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.     

Study of bubble induced flow structure using PIV

An experimental investigation of the flow structure induced by a chain of gas bubbles was carried out in a rectangular bubble column using particle image velocimetry (PIV). It is observed that the bubble rising trajectory changes from one dimension to three dimension as liquid viscosity reduces. The variation of bubble rising trajectory associates with the alternation of bubble motions—with or without oscillatory and rotational motion depending the bubble rising trajectory is 3-D or 1-D. The different behaviors of gas bubbles introduce various instantaneous and averaged liquid flow structures. In general, complex fluid velocity fields present in liquid system of low viscosity where free vortex, cross flow, and irregular circular flow can be observed. The liquid pseudo-turbulence measured in terms of turbulence intensity and Reynolds stress is more intense in liquid of low viscosity. The turbulence is also enhanced by the frequency of bubble formation.     

PIV study of bubble rising behavior

Bubble behaviors are studied in a rectangular bubble column using particle image velocimetry (PIV). Bubbles rise in a chain through a stagnant liquid. As liquid viscosity reduces, bubble rising trajectory changes from one-dimensional to three-dimensional. This transition is due to different bubble wake structures. The bubble shapes also show dissimilar characteristics in liquid of different viscosities. The instantaneous liquid flow fields measured by PIV show the diversities as bubbles rise in different paths. Based on the experimental data obtained in this study, a correlation is proposed to calculate the terminal velocity of bubbles in a chain.     

Bubble Formation at Variously Inclined Nozzles

Bubble formation at variously inclined submerged nozzles, fed with a continuous gas flow rate, is investigated. The average liquid velocity induced by bubble motion is determined, leading to a simple correlation for estimation of the liquid velocities induced by the repeated passage of bubbles. An effective model for the prediction of bubble sizes at their detachment from vertical nozzle orifices is presented which encompasses a bound expansion stage, followed by an accelerated expansion phase, and ends by an original bubble detachment criterion. The existence of a liquid‐phase effective entrainment velocity, generated by the continuous passage of the bubble stream, is quantified and included in the model. Model predictions are found to correlate well with experimental and literature data.     

Liquid flow pattern around Taylor bubbles in an etched rectangular microchannel

Liquid flow around Taylor bubbles and the motion of bubble interface in a rectangular microchannel etched on a microfluidic chip were investigated using a three-dimensional particle tracking method. The Taylor bubbles were generated by releasing the dissolved air in working the liquid (water) through heating the microfluidic chip to 35–55 °C and had low velocities (15–1500 μm/s). Three-dimensional velocity distributions of liquid recirculation flows surrounding the Taylor bubble head and tail were obtained by tracking submicron fluorescent particles seeded in the working liquid and the motion of the bubble interface was analyzed by monitoring the motions of the particles attached on the bubble interface. The high velocity film flow through the microchannel corners acted as a liquid jet in front of bubble head and drainage into the corners behind the bubble tail to drive the liquid recirculation flows. The bubble interface near the microchannel corners was also moved by the strong liquid shear induced from the high velocity liquid flow in the microchannel corners. This high velocity liquid flow through the corners could be considered to be driven by the pressure drop over the Taylor bubble. The pressure drop resulted from the decrease of bubble surface mobility due to tracer surfactant in the gas–liquid interface.     

Numerical investigation on coalescence of bubble pairs rising in a stagnant liquid

In the present study, we preformed a two-dimensional numerical simulation of the motion and coalescence of bubble pairs rising in the stationary liquid pool, using the moving particle semi-implicit (MPS) method. Moving particles were used to describe the liquid phase and the vapor phase was evaluated using real vapor sate equation. The bubble–liquid interface was set to be a free surface boundary which could be captured according to the motion and location of interfacial particles. The behaviors of coalescence between two identical bubbles predicted by the MPS method were in good agreement with the experimental results reported in the literature. Numerical results indicated that the rising velocity of the trailing bubble was larger than that of the leading bubble. Both of the leading bubble and the trailing bubble rose faster than the isolated bubble. After coalescence, the coalesced bubble showed velocity and volume oscillations. The time of the volume oscillations increased with increasing initial bubble diameter. The wake flow and vortex would form behind the coalesced bubble.     

Multiple effects of operating variables on the bubble properties in three-phase slurry bubble columns

Flow properties of gas phase reactants such as size, rising velocity and frequency were investigated in simulated three-phase slurry bubble column reactors. Effects of gas velocity, reactor pressure, liquid viscosity, solid content in the slurry phase and column diameter on the flow properties of a gas reactant were determined. The multiple effects of operating variables on the bubble properties were well visualized by means of contour maps. The effects of operating variables on the flow properties of bubbles changed with changing column diameter of the reactor. The size, rising velocity and frequency of reactant gas bubbles were well correlated in terms of operating variables including column diameter of the reactor. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Unsteady motion of a single bubble in highly viscous liquid and empirical correlation of drag coefficient

The unsteady motion of single bubbles rising freely in a quiescent liquid with high viscosity was measured using a CCD (charge coupled device) camera. Sequences of the recorded frames were digitized and analyzed using image analysis software and the measurements of the acceleration and steady motion of bubbles were obtained. The total drag coefficient was calculated from the accelerating motion to the steady motion with the added mass force and history force included. In virtue of dimensional analysis, the total drag coefficient of single bubbles is correlated as a function of the acceleration number, Archimedes number and Reynolds number based on the equivalent bubble diameter. The proposed correlation represents very well the experimental data of the total drag force in a wide range covering both unsteady accelerating motion and steady motion. The combined added mass and history force coefficient accounting for the accelerating effect on single bubbles was evaluated and correlated.     

Bubble flow mechanisms in trickle beds—an experimental study using image processing

The mechanisms of bubble motion in concurrent gas-liquid down flow through trickle beds are investigated. The laboratory reactor is a structured quasi-two-dimensional porous medium with an average pore diameter close to the values encountered in trickle beds. The accuracy of the reactor design is demonstrated by hydrodynamic investigations on the reactor scale where it is shown that the flow regimes encountered and the experimental pressure drop are comparable to those observed in trickle beds. The investigations on the pore scale are focused on the dispersed bubble flow regime where the liquid flow is continuous and the gas is divided into elongated bubbles. The bubble motion is recorded with the aid of a high-speed video camera and the images are processed and analysed in a quantitative manner. The investigations clearly show that in dispersed bubble flow, the bubbles are frequently pulsing on the pore scale. The mechanism of this flow pattern is discussed.     

Three-dimensional vortex simulation of bubble dispersion in excited round jet

This study is concerned with the three-dimensional numerical simulation of a bubbly jet, injected vertically upward from a circular nozzle in still water, when the axial and helical disturbances are imposed. The water flow is simulated by a vortex method, and the equation of motion for a bubble is solved on the flow by the Lagrangian scheme. The disturbances markedly change the vortical structure of water in the developing region. Since the bubbles accumulate on the high vortical region, their dispersion remarkably varies owing to the disturbances. The single helical disturbance causes the larger dispersion of bubble. The combined two helical disturbances make the bubble concentrate on a line in the jet cross-section. The present simulation suggests the possibility of the active control for the bubble dispersion in bubbly jet.     

Bubble wake visualization by using photochromic dye

Wake structure of a single “clean bubble”, rising in rectilinear, zigzagging or spiraling path, is experimentally investigated. A single nitrogen gas bubble was produced in a silicone oil pool and the wake structure development in the rear of the rising bubble was visualized by using photochromic dye. The flexibility of this visualization method enabled us to distinguish wake from drift easily. Both bubble motion and wake structure were recorded by using stereo high-speed video camera simultaneously. We present the first experimental support for the existence of the standing eddy at the rear of the clean bubble, as predicted by a previous numerical study by Ryskin and Leal [1984. Journal of Fluid Mechanics 148, 19-35], Dandy and Leal [1986. Physics of Fluids 29(5), 1360-1366] and Blanco and Magnaudet [1995. Physics of Fluids 7(6), 1265-1274]. We study motion of a pair of vortex filaments, which is called double-threaded type wake, in the case of bubble rising in an axi-asymmetric path. Visualization results of multiple formations of horse-shoe type vortices in one period of zigzag motion of rising bubble with shape oscillations, which has not been observed in previous studies are also presented.     

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.     

Interaction between partitioning porous plate and rising bubbles in a trayed bubble column

In a trayed bubble column, the structure of the partitioning plate plays an important role on the bubble behavior. This study examined the effect of the opening ratio and pore size of the plate on the bubble break-up frequency and bubble size distribution. The sieve tray was used as the partitioning plate. The opening ratio was closely related to gas cap development. The stagnation of bubble flow and a gas cap were observed with an opening ratio less than 48.5%. The gas cap increased with decreasing opening ratio and increasing superficial gas velocity. The main effect of the sieve tray could be categorized into the additional drag force and bubble break-up depending on the sieve pore size. When the sieve pore size was smaller than the Sauter diameter of the bubble swarm, the movement of rising bubbles was interrupted by the drag force applied by the surrounding mesh lines. On the other hand, when the sieve pore size was larger than the Sauter diameter, the bubbles were affected by the additional bubble break-up. After the bubbles penetrated the sieve tray, the bubble size distribution shifted to a smaller one and the Sauter diameter decreased.     

Generation of uniform small bubbles and hydrodynamic characterization of a bubble column with high pressure jet sparger

Systems generating uniform small bubbles are used in many mineral processing and chemical operations. We investigated the generation of smaller bubbles by using a two fluid jet system. Gas holdup results are reported in terms of the effect of superficial gas and liquid velocities in relation to the pressure in a bubble column with a water jet sparger. Experiments were conducted with hydrostatic head of 80 cm, 100 cm, and 120 cm in the bubble column. The gas velocity varied from 0.122 to 1.22 cm/s, and water flow rate from 33.3 to 333 cm³/s. Experiments were conducted at pressures of 2 atms., 3 atms. 4 atms. and 5 atms., and bubble sizes were measured by a digital camera (bubble compared to a reference wire inside the bubble column). Results show that the gas holdup increases with the pressure and superficial gas velocities; and at pressures of 2, 3, 4 and 5 atms., the gas holdup increases by 8.75%, 9.166%, 10% and 10%, respectively. The maximum gas holdup of 16.4% was observed at a liquid level of 80 cm and pressure of 4 atms. Optimum conditions for generating smaller bubbles with larger gas holdup are increased liquid flow rate, low liquid level, and high gas pressure. Experimental results also indicate that the column operates in both the homogeneous and heterogeneous regimes of gas-liquid flow.     

Effect of gas expansion on the velocity of individual Taylor bubbles rising in vertical columns with water: Experimental studies at atmospheric pressure and under vacuum

This study was designed to determine the effect of gas expansion on the velocity of Taylor bubbles rising individually in a vertical column of water. This experimental study was conducted at atmospheric pressure or under vacuum (33.3 and ) using three different acrylic columns with internal diameters of 0.022, 0.032, and 0.052 m, and more than 4.0 m high. A non-intrusive optical method was used to measure velocity and length of Taylor bubbles at five different locations along the columns. The operating conditions used correspond to inertial controlled regime.In experiments performed under vacuum, there is considerable gas expansion during the rise of Taylor bubbles, particularly when they approach the liquid free surface where the pressure drop (due to the hydrostatic pressure) is of the order of magnitude of the absolute pressure. The liquid ahead of the bubble is displaced upward by an amount proportional to the gas expansion resulting in increased bubble velocity. The calculated Reynolds number suggests a laminar regime in the liquid ahead of the bubble. However, the experimentally determined velocity coefficient C for each column was much smaller than 2, which would be expected for laminar flow. The value of C obtained ranges from 1.13±0.09, for the narrowest column, to 1.40±0.24, for the widest column. This suggests that a fully developed laminar flow in the liquid ahead of the bubble is never achieved due to continuous bubble expansion at a variable rate, regardless of column height.The velocity coefficient C can be used to calculate the contribution of liquid motion to bubble velocity. Subtracting this contribution from the measured bubble velocity defines a constant value which is nearly identical to the bubble rise velocity measured in the same column operated as a constant volume system (two ends closed) where gas expansion is absent.     

Interaction of two in-line bubbles of equal size rising in viscous liquid

The interaction of bubbles is the key to understand gas–liquid bubbling flow. Two-dimensional axis-symmetry computational fluid dynamics simulations on the interactive bubbles were performed with VOF method,which was validated by experimental work. It is testified that several different bubble interactive behaviors could be acquired under different conditions. Firstly, for large bubbles(d: 4, 6, 8, 10 mm), the trailing bubble rising velocity and aspect ratio have negative correlations with liquid viscosity and surface tension. The influences of viscosity and surface tension on leading bubble are negligible. Secondly, for smaller bubbles(d: 1, 2 mm), the results are complicated. The two bubbles tend to move together due to the attractive force by the wake and the potential repulsive force. Especially for high viscous or high surface tension liquid, the bubble pairs undergo several times acceleration and deceleration. In addition, bubble deformation plays an important role during bubble interaction which cannot be neglected.     

The modifications of a plane mixing layer by condensed bubbles

Injecting superheated water vapour bubbles into a sub-cooled shear flow will induce heat and mass transfer in the form of condensation. Of particular interest is finding the characteristic role that the large-scale vortex structures play on bubble condensation. Because the heat transfer and condensation take place mainly due to convection, the bubble is expected to collapse at an earlier stage because of the trapping by large-scale vortices. The aim of this paper is to investigate two-way thermal and momentum interaction between bubbles and the large-scale coherent structures in a plane turbulent shear layer, focusing on the effect of bubble condensation on large-scale vortex structures embedded in plane, free shear layers. The parameters such as initial bubble temperature, bubble injection location, and carrier fluid temperature have been chosen to examine their effects on the condensation and dispersion of bubbles. Because superheated vapour bubbles immersed within a sub-cooled shear flow field experience heat transfer, the gas phase will certainly condense into liquid if these bubbles remain surrounded long enough by this same sub-cooled liquid. Accordingly, bubble condensation is evident within the shear layer and bubble dispersion is influenced by the large-scale vortex structures. The main driving force behind complete bubble condensation was the difference in temperature between the bubble vapour and the carrier fluid. It was revealed that the effect of the large-scale vortex structures is that the condensed bubbles acquire a larger dispersion than bubbles that were not subject to thermal coupling.     

The pressure beneath a growing rising bubble

On the basis of potential theory, the equations of motion and the associated pressure field are derived for an idealized growing spherical gas bubble rising in an inviscid liquid under the influence of gravity from a horizontal plate-orifice and from a free standing nozzle. The dimensionless pressure (p_N?p₀)/πga at the gas source N (where p₀ is the undisturbed ambient pressure, a the instantaneous bubble radius and π the liquid density) is found to rise to a maximum when the dimensionless bubble position h/a = 1·55 for bubbles formed at a plate orifice and (125/48)^{$\text{1}\text{3}$} ? 1·38 for bubbles formed at a free standing nozzle. These positions of maximum pressure correspond well to experimentally observed positions at which the gas supply to bubbles grown at constant flow rate in water is cut off by the collapse of the neck linking bubble and gas source. The volumes of bubbles at this instant are predicted theoretically and compare well with experimentally determined values.     

Improving Gas‐Liquid Mass Transfer in Bubble Columns by Applying Low‐Frequency Vibrations

We show that application of low‐frequency vibrations, in the 50–200 Hz range, to the liquid phase of an air‐water bubble column causes significantly smaller bubbles to be generated at the distributor plate. For bubble column operation in the homogeneous flow regime, measurements of the volumetric mass transfer coefficient using the oxygen absorption technique show that the increase in the k_La values ranges from 50–100 % depending on the flow rate. It is concluded that application of low‐frequency vibration has the potential of improving the performance of bubble columns.     

喷嘴释放单气泡的声发射特性

利用声发射技术在单气泡发生实验装置中研究了气液两相流中单气泡的动力学特性,使用自行开发的采集处理程序进行气泡声信号的参数提取,采用统计分析、小波变换和快速傅里叶变换对声信号进行时域和频域范围的分析。分析结果表明,声发射技术可以检测到管内气泡的声信号,具有较高的信噪比,且声信号随着喷嘴尺寸的增大而增大,随着液相表面张力的减小而减小。比较不同喷嘴直径下气泡的频率谱,发现喷嘴释放气泡发出的声信号频率为150～200 kHz,且随着喷嘴直径的增大,峰值频率相应增大,提出了声信号峰值频率与气泡尺寸之间的关联式。同时得到了气泡上升过程中的连续形态变化,分析了气泡产生声音的机理。研究表明,声发射技术是一种灵敏度高、测量手段方便的方法,可用于气液两相流气泡运动特性的检测。