Daughter Bubble Size Research Articles

Computational study of bubble coalescence/break-up behaviors and bubble size distribution in a 3-D pressurized bubbling gas-solid fluidized bed of Geldart A particles

A computational study was carried out on bubble dynamic behaviors and bubble size distributions in a pressurized lab-scale gas-solid fluidized bed of Geldart A particles. High-resolution 3-D numerical simulations were performed using the two-fluid model based on the kinetic theory of granular flow. A fine-grid, which is in the range of 3–4 particle diameters, was utilized in order to capture bubble structures explicitly without breaking down the continuum assumption for the solid phase. A novel bubble tracking scheme was developed in combination with a 3-D detection and tracking algorithm (MS3DATA) and applied to detect the bubble statistics, such as bubble size, location in each time frame and relative position between two adjacent time frames, from numerical simulations. The spatial coordinates and corresponding void fraction data were sampled at 100 Hz for data analyzing. The bubble coalescence/break-up frequencies and the daughter bubble size distribution were evaluated by using the new bubble tracking algorithm. The results showed that the bubble size distributed non-uniformly over cross-sections in the bed. The equilibrium bubble diameter due to bubble break-up and coalescence dynamics can be obtained, and the bubble rise velocity follows Davidson’s correlation closely. Good agreements were obtained between the computed results and that predicted by using the bubble break-up model proposed in our previous work. The computational bubble tracking method showed the potential of analyzing bubble motions and the coalescence and break-up characteristics based on time series data sets of void fraction maps obtained numerically and experimentally.

Numerical study of asymmetric breakup behavior of bubbles in Y-shaped branching microchannels

Microfluidic technology based on microchannel two-phase flow has been widely used. The precise control of the bubble or droplet size in the channel plays a crucial role in designing the microfluidic systems. In this work, the bubble breakup behavior in Y-shaped microchannel is reconstructed based on the volume of fluid method (VOF), and the effects of bubble dimensionless size (1.2–2.7), outlet flow ratio (1–4) and main channel Reynolds number (100–600) on the bubble breakup behavior are systematically investigated. The bubble asymmetric breakup process is found to be divided into three stages: extension stage, squeeze stage, and rapid pinch-off stage. In the case of small initial bubble size or relatively high outlet flow rate, the bubble does not break, but only experiences the extension stage and the squeezing stage. Four flow patterns of bubble breakup are further revealed for the bubbles with different sizes and outlet flow ratios: tunnel-tunnel breakup, obstruction-obstruction breakup, tunnel-obstruction breakup, and non-breakup. With the increase of outlet flow ratio, the breakup process of the bubble gradually becomes asymmetrical, and the flow pattern shifts along the tunnel-tunnel breakup and the obstruction-obstruction breakup, gradually turns toward the tunnel-obstruction breakup and non-breakup. On this basis, the critical flow ratio of bubble breakup and the variation of daughter bubble volume ratio with outlet flow ratio are obtained for different Reynolds numbers and initial bubble sizes, and the corresponding criterion correlation equation is refined, which can provide theoretical guidance for accurately regulating the daughter bubble size after breakup.

Experimental study on breakup of a single bubble in a stirred tank: Effect of gas density and liquid properties

AbstractBubble breakup plays an important role in gas–liquid flows, but detailed studies are still scarce. In this work, the breakup behavior of a single bubble in a stirred tank was experimentally studied with a high‐speed camera, focusing on the effect of gas density, liquid properties, agitation speed, and mother bubble size. The bubble breakup time, breakup probability, breakup rate, and daughter bubble size distribution were determined. The internal‐flow phenomenon that the gas flowed from the smaller part to the larger part of a deformed mother bubble was observed by a high‐speed camera. The results showed that with increasing gas density, agitation speed, mother bubble size and decreasing surface tension, the bubble breakup rate and probability of equal‐size distribution significantly increased. With increasing liquid viscosity, the bubble breakup rate decreased especially in the high viscosity range. An M‐shaped daughter bubble size distribution was observed, which was consistent with our previous bubble breakup model.

Towards a model of bubble breakup in turbulence through experimental constraints

Complex bubble breakup in turbulence has been studied and modeled extensively by employing the population balance equation. This equation hinges on two quantities, i.e. daughter bubble size distribution and breakup frequency. Since there is no first-principle equation that can be solved to calculate these two quantities, many phenomenological models based on different physical mechanisms have been proposed. A large number of possible mechanisms at play leads to models with drastically different, and even contradictory, predictions. In contrast, experimental measurements of these two quantities, including several previous works and our own results collected in a vertical water tunnel that features a large homogeneous and isotropic region, seem to be consistent with one another. To resolve the difference between models and experiments, rather than following another physical argument, we approach the problem from a different direction by asking how to constrain a model based on experimental results. The specific constraints extracted from eight experimental results include: (i) direct measurements of daughter bubble size distribution; (ii) Super-Hinze-scale bubble size spectrum for constraining breakup frequency; (iii) Sub-Hinze-scale bubble size spectrum for modeling daughter bubble size distribution; (iv) Convergence time to an equilibrium state. Finally, based on these experimental constraints, a new breakup model that incorporates a corrected formulation for breakup frequency as well as a simplified function for daughter bubble distribution is developed to meet all constraints. Although the new model is deliberately not connected to any specific physical arguments for simplification, it appears to be robust and consistent with all experimental constraints mentioned.

Simulation of bubbles behavior in steel continuous casting mold using an Euler-Lagrange framework with modified bubble coalescence and breakup models

A mathematic model considering the interaction of bubbles was developed to simulate bubbles behavior in a continuous slab casting mold. The flow of molten steel is calculated with an Eulerian model coupled with a Lagrangian model for bubbles. Modified bubble coalescence and breakup models are established as user-defined functions of FLUENT software. Collisions between bubbles are described by means of hard-sphere model. The break-up mold is implemented with a break-up constrain based on the critical Weber number augmented a model for daughter bubble size distribution. The daughter bubble fraction is calculated with a tanh function, which has a U-shape distribution with lower bounds. Small bubbles were produced by the breakup of large bubbles. The predicted size and number of captured bubbles obtained with the mathematic model agree well with the plant measurements. The size and number distributions of small bubbles near the advancing solidification shell were obtained with simulation results. The results show that the bubble size and number at both the narrow and wide faces decrease with the increase of distance from the meniscus, and they get close to a constant in the deep region. Meanwhile, the initial bubble size has no obvious effect on the bubble distribution in the mold at the operation conditions in the present work.

Bubble collisions on parallel arranged fibers

We report an experimental and theoretical investigation of bubble collisions on parallel arranged fibers. Using high speed videography, we visualized the impact outcomes and classified them into three modes: capturing, single bubble rising, and splitting. The experimental results demonstrate that the impact mode is mainly determined by the distance between the fibers as well as the rising speed of the bubble. We present the results in a regime map, displaying the dependence of the impact outcomes on dimensionless parameters: Web = ρUb2D/σ, Bo = ρgD2/σ, and d/D in the range of 0 ≤ Web ≤ 25, 0.4 ≤ Bo ≤ 0.7, 0.30 ≤ d/D ≤ 0.75, where ρ is the liquid density, Ub is the collision speed of bubble, D is the bubble diameter, σ is the surface tension, g is the gravitational acceleration, and d is the inter-fiber distance. The observed regime boundaries are explained with scale analysis. We also examined the size of daughter bubbles for splitting cases, and the results unveil the critical dependence of the daughter bubble size on the impact offset rather than on the collision speed. The results are expected to be widely used to control bubble filtration or generation in a variety of engineering systems aimed at improving heat and mass transfer using bubbles.

An improved bubble breakup model in turbulent flow

An improved bubble breakup model has been developed by adequately describing the internal flow inside the deformed bubble, to account for the significant effect of pressure on the bubble breakup behaviors and hydrodynamics in a bubble column. In this new model, several significant conditions and experimental findings, such as static stress analysis, interfacial stress of bubble neck, viscous flow resistance, were taken into account. These issues were not considered and led to under-predictions of the bubble breakup rate in our previous model. The bubble breakup rate and daughter bubble size distribution predicted by this improved model were consistent with available experimental data. Moreover, this bubble breakup model was directly used in the CFD-PBM model to numerically simulate a bubble column operated at different pressures. A good agreement between the simulated and experimental results was obtained under complex operating conditions.

Energy Optimization of Gas–Liquid Dispersion in Micronozzles Assisted by Design of Experiment

In recent years gas–liquid flow in microchannels has drawn much attention in the research fields of analytics and applications, such as in oxidations or hydrogenations. Since surface forces are increasingly important on the small scale, bubble coalescence is detrimental and leads to Taylor bubble flow in microchannels with low surface-to-volume ratio. To overcome this limitation, we have investigated the gas–liquid flow through micronozzles and, specifically, the bubble breakup behind the nozzle. Two different regimes of bubble breakup are identified, laminar and turbulent. Turbulent bubble breakup is characterized by small daughter bubbles and narrow daughter bubble size distribution. Thus, high interfacial area is generated for increased mass and heat transfer. However, turbulent breakup mechanism is observed at high flow rates and increased pressure drops; hence, large energy input into the system is essential. In this work Design of Experiment assisted evaluation of turbulent bubbly flow redispersion is carried out to investigate the effect and significance of the nozzle’s geometrical parameters regarding bubble breakup and pressure drop. Here, the hydraulic diameter and length of the nozzle show the largest impacts. Finally, factor optimization leads to an optimized nozzle geometry for bubble redispersion via a micronozzle regarding energy efficacy to attain a high interfacial area and surface-to-volume ratio with rather low energy input.

Experimental study of bubble breakup process in non-Newtonian fluid in 3-D pore-throat microchannels

Foam regeneration, an important step in foam profile control and foam flooding, is determined by the process of large dispersed mother bubbles breaking into small ones. The bubble behaviors in 3-D pore-throat microchannels were investigated in different polymer solutions by a high-speed digital camera. The initial bubble slug was generated through a co-flowing geometry and then flowed through a microfluidic constriction. Both breakup and non-breakup were observed at the operating conditions of Ca ranging from 0.000068 to 0.0067, and the breakup mechanisms can be divided into two different types: single bubble snap-off and multi-bubbles pinch-off. Emphases are given to the influences of pore-throat structures, capillary number and continuous phase viscoelasticity on the size variation of the daughter bubbles. The average daughter bubble size decreases with the increase of the pore-throat ratio or elasticity of solution. Furthermore, the change of the average size of daughter bubbles with capillary number could be scaled with a power law in breakup type Ι, while in breakup type ΙΙ, the mother bubble size becomes another decisive factor.

Investigation on bubble snap-off in 3-D pore-throat micro-structures

Abstract Bubble behaviors in the single and cascaded three-dimensional (3-D) pore-throat micromodels were studied at the operating ranging from 0.0007 to 0.0249. It was found that the average size of daughter bubbles decreased with the rising of the pore-throat ratio, length of throat and mother bubble velocity. The variation of the average size of daughter bubbles with capillary number could be scaled with a power law. In addition, the bubble breakup mainly occurred in the first pore-throat structure and the size of the daughter bubbles maintained almost stable.

Single Air Bubble Breakup Experiments in Stirred Water Tank

AbstractSingle gas bubbles have been injected into an stirred liquid tank and the eventual breakup process of the bubbles was examined through high-speed imaging. Experimental observations of the breakup probability, breakup time, number of daughter bubbles and daughter bubble size distribution have been collected. The occurrence of non-equal-size breakup events dominated over equal-size breakup events. The frequency of binary and multiple breakup events was about equal. The largest uncertainty is associated with the determination of the breakup time because the bubbles take continuously altering deformed shapes already from the point of injection into the tank. The present experimental data do not support the standard model assumption regarding the number of daughter particles. The active breakup zone in the stirred tank was in the large velocity field close to the radial impeller. It is not evident whether the breakup events are due to time average shear or pressures and velocity fluctuations.

Experimental study and numerical simulation with a coupled CFD–PBM model of the effect of liquid viscosity in a bubble column

The effect of liquid viscosity on the hydrodynamic behavior in a bubble column was investigated by experimental study and numerical simulation with a coupled CFD–PBM (population balance model) model. The total gas holdup and volume fractions of small and large bubbles were determined by the dynamic gas disengagement method. In the low viscosity range, the total gas holdup and the volume fractions of small and large bubbles were almost independent of the liquid viscosity. In the high viscosity range, an increased viscosity gave a decrease in the total gas holdup and volume fraction of small bubbles and an increase in the volume fraction of large bubbles. The simulation captured these features and showed that when the liquid viscosity was <10mPas, the viscosity had negligible effect on the bubble breakup rate and daughter bubble size distribution, while a further increase of the liquid viscosity significantly decreased the bubble breakup rate and increased the probability of equal breakup. The influence of liquid viscosity on bubble coalescence was less important than its effect on bubble breakup for the hydrodynamic behavior. With the use of the multiple bubble breakup and coalescence models and the use of the bubble size distribution to describe the interphase forces, the coupled CFD–PBM model could describe the dependences of the total gas holdup and volume fractions of small and large bubbles on liquid viscosity in both the homogeneous and heterogeneous regimes.