摘要 :
We analyse the controlled generation of bubbles of a given size at a determined bubbling rate in a co-flowing water stream forcing the gas flow. The temporal evolution of the bubble size, R(t), the air flow rate, Q(a)(t), and the ...
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We analyse the controlled generation of bubbles of a given size at a determined bubbling rate in a co-flowing water stream forcing the gas flow. The temporal evolution of the bubble size, R(t), the air flow rate, Q(a)(t), and the pressure evolution inside the bubble, p(b)(t), during the bubbling process are reported. To that aim, the temporal evolution of the bubble shape and the pressure inside the air feeding chamber, p(c)(t), where a harmonic perturbation is induced using a loudspeaker, are obtained from high-speed images synchronized with pressure measurements. A model is developed to describe the unsteady motion of the gas stream along the injection needle, coupled with the Rayleigh-Plesset equation for the growing bubble, allowing us to obtain p(b)(t). Thus, the minimum pressure amplitudes required inside the forming bubble to control their size and bubbling frequency are provided as a function of the gas flow rate, the liquid velocity, u(w), and the forcing frequency, f(f). Two different behaviors have been observed, depending on the liquid-to-gas velocity ratio, Lambda = u(w)/u(a). For small enough values of Lambda, the critical pressure amplitude is given by p(s) similar to rho(a) cu(a) St(f)(3), associated to a rapid pressure increase taking place during an interval of time of the order of the acoustic time. However, for larger values of Lambda, p(s) similar to rho u(w)(2) St(f)(3 )Lambda(-1/5)We(-1/4). Here rho and rho(a) are the liquid and gas densities respectively, c the speed of sound in air and St(f) = f(f)r(0)/u(w) and We = rho u(w)(2)r(o)/sigma the Strouhal and Weber numbers, where tau(o) denotes the outer radius of the injector. (C) 2020 Elsevier Ltd. All rights reserved.
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Gas sparged ultrafiltration experiments are performed using a tubular membrane module with solutions of dextran and human serum albumin (HAS) as the test media. Air is injected, in a controlled manner with the ability to adjust bu...
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Gas sparged ultrafiltration experiments are performed using a tubular membrane module with solutions of dextran and human serum albumin (HAS) as the test media. Air is injected, in a controlled manner with the ability to adjust bubble size and frequency independently, into the membrane module to create a gas-liquid two-phase crossflow operation. The effects of bubble size and frequency on the permeate flux of the spared ultrafiltration are studied experimentally. It is found that the permeate flux increases with the bubbling frequency in the examined range.
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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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Bubble-bubble interactions are relevant to industrial applications, for which air is injected at different flowrates from submerged orifice. These interactions provoke aperiodic bubble formation that affects their departure and si...
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Bubble-bubble interactions are relevant to industrial applications, for which air is injected at different flowrates from submerged orifice. These interactions provoke aperiodic bubble formation that affects their departure and size distribution, which is revealed through our experimental data. Air is injected and regulated at flow rate between 10~(-4) m~3.s~(-1) and 10~(-3) m~3.s~(-1) from a submerged orifice of inner diameter 12 mm, corresponding to a gas Weber number between 0.16 and 16. Image analysis is carried out to track the formation and departing of bubbles, as well as the formation of a globule, which is a large bubble formed after the bubbles coalescence. Upon increasing the flowrate, the different patterns of bubbling observed have been classified according to the variation of the coalescence position. In a conventional method, bubbles volumes are determined by inspecting the departure frequency and assuming single bubbling spherical pattern. Using the same assumptions and method, we find a good agreement with literature models. Owing to the phenomenological description of the flow, the frequency and volume of globules are characterized. A contrast between the conventional and phenomenological approaches is exposed, revealing a weak applicability of conventional models in estimating bubbles sizes at high flow rates.
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This work deals with the study of the coalescence phenomenon (merging of two or more bubbles into a single larger one) during pool boiling on a duraluminium (AU4G) vertical heated wall. Various boiling curves characterising boilin...
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This work deals with the study of the coalescence phenomenon (merging of two or more bubbles into a single larger one) during pool boiling on a duraluminium (AU4G) vertical heated wall. Various boiling curves characterising boiling (with or without coalescence) from three artificial nucleation sites a variable distance apart are presented. The heat flux ranges from 100 to 900 W/cm(2) and the wall superheat from 5 to 35 K. It is pointed out that the coalescence of bubbles growing on three sites results in higher heat transfer coefficients than single-site boiling. However, the highest heat transfer coefficients are obtained for an optimal distance between the sites for which coalescence does not occur. It is also shown that coalescence results in a decrease in the bubble frequency. Lastly, a map is proposed to determine the conditions leading to coalescence or to site activation. (C) 2000 Elsevier Science Inc. All rights reserved. [References: 19]
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Underwater gas bubbles are universal phenomenon in various industries. These bubbles appear under ocean conditions especially rolling conditions, that affect the mass transfer and produce different acoustic signals. The bubble for...
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Underwater gas bubbles are universal phenomenon in various industries. These bubbles appear under ocean conditions especially rolling conditions, that affect the mass transfer and produce different acoustic signals. The bubble formation and oscillation frequency characteristics under rolling conditions were investigated in this study by using a numerical method. Results showed that the bubbles followed two basic steps under rolling conditions, namely, expansion and detachment. The bubble detachment surface area and shape exhibited periodic changes because of the periodical motion of the surrounding water. A correlation was established to predict the bubble detachment surface area under rolling conditions. Similar to the bubble detachment surface area, the bubble oscillation frequency periodically changed. The oscillation mode of the bubbles under rolling conditions was the same as those under motionless condition, that is zeroth mode. A modified Minnaert formula was established to predict the bubble oscillation frequency under rolling conditions. (C) 2020 Elsevier Ltd. All rights reserved.
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The effect of pressure on gas holdup was investigated experimentally with gas-liquid systems, of nitrogen-water, and carbon dioxide-water, in a bubble column with diameter of 45 mm. Single nozzles of 1.4, and 4.0 mm in diameter we...
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The effect of pressure on gas holdup was investigated experimentally with gas-liquid systems, of nitrogen-water, and carbon dioxide-water, in a bubble column with diameter of 45 mm. Single nozzles of 1.4, and 4.0 mm in diameter were used as gas disperser. When the 4.0 mm nozzle was used, no effect of pressure on gas holdup was observed. When single nozzle of 1.4 mm with nitrogen as the gas phase, gas holdup increased with pressure. But when carbon dioxide was used as the gas phase, gas holdup showed a maximum at pressure of 0.6 MPa. The gas holdup at 1.1 MPa was approximately the same value as that at atmospheric pressure. To explain these phenomena by the behavior of bubbles in the column, bubble size near the wall and bubble frequency at the center of column were measured.
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Prediction correlations of air bubble diameter and frequency in stagnant clean water were established. Eleven different orifice diameters were tested under flow rate of 0.05-0.15 SLPM. The resulted bubble size and frequency were t...
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Prediction correlations of air bubble diameter and frequency in stagnant clean water were established. Eleven different orifice diameters were tested under flow rate of 0.05-0.15 SLPM. The resulted bubble size and frequency were traced using high-speed camera. It was found that the mean Sauter diameter and bubble frequency are in the range of 3.7-6.9mm and 6.4-47.2 bubbles per second, respectively. Nonlinear regression was performed to design the new correlations of estimating diameter and frequency with a correlation factor of 0.93 and 0.94, respectively. Flow rate and orifice size had the highest impact on the studied parameters.
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The periodic generation of monodisperse micro air bubbles in water flowing in silicon microchannels having a trapezoidal cross section is investigated experimentally with the aid of high-speed CCD imaging technology. Air and water...
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The periodic generation of monodisperse micro air bubbles in water flowing in silicon microchannels having a trapezoidal cross section is investigated experimentally with the aid of high-speed CCD imaging technology. Air and water, used as the discontinuous and carrier fluid, are first co-flowing in the air microchannel and two adjacent water microchannels separately, and then converge in the main microchannel downstream. After breakup of the air bubble injected from the air microchannel, slug/bubbly flow regimes are formed periodically in the main microchannel. Correlations for the dimensionless bubble length and the dimensionless bubble formation frequency are obtained in terms of appropriate dimensionless parameters respectively.
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In order to investigate the effects of electric fields on the bahavior of a bubble attached to a wall, basic experiments on the deformation and departure of a bubble are carried out under d.c./a.c. applied voltages. In the present...
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In order to investigate the effects of electric fields on the bahavior of a bubble attached to a wall, basic experiments on the deformation and departure of a bubble are carried out under d.c./a.c. applied voltages. In the present study, three types of electrodes are used, to examine the bubble behavior by the nonuniformity of the electric field. For a d.c. electric field, as the applied voltage increases the bubble attached to a wall is more extended in the direction parallel to the imposed electric field, thus the aspect ratio and the contact angle also increase. The bubble departure volume in a nonuniform electric field decreases continuously, while that in a uniform electric field is nearly constant. The present results by the d.c. electric field show that the bubble behavior is significantly affected by the degree of the inhomogeneity of the electrode configuration. For an a.c. electric field, as a preliminary analysis, a general theory on an equivalent dynamic system is extended to derive the bubble oscillation frequency. The analysis predicts that the bubble departure occurs near a resonant frequency. It is observed that the departure process of a bubble in the a.c. electric field is associated with the bubble oscillation which is composed of three different regions. Near the critical voltage, the bubble departure volume drops suddenly. It is also found that the reduction of the bubble departure volume by the a.c. electric field is more effective than in the d.c. electric field case.
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