Gas Bubbles In Liquid Research Articles

On the dynamics of instabilities in two-fluid models for bubbly flows

In this paper we look at instabilities in bubbly gas-liquid flows and investigate the emergence and characteristics of phase heterogeneities. We apply a shared-pressure two-fluid model to low bubble loadings and demonstrate the existence of persistent gas fraction instabilities of a characteristic size larger than the applied computational grid. In particular, we investigate the influence of a virtual mass effect on the stability of the two-fluid model and we demonstrate a change in the emergence and the dynamics of the phase heterogeneities. The change is accounted to a difference in the degree of hyperbolicity due to the inclusion of the virtual mass force. Furthermore, the results indicate that an initial instability, concluded as numerical in its character, evolves into a state with a physical character of the heterogeneities. We discuss implications of the existence and dynamics of the heterogeneities and the importance of the numerical behavior for interpretation of the results. In particular, we argue that underlying characteristics of the model cannot and should not be concealed with additional sub-models (such as momentum exchange terms) but must be acknowledged in the analysis of results from the two-fluid model for bubbly flows.

Measurement method of bubble behavior by optical probe in gas-liquid bubble column reactor

Measurement method of bubble behavior by optical probe in gas-liquid bubble column reactor

Hydrodynamic interaction of weakly nonspherical gas bubbles in a liquid in the three-dimensional problem statement

The influence of the relative location of weakly nonspherical gas (air) bubbles in liquid (water) on their hydrodynamic interaction at the pressure antinode of an ultrasonic standing wave is studied. Three essentially different configurations of the mutual arrangement of the bubbles are considered: the linear, planar, and spatial ones. In the linear configuration, three bubbles are located on one straight line. In the planar configuration, five bubbles are situated on two mutually orthogonal straight lines. In the spatial configuration, seven bubbles are located on three mutually orthogonal straight lines. In all the cases, one of the bubbles is central, the others are equally-spaced from it on those lines. All the bubbles are initially spherical with a radius of 3 ?m. Room conditions are considered, the frequency of the wave is 20 kHz, its amplitude is 1.2 times the static liquid pressure. A mathematical model is used, in which the liquid is assumed weakly compressible, the bubbles being homobaric. It is shown that in each configuration of the bubbles the influence of their interaction on their radial dynamics is insignificant. In each configuration, the central bubble remains stationary, while the others move to it at a speed equal for a fixed configuration. At transition from a linear configuration to a planar one and from a planar configuration to a spatial one, the bubble motion speed increases. In all the cases, the deformations of the central stationary bubbles are smaller than those of the rest. Among the central bubbles, the bubble in a planar configuration is deformed most.

Dynamics of two gas bubbles in liquid in an ultrasonic traveling wave

The influence of the liquid viscosity and compressibility on the dynamics of two air bubbles (with equilibrium radii of 5 μm) in water at room conditions under the action of a plane ultrasonic wave traveling along the line of the bubble centers (the wavelength is 5000 μm, the amplitude is 0.3 bar) is studied. The initial distance between the centers of the bubbles is six bubble radii. A mathematical model is used, which is fourth-order accurate in terms of the ratio of the radius of the bubbles to the distance between them. It is shown that the spatial displacements of the bubbles are determined mainly by their hydrodynamic interaction. The influence of the liquid viscosity and compressibility is generally significant, and the viscosity affects much more. Without account of the liquid viscosity and compressibility, the bubbles collide with each other after the action of 4.5 running-wave lengths. With taking into account the liquid compressibility, the bubbles under the same action remain remote at a distance on the order of their equilibrium radii, while with additionally allowing for the liquid viscosity, their spacing is kept close to the initial one.

Experimental investigation into the drag volume fraction correction term for gas-liquid bubbly flows

Estimates have been made using data from an instrumented pilot-scale bubble column of the volume fraction correction term over a range of local volume fractions (0.03–0.38) and mean bubble diameters (4–10mm) for Bubble Size Distributions (BSDs) of varying ‘broadness’. The value of this correction term was found to depend on both the mean bubble diameter and the dispersity of the BSD, not simply on the local volume fraction. Using the available experimental data, no hindered bubble rise was observed. The results from this study should assist in the development of more accurate predictive CFD models of bubble columns and other gas-liquid systems operated at high local volume fractions.

The effect of electrolyte concentration on counter-current gas–liquid bubble column fluid dynamics: Gas holdup, flow regime transition and bubble size distributions

Electrolytes are well-known to suppress the coalescence, stabilize the homogeneous flow regime and increase the gas holdup; however, there is a lack of studies concerning the influence of electrolyte concentration on counter-current bubble column fluid dynamics. In this paper, we contribute to the existing discussion by experimentally investigating the effect of electrolyte concentration (sodium chloride, NaCl) on gas holdup, flow regime transition and bubble size distributions in a large-diameter and large-scale counter-current bubble column (5.3m in height and 0.24m inner diameter). We considered gas superficial velocities in the range of 0.004–0.20m/s and liquid superficial velocities up to −0.06m/s. Air was used as the dispersed phase and various aqueous solutions of NaCl—up to the critical concentration—were used as the liquid phase. The gas holdup measurements were used to investigate the bubble column fluid dynamics and to analyze the flow regime transition. The image analysis was applied, near the sparger and in the developed region, to study the bubble size distributions in batch and in counter-current modes. The presence of NaCl—up to a critical concentration—stabilizes the homogeneous flow regime, increases the gas holdup and shifts the bubble size distribution toward lower bubble diameters. The counter-current mode destabilizes the homogeneous flow regime when using tap water and stabilizes the homogeneous flow regime when using aqueous solution of NaCl. The results suggest that the changes in the bubble size distributions stabilize/destabilize the homogeneous flow regime and, thus, increase/decrease the gas holdup.

On-line continuous generation of zinc chelates in the vapor phase by reaction with sodium dithiocarbamates and determination by atomic fluorescence spectrometry

The present study shows for the first time that a volatile zinc chelate species can be generated by the on-line continuous merging of an acidified sample solution with an aqueous sodium diethyldithiocarbamate solution followed by rapid separation using a frit-based bubble gas-liquid separator at room temperature. The operating conditions for the generation of the vaporous zinc chelate were preliminarily investigated by non-dispersive atomic fluorescence spectrometry. The possible mechanism of zinc vapor generation is discussed. The study shows that the volatile species is an intermediate species with very similar properties to diethyldithiocarbamic acid and a very short half-life in the acidic solution. Moreover, this species can only be generated by on-line mixing and rapid separation. The efficiency of generation was 33–85% depending on acidity. Under optimal conditions, the flow rates of the sample and Na-DDTC solution were 1.3mlmin−1, the carrier argon flow rate was 225mlmin−1, the acid concentration of the sample solution and the concentration of Na-DDTC were 0.05M and 0.4% (m/v), respectively, the detection limit of zinc was 0.33 (3σ) ngml−1, and the relative standard deviation (RSD) was 1.3%. The accuracy of the method was verified by the determination of zinc in the plant reference materials GBW10015 (spinach) and GBW10045 (rice). The results were in good agreement with the certified reference values.

Ozonation of acid red 18 wastewater using O3/Ca(OH)2 system in a micro bubble gas-liquid reactor

The ozonation of acid red 18 (AR18) wastewater was investigated in an integrated process consisting of O3/Ca(OH)2 system and a newly-developed micro bubble gas-liquid reactor. The effects of operating parameters such as Ca(OH)2 dosage, reactor pressure, liquid phase temperature, initial dye concentration and inlet ozone concentration on decolorization and mineralization (TOC removal) were studied in order to know the ozonation performance of this new integrated process. The decolorization and TOC removal efficiency increased with increasing inlet ozone concentration and increasing Ca(OH)2 dosage before 2g/L, as well as decreasing initial dye concentration. The optimum Ca(OH)2 dosage should exceed Ca(OH)2 solubility in liquid phase. The reactor pressure and liquid phase temperature have little effects on the decolorization and TOC removal efficiency. When Ca(OH)2 dosage exceeded 3g/L, the decolorization and TOC removal of AR 18 almost reached 100% at 6 and 25min, respectively. The intensification mechanism of Ca(OH)2 assisted ozonation was explored through the determination of CO32− and SO42− ions formed in the liquid phase and analysis of the precipitated substances. The mechanism for Ca(OH)2 intensified mineralization of dye solution is the simultaneous removal of CO32− ions, as hydroxyl radical scavengers, due to the presence of Ca2+ ions. Results indicated that the proposed new integrated process is a high efficient ozonation process for persistent organic wastewater treatment.

An analytical model of gas content in fluids considering the distribution of bubble radius

Gas bubbles in liquid can change the properties of liquid, such as density, viscosity and compressibility. Therefore, to predict the liquid properties, it is essential to evaluate the gas content, which depends on the bubble radius and its size distribution. This paper presents an analytical model of gas content with consideration of the distribution of bubble radius. A steady state formula of bubble radius is presented based on force equilibrium of bubble interface. From the experimental data, the distribution of bubble radius is found to agree with the logarithmic normal distribution, so a corresponding distribution function is proposed. Then, both the gas content and the average bubble radius are derived from the distribution function. Specific tests are performed to validate the developed model particularly as concerns the change of bubble radius with respect to the fluid pressure. Combined with the steady state model of bubble radius, the presented analytical model allows an accurate prediction of gas content at different pressures, implying its potentials to describe the fluid properties in two-phase flows. Copyright © 2016 John Wiley & Sons, Ltd.

Interfacial area transport due to shear collision of bubbly flow in small-diameter pipes

In view of the practical importance of the interfacial area transport equation in flow analysis based on the two-fluid model, especially for flow analysis in a reactor core which has a relatively small channel configuration, the dominant mechanism in the interfacial area transport in small diameter pipes was firstly identified by evaluating the experimental data obtained for pressurized bubbly flows under microgravity, where the source and sink terms of the interfacial area concentration due to bubble expansion and wake entrainment collision are negligible. The sink term of the interfacial area concentration of bubbly flow in small diameter pipe was modeled in terms of the shear collision mechanism. The one-dimensional interfacial area transport equation with the developed sink term was evaluated by using 74 datasets obtained for adiabatic gas-liquid bubbly flows, including upward flows and microgravity flows in pipes with inside diameters of 9.0, 5.0 and 3.0mm. A detailed discussion was provided on the interfacial area transport mechanism in small-diameter pipe. The overall standard deviation of the absolute error between predicted and measured interfacial area concentrations was 11.8%. The present model will reproduce the interfacial area transport of bubbly two-phase flows in small-diameter pipes with inside diameters of 3.0–9.0mm, at superficial liquid and gas velocities over the ranges 〈jf〉=0.073–0.949m/s, and 〈jg〉=0.00434–0.0500m/s, and also over a wide range of axial distances, z/D=5–188.

Nonequilibrium all-atom molecular dynamics simulation of the bubble cavitation and application to dissociate amyloid fibrils.

The cavitation of gas bubbles in liquids has been applied to different disciplines in life and natural sciences, and in technologies. To obtain an appropriate theoretical description of effects induced by the bubble cavitation, we develop an all-atom nonequilibrium molecular-dynamics simulation method to simulate bubbles undergoing harmonic oscillation in size. This allows us to understand the mechanism of the bubble cavitation-induced liquid shear stress on surrounding objects. The method is then employed to simulate an Aβ fibril model in the presence of bubbles, and the results show that the bubble expansion and contraction exert water pressure on the fibril. This yields to the deceleration and acceleration of the fibril kinetic energy, facilitating the conformational transition between local free energy minima, and leading to the dissociation of the fibril. Our work, which is a proof-of-concept, may open a new, efficient way to dissociate amyloid fibrils using the bubble cavitation technique, and new venues to investigate the complex phenomena associated with amyloidogenesis.

Numerical simulaion of dynamics of a gas bubble in liquid near a rigid wall during its growth and collapse

A numerical technique of calculating the dynamics of a cavitation bubble near a plane rigid wall is presented. The bubble at its collapse can become toroidal. The liquid is assumed inviscid and incompressible, its flow being potential. The bubble surface movement is determined by the Euler method, the normal component of the liquid velocity on the bubble surface is found by the boundary element method. The technique also includes an algorithm for calculating the velocity and pressure fields in the liquid. The convergence of the numerical solution with refining the temporal and spatial steps is demonstrated. The results of the present technique are compared with some known numerical and experimental data by other authors, their satisfactory agreement is found. To illustrate the capabilities of the present technique the process of growth and collapse of a bubble in water near a wall is considered. The liquid pressure contours in the stage of the bubble collapse are given and the radial liquid pressure profiles on the wall and at a small distance from the wall where the liquid pressure is maximum are shown.

Low-frequency approximations for the radiation damping of a bubble

The recent work by Ainslie and Leighton [J. Acoust. Soc. Am. 126, 2163 (2009)] pointed out that there exist two different expressions in the literature for the scattering cross-section of a spherical gas bubble in liquid. The difference between the two expressions is contained in the term corresponding to the losses due to acoustic radiation. The more common expression, often attributed to R. Wildt [“Acoustic theory of bubbles,” in Physics of Sound in the Sea], is based on a low-frequency approximation and is thus only accurate when the wavelength of the incident wave is large compared to the circumference of the bubble. The purpose of the present work is to investigate specifically the accuracy of this low-frequency approximation for various physical parameters and frequencies of interest. An alternative low-frequency approximation is also suggested, which is both simpler and more accurate than the present approximation. Finally the case of a nonspherical bubble is considered, and an expression for the scattering cross-section is given that is based on the same low-frequency assumptions made by Wildt. [Work supported by the IR&D program at ARL:UT.]

Theoretical prediction of mass transfer coefficients in both gas–liquid and slurry bubble columns

The gas–liquid contact time has been defined in a new way (bubble surface-to-rate of surface formation) and the range of applicability of the penetration theory in both gas–liquid and slurry bubble columns has been examined. In both reactors, the mass transfer coefficients were predicted successfully not only in the homogeneous regime but also in the heterogeneous regime (superficial gas velocities up to 0.08ms−1).The results in the article demonstrate the importance of the geometrical characteristics (length and height) of the oblate ellipsoidal bubbles for the accurate calculation of the contact time and thus the volumetric liquid-phase mass transfer coefficient kLa. The gas–liquid interfacial area has been calculated in both reactors in the classical way, i.e. as a function of the gas holdup and inversely proportional to the Sauter-mean bubble diameter. It was found that in the gas–liquid bubble column (0.095m in ID) the modified penetration theory was applicable to tap water, 9 organic liquids (decalin, nitrobenzene, 2-propanol, 1,4-dioxane, ethanol (99%), tetralin, xylene, 1,2-dichloroethane, ethylene glycol) and two liquid mixtures (water-glycol and tetralin-ethanol). Tetralin was aerated with both nitrogen and helium, whereas xylene was aerated with hydrogen and helium. The correction factor introduced by Calderbank (1967) was found useful for improving the kLa predictions in 1,2-dichloroethane, ethanol (99%), xylene(-hydrogen) and toluene–ethanol 97.2%. In the case of a slurry bubble column, the new approach was found applicable (at low solids concentrations) to four different gas–liquid–solid systems: air–tetralin–Al2O3, air–water–Al2O3, air–water-activated carbon and air–Na2SO4–kieselguhr. It is noteworthy that in some cases (air–water-Al2O3) the new definition of the contact time was found applicable up to solids concentrations of 6.29%. In the case of a slurry bubble column, it was found that when the theoretical kLa value is multiplied by the inverse value of the correction factor the predictions improve with about 5%.Finally, in the slurry bubble column the contact time was defined on the basis of the length of the micro-eddies and the kLa values in both air–water-alumina and air–water-activated carbon systems were successfully predicted. This is also a potentially good approach.

The necking time of gas bubbles in liquids of arbitrary viscosity

We report an experimental and theoretical study of the collapse time of a gas bubble injected into an otherwise stagnant liquid under quasi-static conditions and for a wide range of liquid viscosities. The experiments were performed by injecting a constant flow rate of air through a needle with inner radius a into several water/glycerine mixtures, providing a viscosity range of 20 cP ≲ μ ≲ 1500 cP. By analyzing the temporal evolution of the neck radius, R0(t), the collapse time has been extracted for three different stages during the collapse process, namely, Ri/a = 0.6, 0.4, and 0.2, being Ri = R0(t = 0) the initial neck radius. The collapse time is shown to monotonically increase with both Ri/a and with the Ohnesorge number, Oh=μ/ρσRi, where ρ and σ represent the liquid density and the surface tension coefficient, respectively. The theoretical approach is based on the cylindrical Rayleigh-Plesset equation for the radial liquid flow around the neck, which is the appropriate leading-order representation of the collapse dynamics, thanks to the slenderness condition R0(t) r1(t) ≪ 1, where r1(t) is half the axial curvature of the interface evaluated at the neck. The Rayleigh-Plesset equation can be integrated numerically to obtain the collapse time, τcol, which is made dimensionless using the capillary time, tσ=ρRi3/σ. We present a novel scaling law for τcol as a function of Ri/a and Oh that closely follows the experimental data for the entire range of both parameters, and provide analytical expressions in the inviscid and Stokes regimes, i.e., τcol(Oh→0)→2lnC and τcol(Oh → ∞) → 2Oh, respectively, where C is a constant of order unity that increases with Ri/a.

Target strength observations of wobbly bubbles

Large methane bubbles released from shallow water seabed are of interest because bubbles facilitate the transport process of gas through their journey in the water column, making them more susceptible to reaching the atmosphere than those released from deep water. Several models that relate bubble radius and acoustic backscattering for ideal, spherical bubbles have been developed and are typically used by researchers attempting to invert acoustic backscatter measurements for bubble size and/or gas flux. However, based on theory and field experimentation, it has been shown that large free gas bubbles in liquids (large Eotvos and/or large Reynolds numbers) are non-spherical. A shallow water (<6 m) tank experiment has been designed to understand the influence of bubble shape on acoustic backscattering. This experiment was conducted for bubbles between 1 and 5 mm in radius (Eotvos number between 0.5 and 13, Reynolds numbers between 300 and 1500, and a Morton number of approximately 7.2 × 10−11) and at frequen...

Influence of internals on counter-current bubble column hydrodynamics: Holdup, flow regime transition and local flow properties

Bubble columns are frequently studied without considering internals (open tube bubble columns). However, in most industrial applications, internal devices are often added to control heat transfer, to foster bubble break-up or to limit liquid phase back mixing. These elements can have significant effects on the multiphase flow inside the bubble column reactor and the prediction of these effects is still hardly possible without experimentation. In this paper, we study experimentally a counter-current gas–liquid bubble column in the open tube and annular gap configurations. In the annular gap bubble column, two vertical internal tubes are considered. The column has an inner diameter of 0.24m, and the global and local hydrodynamic properties are studied using gas holdup measurements and a double-fiber optical probe. The gas holdup measurements are compared with the literature and used to investigate the flow regime transition. A double-fiber optical probe is used to acquire midpoint data and radial profiles of the local properties to study the flow properties and to further investigate the flow regime transition. The counter-current mode is found to increase the holdup, decrease the bubble velocity and cause regime transition at lower superficial gas velocity. The holdup curves in the annular gap and open tube configurations are similar in shape and values, suggesting that the presence of internals has a limited influence on the global hydrodynamic. In addition, it is found that the presence of the internals stabilizes the homogeneous regime in terms of transition gas velocity and holdup.

Magnetic field induced motion behavior of gas bubbles in liquid.

The oxygen evolution reaction generally exists in electrochemical reactions. It is a ubiquitous problem about how to control the motion of oxygen bubbles released by the reaction. Here we show that oxygen bubbles during oxygen evolution reaction exhibit a variety of movement patterns in the magnetic field, including directional migration and rotational motion of oxygen bubbles when the magnet in parallel with the electrode, and exclusion movement of oxygen bubbles when the magnet perpendicular to the electrode. The results demonstrate that the direction of oxygen bubbles movement is dependent upon the magnet pole near the electrode, and the kinetics of oxygen bubbles is mainly proportional to intensity of the electromagnetic field. The magnetic-field induced rotational motion of oxygen bubbles in a square electrolyzer can increase liquid hydrodynamics, thus solve the problems of oxygen bubbles coalescence, and uneven distribution of electrolyte composition and temperature. These types of oxygen bubbles movement will not only improve energy saving and metal deposition for energy storage and metal refinery, but also propel object motion in application to medical and martial fields.

CFD simulation of bubbly turbulent Tayor–Couette flow

Bubbly gas–liquid Taylor–Couette vortex flow has been the subject of several recent investigations both because of interest in bubble-induced drag reduction and because such devices have potential applications to a variety of chemical and biochemical processing problems. In order to quantitatively describe the hydrodynamics of highly turbulent two phase Taylor–Couette flow, a rigorous two-fluid computational fluid dynamics (CFD) model was developed and compared with previously published experimental data. This model includes a comprehensive description of the constitutive closure for inter-phase forces and turbulence was simulated using both the k–ε and k–ω models. In addition, the mechanism by which the dispersed fluid attains a non-uniform radial and axial distribution is analyzed and the relative importance of various interphase forces is discussed. Lastly the model was validated by comparison of simulation predictions with experimental data, and it is shown that the CFD model correctly predicts phase velocity, velocity fluctuation, and gas distribution, and may provide guidance for reactor design and scale-up.

Simulation of Convective Heat Transfer of Gas–Liquid Bubble Train Flow in Wet Microtube

In this paper, a direct numerical simulation of a two‐phase incompressible gas–liquid flow for simulation of bubble motion and convective heat transfer in a microtube is presented. The microtube radius is 10 μm. The interface between the two phases is tracked by the volume of fluid method with the continuous surface force model. Newtonian flows are solved using a finite volume scheme based on the PISO algorithm. Numerical simulation is done on an axisymmetric domain with a periodic boundary condition for different values of pressure gradient, void fraction, and bubble period. Mean pressure gradient is fixed for each simulation. The superficial Reynolds numbers of gas and liquid phases studied are 0.3 to 7 and 5 to 210, respectively. Numerical results are coincident with the Serizawa regime map, and there is a linear relation between the void fraction and gas flow ratio. Simulation shows local Nusselt number increases in the presence of a gas bubble.