Dynamics of bubble breakup in a microfluidic T-junction divergence

The aim of this work is to investigate experimentally the bubble breakup in a microfluidic T-junction divergence using a high-speed digital camera and a micro-Particle Image Velocimetry (micro-PIV) system. The breakup and non-breakup of N₂ bubbles in glycerol–water mixtures with several concentrations of sodium dodecyl sulphate (SDS) as surfactant were studied with capillary number ranging from 0.001 to 0.1. The cross section of PMMA square microchannel is 400 μm wide and 400 μm deep. Four various flow patterns were observed at the T-junction by changing gas and liquid flow rates. The dynamics of three various types of symmetric breakup of bubbles were investigated. The symmetric breakup of bubbles type I is mainly controlled by the augmented pressure in liquid phase. The symmetric breakup of bubbles type II is controlled by both the increased pressure and viscous forces. In the symmetric breakup of bubbles type III, a scaling law for the minimum bubble neck and the remaining time during bubble breaking process were found. The transitions between breakup and non-breakup of bubbles were investigated, and a power–law relationship between bubble extension and capillary number was proposed to predict the transitions between adjacent regimes. Our experimental results reveal that the bubble breakup in a microfluidic T-junction divergence is similar to the droplet behaviours in such a device ( [Jullien et al., 2009] , [Leshansky and Pismen, 2009] and [Link et al., 2004] ).     

Bubble formation in non-Newtonian fluids in a microfluidic T-junction

The aim of this work is to investigate the bubble formation in non-Newtonian fluids in a microfluidic T-junction by crossflowing rupture technique, using a high-speed digital camera. Experiments were conducted in a glass microchannel with 120 μm wide and 40 μm deep. N₂ bubbles were generated in different concentrations of polyacrylamide (PAAm) solutions. Various flow patterns were observed at the T-junction by varying gas and liquid flow rates. The breakup mechanism for bubbles was investigated to gain insight into the effects of flow rates and concentrations of PAAm solutions on bubble size. The gaseous thread collapses at a constant speed in the collapse stage; while during the final pinch-off stage, the variation of the minimum width W_m of the gaseous thread with the remaining time (T − t) could be scaled as W_m ∼ (T − t)^0.21. The bubble size increases non-linearly with the gas/liquid flow rates ratio, and decreases with the concentration of PAAm solutions.     

T-型微通道中液滴形成机制的CFD模拟

对微米级窄型T-型微通道中微液滴的形成机制进行了CFD模拟,验证了随毛细准数Ca的增加,液滴的形成会经历"squeezing"和"dripping"机制,且2个机制之间明显的存在着一个"transient"机制。通道壁的润湿性能对液滴的形成过程有显著影响,只有当通道壁更亲连续相时,微液滴才能形成。但与"dripping"机制不同,在"squee-zing"机制下,通道壁的润湿性对形成液滴的体积有明显的影响。     

Gas-liquid mass transfer in bubble columns with a T-junction nozzle for gas dispersion

Bubble break-up, gas holdup, and the gas-liquid volumetric mass transfer coefficient are studied in a bubble column reactor with simultaneous injection of a gas and liquid through a T-junction nozzle. The theoretical dependence of bubble break-up and the volumetric mass transfer coefficient on liquid velocity in the nozzle is developed on the basis of isotropic turbulence theory. It is shown that correlations which are developed based on liquid jet kinetic power per nozzle volume explain average gas holdup and the volumetric mass transfer coefficient within an error of 15% for all gas and liquid flow rates and nozzle diameters used. Experiments with a larger scale column, height 4.64 m and diameter 0.98 m, show a transition from homogeneous to heterogeneous flow at a certain liquid flow rate through the nozzle. Liquid composition was found to have a significant effect on gas-liquid mass transfer. A phenol concentration of 10–30 mg/l in water increases the volumetric mass transfer coefficient of oxygen by 100%. This phenomenon may have significance in the chemical oxidation of wastewater.     

Bubble formation and breakup mechanism in a microfluidic flow-focusing device

The aim of this study is to investigate the bubble formation mechanism in a microfluidic flow-focusing device using a high-speed digital camera and a micro-particle image velocimetry (μ-PIV) system. Experiments were conducted in a PMMA square microchannel with 600 μm wide and 600 μm deep. Gas bubbles were generated in glycerol-water mixtures with several concentrations of surfactant sodium dodecyl sulfate (SDS). Various flow patterns were obtained at the cross-junction by changing gas and liquid flow rates. The formation mechanism of slug bubble at the cross-junction was investigated to gain insight into the effects of liquid and gas flow rates, and viscosity of the liquid phase on the breakup rate of the gas thread, and on the collapse time. The velocity fields in the liquid phase around the thread were determined by μ-PIV measurements. The experimental data of the breakup rate and the collapse time of the gas thread were described as a function of the liquid superficial velocity u_l, the ratio of the gas and liquid flow rates Q_g/Q_l and Reynolds number Re=ρul/μ.     

Studies of ethylene chlorohydrin formation in a bubble column reactor

An experimental study of the reactions of ethylene, chlorine, and water in a bubble column reactor, on a laboratory scale, has been carried out. The effect of gas flow rates on the yield of the products in an unbaffled reactor is reported. Baffles were introduced into the column to reduce axial mixing. A significant improvement in the conversion and yield of ethylene to ethylene chlorohydrin was observed.     

On the mechanism of bubble formation in a fluidized bed

A mechanism of bubble formation in a nonhomogeneous fluidized bed, based on instability theory, is suggested. In this mechanism it is assumed that the “bubbles” are formed at the bottom of the bed by the growth of prominences appearing because of the instability to perturbations of the lower surface of the bed. A bubble breaks off when the volume of the prominence becomes sufficiently large so that the buoyant force equals the sum of the bed resistance opposing its growth and the inertial forces. An equation for the diameter of the departing bubble is established.     

Stability analysis of bubble columns: Predictions for regime transition

A criterion for the transition from the homogeneous to the heterogeneous regime in a bubble column is developed based on the theory of linear stability. Hydrodynamics of bubble column is described by two-fluid model incorporating the interphase forces like drag force and added mass force. Added mass force affects the hydrodynamics of gas-liquid flows significantly and is formulated by taking into account the bubble deformation. A proper understanding of the nature of gas-liquid interface (clean or contaminated) is desired for the reliable predictions of the added mass coefficient. Data from the literature on the transition in bubble columns is critically analyzed. A good agreement has been obtained between the experimental transition gas hold-up and the predictions of the same obtained by the theory developed in this work.     

Small bubble formation via a coalescence dependent break-up mechanism

A mechanism has been elucidated for the coalescence-mediated break-up of bubbles in gas-liquid systems. Images taken from dynamic systems (a coalescence cell and laboratory-scale bubble columns) show that in some instances the coalescence of two bubbles is accompanied by the formation of a much smaller daughter bubble. Following the coalescence process an annular wave is formed due to the very rapid expansion of the hole following the instant of film rupture. As the wave moves along the length of the bubble, away from the point of rupture it causes a rippling effect which distorts the newly coalesced bubble and may result in the formation of an unstable extension. Instabilities due to the annular wave pinch off a portion of this extension, resulting in the generation of a small daughter bubble. In coalescence dominated systems the process results in the generation of significant numbers of bubbles much smaller (100- diameter) than the Sauter mean diameter (3-).     

Study on the kinetics of hydrate formation in a bubble column

Gas hydrate formation experiments were performed using methane in the presence of tetrahydrofuran (THF) in aqueous solution in a transparent bubble column in which a single pipe or a sintered plate was used to produce bubbles. The mole fraction of THF in aqueous solution was fixed at 6%. The hydrate formation kinetic behaviors on the surface of the rising bubble, the mechanical stability of hydrate shell formed on the surface of the bubble, the interactions among the bubbles with hydrate shell were observed and investigated morphologically. The rise velocities of individual bubbles with hydrate shells of different thickness and the consumption rates of methane gas were measured. A kinetic model was developed to correlate the experimentally measured gas consumption rate data. It was found that the hydrate formation rate on the surface of the moving bubble was high, but the formed hydrate shell was not very easy to be broken up. The bubbles with hydrate shells tended to agglomerate rather than merge into bigger bubble. This kind of characteristic of hydrate shell hindered the further formation of hydrate and led to the lower consumption rate of methane. The consumption rate of methane was found to increase with the decrease of temperature or increase of pressure. The increase of gas flux led to a linear increase in consumption rate of methane. It was demonstrated that the developed kinetic model could be used to correlate the consumption rate satisfyingly.     

Shape oscillations and path transition of bubbles rising in a model bubble column

We investigate experimentally the occurrence of shape oscillations accompanied by path transition of periodically produced air bubbles rising in water. Within the period of bubble formation, the induced velocity is measured to examine bubble-liquid and bubble-bubble interactions. The flow is produced in a small-scale bubble column with square-shaped cross section. A capillary aerator produces bubbles of size 3.4 mm at a frequency of 5 Hz. Measuring techniques employed are high-speed imaging to capture bubble shape oscillations and path geometry, and laser-Doppler anemometry (LDA) to measure the velocity in the liquid near the rising bubbles. The experimentally obtained bubble shape data are expanded in Legendre polynomials. The results show the occurrence of oscillations by the periodicity of the expansion coefficients in space. Significant shape oscillations accompanied by path transition are observed as the second-mode oscillation frequency converges to the frequency of the initial shape oscillations. The mean velocity field in the water obtained by LDA agrees well with potential theory. An analysis of the decay of the induced flow shows that there is no interaction between the flow fields of two succeeding 3.4 mm bubbles in the rectilinear path when the bubble production frequency is lower than 7.4 Hz.     

Modelling the bubble formation dynamics in non-Newtonian fluids

A theoretical model is developed for modelling the non-spherical bubble formation at an orifice submerged in non-Newtonian fluids under constant flowrate conditions. The equations of motion are, respectively, the radial expansion and vertical ascension of the bubble interface. They are combined with the thermodynamic equations for the gas in the bubble and the chamber below the orifice as well as the fluid rheological equation. In particular, the influence of in-line interactions between bubbles due to the fluid memory effects of the viscoelastic characteristics is taken into account for the first time. The present model is able to compute the instantaneous growing shape of the bubble during its formation and determine the final size of detachment as well as the frequency of bubble formation. The values predicted by this model compare satisfactorily with the experimental results obtained under different operating conditions.     

An improved model for bubble formation using the boundary-integral method

A model for bubble formation from a single submerged orifice is developed using the boundary-integral method. Since the flow field is assumed to be irrotational, potential-flow theory is used to predict the growth of the bubble. The effects of the surface tension and the liquid circulation around the bubble are included in the calculation. Predictions of bubble shape, chamber pressure and the effect of surface tension are presented to compare with reported experimental data.     

A model for bubble formation and weeping at a submerged orifice with liquid cross-flow

A new theoretical model to predict bubble frequencies and weeping rates at a submerged orifice with liquid cross-flow has been developed. The model predicts a significant influence of liquid cross-flow velocity on bubble formation frequency, and especially on liquid weeping. Simulated values of weeping rates for different orifice diameters, gas flowrates and liquid cross-flow velocities show good agreement with experimental data.     

T型微通道内流体流动的FLUENT模拟

在流体力学研究中,微流控技术是当今研究的一个热点。利用微尺度下流体流动的一些特性可以提高流体间传质、传热以及反应速率。T型微通道是一个比较常见的微流控装置,利用ICEM CFD软件建立T型微通道的几何模型;然后通过FLUENT软件对微通道内流体流动进行模拟仿真。模拟实验结果表明,通道内两相流体的速度和粘度对流型有显著影响。     

Experimental study of the slug/churn flow transition in a single Taylor bubble

A study is presented of the hydrodynamic behaviour of very long gas slugs, rising co-currently with water along a 20 mm i.d. vertical tube. Visual observation (supported by pictures from video and still cameras) and the signals from a set of three fast response differential pressure transducers, were used to elucidate the flow behaviour of individual slugs of argon, with densities in the range 6.6-21.5 kg/m³ (corresponding to operating pressures in the range 0.4-1.3 MPa), as they rose in water that moved up the tube, with constant average velocity in the range 0.17-1.4 m/s. For the lower gas densities and liquid velocities the slugs were stable, but for the higher liquid velocities and/or gas densities, the slugs would become unstable, as a result of flooding in the wetted wall flow around them. The video sequences show clearly that, at the higher pressures, liquid from the film was dragged up by the gas, while the still pictures document the corresponding transition to churn flow in the lower regions of the rising slugs.