Bubble Wake Research Articles

Hydrodynamics and gas-liquid mass transfer around a confined sliding bubble

An experimental investigation of gas-liquid mass transfer in the wake of a confined air bubble sliding under an inclined wall in a 2D Hele-Shaw cell is reported. A colorimetric technique based on an oxygen-sensitive dye was used to visualize the oxygen transfer. Bubble velocities, shape eccentricities, interfacial areas and, for the first time, the instantaneous spatio-temporal distribution of oxygen concentration fields in the bubble wake, have been investigated for upper wall inclination angles of 10° ≤ α ≤ 60° and Archimedes numbers of 783 ≤ Ar ≤ 3221. Image processing has allowed, through a specific approach, a quantification of mass transfer. The calculation of the mass flux allowed the deduction of the liquid-side mass transfer coefficient kL. Experiments reveals that, at low angles of inclination, bubble velocities decelerates, shape eccentricities increased, and the instantaneous spatial and temporal distribution of oxygen concentration fields illustrated two distinct regions underneath the sliding bubble: a single vortex loop enclosing the near wake where oxygen is transferred, and a far wake containing oxygen in the form of a single long strip. When inclination angles and bubble sizes were increasing, velocities were increasing, the vortex elongated gradually until it disappears at high angles where total mass fluxes increased. This increase of bubble velocities has increased liquid-side mass transfer coefficient kL allowing a scaling law between the Sherwood number and the modified Archimedes number Ar.sin(α) to be proposed.

Wake volume of injected bubbles in fluidized beds: A magnetic resonance imaging velocimetry study

Rapid magnetic resonance imaging (MRI) was used to determine the volume of wakes of bubbles injected into incipiently fluidized beds. MRI was used to generate 2D maps of particle velocity surrounding bubbles with a temporal resolution of 18 ms. The bubble rise velocity, ub, was determined from the change in bubble position in successive maps, and pixels of particles which had an upward velocity greater than or equal to 0.8 ub were considered to be in the “wake” if they were below the bubble and in the “crown” if they were above the bubble. Results show that (i) the wake volume is much larger than the crown volume and (ii) the ratio of the wake volume to bubble volume increases as the bubble rises in the bed but decreases with increasing bubble volume. The influence of two bubbles interacting on bubble wake volume is also investigated. Comparison with geometric theory for wake volume exposes flaws in this theory for fully developed bubbles, bubbles near the distributor and interacting bubbles.

Multi-element coupling model of rising bubble in water

Multi-factor coupling modeling is carried out in the process of floating bubbles in water in this paper, and the model is employed to analyze the full force of bubbles in water, and to consider the change of pressure inside bubbles and the mass exchange of gas and liquid during bubble floatation. By numerical simulation of the coupling model, it is confirmed that the Non-equilibrium mass transfer condition is more consistent with the actual situation, and the relationship between the elements in the process of bubble floatation is analyzed, which provides a theoretical basis for the subsequent study of the acoustic characteristics of the bubble wake of the ship.

Experimental and numerical investigation of reactive species transport around a small rising bubble

In this article, we present experimental and numerical techniques to investigate the transfer, transport, and reaction of a chemical species in the vicinity of rising bubbles. In the experiment, single oxygen bubbles of diameter db=0.55…0.85mm are released into a measurement cell filled with tap water. The oxygen dissolves and reacts with sulfite to sulfate. Laser-induced fluorescence is used to visualize the oxygen concentration in the bubble wake from which the global mass transfer coefficient can be calculated. The ruthenium-based fluorescent dye seems to be surface active, such that the rise velocity is reduced by up to 50% compared to the experiment without fluorescent dye and a recirculation zone forms in the bubble wake. To access the local mass transfer at the interface, we perform complementary numerical simulations. Since the fluorescence tracer is essential for the experimental method, the effect of surface contamination is also considered in the simulation. We employ several improvements in the experimental and numerical procedures which allow for a quantitative comparison (locally and globally). Rise velocity and mass transfer coefficient agree within a few percents between experiment, simulation and literature results. Because the fluorescence tracer is frequently used in mass transfer experiments, we discuss its potential surface activity.

Large-scale simulation of ship bubbly wake during a maneuver in stratified flow

We study the stratified bubbly wake of the research vessel Athena R/V during an overshoot maneuver using a polydisperse two-fluid model. The motions of ship, rudders and propellers are handled with a dynamic overset approach. The demands of the resolution of the flow and the small time step imposed by the rotating propellers make this problem very demanding and challenging. We discuss the dynamic response of the ship, focusing on trajectory, motions, forces and moments. Vortical structures and their effects on bubble and density transport are studied. Efforts are then focused on studying the bubbly flow and density distribution, and how vortices and flow field affect bubble entrainment, density and bubble transport, and size distribution. Results indicate that the void fraction and maximum depth in the wake increase during the maneuver, as do the disturbances to the stable stratification. Bubble entrainment during the maneuver increases in the transom stern but decreases around the hull, resulting in more bubbles in the wake. Due to transport of bubbles entrained around the hull into the inboard side of the turn, the resulting wake is also wider than in straight self-propulsion operation.

Distribution Regularity of Dynamic Viscosity Blind Region behind the Bubble in Shear-Thinning Fluids under Different Gravity Levels

The gas-liquid two-phase flow has been widely applied to the field of space technology, such as thermal energy and power generation, long duration life support systems, transportation for propulsion, etc. (Microgravity Sci. Technol. 21, 175-183 (2009); Microgravity Sci. Technol. 22, 87-96 (2010); etc.). Through reviewing previous researches, it can be found that a dynamic viscosity blind region (almost not influenced by the shear-thinning effect) appears behind it when one bubble rises in the shear-thinning fluid (J. Non-Newton Fluid. Mech. 165, 555-567 (2010); Phys. Fluids. 29, 033103 (2017), J. Non-Newton. Fluid. Mech. 239, 53 -61 (2017); etc.). As a matter of fact, the occurrence of the viscosity blind region has the important effect on heat and mass transfer between bubble and liquid phases. Therefore, volume of fluid method is implemented to study the formation and evolution mechanisms of the viscosity blind region in the shear-thinning fluid. The results show that the viscosity blind region can be split into two regions, namely the first and the second viscosity blind region. The stronger the gravity level and the shear-thinning effect are, or the smaller the surface tension is, the more easily the viscosity blind region appears in the bubble wake. The viscosity blind region appears much easily in the wake of a skirt bubble under the same conditions. And in the rising process of the bubble, there is a stagnant flow region in the bubble wake, where the shear rate is very small, so the viscosity blind region appears in that region. In addition, the appearance of double wake vortices can split the stagnant flow region into two parts, which leads to the formation of the second viscosity blind region.

Application of analytical and CFD models of liquid fuels combustion in a fluidized bed

In Laboratory for Thermal Engineering and Energy of Institute Vinca, University of Belgrade, a 2-D CFD modeling procedure of numerical simulation of unconventional liquid fuel combustion in bubbling fluidized bed has been developed. This procedure is based on a two-fluid Euler-Euler approach modeling a fluidized bed with the determination of the velocities field of gas and particulates in two-phase, granular flows, analog to the kinetic theory of gases. This model describes in detail the unsteady motion of gas and solid phases, the formation and movement of bubbles with the combustion process in the fluidized bed, but temperature profiles calculated by the bed height differ to some extent from the experimentally obtained profiles. This discrepancy is probably due to the inability of a two-fluid CFD model to give a realistic simulation of the liquid fuel mixing in a fluidized bed. Therefore, an analytical model has been developed, where one of the basic assumptions is that the particles are mixed in the vertical direction of fluidized bed mainly by the bubble wakes. The proposed zonal type of calculating procedure is based on Davidson and Harrison two-phase model of the bubbling fluidized bed, where fluidized bed is divided into zones within which material and energy balances are set.

Simulation and Statistical Modeling of Acoustic Scattering of Bubble Wakes

Ship wakes are large, exist for a long time, and are difficult to disguise or conceal. These characteristics can be used as an important basis for ship tracking and recognition. However, distinguishing between wakes, underwater targets, and sea surface is a difficult problem that currently limits acoustic wake homing technology. To solve this problem, in this study, from the viewpoint of feature recognition, the effects of bubble radius, air volume fraction, frequency, depth, and other parameters on the group bubble were first investigated. The volume scattering intensity of acoustic wakes at different frequencies and depths was also calculated and analyzed, and the results of our theoretical calculations were verified through an experiment using a multifrequency single wave sonar dock. Subsequently, through the single frequency and multibeam sonar sea trial test, a statistical model of target characteristics with a clear physical mechanism was developed. The developed model can be utilized for the guidance and recognition of acoustic wake targets. Thus, this study lays the foundation for the practical application of acoustic wake guidance.

Axial meso-scale modeling of gas-liquid-solid fluidized beds

The meso-scale theory, based on the energy minimization multi-scale principle, has recently inspired the accurate modeling of the multi-phase flow. In this study, an axial meso-scale flow model for the gas-liquid-solid fluidized bed is established and verified by the literature models and data. Compared with the reported axial models, this model predicts the axial distribution of all three-phase hydrodynamic parameters with fewer empirical corrections. Predictions of the present model show that the operation conditions and physical properties of solid particles obviously affect the flow behaviors of phases, such as the slip velocity, entrainment of solid particles, wake shedding parameters. Solid particles entrained by bubble wake determine the formation of axial distribution in the gas-liquid-solid fluidized bed. Region with the axial structure is the transition section, length of which depends on the entrainment of solid particles. Furthermore, behaviors of the gas bubbles and the solid particles are mutually influential.

Fluid Flow and Inclusion Behavior Around Spherical-Cap Bubbles

The fluid flow and inclusion behavior around a spherical-cap bubble in molten steel is investigated by the computational model in a two-dimensional axisymmetric domain. Discrete phase model is employed to describe the trajectory of inclusion through the flow field around the bubble, and a stochastic tracking model is adopted to account for the dispersion of inclusions due to turbulence. Increasing the level of turbulence kinetic energy causes the effective attachment radius and the overall attachment probability to increase. Larger inclusions have a larger attachment probability. The wake region grows larger as the turbulence becomes weaker. The entrapment of inclusions by the bubble wake is also considered and the entrapment probability is calculated.

Mass Transfer and Diffusion of a Single Bubble Rising in Polymer Solutions

Based on the planar laser-induced fluorescence with inhibition (PLIF-I) experiments, the mass transfer and diffusion phenomena in the wake of single air bubbles (equivalent diameter ∼ 1–1.4 mm) rising in various aqueous polymer solutions (PAAm: 0.1–0.5 wt %; Breox: 2–9.1 wt %) are investigated. For each fluid medium, liquid-side mass transfer coefficient and the diffusion coefficient were determined and analyzed by considering multiple factors: the rheological properties of the fluid, the concentration of the solute, the hydrodynamics of the bubble, and the contamination effect. The results were compared with cases implemented in nonpolymer solutions to clearly identify the characteristics of mass transfer in the polymer media. In addition, a new method is proposed to determine the diffusion coefficient in the bubble wake. The present research proves that PLIF-I is a promising tool for investigating local gas–liquid mass transfer, even in complex liquid media.

Numerical investigation on bubble size distribution around an underwater vehicle

The interaction of bubbles with flow boundaries has been of high interest for marine engineering; for example, when a propeller is cavitating or air is entrained in the wake of a maneuvering ship, the strong interaction with the boundary layer will lead to the formation of a bubbly wake. To be able to develop the best mitigation strategy, a deep understanding of the associated physics is required. Only a few articles published in open literature address this two-phase fluid system. In the present study, a 3D numerical simulation has been performed to model a bubbly two-phase flow around the DARPA SUBOFF, in which exhaust gas is discharged into the flow around the object to provide a platform for investigating the distribution of the bubbles around a curved body. The two-phase flow is modelled using the Eulerian-Eulerian approach coupled with the MUSIG model. The bubble distribution is characterized based on different gas discharge configurations. It has been found that the boundary layer flow has a strong effect on bubble formation process, particularly encourages the bubble fragmentation. As a result, many small bubbles will be trapped at the aft of the vehicle.

Influence of Breaking Waves and Wake Bubbles on Surface-Ship Wake Scattering at Low Grazing Angles**Supported by the National Natural Science Foundation of China under Grant No 61571355.

A surface-ship wake model is proposed for calculating the scattering of ship wake from a nonlinear sea surface at a high sea state. Ship waves are simulated based on the Kelvin wave model by the point-source method. A Creamer II sea surface based on the Elfouhaily sea spectrum is generated, and breaking waves and foam layer effects are taken into account for the background sea scattering at slight, moderate and high wind speeds. Turbulent bubbles scattering from the ship, which is different from wind-driven bubble breaking, is taken into account with a different concentration distribution using a polynomial fitting function combined with measured data. The surface-ship wake scattering is presented for different wind speeds. Numerical simulations show that ship wake scattering results will be higher when wake bubbles are taken into account. The ship beam is a key parameter that influences the width of the turbulent wake, and results in different scattering characteristics on the scattering image. The wind-induced surface in the presence of breaking waves and whitecaps results in scattering enhancement. This will cause the ship wake signal to be submerged in the back-ground of sea noise, leading to false alarms.

Electron energy optimization by plasma density ramp in laser wakefield acceleration in bubble regime

AbstractEnergy gain of electron beams in bubble regime of the laser wakefield accelerator can be optimized by improving the acceleration length, radial accelerating and focusing force, number of monoenergetic electrons trapped inside the bubble, and increasing dephasing length. In order to enlarge the dephasing length, the phase velocity of the plasma wave can be increased by optimizing the plasma density profile. We report the estimation of dephasing length using plasma density distribution with the flat and linear-upward profile using two-dimensional particle-in-cell simulations. The size of wakefield bubble depends on the plasma density. With a positive plasma density gradient, the size of bubble decreases. The front and trail part of wake bubble will have different phase velocity in plasma density gradient region. After density transition in constant density region, the bubble elongates and the velocity of the back part of the bubble increases so that the accelerated electron phase synchronizes with the phase of the plasma wave. In a result, the electron acceleration length enhances to improve the beam quality.

Flow regimes in gas-liquid-solid mini-fluidized beds with single gas orifice

Flow regimes and their transitions in the gas-liquid-solid mini-fluidized beds (MFBs) with single gas orifice were studied experimentally in this paper. The diameter of the fluidized beds varied from 3 mm to 5 mm and the vertical column height was 50 mm. The ranges of superficial gas velocity and liquid velocity were 1.96 × 10−4–4.73 × 10−3 m/s and 5 × 10−6–4.2 × 10−2 m/s, respectively. The solid particles ranged in size from 50 μm to 300 μm. Half-fluidization, slug, dispersed bubble and transport flow regimes in the three-phase MFBs were identified by analyzing minimum fluidization velocity, pressure drop, and entrainment velocity of solid particles based on experimental data of pressure drop and fluidization velocity as well as flow observations. The effects of solid particle, liquid properties, bed, gas orifice sizes, and static bed height on the flow regimes and transitions were investigated. Results showed that the behavior of gas bubbles and the wall effect affected the flow regimes and transitions. Obvious wall effect increased minimum fluidization liquid velocity in the liquid-solid mini-fluidized beds, which increased the transition liquid velocity from the half-fluidization to slug flow regimes. Moreover, wall effect made Taylor bubbles bigger at lower superficial liquid velocities due to the bubble coalescence and the transition boundaries expansion. Solid particles aggregation, wall effect and bubble wake behavior were responsible for increment of the minimum entrainment velocity of solid particles. Flow regime maps of the gas-liquid-solid MFBs were presented and correlations were suggested for the flow regime boundaries.

Numerical study on dynamics of single bubble rising in shear-thinning power-law fluid in different gravity environment

The bubble dynamics in non-Newtonian fluids play an important role in industrial processes, such as petroleum process, fermentation, wastewater treatment, boiling heat transfer, etc. To understand the bubble dynamics, the single bubble rising in the shear-thinning non-Newtonian fluid is detailedly studied with the volume of fluid (VOF) method, in which the continuous surface tension model and the Carreau model are used to calculate surface tension and liquid viscosity, respectively. Studies are performed on the influence of the rheological index, the Eötvös and Galilei numbers on bubble shape, wake characteristic, terminal velocity and liquid apparent viscosity. The present results show that, the bubble deformation and the circulating region in the bubble wake increase with increasing the Eötvös and Galilei numbers or with decreasing the rheological index. With the decrease of the rheological index, one confined region with the high viscosity exists in the bubble rear, and this region gradually detaches from the bubble rear. The detachment becomes fast with the increase of the Galilei number. In addition, the bubble terminal velocity depends on the bubble shape, the shear-thinning effect of liquid and the gravity level.

Experimental Analysis of a Bubble Wake Influenced by a Vortex Street

Bubble column reactors are ubiquitous in engineering processes. They are used in waste water treatment, as well as in the chemical, pharmaceutical, biological and food industry. Mass transfer and mixing, as well as biochemical or chemical reactions in such reactors are determined by the hydrodynamics of the bubbly flow. The hydrodynamics of bubbly flows is dominated by bubble wake interactions. Despite the fact that bubble wakes have been investigated intensively in the past, there is still a lack of knowledge about how mass transfer from bubbles is influenced by bubble wake interactions in detail. The scientific scope of this work is to answer the question how bubble wakes are influenced by external flow structures like a vortex street behind a cylinder. For this purpose, the flow field in the vicinity of a single bubble is investigated systematically with high spatial and temporal resolution. High-speed Particle Image Velocimetry (PIV) measurements are conducted monitoring the flow structure in the equatorial plane of the single bubble. It is shown that the root mean square (RMS) velocity profiles downstream the bubble are influenced significantly by the interaction of vortices. In the presence of a vortex street, the deceleration of the fluid behind the bubble is compensated earlier than in the absence of a vortex street. This happens due to momentum transfer by cross-mixing. Both effects indicate that the interaction of vortices enhances the cross-mixing close to the bubble. Time series of instantaneous velocity fields show the formation of an inner shear layer and coupled vortices. In conclusion, this study shows in detail how the bubble wake is influenced by a vortex street and gives deep insights into possible effects on mixing and mass transfer in bubbly flows.

Numerical Investigation of Flow Structure and Heat Transfer Produced by a Single Highly Confined Bubble in a Pressure-Driven Channel Flow

A numerical investigation of a single highly confined bubble moving through a millimeter-scale channel in the absence of phase change is presented. The simulation includes thermal boundary conditions designed to match those of completed experiments involving bubbly flows with large numbers of bubbles. The channel is horizontal with a uniform-heat-generation upper wall and an adiabatic lower boundary condition. The use of a Lagrangian framework allows for the simulation of a channel of arbitrary length using a limited computational domain. The liquid phase is a low-Reynolds-number laminar flow, and the phase interactions are modeled using the volume-of-fluid (VOF) method with full geometric reconstruction of the liquid/gas interface. Results are presented for three bubble diameters, which include two levels of confinement within the channel and two liquid flow rates. Bubble shape and speed closely match experimental observations for each bubble size and liquid flow rate. Nusselt numbers in the bubble wake for all configurations follow a power law relationship with distance behind the bubble. Important dynamical structures include a pair of vortical structures at the rear of the bubble associated with the primary heat transfer enhancement and a pair of prominent liquid jets oriented in the transverse direction on either side of the bubble.

Dual emission LIF technique for pH and concentration field measurement around a rising bubble

Mass transfer plays an important role in chemical engineering applications involving multiphase systems. Several techniques have been developed in the field to measure the global mass concentration in multiphase chemical reactors. However, because of the complexity of multiphase processes, only few techniques are able to provide local and quantitative information. In this study a dual-emission Laser Induced Fluorescent technique (LIF) has been developed to locally measure the mass transfer of a rising CO2 bubble in a quiescent fluid. Details of the pH field with an overall precision of ±0.04 pH units, dissolved CO2 concentration, and the mass transfer evolution in the bubble wake can now be measured quantitatively. It is capable to provide quantitative comparison with simulation results and as such experimental validation of multiphase CFD models.

Discrete element method simulation of the mixing process of particles with and without cohesive interparticle forces in a fluidized bed

The discrete element method (DEM) is used to simulate two types of mixing processes (radial mixing and axial mixing) of particles with and without cohesive interparticle forces in a two-dimensional (2D) fluidized bed. By using the Ashton index, the mixing degree is quantified and the differences between the mixing characteristics of particles with and without cohesive interparticle forces are compared. The cohesive interparticle forces, which impair bubble behaviors, can apparently weaken the mixing process and prolong the mixing time. This effect increases with increasing cohesive interparticle force. Radial mixing, which is mainly dependent on the convective mixing caused by bubbles, is relatively sensitive to cohesive interparticle force and shows a long mixing time, while axial mixing, which is mainly induced by the bubble wake, is less sensitive to cohesive interparticle force and presents a fast mixing process. Finally, to weaken the effect of cohesive interparticle force, an uneven air distribution is introduced, which greatly improves the mixing process by changing the bubble movement. This method may be helpful for the reduction of particle agglomeration in high-temperature fluidized beds.