Bubble formation and dynamics in a quiescent high‐density liquid

Gas bubble formation from a submerged orifice under constant‐flow conditions in a quiescent high‐density liquid metal, lead–bismuth eutectic (LBE), at high Reynolds numbers was investigated numerically. The numerical simulation was carried out using a coupled level‐set and volume‐of‐fluid method governed by axisymmetric Navier–Stokes equations. The ratio of liquid density to gas density for the system of interest was about 15,261. The bubble formation regimes varied from quasi‐static to inertia‐dominated and the different bubbling regimes such as period‐1 and period‐2 with pairing and coalescence were described. The volume of the detached bubble was evaluated for various Weber numbers, We, at a given Bond number, Bo, with Reynolds number . It was found that at high values of the Weber number, the computed detached bubble volumes approached a 3/5 power law. The different bubbling regimes were identified quantitatively from the time evolution of the growing bubble volume at the orifice. It was shown that the growing volume of two consecutive bubbles in the period‐2 bubbling regime was not the same whereas it was the same for the period‐1 bubbling regime. The influence of grid resolution on the transition from period‐1 to period‐2 with pairing and coalescence bubbling regimes was investigated. It was observed that the transition is extremely sensitive to the grid size. The transition of period‐1 and period‐2 with pairing and coalescence is shown on a Weber–Bond numbers map. The critical value of Weber number signalling the transition from period‐1 to period‐2 with pairing and coalescence decreases with Bond number as , which is shown to be consistent with the scaling arguments. Furthermore, comparisons of the dynamics of bubble formation and bubble coalescence in LBE and water systems are discussed. It was found that in a high Reynolds number bubble formation regime, a difference exists in the transition from period‐1 to period‐2 with pairing and coalescence between the bubbles formed in water and the bubbles formed in LBE. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3996–4012, 2015     

Computational Fluid Dynamics Simulation of Generation and Coalescence of Bubbles in Non‐Newtonian Fluids

The successive generation and coalescence behaviors of bubbles from two parallel nozzles in non‐Newtonian fluids were numerically simulated by using the volume of fluid (VOF) method. Three flow patterns for bubbles and the related flow regime transition lines were obtained. Two critical nozzle intervals exist: one for the bubble coalescence before pinch‐off, and another for alternating bubble formation then in‐line coalescence under different conditions. Two correlations were proposed to predict the dimensionless critical nozzle intervals for the transition of bubble‐flow patterns. The influences of nozzle diameter, gas flow rate, nozzle interval, and rheological properties of fluid on bubble‐flow patterns were investigated systematically.     

Bubble breakup with permanent obstruction in an asymmetric microfluidic T‐junction

Bubble breakup with permanent obstruction in an asymmetric microfluidic T‐junction is investigated experimentally. The breakup process of bubbles can be divided into three stages: squeezing, transition, and pinch‐off stages. In the squeezing stage, the thinning of the bubble neck is mainly controlled by the velocity of the fluid flowing into the T‐junction, and the increase of the liquid viscosity can promote this process. In the transition stage, the minimum width of bubble neck decreases linearly with time. In the pinch‐off stage, the effect of the velocity of the fluid flowing into the T‐junction on the thinning of the bubble neck becomes weaker, and the increase of the liquid viscosity would delay this process. The evolution of the minimum width of the bubble neck with the remaining time before the breakup can be scaled by a power–law relationship. The bubble length has little influence on the whole breakup process of bubbles. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1081–1091, 2015     

Novel aeration of a large‐scale flat sheet MBR: A CFD and experimental investigation

Having previously established that the hydrodynamic effect introduced by slug bubbles is more effective and economic in fouling amelioration in flat sheet MBRs (FSMBR) than conventional bubbling, this work is focused on its implementation in a commercial FSMBR. The overall objective is to enhance the hydrodynamic effect on fouling control through the use of two‐stage large‐sized bubble development (coalescence and split). Computational Fluid Dynamics (CFD) was used to predict hydrodynamic features and substantial agreement was observed with experimental measurements. The critical height for bubble development space was determined to be circa 250 mm. Slug bubbles could be introduced into 14 channels, resulting in six‐fold stronger shear stress than that from single bubbles. Energy demand could be reduced by circa 50% compared with industry average usage and the shear stresses developed would, for most applications, be sufficient to ameliorate fouling. Furthermore, the specific air demand per permeate would be halved. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2721–2736, 2018     

幂律流体中平行上升双气泡间相互作用研究

采用流体体积(VOF)法对非牛顿幂律流体中平行上升双气泡间相互作用进行了模拟研究.结果表明:剪切变稀效应及气泡周围流场结构对气泡间相互作用的贡献分别随着气泡间距增大而减小,气泡间相互作用主要受气泡间流体涡旋结构控制.当初始间距很小时,气泡平行聚并发生;随着初始间距增大,双气泡行为转变为相互影响的平行上升;当初始间距增大...     

Characterization of Bubble Shapes in Non‐Newtonian Fluids by Parametric Equations

Based on experiments with single air bubbles rising in stagnant non‐Newtonian fluids, an innovative model containing the aspect ratio (E) and two parameters (α, β) was proposed and proved to be capable of characterizing the bubble shape from spherical/ellipsoidal to prolate/oblate‐tear with good accuracy. Several impacts on bubble deformation were investigated, involving the rheological properties of the fluids and different forces exerted on the bubble, which were quantified by multiple dimensionless numbers (e.g., Reynolds, Eötvös, and Deborah number). Within a wide range, the empirical correlations were obtained for parameter β, and between α and β. Together with the shape model, a complete system was set up for bubble shape characterization and prediction that will provide new ideas for future studies on bubble hydrodynamics.     

Direct simulation of the buoyant rise of bubbles in infinite liquid using level set method

In this study, 3‐D level set method is applied to investigate the rise of gas bubbles in infinite liquid domain due to the buoyancy force. A number of typical regimes for single bubble rising are studied, including the ellipsoidal, ellipsoidal cap, spherical cap, and skirted bubbles. The bubble shape and rise velocity predicted by the simulation are compared with the graphical correlations of Grace, Trans. Inst. Chem. Eng., 51 , 116–120, (1973) and Bhaga and Weber, J. Fluid Mech., 105 , 61–85, (1981). Good agreement is found between the simulation results and the correlations. These simulations cover a wide range of the parameters, including Eo, Mo, and Re, and demonstrate the capability and accuracy of level set method for simulation of bubbles under various conditions with considerable deformation. Finally, simulation results for the coalescence of two bubbles are also presented.     

Velocity Evolution for the Coalescence of Two In-Line Bubbles Rising in Non-Newtonian Fluids

This study experimentally investigates velocity evolution for the coalescence of two in-line bubbles rising in non-Newtonian fluids by using a high speed camera. Due to the wake of the leading bubble and the shear-thinning effect of non-Newtonian fluids, the following bubble is accelerated to approach the leading bubble, leading to the coalescence of the two bubbles. Based on the Newton’s second law and Schlichting’s wake theory, a theoretical model was developed to estimate the instantaneous rising velocity of the following bubble in the coalescence process of the two bubbles. The predicted values by the present model showed a good agreement with the experimental data.     

Three‐Dimensional Simulation of Bubble Formation Through a Microchannel T‐Junction

Three‐dimensional simulations of bubble formation in Newtonian and non‐Newtonian fluids through a microchannel T‐junction are conducted by the volume‐of‐fluid method. For Newtonian fluids, the critical capillary number Ca for the transition of the bubble breakup mechanism is dependent on the velocity ratio between the two phases and the microchannel dimension. For the power law fluid, the bubble diameter decreases and the generation frequency increases with higher viscosity parameter K and power law index n. For a Bingham fluid, the viscous force plays a more important role in microbubble formation. Due to the yield stress τ_y, a high‐viscous region is developed in the central area of the channel and bubbles deform to a flat ellipsoid shape in this region. The bubble diameter and generation frequency are almost independent of K.     

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     

Towards the understanding of bubble interactions and coalescence in non-Newtonian fluids: a cognitive approach

The present work provides new insights into the behavior of air bubbles in non-Newtonian fluids. The interactions and coalescence between bubbles rising in non-Newtonian fluids were simultaneously investigated by means of birefringence measurements and particle image velocimetry for a chain of bubbles formed from a submerged orifice. Two aspects are identified for the first time as central to interactions and coalescence: (i) the stress creation by the passage of bubbles, and (ii) their relaxation due to the fluid's memory. This competition displays complex nonlinear dynamics, from periodic phenomena to deterministic chaos. From these fundamental mechanisms, a cognitive model based on behavioral rules has been developed to describe collective behaviors of a group of bubbles. By simulating bubbles as adaptive agents with their fluid via residual stresses, model predictions for consecutive coalescence between a great number of bubbles compare very satisfactorily with the experimental investigation.     

Multi‐Phase‐Multi‐Size‐Group Model for the Inclusion of Population Balances into the CFD Simulation of Gas‐Liquid Bubbly Flows

A CFD model for the simulation of gas‐liquid bubbly flow is developed. In the model, the multi‐phase flow is simulated by an Eulerian‐Eulerian approach using several phase definitions (from 3 to 10). The bubble size distribution is simulated by a solution of the discretized population balance equation with coalescence and break‐up of bubbles. The number of the discretized population balance equations in the model is larger than the number of the phases used in the flow field simulation. A desired accuracy in the simulation can be achieved by choosing a suitable number of phases as a compromise between accuracy and computational cost. With this model, more detailed flow hydrodynamics and bubble size distribution can be obtained. The model was tested with different operating conditions and for different numbers of dispersed phases in a bubble column, and was verified with a bubble size distribution obtained experimentally.     

Motion and coalescence of a pair of bubbles rising side by side

Bouncing and coalescence of a pair of slightly deformed bubbles rising side by side in a quiescent liquid are experimentally studied. The trajectories and shapes of the bubbles are investigated in detail by using a high-speed video camera. The wakes of bubbles are visualized by using a photochromic dye that is colored with UV light irradiation. We observe that the patterns of the trajectories of rising bubbles are strongly dependent on the Reynolds number. When the Reynolds number is over the critical region, two bubbles approach each other and then collide. After the collision, two types of motions are observed—coalescence and bouncing. We investigate the critical Reynolds number and Weber number over which the bubbles bounce. In the definitions of these numbers, we use vertical velocity, instead of horizontal one, as the characteristic velocity. We clarify that the critical Weber number is around 2 regardless of the Morton number. The critical Reynolds number decreases with an increase in the Morton number. Moreover, the visualization of the wake of bubbles enables us to observe the vortex separation from the rear surface of the bubbles on collision. We find that the vortex separation from the rear of bouncing bubbles causes a decrease in the rising velocity and an increase in the horizontal speed after their collision. We also observe that the behavior of repeatedly bouncing bubbles is significantly influenced by the wake instability of a single bubble rather than by the bubble-bubble interaction. By applying an existing model for spherical bubble-bubble interaction, we clarify that the revised model accurately describes the trajectory of a pair of slightly deformed bubbles using the restitution coefficient and velocity fluctuation from the experimental results.     

Shear‐rate‐dependent rheology effects on mass transport and surface reactions in biomicrofluidic devices

Consideration is given to shear‐rate‐dependent rheology effects on mass transport in a heterogeneous microreactor of rectangular cross section, utilizing both numerical and analytical approaches. The carrier liquid obeys the power‐law viscosity model and is actuated primarily by an electrokinetic pumping mechanism. It is discovered that, considering the shear‐thinning biofluids to be Newtonian fluids gives rise to an overestimation of the saturation time. The degree of overestimation is higher in the presence of large Damkohler numbers and electric double layer thicknesses. It is also increased by the application of a favorable pressure gradient, whereas the opposite is true when an opposed pressure gradient is applied. In addition, a channel of square cross section corresponds to the maximum fluid rheology effects. Finally, the numerical results indicate the existence of a concentration wave when using long channels. This is confirmed by analytical solutions, providing a closed form solution for wave propagation speed. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1912–1924, 2015     

CFD studies coupling hydrodynamics and solid‐liquid mass transfer in slug flow for matter removal from tube walls

Organic matter deposition on internal surfaces constitutes a drawback that impairs the efficiency of several industrial processes. To overcome this problem, sparging a train of bubbles could be useful since its presence strongly increases the wall shear stress. A detailed numerical mass‐transfer study between a finite soluble wall and the liquid around a rising Taylor bubble was performed, simultaneously solving velocity and concentration fields. The bubble passage throws solute backward and is responsible for radial dispersion. There is also an increase in the transfer rate with enhancements between 10 and 20% (depending on liquid average velocity and bubble length) compared to single‐phase flow. Mass‐transfer coefficients along the different hydrodynamic regions around the bubble nose, liquid film, and wake were characterized and their values compared with those from literature. The results suggest a promising potential of bubble train flow to enhance organic matter removal from walls in biological systems. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2420–2439, 2017     

液相中气泡上升行为与界面传质:实验研究与数值计算

在工业生产过程中，气泡在液相中的上升行为及气液界面的传质行为极为常见。本文针对不同条件下气泡上升过程的实验研究方法以及数值计算方法进行了总结。从实验与数值计算的角度，综述了单气泡上升过程的影响因素、多气泡上升过程聚并与破裂的现象和机理以及工业装置中气液两相流型和气泡特性，并对传质模型进行了归纳，主要关注了气侧-界面传质模型的研究现状。综述结果表明：当前对于单气泡上升行为的研究较为充分，而对于多气泡的行为机理的研究尚需深入。此外，受到研究手段的限制，进行气侧-界面传质模型研究具有一定挑战性。针对当前的相关研究进展和存在的问题，对今后气泡上升行为和传质行为的研究提出以下建议，即开展气泡聚并与破裂可控性研究，强化对气侧-界面传质过程的研究，包括泡内流体行为可视化研究和相关传质模型的建立。     

Bubble splitting under gas–liquid–liquid three‐phase flow in a double T‐junction microchannel

Gas–aqueous liquid–oil three‐phase flow was generated in a microchannel with a double T‐junction. Under the squeezing of the dispersed aqueous phase at the second T‐junction (T2), the splitting of bubbles generated from the first T‐junction (T1) was investigated. During the bubble splitting process, the upstream gas–oil two‐phase flow and the aqueous phase flow at T2 fluctuate in opposite phases, resulting in either independent or synchronous relationship between the instantaneous downstream and upstream bubble velocities depending on the operating conditions. Compared with two‐phase flow, the modified capillary number and the ratio of the upstream velocity to the aqueous phase velocity were introduced to predict the bubble breakup time. The critical bubble breakup length and size laws of daughter bubbles/slugs were thereby proposed. These results provide an important guideline for designing microchannel structures for a precise manipulation of gas–liquid–liquid three‐phase flow which finds potential applications among others in chemical synthesis. © 2017 American Institute of Chemical Engineers AIChE J, 63: 376–388, 2018     

Discrete Bubble Model for Prediction of Bubble Behavior in 3D Fluidized Beds

A discrete bubble model has been developed taking into account multiple bubble‐bubble interactions and a delayed coalescence method. The obtained simulation results were compared with experimental data reported in literature. The simulation results predicted by the developed model indicate clearly that the multiple interactions of bubbles lead to more reasonable results than those predicted by a binary interaction model. In addition, two types of interaction models were applied and predicted results were compared. The frequency of gas bubbles passing through the bed cross section versus bed height follows the same trend as the experimental data.     

Experimental investigation of interfacial mass transfer mechanisms for a confined high‐reynolds‐number bubble rising in a thin gap

Interfacial mass transfer is known to be enhanced for confined bubbles due to the efficiency of the transfer in the thin liquid films between them and the wall. In the present experimental investigation, the mechanisms of gas–liquid mass transfer are studied for isolated bubbles rising at high Reynolds number in a thin gap. A planar laser induced fluorescence (PLIF) technique is applied with a dye the fluorescence of which is quenched by dissolved oxygen. The aim is to measure the interfacial mass fluxes for pure oxygen bubbles of various shapes and paths rising in water at rest. In the wakes of the bubbles, patterns due to the presence of dissolved oxygen are observed on PLIF images. They reveal the contrasted contributions to mass transfer of two different regions of the interface. The flow around a bubble consists of both two thin liquid films between the bubble and the walls of the cell and an external high‐Reynolds‐number in‐plane flow surrounding the bubble. Mass transfer mechanisms associated to both regions are discussed. Measurement of the concentration of dissolved oxygen is a difficult task due to the nonlinear relation between the fluorescence intensity and the concentration in the gap. It is however possible to accurately measure the global mass flux transferred through the bubble interface. It is determined from the fluorescence intensity recorded in the wakes when the oxygen distribution has been made homogeneous through the gap by diffusion. Assuming a reasonable distribution of oxygen concentration through the gap at short time also allows a measurement of the mass fluxes due to the liquid films. A discussion of the results points out the specific physics of mass transfer for bubbles confined between two plates as compared to bubbles free to move in unconfined flows. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2394–2408, 2017     

Binary coalescence of air bubbles in viscous liquids in presence of non‐ionic surfactant

Coalescence of air bubbles is important in gas–liquid reactors and food processing operations. Bubbles can be stabilized by using non‐ionic surfactants. Binary coalescence of air bubbles in ethylene glycol and aqueous glycerol solutions were studied in this work in presence of Span 80. A novel set‐up was developed to study long coalescence times. Coalescence time was observed to follow broad stochastic distributions in all systems. The distributions were fitted with a stochastic model developed earlier. The surface tension of ethylene glycol and glycerol solutions was measured at various concentrations of Span 80. These data were fitted using a surface equation of state derived from the Langmuir isotherm. The effect of surfactant concentration on coalescence time was explained in terms of the surface excess of the surfactant and the repulsive force generated at the air–liquid interface. The results from this work illustrate the stochastic nature of bubble coalescence in viscous liquids. This work also demonstrates how non‐ionic surfactants can stabilize bubbles in such liquids.