大孔径高气速单孔气泡形成

在内径为190mm的鼓泡塔内,研究了空气-去离子水系统在大孔径高气速条件下的单孔气泡形成。考察了五个不同的孔径,分别为4、8、10、15及21mm,孔口气速范围为0.8~154.8m·s^-1。以CCD摄像记录气泡的形状及尺寸,根据气泡长径比的变化,得到气泡初始形态转变时的临界孔口气速：当孔口气速低于20m·s^-1时,孔口气泡近似于球形,长径比小于1.1;当孔口气速大于50m·s^-1时,气泡呈现椭球形,长径比大于1.5。并对气泡尺寸与孔径及孔口气速进行关联,所得关联式对孔径大于3mm、孔口气速在10~80m·s^-1范围内所形成的气泡尺寸预测效果较好。     

Squeezing-to-dripping transition for bubble formation in a microfluidic T-junction

The aim of this paper is to investigate the squeezing-to-dripping transition for bubble formation in a microfluidic T-junction by cross-flowing rupture technique using a high-speed digital camera. Experiments were conducted in a glass microfluidic T-junction with the cross-section of the microchannel of 120 μm wide and 40 μm deep. N₂ bubbles were generated in glycerol–water mixtures with several concentrations of surfactant sodium dodecyl sulfate (SDS). Three different regimes were identified for generating different kinds of bubbles: squeezing, dripping and transition regimes. Various forces exerted on the gaseous thread in different regimes were analyzed. Long slug bubbles were formed in the squeezing regime, while dispersed bubbles in the dripping regime. The transition regime formed short slug bubbles. The bubble sizes in various regimes could be correlated with several dimensionless numbers such as the ratio of gas/liquid flow rates and capillary number. The two-step model for droplets (Steegmans et al., 2009) was extended to describe the bubble formation.     

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 from a free-standing tube in microgravity

The bubble characteristics and the bubble detachment mechanisms during injection of air from a free-standing capillary tube submerged in water were studied in microgravity. The experiments were conducted in the 2.2-s drop tower at the NASA Glenn Research Center. A tube, 0.51 mm in diameter and 150 mm long, in a rectangular test section ( long) served as the injector. Images of the bubbles during the drops were acquired using a high-speed camera for various gas flow rates. Bubble detachment was observed for all the Weber numbers tested (0.28-31.12). This observation was different from previous studies using plate orifices in microgravity when bubble detachment was observed only for Weber numbers larger than 8. In order to resolve these differences, experiments were carried out using plate orifices. It was found that the bubbles detached from the orifice for all Weber numbers and that the bubbles formed were larger than those formed with the tube injector, particularly at low gas flow rates. The availability of a large area for the bubble to anchor itself and the presence of the chamber underneath the orifice could cause these differences. The effects of the chamber volume on the unsteadiness of bubble formation in plate-orifices at low gas flow rates are discussed.     

Ammonia absorption from a bubble expanding at a submerged orifice into water

To investigate the mechanism of gas absorption from a bubble containing soluble and insoluble components, a gaseous mixture of ammonia and nitrogen was bubbled into water. The growth curve, volume, surface area and shape of the growing bubbles were measured with parameters such as inlet gas composition, gas flow rate and gas chamber volume. The bubble volume decreased with the increasing composition of ammonia in a bubble, decreasing gas chamber volume and decreasing gas flow rate.To reasonably express the mass transfer from the bulk of a gas in a bubble to the bulk of a liquid, the overall mass transfer resistance was evaluated by the mass transfers in the gas phase, interface and liquid phase.The non-spherical bubble formation model combined with the overall mass transfer resistance estimated well experimental bubble shape, bubble volume at its detachment from an orifice, growth rate and mass transfer rate.Moreover, the change of concentration with bubble growth time and the fractional absorption during bubble formation were simulated.     

Bubble formation in flowing liquid

Initial bubbles in flowing liquid from a nozzle were observed from two mutually perpendicular directions. Two nozzles of 0.086 cm and 0.305 cm in diameter were used. The gas flow rate and the superficial liquid velocity ranged from 0.33 cm³/s to 36.2 cm³/s and from 0 cm/s to 154.9 cm/s, respectively. The bubble size formed in flowing liquid decreased with decreasing gas flow rate and with increasing superficial liquid velocity. Three types of bubble formation, i.e. single bubbles, coalescent bubbles and gas jets, were observed depending upon the gas rate and the liquid velocity. Two empirical equations of the bubble sizes are given.     

Experimental and numerical studies on the effects of pressure release rate on number density of bubbles and bubble growth in a polymeric foaming process

A simulation of simultaneous bubble nucleation and growth was performed for a batch physical foaming process of polypropylene (PP)/CO₂ system under finite pressure release rate. In the batch physical foaming process, CO₂ gas is dissolved in a polymer matrix under pressure. Then, the dissolved CO₂ in the polymer matrix becomes supersaturated when the pressure is released. A certain degree of supersaturation produces CO₂ bubbles in the polymer matrix. Bubbles are expanded by diffusion of the dissolved CO₂ into the bubbles. The pressure release rate is one of the control factors determining number density of bubbles and bubble growth rate.To study the effect of pressure release rate on foaming, this paper developed a simple kinetic model for the creation and expansion of bubbles based on the model of Flumerfelt's group, established in 1996 [Shafi, M.A., Lee, J.G., Flumerfelt, R.W., 1996. Prediction of cellular structure in free expansion polymer foam processing. Polymer Engineering and Science 36, 1950-1959]. It was revised according to the kinetic experimental data on the creation and expansion of bubbles under a finite pressure release rate. The model involved a bubble nucleation rate equation for bubble creation and a set of bubble growth rate equations for bubble expansion. The calculated results of the number density of bubbles and bubble growth rate agreed well with experimental results. The number density of bubbles increased with an increase in the pressure release rate. Simulation results indicated that the maximum bubble nucleation rate is determined by the balance between the pressure release rate and the consumption rate of the physical foaming agent by the growing bubbles. The bubble growth rate also increased with an increase in the pressure release rate. Viscosity-controlled and diffusion-controlled periods exist between the bubble nucleation and coalescence periods.     

Pressure drop in monolith reactors

A theoretical model for the computation of pressure drop in bubble-train flow inside capillaries of square cross-section was developed. The model is based on three contributions: hydrostatics, viscous pressure drop, and capillary pressure drop. Capillary pressure drop is related to the shape of the fronts and ends of the bubbles. The model does not include entrance or exit effects, has no adjustable parameters, and agrees very well with available experimental data.

For a given set of flow parameters, bubble velocity and liquid slug average velocity are computed as a function of gas and liquid superficial velocities. The length of the unit cell determines the number of bubbles inside the capillary for a given flow situation. The model requires experimental information of average bubble lengths to compute the length of a unit cell consisting of a bubble and a liquid slug.

The three pressure contributions for a unit capillary length are linear functions of the number of bubbles inside the capillary. The length of the bubbles in bubble-train flows is a critical parameter in the computation of pressure drop.     

鼓泡塔反应器气液两相流CFD数值模拟

对圆柱形鼓泡塔反应器内的气液两相流动进行了三维瞬态数值模拟,模拟的表观气速范围为0.02～0.30 m•s^-1; 模拟采用了双流体模型,并耦合了气泡界面密度单方程模型预测气泡尺寸,该模型考虑了气泡聚并与破碎对气泡尺寸的影响。液相湍流采用考虑气相影响的修正k-ε模型,两相间的动量传输仅考虑曳力作用。模拟获得了轴向气/液相速度分布、气含率分布、湍流动能分布以及气泡表面面积密度等,对部分模拟结果与实验值进行了定量比较,结果表明模拟结果与实验结果吻合较好。     

Bubble phenomenon of ZrB2 based composites at high temperatures

Bubble phenomenon is common for ultra-high temperature ceramics (UHTCs) during oxidation or ablation processes, which will impair the oxidation/ablation resistant properties. This work is aiming to illuminate the formation and rupture processes of bubbles. In this work, ZrB₂-SiC-WB composite coatings were prepared via vacuum plasma spray technique and oxidized at 1500?°C for different durations. Obvious bubble phenomenon was observed. The morphology and distribution of bubbles were characterized. The formation mechanism of bubbles was calculated and analyzed based on thermal dynamics. The results showed that B₂O₃ gas played a key role in affecting the bubble behaviors. Bubbles tended to nucleate near the interface between the solid and liquid oxide layers. Small bubbles aggregated to large bubbles near the outmost liquid layer. Large bubbles near the surface were easy to rupture. The calculated results were consistent with the observed results.     

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.     

Hydrodynamics of slug flow inside capillaries

Understanding the motion of long gas bubbles (gas slugs) inside capillaries is a challenging problem that is relevant in many processes of chemical and biological interests. It has been proved by many workers that such long gas bubbles can be used successfully in enhancing mass and heat transfer in many chemical and biological processes. In order to quantify and understand this enhancement a light was shed on the hydrodynamics of such a flow pattern. The volume of fluid method implemented in the commercial CFD package, Fluent, is used for this numerical study. Velocity and bubble profile were obtained as functions of capillary number. Computed values of the bubble velocity and diameter were in excellent agreement with published experimental measurements. The detailed velocity field around the bubble was also computed and compared favourably with those experimental results reported in literature.     

Rheological behavior of polydispersed bubble suspensions in shear flows

Effects of the capillary number and size distribution of bubbles on rheological properties of bubble suspensions with different volume fractions were investigated through both experimental and theoretical analyses. Polydispersed bubble suspensions were prepared by using a bubble making device and characterized in steady and dynamic oscillatory shear flows to examine the effect of the bubble size distribution. To predict the rheological properties of the polydispersed bubble suspension, a constitutive equation was modified by applying superposition schemes, i.e., a probability density function was introduced to reflect the size distribution of bubbles. Dilute bubble suspensions showed shear thinning and linear viscoelastic behavior. At small capillary numbers, relative viscosity increased with increasing bubble volume fraction, whereas it decreased as bubble volume fraction increased at relatively large capillary numbers. It was found that rheological properties had been affected significantly by polydispersity of bubble suspensions and theoretical results were in good agreement with the experimental results. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Spontaneous Bubble Formation in Silicate Melts at High Temperatures

Heating previously melted glasses usually produces gas bubbles in the melt at constant pressure. Bubble formation generally occurs in a narrow temperature range; if experimental conditions are carefully controlled the temperature at which gas bubbles form and the characteristics of the bubbles can be reproduced with reasonable accuracy. Extensive measurements were made with binary silicates of lithia, soda, and potash. Significant bubble evolution occurred only when there was sulfur in the glass. Bubbles formed at quite high temperatures (1400° to 1500°C) in dry oxygen but did not form in reducing atmospheres and quite stable foams resulted. In dry reducing atmospheres some bubble formation was observed at lower temperatures (1200° to 1300°C). The behavior was considerably altered in atmospheres containing water vapor and by changes in base glass composition or sulfur content.     

FORMATION AND FLOW OF GAS BUBBLES IN A PRESSURIZED BUBBLE COLUMN WITH A SINGLE ORIFICE OR NOZZLE GAS DISTRIBUTOR

The characteristics of gas bubbles in a 5 cm diameter bubble column equipped with a single orifice of 1,3 or 5 mm diameter were investigated under system pressure of 0.1-15 MPa. The formation of gas bubbles was strongly affected by the system pressure. Under high pressures a dispersed gas jet was formed at gas velocities where spherical gas bubbles would have been formed at atmospheric pressure. The critical gas velocity between the bubbling regime and the jetting regime was correlated with the liquid phase Weber number and the gas phase Reynolds number based on the gas velocity at the orifice. Bubble size and gas holdup in the main part of the bubble column were also affected by the bubble formation pattern at the distributor     

Evolution of cake batter bubble structure and rheology during planetary mixing

The incorporation of air into a high ratio cake batter by planetary mixing was studied using two bench mixers, a Kenwood KM250 and a Hobart-N50. Power draw (expressed as specific mechanical power input) and air volume fraction, ?, were monitored over time for batters prepared in each mixer for heat-treated and un-treated cake flours. The two flour types gave very similar results, indicating that the benefit of heat treatment for cake manufacture is manifested during the baking step. Both mixers gave a rapid initial increase in ? up to ～0.50 followed by a gradual decay. Bubble size distributions of batters prepared in the Hobart showed the initial aeration stage to be accompanied by the formation of a large number of small bubbles, with diameters around 5 μm: the average bubble size increased and number of bubbles decreased with extended mixing. Batters prepared without emulsifier exhibited a steady increase in ? to a plateau at ～0.2 and contained larger bubbles which did not change in average size significantly over time. All batters exhibited power-law shear-thinning behaviour with power law indices and consistency strongly dependent on ?. At low shear rates, the apparent viscosity showed the non-linear dependency on ? expected for dense suspensions. At higher shear rates, elastic forces generated by the bubble phase became significant. The non-Newtonian nature of the liquid phase and the elasticity generated by the bubbles meant that shear-thinning and bubble break-up could not be predicted by analyses based on the capillary number. The results support a qualitative model of bubble formation and break-up caused by extensional deformation generated by shearing these bubbly liquids: the bubbles cream out slowly owing to the high viscosity of the continuous phase at rest.     

Coalescence behaviour of gas bubbles in aqueous solutions of n-alcohols and fatty acids

Coalescence frequency and coalescence times of bubble pairs formed on two adjacent capillary tubes were determined in aqueous solutions of n-alcoho and fatty acids. The results show, that coalescence times of bubbles are proportional to the surface excess concentration of the solute. Coalescence ti increase with the polarity of the hydrophilic group of the solute, its chain length and the bubble size. Coalescence frequency decrease drastically fro 100% to 0% at a distinct solute concentration. The transition in coalescence behaviour occurs, if the coalescence times becomes greater than the availa contact times of the bubbles. The transition concentration therefore decreases with increasing rate of bubble formation.     

单孔及微孔曝气低气速鼓泡床内气泡行为比较

引言鼓泡床反应器被广泛应用于吸收、液相氧化、好氧生化等气液反应过程,气体在液相中的分散情况对鼓泡床的反应和传质特性都有很大影响.为了提高气液传质效率,增加生产强度,工业反应器很多都是在高气速下操作(Ug>0·05m·s-1),很多研究都集中在高气速湍动鼓泡区[1~3].但对有机     

Bubble formation in cocurrently upward flowing liquid

The effects of liquid velocity, nozzle diameter, gas chamber volume and gas flow rate on volumes, shapes and growth curves of bubbles formed at a nozzle submerged in a cocurrently upward flowing liquid in a bubble column were experimentally investigated. The bubble volume decreases with increasing liquid flow velocity. The effect of liquid flow velocity on the volume of bubble increases with an increase in the gas flow rate. To simulate bubble formation at a nozzle submerged in cocurrently upward flowing liquid, a revised non-spherical bubble formation model was proposed. Bubble volumes, bubble growth curves and shapes experimentally obtained in this study, as well as in previous experimental studies, are well predicted by the present model.     

Effect of particles on hydrodynamics and mass transfer in a slurry bubble column: Correlation of experimental data

The effects of particle concentration and size on hydrodynamics and mass transport in an air–water slurry bubble column were experimentally studied. When the particle concentration α_s increased from 0% to 20%, the averaged gas holdup decreased by ~30%, gas holdup of small bubbles and gas–liquid volumetric mass transfer coefficient decreased by up to 50%, while the gas holdup of large bubbles increased slightly. The overall effect of particle size was insignificant. A liquid turbulence attenuation model which could quantitatively describe the effects of particle concentration and size was first proposed. Semi-empirical correlations were obtained based on extensive experimental data in a wide range of operating conditions and corrected liquid properties. The gas holdup and mass transfer coefficient calculated by the correlations agreed with the experimental data from both two-phase and three-phase bubble columns, with a maximum error <25%.