Stage Of Bubble Formation Research Articles

Influence of wettability and surface tension on bubble formation from a needle

AbstractThe multiphase flow accompanied by contact line motion is a common flow phenomenon in nature and industry. When investigating the bubble formation from a needle, it is generally assumed that the contact line is fixed at the inner diameter of a needle. Namely, the influence of contact line motion on bubble formation is ignored. In this paper, the effects of wettability and surface tension on the characteristics of bubble formation were studied using volume of fluid method. The changes of bubble volume, instantaneous contact angle, contact line radius, and the motion mechanism of contact line were analyzed in detail. It is found that when surface wettability and/or surface tension are different, the motion type of contact line during bubble formation is different, and the stage of bubble formation shows a great difference too. The motion of contact line is mainly affected by the pressure inside a bubble, the adhesion force, and the hydrostatic pressure of liquid outside a bubble. The sum of all forces driving contact line to move inward or outward controls the motion type of contact line.

Mass transfer of chemical absorption of CO2/N2 mixed gas in a microchannel

In the chemical industry, the gas phase is not always pure gas generally. The mass transfer (MT) of mixed gas in a microchannel was investigated by using organic amine aqueous solution to absorb CO2/N2 mixed gas. MT in the bubble formation stage was calculated, and the influence of operating conditions on it was investigated. When the gas contacts the liquid, the CO2 in the bubble is continuously absorbed. Combined with the reduction of bubble volume, the instantaneous MT driving force of the bubble along the microchannel is calculated. On this basis, the MT coefficient of the mixed gas is obtained. The results show that after mixed gas in a certain concentration enters the microchannel, the instantaneous MT driving force and instantaneous MT coefficient are constantly decreased during the bubble moves along the microchannel.

The research of gas-solid fluidized bed bubbling behavior based on CFD-DEM coupled simulation

Fluidized bed is an efficient reactor. For fluidized beds, the changes and bubbling behavior of the bed are important indicators of fluidization quality. Different operating conditions can affect the bed state and bubbling behavior. Moreover, the generation and rupture laws of bubbles under different operating conditions have not been thoroughly studied. Based on the CFD-DEM method, this paper investigates the bubbling behavior and bed state of fluidized beds with different boiler shapes. The simulation results indicate that the inclined fluidized bed has the most significant range of variation and the largest cross-sectional area of bubbles, with a maximum area of 231.76 mm2. The bubbling interface area of the conical fluidized bed without corners is only 83.76 mm2. Smaller bubbles tend to have a larger aspect ratio and a form factor closer to 1. In addition, the merging phenomenon in the early stage of bubble formation makes it more difficult to track the rupture. These results will help improve the contact between gas and solid phases, providing a theoretical basis for improving the performance of fluidized beds.

Experimental and numerical investigation of bubble evolution and bubble wall fluctuation mechanism during water entry of flared cavity

Most of the previous research on water entry focused on simple objects with “closed surface and regular shape”, such as wedges and disks. Only a few studies have investigated the water entry of complex structures with “surface cavity and irregular shape”, such as typical spacecraft, which are summarized as flared cavity in this paper. The water entry of flared cavity is a strong nonlinear process coupled with multi phase media, during which the liquid flows into or out of the flared cavity, and the obvious phenomenon of bubble wall fluctuation will appear. In this study, we experimentally investigate the bubble evolution process and bubble fluctuation law in a flared cavity under different impact speeds. The evolution process of bubble shape is photographed by a high-speed camera, and the bubble evolution process is divided into four stages: bubble formation stage, bubble development stage, bubble contraction stage and bubble shedding stage. It is found that there is an obvious fluctuation phenomenon on the bubble wall during the bubble development stage, which is mainly because of gas expansion and compression in the structural cavity, and the wall fluctuation law is basically consistent with the gas expansion and contraction trend. It is also established that the dimensionless parameters R/H1 and H1/H2 can be good indicators to describe the bubble expansion and bubble shedding, the dimensionless bubble expansion parameter R/H1 is stable between 0.033 and -0.042, and the dimensionless bubble shedding parameter H1/H2 is stable between 0.062 and -0.069.

Taylor bubble formation and flowing in a straight millimetric channel with a cross-junction inlet geometry Part II: Gas-liquid mass transfer

• Mass transfer was studied during bubble formation at a cross-junction. • Accuracy of resazurin-based technique was improved by advanced image systems. • Different mechanisms were observed for the developing O 2 concentration fields. • Oxygen concentration fields were measured during and after bubble pinch-off. • A scaling law relating k L a and f c was proposed to gather literature data. Using the resazurin-based colorimetric technique and advanced image acquisition, the equivalent oxygen concentration fields inside the liquid slugs were measured during and after the bubble formation stage at a cross-junction in a straight millimetric channel. Firstly, two different mechanisms were identified for the development of oxygen concentration fields depending on the two-phase Reynolds numbers (Re TP ). Under low Re TP , a jet-like central oxygen concentration ‘finger’ occurred between the newly formed bubble and the gas finger at the bubble pinch-off point. Right after the bubble pinch-off, the dissolved oxygen was transported first by the entering liquid from two side inlets, and later by the developing recirculation loop inside the liquid slug. Under higher Re TP , two highly concentrated oxygen concentration spots were formed near the bubble rear and the channel wall region, and a much more complex flow structure in the liquid slug appeared. Then, whatever the operating conditions, it was observed that the averaged oxygen concentrations inside the liquid slug followed a nearly linear relation as a function of the axial position in the channel, and that the related mass flux density decreased as far as the bubbles flowed along the channel’s length. The bubble formation process could be decomposed into three stages when considering the temporal evolution of the cumulated oxygen mass and depending on the development of the liquid slug. At last, the overall volumetric liquid side mass transfer coefficients ( k L a ) were deduced from the concentration fields, and found to linearly increase with the recirculation frequencies, leading to a scaling law.

Nucleation and relaxation processes in weak solutions: molecular dynamics simulation

ABSTRACT The kinetics of homogeneous nucleation of the vapour-gas phase was studied by the molecular dynamics (MD) method in Lennard-Jones systems simulating solutions with partial (A) and complete (B) solubility of components. The calculations were carried out at a temperature close to the normal boiling point of the solvent, two concentrations of the solute, and negative pressures. The pressure, as a function of density and concentration, in stable and metastable states, the parameters of phase equilibrium at a flat interface were determined. The ultimate stretching Δpn = ps –pn exceeded than those calculated from the classical nucleation theory at concentrations of solute c > 0.14 and were less theoretical ones at c < 0.2 in solutions A and B, respectively. A qualitative difference in the formation and growth of the vapour-gas nucleus in solutions A and B was established. In solution A, the partial density of the solvent sharply decreases and the density of the volatile component increases at the region of the bubble formation. In solution B, the density of the volatile component inside the bubble remains close to the density in the surrounding phase, and the density of the solvent decreases at the initial stage of bubble formation.

Numerical modeling for characterization of CO2 bubble formation through submerged orifice in ionic liquids

Industrial applications of Ionic liquids (ILs) in CO2 capture are still limited due to lack of research on CO2-ILs bubble dynamics. A CFD model was developed to understand CO2 bubble formation in the pool of ILs by coupling improved VOF (Volume of Fluid) method with FW–H (Ffowcs Williams and Hawkings) acoustic formulation. Conventional VOF method was improved by adding suitable drag force for CO2-ILs interaction and ILs viscosity was considered as a variable with dissolved CO2. The predicted results are in good agreement with experimental measurements. Based on current CFD model, distinct acoustics pressure signals were examined to characterize CO2 bubble formation stages (growth, necking, detachment). It was investigated that bubble diameter and detachment time were increased with increasing of orifice diameter. Moreover, effects of gas and ILs inlet velocities were explained by fluctuations in Reynolds number and acoustic pressure signals respectively. Higher dissolved CO2 quantity and earlier bubble detachment time were obtained in aqueous ILs than pure ILs. The results have potential to give some useful guidelines to design industrial gas distributor (multiple-holes) for CO2-ILs system.

Influence of Key Factors on the Characteristics of Flue Gas Desulfurization of Basic Aluminum Sulfate by Bubbles.

Temperature, reactant concentration, bubble flow field characteristics, and mass-transfer characteristics play an important role in flue gas desulfurization of basic aluminum sulfate (aluminum base). The influence of various factors on desulfurization efficiency (ηabs) was determined from the macroscopic and microscopic levels through experiments and numerical simulations. The temperature of absorption solution had a significant effect on ηabs, and low temperature was advantageous for SO2 absorption performance of aluminum base. The value of ηabs increased with increasing reactant concentration. When the aluminum base concentration was higher than 50%, ηabs remained above 90%. The low aluminum base shortened the beginning time of the rapid decline of ηabs. This outcome was related to the low concentration of active Al2O3. At a low concentration range of 2000–5000 ppm of inlet SO2, the reaction rate determined the ηabs level, and gas-phase mass-transfer resistance was the main factor restricting the increase in ηabs. However, reaction rate and gas-phase mass-transfer resistance determined the ηabs level when the inlet SO2 concentration was as high as 50 000 ppm. The aeration rate affected the bubble size, number, and diffusion state. In addition, a higher aeration rate reduced the gas–liquid contact time and speeded up the consumption of aluminum base. As a result, bubbles along the absorber height showed different desulfurization characteristics, that is, 0–0.1 m was the initial stage of bubble formation and desulfurization (the ηabs was lower than 15%), 0.1–0.34 m was the main stage of bubble diffusion and desulfurization (the ηabs rapidly increased to 86.4%), and 0.34–0.4 m was the stable stage of desulfurization (the ηabs slowly increased to 92.5%).

Molecular gas in the H ii-region complex RCW 166: Possible evidence for an early phase of cloud–cloud collision prior to the bubble formation

Abstract Young H ii regions are an important site for the study of O star formation based on distributions of ionized and molecular gas. We reveal that two molecular clouds at ∼48 km s−1 and ∼53 km s−1 are associated with the H ii regions G018.149−00.283 in RCW 166 by using the JCMT CO High-Resolution Survey (COHRS) of the 12CO(J = 3–2) emission. G018.149−00.283 comprises a bright ring at 8 μm and an extended H ii region inside the ring. The ∼48 km s−1 cloud delineates the ring, and the ∼53 km s−1 cloud is located within the ring, indicating a complementary distribution between the two molecular components. We propose a hypothesis that high-mass stars within G018.149−00.283 were formed by triggering during cloud–cloud collision at a projected velocity separation of ∼5 km s−1. We argue that G018.149−00.283 is in an early evolutionary stage, ∼0.1 Myr after the collision according to the scheme detailed by Habe and Ohta (1992, PASJ, 44, 203), which will be followed by a bubble formation stage like RCW 120. We also suggest that nearby H ii regions N21 and N22 are candidates for bubbles possibly formed by cloud–cloud collision. Inoue and Fukui (2013, ApJ, 774, L31) showed that the interface gas becomes highly turbulent and realizes a high-mass accretion rate of 10−3–10−4 M⊙ yr−1 by magnetohydrodynamical numerical simulations, which offers an explanation of the O-star formation. The fairly high frequency of cloud–cloud collision in RCW 166 is probably due to the high cloud density in this part of the Scutum arm.

CFD simulation of electrostatic effect on gas interchange, vortex and heat transfer in the gas-solid fluidized bed

Vortex and electrostatic charges in the gas-solid fluidized bed have a significant influence on its transport abilities and hydrodynamics. In this work, the electrostatic model coupled with energy model has been applied to reveal the electrostatic effect on hydrodynamics, vorticity and local heat transfer coefficients based on the kinetic theory of granular flow. The results indicate that particle vortices change the gas and solid phase interaction around the bubble and enhance the local heat transfer coefficients. Gas interchange decreases by 6.5% compared to Davidson model at the jet velocity of 10 m·s−1 and 13% of 5 m·s−1. After adding electrostatic charges, bubble diameter decreases with the increasing specific charges. Furthermore, vorticity at the initial stage of bubble formation is larger and the particle vortex diffuses to a large extent. The simulation results can be applied to modify and estimate the overall heat transfer coefficient of the fluidized bed reactor and provide the basis for studying the effect of electrostatic effect on heat transfer.

Visualization and characterization of gas–liquid mass transfer around a Taylor bubble right after the formation stage in microreactors

The gas–liquid mass transfer occurring in Taylor flows right after the bubble formation stage were investigated in a flow-focusing microreactor. The colorimetric technique proposed by Dietrich et al. (2013) was used for locally visualizing and characterizing the gas–liquid mass transfer. Thanks to this method, the liquid-side mass transfer coefficients kL were measured at the moment right after the bubble is detaching from the gas film near the cross-junction of the microreactor. Experiments were carried out for several flow conditions (95.7<Re<226.1,0.0043<Ca<0.010,0.4<We<2.3,Bo=0.044) and bubble size (2.34<Lb/l<5.59). The results have demonstrated that the contribution of mass transfer right after the bubble formation stage is reasonably larger to those obtained at the bubble flowing-stage.

Three stages of bubble formation on submerged orifice under constant gas flow rate

Bubble formation is involved in many engineering applications. It is important to understand the dynamics of bubble formation. This work reports experimental and numerical results of bubble formation on submerged orifice under constant gas flow rate. Compressible large eddy simulation combined volume of fluid (VOF) was adopted in simulation and results was validated by experiment. Bubble formation is divided into three stages in this paper, expansion stage, elongation stage and pinch-off stage. In expansion stage, The bubble grows radially due to the incoming gas flux, but the bubble base remains attached to the orifice. But as gas injected, the spherical bubble will go into the elongation stage when the downward resultant force is lager than upward resultant force. And when bubble neck's length is bigger than √2Ro the bubble will go into pinch-off stage. Cylindrical Rayleigh-Plesset equation can be used to describe the pinch-off stage. Uncertain parameter r in it is given reference value in this paper.

Effect of orifice size and bond number on bubble formation characteristics: A CFD study

The volume of fluid with the continuum surface force (VOF‐CSF) model has been used in the present numerical study to investigate bubble formation and shapes in a bubble column. The effects of orifice sizes ranging from 0.5–1.5 mm on the bubble formation stages (i.e., expansion, elongation, and pinch‐off), bubble contact angle, departure diameter, time, and shape of bubble are investigated under a constant inlet velocity (0.2 m/s) boundary condition (BC). It is found that the initial formation of a bubble is dependent on the orifice diameter. Consequently, the formation of the bubble's hemispherical shape at the orifice is faster for a smaller orifice diameter than for a larger orifice diameter which forms a bigger bubble. A leading bubble requires a longer time to detach itself from an orifice in comparison to the next bubble (the second bubble), but interestingly the third bubble detaches much faster than the second. This model was also used to investigate the effect of the Bond number (Boσ). An increase of the Bond number (Boσ) from 0.047 to 0.16 speeds up the bubble pinch‐off, but a further increase in the Boσ (e.g. 0.17–0.47) slows it down. Based on the simulated cases used in this study, the findings demonstrate the capacity and accuracy of the CFD method in predicting bubble formation characteristics. The findings may be useful in the design of spargers for bubble column reactors.

Gas-liquid flow and mass transfer in a microchannel under elevated pressures

Flow and mass transfer of gas-liquid slug flow under elevated pressures up to 3.0MPa in a microchannel are investigated with CO2-water system. The results show that the ratio of the initial bubble length to the unit cell length is linear with the injection gas volume fraction under each pressure condition, but the slope decreases with an increase in the system pressure. The mass transfer coefficients are calculated with a unit cell model based on the dissolution rate of gas bubbles. Increasing pressure leads to larger mass transfer coefficients, as well as higher amount of gas absorption during the bubble formation. But the fraction of gas absorption during the bubble formation stage is only about 1.5~4.0% of feeding gas. For the bubble dissolution in the main channel, the dissolution rates at different flow rates differ very little for short contact distances from the T-junction, whereas the balance limitation of dissolution at large contact distances only depends on the amount of liquid in a unit cell.

Effect of orifice geometry on bubble formation in melt gas injection to prepare aluminum foams

The bubble formation process at submerged orifices with different geometry is investigated in the preparation of aluminum foams by gas injection method. The bubble profile on a horizontal plate is calculated by quasi-static analysis through Laplace equation. The bubble formation process is then distinguished into three stages: nucleation stage, growth stage and detachment stage in wetting and less wetting conditions based on the force balance analysis. In addition, the bubble size at high Reynolds number is obtained by considering the contribution of buoyancy, pressure force, inertial force, drag force and surface tension based on the three stages of bubble formation. The bubble size is confirmed to be sensitive to the equivalent contact angle, which consists of two terms including the contact angle and the wedge angle. Therefore, the wedge angle is introduced in the design of gas outlet orifices for the purpose of decreasing bubble size generated. The experimental study is conducted at three different types of stainless steel orifices under constant gas flow rates (0.05–2 L/min). It is clarified that the orifice geometry and the orifice size are both responsible for the cell size of aluminum foams. The experimental results for three different types of orifices show a consistent trend with the theoretical predictions at various gas flow rates. In the design of orifices to generate small bubbles in the melt, the wedge angle that coordinates with the contact angle is thus suggested.

Validity of critical coalescence concentration in dynamic conditions

The aim of the study was to confirm that the concept of the critical coalescence concentration (Cho and Laskowski, 2002) is only applicable to static conditions.A new experimental setup was developed. The experimental idea was to sample the same batch of bubbles at their formation point and later, during the bubble rise in static and dynamic conditions. The difference in the measured bubble size in the two sampling areas gives indication on the effect of frothers on bubble coalescence and breakup that takes place after the bubble creation due to the dynamic conditions.Three commercial frothers, DF200, NF240 and DF250, were tested in the experiments.In static conditions, the size of the forming and rising bubbles became equal at the frother concentration equal to the critical coalescence concentration (CCC) observed previously in mechanical flotation cell. On the other hand, in dynamic conditions, the size of rising bubbles was significantly larger than the size of freshly formed bubbles not only below the CCC as expected based on the literature. This finding confirms the hypothesis that the CCC is only applicable in static conditions, or in conditions where hydrodynamic forces do not contribute to bubble coalescence or breakup. The results showed that the frothers have a major effect on bubble size already at the early stage of bubble formation.

On the transition stage of bubble formation on the orifice of a submerged vertical nozzle

AbstractThe transition stage (self‐accelerating necking process) generally takes place before a bubble collapses. However, the mechanism is still not well understood. In this article, seven existing experiments dealing with bubble formation on the orifice of a submerged vertical nozzle are examined by solving the Young–Laplace equation. Multiple solution modes are found. Bubble in solution mode 2 has a neck and thus taller than mode 1 at the same volume. The present numerical result along with an experiment from Longuet‐Higgins et al. [J. Fluid Mech. 230, 365–390 (1991)] evidences an isometric transition from solution modes 1 to 2. This might account for the self‐accelerating necking process before a bubble collapses. Surprisingly, all of the seven existing experiments agree excellently with the bubble shapes from the Young–Laplace equation without the dynamic effect even when the bubble growth rate in the experiment is 2.25 times as large as the critical value. The gas flow rate (the dynamic effect) seems to play a role only after the transition stage. © 2011 Canadian Society for Chemical Engineering

Bubble growth rate from stainless steel substrate and needle nozzles

The formation of bubbles from a substrate nozzle immersed in water is studied experimentally and compared with bubble formation from needle nozzles under the same conditions. Nozzles with different internal diameter sizes in the range of 0.11–0.84 mm and low gas flow rates from 0.015 to 0.85 ml/min are used. The bubble formation is recorded by a high speed video camera and detailed characteristics of bubble formation are obtained. Interestingly, it is realized that the bubble volume expansion rate follows a cyclic behavior for the substrate nozzles while it shows a smooth decrease after an initial increase for the needle nozzles. Force balance analysis suggests that fluctuations in the substrate are related to the rapid increase of the attaching inertial force at the initial stages of bubble formation. In contrast, the inertial force in the case of the needle is smaller and takes longer to acquire its maximum value. The results also show that bubbles emerge earlier on top of the substrate than on the needle but the initial bubble volume is smaller in the former case. However, the bubble expansion rate is larger for the substrate than for the needle and as a result the final bubble volume is similar for both cases. By using experimentally captured values of bubble height and radius of contact line, the Young–Laplace equation predicts well the bubble evolution until just before the bubble detachment. Other bubble characteristics follow similar trends for both inlets with time and volume.

Investigation on the effect of temperature excursion on the helium defects of tungsten surface by using compact plasma device

The effects of temperature excursion on the helium defects of tungsten surface have been investigated by using compact plasma device AIT-PID (Aichi Institute of Technology – Plasma Irradiation Device). An initial stage of bubble formation has been identified with an order of smaller (sub-micron) bubbles and holes than those in the past in which the micron size is the standard magnitude. The radiation cooling has been detected when a blacking of tungsten surface coming from nanostructure formation is proceeding due to an increase in the emissivity. The temperature increase to the domain (∼1600K) in bubble/hole formation from that in nanostructure formation has been found to bring a constriction in diameter and a reduction in length of fiber-form nanostructure.

Theoretical Model of the Transient Combustion of Organic-Gellant-Based Gel Fuel Droplets

Experimental evidence of the combustion process of an all-organic gel fuel droplet indicates that at a certain time after ignition, evaporation of the liquid fuel results in the formation of an elastic layer of high-viscosity gellant around the droplet, which prevents further vaporization. As a result, constantly expanding vapor bubbles are produced within the droplet. Eventually, the layer ruptures and jets of fuel vapor are released. A theoretical, time-dependent model of organic-gellant-based gel droplet combustion has been developed and numerically solved. The results indicate that the evaporation rate of the liquid fuel from the droplet surface depends on droplet size and strongly affects the thickness of the gellant layer. The tensile stress, applied to the gellant layer during the formation of the bubbles, reaches high levels in short periods of time and causes the droplet to rupture when it exceeds the layer material rupture stress. The stage during which the gellant layer is formed is almost three orders of magnitude longer than the stage of bubble formation and layer rupture.