerical study of the dynamic behaviors of a coalesced bubble near a free surface is conducted, which shows quite different physical features from single bubble dynamics. Firstly, two similar sized underwater discharge bubbles are generated simultaneously beneath a free surface and their complex interactions are experimentally studied with high-speed photography imaging. A strong interaction between two bubbles and the subsequent coalescence are observed when the initial distance between two bubbles is smaller than the maximum equivalent bubble radius. Secondly, both axisymmetric and three-dimensional (3D) boundary integral models are used to simulate the pre-coalescence and post-coalescence of two bubbles. The results obtained by the two models agree well in axisymmetric conditions. The essential physical phenomena in representative experiments are well reproduced by the present 3D model. The pressure field is calculated by the auxiliary function method, which helps to reveal the underlying mechanisms of bubble collapse patterns and jetting behaviors. A parametric study reveals the dependence of the coalesced bubble dynamics and free surface motion on the governing dimensionless quantities.
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