Configurations and dynamics of single air/alcohol gas-liquid compound drops in vegetable oil

Gas-liquid compound drops can be used as a low energy liquid-in-liquid disperser, mixer and transporter in place of the energy intensive mechanical agitator. This paper presents an experimental investigation of single air/methanol and air/ethanol gas-liquid compound drops (alcohol-floated bubbles) in sunflower oil at room temperature. Dispersed methanol (ethanol) droplets rise and dissolve slowly in vegetable oil. The air bubbles can float such droplets and increase their rise velocities. The shape of alcohol droplet on the bubble surface was changed to be an alcohol film covering around the bubble and was continuously thrown into the sunflower oil in the form of a toroidal-shaped film, which increases a huge contact area between the alcohol and the oil. A double tube nozzle was used to inject the air bubble (internal tube) and the alcohols (external tube) into the oil. The images showing the configurations of the compound drops during their rise are shown and discussed. The experimental results of the rise velocity and the drag coefficient of the compound drops are presented and compared to those of the single air bubbles and the single alcohol droplets. Rise velocities and drag coefficients predicted by theory of immobile and mobile surface air bubble in sunflower oil are also included for comparison.     

Visualization of compound drops formation in multiphase processes for the identification of factors influencing bubble and water droplet inclusions in oil drops

An innovative methodology for visualizing and identifying some mechanisms by which complex structures such as air-in-oil-in-water (A/O/W) and water-in-oil-in-water (W/O/W) may be formed inside mixing tanks dispersing various phases is described. In the case of A/O/W inclusions, isolated inclusion events could be observed by the first time with an experimental setup designed to produce sudden turbulence in a small confined space simulating a three-phase fermentation system. It was observed that high-energy direct-collisions of the bodies are not required for inclusions to occur; rather, a gentle contact between the phases was needed. Then, by maintaining an oil drop in a fixed position while it was impacted by single air bubble, it was feasible to calculate the percentage of air-bubble inclusions into oil drops for different compositions of the continuous phase. By adding biomass as a solid phase, the inclusion occurrence reached 61%; likely this was caused by a mechanical effect of the added biomass (making the interface breakable or unstable) with a minor contribution by the decreased surface tension. In the case of W/O/W, a basic mechanism by which the inclusion of water droplets in oil drops may occur is described. This was derived from the analysis of the hydrodynamic process of the formation of a water drop inside a volume of oil where the differential pressures occurring along the water–oil interface were mapped. This is the first time that factors influencing water and air inclusions in oil drops are identified, and possible mechanisms behind their occurrence are proposed, based on visual evidence.     

An optical approach for identifying the nature and the relative 3D spatial position of components of complex structures formed in multiphase dispersion systems

This paper describes a novel method that makes possible the identification of the nature and the 3D relative spatial position (free or embedded) of the components of complex structures (oil drops, water droplets, air bubbles and multiphase drops) formed in dispersions occurring in fermentation systems, without disturbing (either chemically or physically) the dispersion. Using the refraction index differences between each phase and the image-forming properties of the complex objects formed in addition to the relative size of the bright part of the spheres, it was possible to determine the nature of each type of structure, as well as to discern whether these structures were located inside or outside of the multiphase oil drops. This method allowed determining unequivocally that the small droplets observed within the complex oil drops are part of the aqueous phase and are trapped inside the oil drops, together, in some cases, with air bubbles.     

The behaviour of large gas bubbles at a liquid-liquid interface. Part 2: Liquid entrainment

The roles played by large gas bubbles in the generation and coalescence of liquid drops at a liquid-liquid interface are elucidated. The amount of lower liquid entrained by individual bubbles and the resulting drop size distributions in the upper liquid phase are quantified for the three phase system: sunflower oil + 50 wt % decane, water + 50 wt % sugar, air, and qualitative theoretical models are presented. Drops settling to the interface were found to coalesce rapidly and bubble flux had no apparent effect on the rate of drop coalescence at the liquid-liquid interface.     

Dispersed oil–water–gas flow through a horizontal pipe

An experimental study of three‐phase dispersed flow in a horizontal pipe has been carried out. The pressure drop over the pipe strongly increases with increasing bubble and drop volume fraction. Because of the presence of drops the transition from dispersed bubble flow to elongated bubble flow occurs at a lower gas volume fraction. The gas bubbles have no significant influence on the phase inversion process. However, phase inversion has a strong effect on the gas bubbles. Just before inversion large bubbles are present and the flow pattern is elongated bubble flow. During the inversion process the bubbles break‐up quickly and as the dispersed drop volume fraction after inversion is much lower than before inversion, a dispersed bubble flow is present after inversion. (When inversion is postponed to high dispersed phase fractions, the volume fraction of the dispersed phase can be as high as 0.9 before inversion and as low as 0.1 after inversion.) © 2009 American Institute of Chemical Engineers AIChE J, 2009     

BUBBLE MOTION IN A THREE-PHASE LIQUID FLUIDIZED BED

This paper presents results for the rise velocities of air bubbles in liquids and liquid-solid fluidized beds. The bubble sizes ranged from approximately 0.03 to 0.45 cm radius. Tap water and distilled water were used as the fluidizing liquids. The solid phase consisted of low density alginate gel beads of mean radius 0.04 cm. The gel beads were translucent which permitted observation of bubbles inside the bed even at large solids volume fractions. Experiments were conducted for solids volume fractions ranging from 15% to 52% and in clear liquids. The goal of the experiments was to determine rise velocities of bubbles and to develop and evaluate correlations of bubble rise velocity based on bubble size, solids volume fraction and liquid properties. It was determined that, for moderate solids fractions (ranging from 28% to 45% solids), a semi-empirical correlation that treated the fluidized bed as a pure liquid with a higher viscosity than the liquid phase could be used to represent the data. The Thomas effective viscosity model was used to predict the viscosity. Provided that one restricts attention to a water fluidized bed, a second empirical correlation can be used to represent the data over a broader range of solids fractions.     

FACTORS THAT INFLUENCE THE COALESCENCE OF BUBBLES IN OILS THAT CONTAIN SILICONE ANTIFOAMANTS

This paper deals with several factors that contribute to the dual nature of silicone antifoamants. For example, soluble silicones can concentrate at the air/oil interface to stabilize bubbles, while dispersed drops of silicone can accelerate the coalescence process by rapidly spreading at the gas/liquid interface of a bubble causing film thinning by surface transport. In this paper, experiments are described that show that the apparently low solubility of silicone in different oils is actually a slow rate of dissolution that depends on the viscosity of the oil and the concentration of dispersed drops. Furthermore, it is shown that there is a critical size for the coalescence of air bubbles in different oils and with different amounts of silicone. Insight into the mechanisms of the critical bubble size and the reason why significantly faster coalescence occurs at a lower concentration of silicone can be explained in terms of higher interfacial mobility (as determined by bubble rise velocities).     

Study of Air‐Liquid Flow Patterns in Hydrocyclone Enhanced by Air Bubbles

In order to improve the oil‐water separation efficiency of a hydrocyclone, a new process utilizing air bubbles has been developed to enhance separation performance. Using the two‐component phase Doppler particle analyzer (PDPA) technique, the velocities of two phases, air and liquid, and air bubble diameter were measured in a hydrocyclone. The air‐liquid mixing pump can produce 15 to 60 μm‐diameter air bubbles in water. There is an optimum air‐liquid ratio for oil‐water separation of a hydrocyclone enhanced by air bubbles. An air core occurs in the hydrocyclone when the air‐liquid ratio is more than 1 %. The velocities of air bubbles have a similar flow pattern to the water phase. The axial and tangential velocity differences of the air bubbles at different air‐liquid ratio are greater near the wall and near the core of the hydrocyclone. The measured results show that the size distribution of the air bubbles produced by the air‐liquid mixing pump is beneficial to the process where air bubbles capture oil droplets in the hydrocyclone. These studies are helpful to understand the separation mechanism of a hydrocyclone enhanced by air bubbles.     

The dynamics of single air bubbles and alcohol drops in sunflower oil at various temperatures

Steady rises of a single air bubble, a methanol drop, and an ethanol drop in a vertical glass column of refined sunflower oil at temperatures of 25, 30, 40, and 50°C are investigated experimentally using photography. The Reynolds numbers obtained are 0.07–16, 0.02–13.43, and 0.017–11.18 for the air bubbles, methanol drops, and ethanol drops, respectively. Results for terminal velocity and drag coefficient are compared with the selected existing correlations for bubble and drop motions in immiscible liquids. Correlations by Rodrigue show good agreement for various bubble sizes and system temperatures. Experimental drag coefficients of methanol and ethanol drops show a systematic deviation from the Oliver and Chung and the Darton and Harrison correlations, respectively. Considering the effect of dissolution of alcohol in vegetable oil, which varies with temperature, on the drop dynamics, semiempirical correction factors are applied to the last two correlations to fit the experimental results. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

支流电场中的单气泡行为

The investigation on bubble behavior in electric field helps to analyze the mechanism of electric enhancement of boiling heat transfer. Experiments were performed to investigate the bubble deformation in direct current （DC） electric field with bubbles attached to the orifice. The air bubbles were slowly generated in the transformer oil pool at different orifices, so that the effect of flow on bubble shape was eliminated. The results showed that the bubbles were elongated and the departure volume decreased when the electric field was intensified. The major and minor axes, aspect ratio and departure volume increased with increasing the orifice diameter. Both the electric field and orifice size have great influence on bubble behavior. The bubble deformation was also simulated to compare with the experimental results. The numerical and experimental data qualitatively agree with each other.     

A numerical study of mass transfer of ozone dissolution in bubble plumes with an Euler-Lagrange method

In this paper we study the mass transfer process of ozone dissolution in a bubble plume inside a rectangular water tank, as a model problem for a water purification system. The effect of bubble diameter and plume structure on mass transfer efficiency of ozone in bubble plumes is investigated numerically. In order to capture the detailed plume structure, the interaction between liquid and bubbles is treated by a two-way coupling Euler-Lagrange method. The motion of the continuous phase (a mixture of liquid and gas bubbles) is solved using a finite difference method in an Eulerian framework. The motion of the dispersed phase (bubbles) is tracked individually in a Lagrangian approach. The ozone transfer process from bubbles to liquid is computed by modelling the mass transfer rate of individual bubbles. Our numerical results show a nonlinear dependence of the ozone dissolution efficiency on the initial bubble size. The dissolution efficiency varies rapidly when the initial bubble size reaches certain value while the change of efficiency is much slower at other bubble sizes. Therefore, for a given tank size it is not necessary to generate bubbles much smaller than the optimal size. This result is of importance for engineering since it is difficult to generate small bubbles in practice. Our results also show that the instantaneous dissolution rate of ozone could be increased by increasing the initial volumetric fraction of ozone inside bubbles even up to 20% while maintaining the dissolution efficiency.     

高气液密度比的传热相变复合模型

为研究两相流动中热量传递机制,基于格子Boltzmann热模型及大密度比模型,将相变源项引入到控制两相密度分布函数中,来描述温度场对气液相变的影响,提出了一个可以描述气液密度比达到2778的传热相变复合模型。通过对压力速度分布函数的回归修正克服了气液密度比过大造成的数值不稳定问题。模拟了溴化锂水溶液中双气泡的上升运动过程及周围的温度场分布,研究发现：双气泡上升时,碰撞前上方气泡温度高于下方气泡,碰撞时,两气泡间液桥打开,发生热量传递,气泡内部温度变得均匀;双气泡体积先减小再增大,碰撞时体积达到最大值,在融合成一个气泡后体积逐渐缩小,最终趋于稳定;初始气泡的体积越大,气泡上升过程中的速度越大。     

相互重叠的油气泡对流量计敏感场影响分析

运用有限元软件ANSYS对电磁流量计中存在相互重叠的油气泡建立仿真模型,在此模型下对电磁流量计敏感场进行了数据分析,研究了流量计中含有两个相互重叠的球形油气泡与一个与其等大的油气泡对电磁流量计的响应特性之间的关系,进而揭示多个重叠油气泡与一个与其等大的油气泡对电磁流量计响应特性的影响,研究结果可为电磁感应测量多相流中的气包油、油包气及油气包水等情况提供一定参考依据。     

Transport of solids by gas-liquid mixtures in horizontal pipes

Two sizes of particles of 500μ and 100μ mean diameter were conveyed as water slurries through 1 inch and 2 inch horizontal pipelines. Pressure drops and saltation velocities were measured over a range of slurry concentrations to 14% by volume with and without concurrently flowing air. In the former case, the principal flow patterns studied were in the bubble and plug flow regimes. Although small effects on saltation point were noted due to the induced turbulence from the air bubbles, these were not significant, and a reasonable estimate of saltation velocities could be made from Durand's correlation. Pressure drops were found to be correlated reasonably well by modified Lockhart-Martinelli parameters as used for turbulent-turbulent horizontal gas-liquid flow except at conditions near the saltation point. Pressure drops for slurry flow alone were predicted reasonably well by the modified Durand equation.     

Work of adhesion of water to a Teflon surface and stability of the system Teflon-air bubble-water

Measurements of water drops beginning to slide down on a vertical Teflon plate and the detachment force of air bubbles of the same volume from the face surface of cylindrical Teflon segments (rods) of various diameters in water were made. Simultaneously, the radius of the contact plane between the air bubble and segment was measured at the moment of detachment, as well as the radius of the small air bubble contact plane which was left after disruption of the Teflon-large air bubble-water system. On the basis of the results obtained, the work of adhesion of water to a Teflon surface was determined using equations known from the literature as well as the equation derived in this paper. Agreement between the values of the work of adhesion calculated from these different equations was obtained. It was found that measurements of the air bubble detachment force from a solid surface may be useful for determination of the work of adhesion of a liquid to a solid surface, at least for a solid for which vapor adsorption may be neglected.     

Analysis of deformation and internal flow patterns for rising single bubbles in different liquids

Gas–liquid multiphase flow is a significant phenomenon in chemical processes. The rising behaviors of single bubbles in the quiescent liquids have been investigated but the internal flow patterns and deformation rules of bubbles, which influence the mass transfer efficiency to a large extent, have received much less attention. In this paper, the volume of fluid method was used to calculate the bubble shapes, pressure, velocity distributions,and the flow patterns inside the bubbles. The rising behavior of the bubbles with four different initial diameters,i.e., 3 mm, 5 mm, 7 mm and 9 mm was investigated in four various liquids including water, 61.23% glycerol,86.73% glycerol and 100% glycerol. The results show that the liquid properties and bubble initial diameters have great impacts on bubble shapes. Moreover, flow patterns inside the bubbles with different initial diameters were analyzed and classified into three types under the condition of different bubble shapes. Three correlations for predicting the maximum internal circulation inside the bubbles in 86.73% glycerol were presented and the R-square values were all bigger than 0.98. Through analyzing the pressure and velocity distributions around the bubbles, four rules of bubble deformation were also obtained to explain and predict the shapes.     

Characteristics of a countercurrent reciprocating plate bubble column. I. Holdup,pressure drop and bubble diameter

The hydrodynamics of countercurrent air/water flow in a 5 cm diameter reciprocating plate bubble column have been studied; the plates contained 14 mm diameter perforations and had a fractional open area of 0.57. The ranges of superficial velocities of air and water were respectively 0-0.99 cm/s and 0-3.95 cm/s. The stroke was in most cases 4.5 cm and the reciprocation frequency was in the range 0–5 Hz. The pressure drops were measured in the absence of reciprocation for single phase and two phase flow conditions. Pressure fluctuations and time-averaged pressure drops were measured with plate reciprocation under single and two-phase conditions. The results were described in terms of the simple quasi-steady state model; the effective orifice coefficients of the perforations were within the range 0.4 to 0.97 depending on the reciprocation conditions. The Sauter mean diameters of the bubbles decreased with agitation; they were about twice the values predicted from an earlier correlation developed for liquid-liquid systems. The gas holdups were also substantially greater than predicted from correlations based on liquid-liquid systems. Both these effects were explained as due to the tendency for bubbles to cluster in the plate region.     

Buoyancy-driven motion of bubbles in square channels

The motion of air bubbles in square capillaries moving under the influence of gravity is studied over a range of Reynolds numbers. The steady shapes and velocities of the bubbles as a function of the bubble size are determined experimentally at moderate Bond and capillary numbers. Bubbles are nearly spherical at lower bubble volumes and become prolate losing their fore and aft symmetry at larger bubble volumes. At higher Weber numbers, a reentrant cavity develops at the rear of bubble. The critical Weber number at which this shape transition occurs lies between 0.89 and 1.38. At small Weber numbers, the terminal velocity of bubbles increases monotonically with bubble volume and eventually reaches a plateau value, which is independent of the bubble size. At higher Weber numbers, a maxima develops in the velocity-volume curve at moderate bubble sizes which grows in magnitude as the Weber number increases. Even at small bubble volumes with nearly spherical shape, the terminal velocity of the bubbles is less than the Hadamard-Rybczynski velocity due to the wall drag. The speed and the maximum bubble width for air bubbles rising in a square channel is higher than that of an air bubble rising in a circular channel with the same hydraulic diameter. The experimental data compares well with predicted trends in the viscous and inertial limits for long bubbles.     

Numerical simulations and kinetic theory of bubbly liquids rheology under microgravity

Simple shear flows of dilute suspensions of spherical bubbles at large Reynolds numbers are studied by using numerical simulations and kinetic theory. It is shown that the mean-square bubble velocity is very sensitive to the volume fraction and Reynolds number of the bubbles as well as on initial conditions. The balance of energy contained in bubble velocity fluctuations plays an important role in the rheology of the dispersed phase, which is generally non-Newtonian.     

Effects of floating contactors on oxygen transfer and hydrodynamics in a bubble column

This study investigates the adoption of floating contactors to promote the rate of oxygen transfer from non-uniform air bubbles to liquid in a bubble column with continuous operation. The volumetric oxygen transfer coefficient and axial dispersion coefficient of a liquid phase have been analyzed based on the axial dispersion model. Attention was focused on the effects of the volume fraction of the floating contactors on the volumetric oxygen transfer coefficient, axial dispersion coefficient of a liquid phase, and gas phase hold-up in the bubble column. The results have shown that the volumetric oxygen transfer coefficient and gas phase hold-up can increase by up to 25% and 13%, respectively, while the axial dispersion coefficient of a liquid phase decreases by up to 30% by adding floating contactors in the column.