Bubble Swarm Research Articles

VOF-based modeling study of surface oscillation behavior and gas–liquid stirring dynamics in the bottom-blown reactors

Gas bottom-blown reactors are widely employed in the chemical, metallurgical, and biochemical industries due to their superior mixing capabilities. This study numerically investigates the gas–liquid flow hydrodynamics and bubble dynamics in a bottom-blown system with a circular boundary using the volume of fluid approach. Following the experimental validation of the mathematical model, the study evaluates the effects of gas flow rates, liquid heights, and fluid medium properties on both the dynamics of the bubble swarm and the mixing characteristics of the flow field. The results indicate that: (1) Rising bubbles disturb the fluid, generating numerous vortices that facilitate momentum transfer for effective gas stirring; (2) The height of the liquid surface oscillation during the bottom-blown process exhibits significant temporal variation, with the peak showing clear lateral swings; (3) Due to the nonlinear interactions involving bubble coalescence and breakup during their ascent, along with constructive and destructive interference between liquid surface waves, the liquid surface can exhibit violent fluctuations at certain moments; (4) Bubble size and aspect ratio significantly influence liquid surface oscillations; (5) The bubble rising path displays instability due to vortex shedding, with some vortices forming at locations where the bubble shape changes abruptly. A bubble velocity prediction equation is proposed: uc=σ5.2737Lcρc4.2737/16156μl9.5475.

Column flotation hydrodynamics operating in the heterogeneous regime estimated by electrical resistance tomography and multi-phase CFD model

ABSTRACT This paper investigates the two-phase flow hydrodynamics of a column flotation operated from a homogeneous to a heterogeneous flow regimes using experimental and computational fluid dynamics (CFD) techniques. Experiments were conducted in a lab scale column flotation of 0.1 m internal diameter and 2.5 m length at various superficial gas velocities using a dual plane electrical resistance tomography (ERT). The mean gas holdup increased from 2 to 18% with the increase in superficial gas velocity from 0.6 to 7.2 cm/s. The gas holdup-based bubble swarm velocity method identified four distinct flow regimes, i.e. homogeneous bubbling regime, narrow discrete bubbling regime, a sustained helical flow regime and churn turbulent flow regime in the column. Further, transient simulations were conducted using a two-fluid model coupled with a population balance model to assess the flow behavior inside the column. Virtual mass, lift and drag forces were incorporated into the model to account for the interphase forces. Numerical simulations predicted mean gas holdup was matching with the experimental data at low gas velocities (<2.4 cm/s), and a marginal deviation was noted at higher gas velocities (>2.4 cm/s). The predicted radial gas holdup profiles were found to change from flatter to parabolic with the increase in gas velocities, i.e. similar to the experiments. The effect of particle size on the rate constant and recovery was estimated with Luttrell and Yoon’s performance model using the particle floatability input data and the two-phase experimental data. The developed CFD model can be useful to optimize the operating and design parameters for efficient operation of the column flotation process.

Heat transfer characteristics during bottom-blown bubble coalescence

Bubble swarm evolution and behaviors are the key to study the heat transfer between a bubble swarm and its surrounding flow field, and are of significant and systematic theoretical guidance for exploring the process reinforcement technology and processes. In this study targeted at Therminol®66 and R245fa, we analyzed the three factors affecting gas-liquid two-phase flow. The results show that the flow rate of the dispersed phase has a significant effect on the heat transfer coefficient. When the flow rate of the working medium increases from 0.4 kg s-1 to 0.8 kg s-1, the heat transfer coefficient increases by about 1.25 times. The initial temperature difference is positively correlated with the heat transfer coefficient, but the continuous phase height increases the heat transfer coefficient first and then decreases. The heat transfer coefficient model is in good agreement with the results of Kwon, and the deviation is within 20 %, accounting for 92.464 %, which can be further used to describe the heat transfer process of heat exchanger or bottom-blown melting pool. Correlation equations for the Nusselt number (Nu) is developed, with average absolute deviation of 18.49 %.

3D detection and tracking of deformable bubbles in swarms with the aid of deep learning models

A 3D tracking of individual bubbles in bubble swarms is essential for revealing and understanding bubble interactions and clustering mechanisms in bubbly flows. In this work, we address this issue and present a new method for tracking deformable bubbles in 3D based on deep learning models. We also present a new dataset of artificial bubbly flow sequences to test the tracker, which could also be used to train future detection or tracking models. Although the developed tracker had difficulties in cases with a large number of bubbles, it showed an overall good performance on the complete dataset and demonstrates the potential of deep learning models for this task. We hope that this work fosters further developments as well as applications of 3D bubble tracking that at the end lead to a deeper understanding of how deformable bubbles interact.

Capture of the Morphology Transfer Process in a High-Momentum Centrally Aerated Bubble Column with a Hybrid Multifield Two-Fluid Model

The role of pool scrubbing in attenuating radioactivity release after severe accidents has been explored extensively. It is known that scrubbing efficiency is largely determined by the hydrodynamic phenomenology in pools, which is still insufficiently understood. Aerosol gas forms large globules at the nozzle exit, which subsequently break up into a swarm of stable bubbles, where the change of bubble size can reach more than two orders depending on the injection rate. Furthermore, with the increase of flow rates, the injection regime changes from globule to jet characterized by a continuous gas structure. The flow field in the pool can be divided into two zones, injection and rise (swarm), according to the gas-liquid interface morphology. In different zones, scrubbing is governed by different mechanisms such as inertial impact, diffusion, and gravity, whereby bubble rise velocity is one major influential parameter. So far, numerical analysis of pool scrubbing is routinely based on system codes, which rely on empirical correlations for the determination of hydrodynamic parameters as well as scrubbing zones. More recently, owing to the increasing availability of computational resources, knowledge is improved through three-dimensional computational fluid dynamics simulations. Nevertheless, morphology and regime change still present a challenge for both two-fluid and interface-tracking models. Because of the limitation of closures, the conventional two-fluid model (TFM) is generally effective for bubble size smaller than cell size while interface-tracking (capturing) methods demand dozens of cells per bubble size. Combining the two kinds of approaches into one simulation is not straightforward. The present work aims to present a hybrid multifield TFM with extensions for capturing the transfer between different morphologies and for considering the effect of mesh resolution in momentum exchange. The developments are validated by simulating the complex hydrodynamic process in a pool-scrubbing column, which operates in the globule regime with an injection Weber number between 103 and 105. By comparison with experimental data, the results are shown to be promising, which provides a versatile framework for investigation of particle scrubbing in the future. The most attractive feature of the model is that compared to one-fluid interface-tracking models, it reliably predicts the bubble rise velocity. Furthermore, it is able to capture the lateral gas distribution without the need of applying a population balance model since most large bubbles are resolved.

Spatiotemporal-Dependent Confinement Effect of Bubble Swarms Enables a Fractal Hierarchical Assembly with Promoted Chirality.

The submarine-confined bubble swarm is considered an important constraining environment for the early evolution of living matter due to the abundant gas/water interfaces it provides. Similarly, the spatiotemporal characteristics of the confinement effect in this particular scenario may also impact the origin, transfer, and amplification of chirality in organisms. Here, we explore the confinement effect on the chiral hierarchical assembly of the amphiphiles in the confined bubble array stabilized by the micropillar templates. Compared with the other confinement conditions, the assembly in the bubble scenario yields a fractal morphology and exhibits a unique level of the chiral degree, ordering, and orientation consistency, which can be attributed to the characteristic interfacial effects of the rapidly formed gas/water interfaces. Thus, molecules with a balanced amphiphilicity can be more favorable for the promotion. Not limited to the pure enantiomers, chiral amplification of the enantiomer-mixed assembly is observed only in the bubble scenario. Beyond the interfacial mechanism, the fast formation kinetics of the confined liquid bridges in the bubble scenario endows the assembly with the tunable hierarchical morphology when regulating the amphiphilicity, aggregates, and confined spaces. Furthermore, the chiral-induced spin selectivity (CISS) effect of the fractal hierarchical assembly was systematically investigated, and a strategy based on photoisomerization was developed to efficiently modulate the CISS effect. This work provides insights into the robustness of confined bubble swarms in promoting a chiral hierarchical assembly and the potential applications of the resulting chiral hierarchical patterns in solid-state spintronic and optical devices.

Numerical study of the drag force, interfacial area and mass transfer in bubbles in a vertical pipe

A systematic numerical study of drag force and mass transfer in gravity-driven bubbles in a vertical pipe is performed at Re∼O(100−1000). This research employs a parallel multi-marker unstructured conservative level set method for interface capturing, avoiding the numerical coalescence in bubble swarms. The finite volume method discretizes transport equations on 3D collocated unstructured meshes. Unstructured flux limiter schemes solve the convective term of transport equations, preserving the numerical stability at high Reynolds numbers and high-density ratios. Thermodynamic equilibrium is assumed at the interface for the concentration of chemical species. The hydrodynamics and mass transfer of bubbles are validated against classical correlations from the literature. Finally, direct simulations are executed to develop new correlations for the drag force, normalized bubble surface and Sherwood number for bubbles in a vertical pipe.

Thermal-hydraulics performance evaluation of bubble swarms sweeping tube bundles in a direct contact gas-liquid-solid reactor

Direct contact gas-liquid-solid reactor pertains to an extremely high efficiency heat-exchange facility in industrial application. Nevertheless, the scarcity of quantitative analysis data hinders the optimal design of this type of reactor. In the present study, a novel variable bubbles size modeling approach which considers the bubble swarms breakup and coalescence rates, is proposed to evaluate the complex thermal-hydraulics performance of bubble swarms sweeping tube bundles. On the basis of verifying the responsibility of the numerical modelling with published experimental data, the intrinsic link of hydrodynamics and thermodynamics parameters on reactor features, including field characteristics, gas holdup, bubble swarms diameter distribution, gas–liquid interfacial area, temperature difference and heat flux inside different tube bundles regions, are deeply revealed. Results demonstrate that the temperature difference between bubble swarms and tube bundles walls is approximately equal to 2 K. Interestingly, different positions of heat exchanging tubes affect the peak value of circumferential heat transfer coefficient. For typical Tube 1–2, the maximum value of 3146.28 W/m2·K appears around 162°. The presence of tube bundles gives rise to the well-distributed iso-surfaces of bubble swarms which contributes to enhance heat transfer. When superficial gas velocities are respectively 0.09 m/s, 0.12 m/s and 0.14 m/s, the average bubble warms diameters are 13.47 mm, 16.22 mm and 17.24 mm. When initial liquid phase height increases from 342 mm to 378 mm, the corresponding gas–liquid interfacial area decreases by 16.2 % in total. Finally, two new modified dimensionless correlations were developed for the prediction of gas holdup and Nusselt number. The prediction errors are respectively 10 % and 6 %. The findings can improve the awareness for promising applications in design and scale-up processes of direct contact gas-liquid-solid reactor.

Hollow drop bubbles: A preliminary study of simplified prototype for improving copper(II) extraction with ACORGA® M5640 using coated bubble swarm

There are two well-defined processes for the types of copper ores. One for the processing of copper sulfides and another for the processing of the copper oxides. There are similarities between the two, particularly in the preparation stage of the run of mine (ROM) such as the crushing. However, significant differences exist in how to concentrate the elements of interest. Flotation is the most used concentration process to separate copper sulfide minerals from other minerals using air bubbles. For oxides, after the dissolution of copper, the solvent extraction (SX) process is the preferred path using two immiscible liquids to separate the copper.A line of investigation for solvent extraction is the use of a bubble coated with solvent to extract the metal of interest from the aqueous solution, some devices have been built and tested for this purpose using different principles to generate a coated bubble swarm. However, those equipments have been tested on laboratory and have not been scaled up to an industry level.The Hollow Drop (HD) concept was born from the idea of building a device to generate coated bubbles in a continuous swarm that could be scalable to an industry level. In this paper two columns were built and operated: a proof-of-concept column and a scale-up attempt for the extraction of Cu(II) fom an aqueous solution of 2.5 g L−1 using ACORGA® M5640 (25% v/v) in the Kerosene.The results show that we could generate a bubble swarm and conduct the solvent extraction process at a 97% recovery using our proposed coated bubble generator. However, in our scaled prototype test only a 70% recovery was achieved, which shows that our column is working but the scaling-up needs more investigation regarding the dimensions and flows of the process.

Unstructured Conservative Level-Set (UCLS) method for reactive mass transfer in bubble swarms at high density ratio

This research presents a parallel Unstructured Conservative Level-Set (UCLS) method for reactive mass transfer in bubbles with a high-density ratio. This method uses the multiple-marker UCLS approach to circumvent the numerical coalescence of bubbles, combined with the finite-volume method to discretize transport equations on 3D collocated unstructured meshes. The fractional-step projection method solves the pressure-velocity coupling. The central difference scheme discretizes the diffusive term, whereas convective term within the momentum transport equation, level-set advection equations, and mass transfer equation are discretized by unstructured flux-limiters schemes. Such a combination of numerical techniques preserves the numerical stability in bubbly flows with a high Reynolds number and high-density ratio. Numerical and physical findings on the effect of physical properties ratios on the reactive mass transfer are reported, including their effect on the Reynolds number, interfacial area and Sherwood number.

Turbulence induced by a swarm of rising bubbles from coarse-grained simulations

We performed numerical simulations of a homogeneous swarm of bubbles rising at large Reynolds number, $Re=760$ , with volume fractions ranging from 1 % to 10 %. We consider a simplified model in which the interfaces are not resolved, but which allows us to simulate flows with a large number of bubbles and to emphasize the interactions between bubble wakes. The liquid phase is described by solving, on an Eulerian grid, the Navier–Stokes equations, including sources of momentum which model the effect of the bubbles. The dynamics of each bubble is determined within the Lagrangian framework by solving an equation of motion involving the hydrodynamic forces exerted by the fluid accounting for the correction of the fictitious self-interaction of a bubble with its own wake. The comparison with experiments shows that this coarse-grained simulations approach can reliably describe the dynamics of the resolved flow scales. We use conditional averaging to characterize the mean bubble wakes and obtain in particular the typical shear imposed by the rising bubbles. On the basis of the spectral decomposition of the energy budget, we observe that the flow is dominated by production at large scales and by dissipation at small scales and we rule out the presence of an intermediate range in which the production and dissipation are locally in balance. We propose that the $k^{-3}$ subrange of the energy spectra results from the mean shear rate imposed by the bubbles, which controls the rate of return to isotropy.

Numerical simulations of bubbly flows in a vertical periodic channel

In this study, numerical simulations of bubbly flows in an infinitely long vertical channel were performed. Generally, the upper and lower boundaries of such channels are simplified as periodic boundary conditions. Unlike horizontal channel flows, the presence of gravity in the streamwise direction complicates the establishment of periodic boundary conditions. Therefore, a specialized treatment is required to prevent fluid acceleration. Here, we developed a novel treatment for the imposition of periodic boundaries and proposed a new microbubble model to consider the surface tension effect of microbubbles. To detect each bubble in the flow field, we implemented a bubble identification algorithm, which facilitates a thorough statistical analysis of bubble number, size, and spatial distribution. Validation tests were conducted, and good agreement was achieved between our results and reference data. We also confirmed that the results obtained with periodic boundaries are consistent with those achieved without them. Finally, we simulated the evolution of rising bubble swarms in a quiescent liquid. The method presented here contributes to the numerical simulations of bubbly flows in industrial systems, including oil-gas transportation, bubble columns, and nuclear reactors.

Bubble size distribution and turbulence characterization in a bubbly flow in the presence of surfactant

This study aimed to quantify the less explored complex multiphase hydrodynamics of a bubble swarm in the presence of surfactant which is the key to flotation process widely used in the resources and environmental engineering applications.Experiments were conducted in a rectangular column (cross-section: 100 mm × 100 mm) in batch mode by varying the gas flux (0.02 to and 0.08 cm/s) in the presence of an anionic surfactant sodium dodecyl sulphate (5 to 15 ppm). High-speed imaging was then used to visualise the unsteady bubble plume dispersion behaviour and estimate the bubble size distribution (BSD) which showed a reduction in the mean bubble size with the increasing gas flux and surfactant concentration.Next, particle image velocimetry (PIV) was utilised to measure the instantaneous velocity field which was used to determine the turbulence characteristics. It was shown that the bubble plume contributes to significant anisotropy in the flow field which increased in the higher surfactant concentration and gas flux cases. The energy containing turbulence length scale was characterized by the integral length scale, which was observed to increase linearly with both the gas flux and surfactant concentration. Also, the local turbulence energy dissipation rate exhibited a strong linear correlation with the bubble surface area flux parameter. In the presence of surfactant, the turbulence energy spectrum of the system exhibited a less steep slope in the inertial subrange regime compared to the Kolmogorov −5/3 slope. The spectrum also showed a leftward shift indicating energy addition to the larger turbulence length scales which was reflected in the formation of large recirculation zones around the bubble plume.

Polydisperse particle inline image method and its application onto gas–liquid flow in a stirred tank

AbstractBubbles of different sizes exhibit distinct hydrodynamic behaviors and their dense coexistence complicates seriously the multiphase flow. The commonly‐used arithmetic average method (e.g., PIV) treats every bubble equally and consequently erases many physical details. In this study, a polydisperse particle inline image method is developed to simultaneously measure comprehensive feature set of every bubble in bubble swarms and carrier phase velocity. By harnessing the optimal frame rate capture alongside assigning IDs for tracking, complete identification of polydisperse swarm in single frame is integrated with the accurate acquisition of transient dynamics across multiple frames. In gas–liquid stirred tanks, the bubble deformation is notably intensified owing to the presence of collisions driven by velocity differences. A novel analysis of bubble hydrodynamics decomposed by bubble size reveals they are jointly commanded by the inertial effect and size effect. Actual flow regime of the system is essentially the superposition of multiple dissimilar flow patterns.

Bubble formation and swarm dynamics: Effect of increased viscosity

Aiming to make the bridge between the studies of bubble behaviour as single bubble and in bubble columns, the characteristics of bubbles in an aerated column with a low gas content were investigated: bubbles formed at one single 0.5 mm orifice in a partially sparged small lab-scale bubble column were visualized by high-speed camera. The formation process and bubble behaviour at heights of 20 and 40 cm above the orifice were investigated. The bubble size, deformation and velocity were investigated in aqueous glycerol solutions (1 to 10 mPa.s). Average characteristics of the bubbles in swarms were obtained by statistical data processing. It was found that, the bubble formation mechanism governs the size and velocity of bubbles in swarms and that, in the investigated range, viscosity has little effect on the bubble behaviour. Individual bubbles in the swarm differ significantly in their size, shape, and velocity without correlation between these quantities.

Enhancing bubble bize prediction in flotation processes: a drift flux model accounting for frother type

This communication presents a methodology, based on a modified drift flux model, to determine bubble size distribution in column flotation. The modified drift flux model incorporates a surfactant-type parameter. This parameter considers the impact of surfactant on bubble hydrodynamics. The methodology aims to improve the accuracy of bubble size distribution prediction, which presents deviation depending on surfactant type (i.e. polyglycolic based or alcoholic base). Many authors have proposed different mathematical improvements to reduce de experimental data deviations in the presence of different surfactants. However, from 1988 to 2022, the determination coefficient, or the quality of the adjustments, from the proposed mathematical models is, at the most, 92% (relative error). The proposed methodology improves the quality of the adjustments to 98.6, adding a single parameter for groups of surfactants. This methodology incorporates a single parameter in the terminal velocity calculation that can compensate for the impact of surfactant type in bubble hydrodynamic (bubble skin friction or drag coefficient, bubble wake, bubble shape, bubble rigidity). This parameter is a function of the gas holdup calculated from gas velocity measured and the bubble size distribution calculated (deviated) from gas holdup and gas velocity measured. The methodology is validated with reported experimental results and proposed modifications from various authors. The confidence interval (2 σ) is reduced from 0.11mm to 0.05mm in the case of (Yianatos, Banisi, Ostadrahimi). In the case of the recently reported experimental results from Maldonado and Gomez, the confidence interval is reduced from 0.31 mm to 0.09 mm. These results improve bubble size estimation based on drift flux in column flotation, contributing to a better understanding of surfactant impact on bubble swarm hydrodynamics.

General prediction models for void fraction and specific surface area of gas–liquid microflows

AbstractThe flow characteristics of void fraction and specific surface area are important to predict the mass transfer and reaction performances in gas–liquid microreactors. However, the models and trends for the two parameters in different flow patterns remain ununified and insufficient. Accordingly, this work first time studies the void fraction and specific surface area in a very wide flow pattern range (Taylor, short bubble, bubbly, and bubble swarm flow). The results show that at the same gas–liquid flow rates, the void fraction decreases with bubble size in Taylor and short bubble flow but is totally opposite in bubbly and bubble swarm flow. Importantly, the variation of specific surface area with bubble size is independent of the flow pattern and decreases with bubble size. Finally, based on bubble size, two general models for the two parameters are developed. This work provides a new direction for the unification of flow characteristics.

Investigation into drag coefficient for sub-millimeter bubbles in gas–liquid bubbly flow: Experiments and CFD simulations

The sub-millimeter bubble technique can enhance the gas–liquid inter-phase mass transfer and show immense potential to increase production efficiency. The two-fluid model (TFM), suitable for the CFD simulations on the scale of actual reactors, is a powerful tool for exploring the application of the sub-millimeter bubble technique. Although the accuracy of TFM is largely affected by the drag models, the applicability of the drag models for gas–liquid two-phase flow of sub-millimeter bubbles is scarcely investigated. In the present work, the applicability of the single-bubble drag models was apprised with experimental data of single sub-millimeter bubbles. Then, the bubble swarm effect and its influence on the calculation of drag coefficients were investigated for the gas–liquid two-phase flow of sub-millimeter bubbles under a wide range of gas holdups (0.02–0.21). The accelerating effect was observed in the experiments of gas–liquid two-phase flows. A reasonable explanation for the accelerating effect was discussed based on the bubble size distribution obtained by applying our original image processing method. Based on the experiments, an equation for the drag modification factors was proposed to include the bubble swarm effect. The accuracy and applicability of the proposed equation were validated by applying it together with the accurate single-bubble drag model in the CFD simulation by TFM. The present work investigated the hydrodynamic characteristics of sub-millimeter bubbly flows experimentally and provided an accurate CFD method for further investigation.

Motion of Air Bubbles in a Cement Slurry.

The dynamics of air (gas) bubbles in a column of cement slurry is examined numerically. The air injected at the bottom of a laboratory-scale column through a porous distributor plate spatially distributes and migrates as a swarm of bubbles throughout the slurry toward the freeboard. The two-phase system of the cement slurry and the air bubbles is modeled using the conservation equations of mass and linear momentum in the framework of the volume-of-fluid (VOF) approach. The cement slurry is modeled using the Herschel-Bulkley and Bingham fluid models. Results show that the mean Sauter diameter and the mean rise velocity of the bubbles decrease with the gas flow rate. Meanwhile, it is found that the rising of the bubbles is controlled by breakup events, along with relatively weak path instabilities of the bubbles resulting in relatively straight trajectories, independent of the gas flow rate. The extent of the yielded region appears larger for the Herschel-Bulkley model compared to the Bingham fluid model (by approximately 10%).

Design and Analysis of a Novel Air Bubble Curtain for Mixing and Aeration in Process Vessels

This article focuses on designing and analyzing a new method for mixing in process vessels. We propose using an air bubble curtain to accomplish the desired mixing. The air bubble curtain used in this study is made of an empty pipe with several holes through which air is pumped, which in turn produces a swarm of bubbles that causes pseudo-turbulent mixing in the tank filled with a liquid solution. A uniform distribution of bubbles within the tank was assured by maintaining a see-saw movement of the air bubble curtain. Mixing inside the tank was quantified by measuring the salt concentration in a laboratory model tank (30 cm × 30 cm × 30 cm) and a pilot-scale tank (200 cm × 100 cm × 100 cm). The optimal flow rates of air and the mixing time in both tanks that can achieve complete mixing were obtained from the experimental results. The proposed air bubble curtain can be of potential use in wastewater treatment, biotechnology, and aquaculture where turbulent mixing is required with minimum energy dissipation per unit mass.