Change In Bubble Shape Research Articles

Behavior of bubble with high density ratio in a microchannel with asymmetric obstacles

Bubbles are existent everywhere and of great importance for the daily life and industry process, such as heat exchange rate influenced by bubbles in the tube, battery life partially decided by bubbles of chemical reaction in it, etc. With the further requirement for miniaturization, physical mechanisms behind bubble behaviors in microchannels become crucial. In the present work, the lattice Boltzmann method is used to investigate the behavior of bubbles as they rise in complex microchannels under the action of buoyancy. The channel is placed with two asymmetric obstacles on its left and right side. Initially, the lattice Boltzmann model is tested for its reliability and accuracy by Laplace law. Then a few parameters of flow field, i.e. the Eötvös number, the viscosity ratio, the vertical distance between the obstacles, the horizontal distance between the obstacles, are employed to study the characteristics of the bubble during the movement, including the deformation, the rising speed, the residual mass, and the time of bubble passing through the channel. The results are shown below. First, the trend of the bubble's velocity changing with time in the process of passing through the channel corresponds to the change process of the dynamic behavior of the interface, i.e. the bubble velocity decreases when the bubble shape changes significantly under the same channel width. Second, with the increase of <inline-formula><tex-math id="M1">\begin{document}$ Eo $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20211328_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20211328_M1.png"/></alternatives></inline-formula> number, the bubble deformation as well as the bubble velocity increases and the bubble residual mass decreases. Besides, the gas-to-liquid viscosity ratio has a significant effect on the bubble velocity. Under the condition of high viscosity ratio, the bubble shape is difficult to maintain a round shape, while the bubble rise velocity increases and the residual mass of the bubble decreases with the viscosity ratio. What is more, when the obstacle setting is changed, the longer the vertical distance between the two asymmetric obstacles, the shorter the bubble passing time is, and the faster it will return to the original shape after passing through the obstacle, while the residual mass of the bubble shows a change trend of approximately unchanged-increase-decrease-increase with the augment of the vertical distance between the obstacles. In the study of changing the horizontal spacing, two cases: the two obstacles are changed at the same time (Case A) and only the one-sided obstacle is changed (Case B), are considered. The results show that under the same small horizontal interval, the obstruction effect caused by changing only the length of one side obstacle is stronger. Finally, the study shows that when the width of the right obstacle is long enough, although the width of the obstacle continues to increase, the passing time of the bubble increases slowly, and the position of the bubble leaving from the obstacle is always approximately the same.

Experimental and theoretical bubble growth comparison at the initial stages of horizontal injection

Two-phase liquid-gas injection constitutes an important industrial process that is used in most separators. At the early step of injection, a cylindrical bubble is formed. As time elapses, the bubble shape becomes more complex and very difficult to analyze. In this study, a simple analytical model is developed to explain bubble shape changes. The analytical model was developed based on water flow inertia that continually pushes the bubble while the drag force resists it so that the frontal area of the bubble increases. The bubble size and frontal area were estimated using the assumption of the equilibrium between inertia force and drag force neglecting viscous force. From the estimation, the role of the vortex ring from the difference between theoretical and experimental results can be identified. The analytical model was verified through experimental data collected on the shape deformation induced by bubble motion at the beginning of injection. The experimental data used as verification were measured from the bubble nose image with ten times repetition having the uncertainty of ±6 %. The experimental method is conducted by injecting a bubble along the horizontal direction into a water pool. The inertial force of the water flow in front of the bubble nose generates the bubble. The bubble suddenly changes its shape, moves in the form of a bubble jet, and undergoes gradual shape changes. The frontal area of the bubble increases and reaches a maximum at the terminal velocity point. The bubble shape deformation is affected by the inertial force of the water flow that pushes the bubble forward. Accordingly, the bubble changes its shape from cylindrical to spherical, and then to an ellipsoidal disk. When the bubble attains terminal velocity, the inertial force becomes equal to the drag force. The edge of the ellipsoidal disk bubble exhibits increased surface tension. The difference between experimental data and the analytical model is due to the complex fluid and dynamic flow surrounding the bubble. The mathematical framework proposed in this work is envisaged to be an important tool for the prediction of the bubble frontal area

Understanding the Influence of Rheological Properties of Shear‐Thinning Liquids on Segmented Flow in Microchannel using CLSVOF Based CFD Model

AbstractIn this study, two phase gas‐shear‐thinning liquid flow in a square microchannel is numerically investigated using the coupled level set and volume of fluid (CLSVOF) methods. A systematic investigation is carried out to explore the influence of polyacrylamide (PAM) concentration, surface tension, velocity ratios, and contact angle on the gas slug length, volume, unit cell length, and pressure drop. Three different concentrations of PAM solutions, which exhibit shear‐thinning behaviour are considered as the continuous phase. Gas slug length, volume, and unit cell length decreased with increasing the PAM concentration. Velocity and non‐homogeneous viscosity distributions in the liquid slug for three different PAM concentration solutions are reported. Gas slug length decreases with an increase of the contact angle and the bubble shape change from convex to concave. This numerical work provides the fundamental insights in segmented flow formation and two‐phase flow characteristics comprising shear‐thinning liquids.

Dynamical transition in a jammed state of a quasi-two-dimensional foam.

The states of foam are empirically classified into dry foam and wet foam by the volume fraction of the liquid. Recently, a transition between the dry foam state and the wet foam state has been found by characterizing the bubble shapes [Furuta et al., Sci. Rep. 6, 37506 (2016)2045-232210.1038/srep37506]. In the literature, it is indirectly ascertained that the transition from the dry to the wet form is related to the onset of the rearrangement of the bubbles, namely, the liquid fraction at which the bubbles become able to move to replace their positions. The bubble shape is a static property, and the rearrangement of the bubbles is a dynamic property. Thus, we investigate the relation between the bubble shape transition and the rearrangement event occurring in a collapsing process of the bubbles in a quasi-two-dimensional foam system. The current setup brings a good advantage to observe the above transitions, since the liquid fraction of the foam continuously changes in the system. It is revealed that the rearrangement of the bubbles takes place at the dry-wet transition point where the characteristics of the bubble shape change.

Experimental study of bubble dynamics of isolated bubbles in water pool boiling at subatmospheric pressures

The purpose of this work is to expose specific features of bubble dynamics in water pool boiling at low pressure in comparison with the atmospheric pressure. At subatmospheric pressures, the boiling environment is known to be very non-homogeneous, both in terms of pressure and subcooling degree. As a consequence, thermophysical properties of the fluid are not homogeneous either. These non-homogeneities are supposed to be responsible for the particular shape, size and departure frequency of bubble observed at low pressure.Experiments of water pool boiling were conducted at saturation state from atmospheric pressure (Tsat=100°C) down to 4.2kPa (Tsat=30°C) with a single imposed heat flux at the heater. Videos of each experiment were recorded with a high speed camera in order to analyze the bubble dynamics for different pressures.Increases of the bubble size and of the detachment frequency are observed as the pressure decreases while the bubble shape changes: a near-spherical shape at atmospheric pressure to an oblate spheroidal shape in the lowest investigated pressure range through a mushroom shape and/or vapor columns due to the rapid growth of successive bubbles sucked into the wake of the first one in the range of intermediate pressures. Recondensation of bubbles occurs rapidly after departure because of the subcooling degree. A home-made image processing software allowed calculating some quantitative parameters of the bubble dynamics such as the instantaneous volume and the departure frequency from the video records.

Measurement of sliding bubble behavior on a horizontal heated tube using a stereoscopic image processing technique

The significance of the sliding bubble effect on boiling heat transfer has been reported by a number of precedent studies through experiments and numerical analyses. However, the experimental data and mechanistic boiling heat transfer models considering sliding bubbles on horizontal tube outer surfaces are insufficient. In the present study, therefore, the behavior of sliding bubbles on a horizontal tube heater is measured at nearly saturation temperature and atmospheric pressure for various flow conditions; liquid velocity (14.7–27.6mm/s), wall heat flux (67–129kW/m2), and the location of nucleation site (45° and 90° from the bottom of the heater). A thin film heater with a narrow heating strip was proposed to generate boiling bubbles in a restricted region on a horizontal heater. This improved the quality of the visualization of the sliding bubbles. Two synchronized high-speed cameras were used to capture the behavior of the bubbles from two perpendicular measurement angles. The configurations of the bubbles were identified by an image processing method based on shadowgraphy from the two images, and the results of the image processing elicited various boiling bubble parameters, such as the bubble diameter transient, bubble velocity, bubble frequency, etc. In particular, the volume of the nonspherical bubble was measured by a three-dimensional reconstruction method which defines a specific cross-section configuration at each elevation using stereoscopic images. Two verification steps confirmed that the reconstruction method has allowable errors and also indicated the limitation of the monoscopic visualization method for deformed bubble measurement. According to the result of this measurement, the location of the nucleation site is a principal factor affecting the life cycle of a sliding bubble on a horizontal tube. Under the present experimental conditions, a bubble generated on the lower half of the tube showed the lift-off from the heating surface twice during its life cycle; the first is due to the contact pressure force and the inertia of the surrounding liquid caused by bubble shape change shortly after the nucleation, and the second is mainly due to the lift force after being accelerated by the sliding motion. In contrast, bubbles occurring at the side of the tube did not show sliding motion, but rather rose without confinement by the wall after the lift-off. The effects of the bulk liquid velocity and wall heat flux on bubble behavior were investigated and the results are discussed in this paper. This experimental observation can be used to improve understanding of the boiling heat transfer mechanism on a horizontal tube and the bubble dynamics of a sliding bubble on a downward heating surface.

A Simple Parameterization for the Rising Velocity of Bubbles in a Liquid Pool

The determination of the shape and rising velocity of gas bubbles in a liquid pool is of great importance in analyzing the radioactive aerosol emissions from nuclear power plant accidents in terms of the fission product release rate and the pool scrubbing efficiency of radioactive aerosols. This article suggests a simple parameterization for the gas bubble rising velocity as a function of the volume-equivalent bubble diameter; this parameterization does not require prior knowledge of bubble shape. This is more convenient than previously suggested parameterizations because it is given as a single explicit formula. It is also shown that a bubble shape diagram, which is very similar to the Grace's diagram, can be easily generated using the parameterization suggested in this article. Furthermore, the boundaries among the three bubble shape regimes in the Eo–Re plane and the condition for the bypass of the spheroidal regime can be delineated directly from the parameterization formula. Therefore, the parameterization suggested in this article appears to be useful not only in easily determining the bubble rising velocity (e.g., in postulated severe accident analysis codes) but also in understanding the trend of bubble shape change due to bubble growth.

Volume-of-fluid simulations of bubble dynamics in a vertical Hele-Shaw cell

Bubbles in confined geometries serve an important role for industrial operations involving bubble-liquid interactions. However, high Reynolds number bubble dynamics in confined flows are still not well understood due to experimental challenges. In the present paper, combined experimental and numerical methods are used to provide a comprehensive insight into these dynamics. The bubble behaviour in a vertical Hele-Shaw cell is investigated experimentally with a fully wetting liquid for a variety of gap thicknesses. A numerical model is developed using the volume of fluid method coupled with a continuum surface force model and a wall friction model. The developed model successfully simulates the dynamics of a bubble under the present experimental conditions and shows good agreement between experimental and simulation results. It is found that with an increased spacing between the cell walls, the bubble shape changes from oblate ellipsoid and spherical-cap to more complicated shapes, while the bubble path changes from only rectilinear to a combination of oscillating and rectilinear; the bubble drag coefficient decreases and this results in a higher bubble velocity caused by a lower pressure exerted on the bubble; the wake boundary and wake length evolve gradually accompanied by vortex formation and shedding.

Bubble Behaviors of Large Cohesive Particles in a 2D Fluidized Bed

Fluidization hydrodynamics is greatly influenced by interparticle cohesive forces. In this paper, we study the bubbling behaviors of cohesive Geldart B particles in a 2D fluidized bed, using the “polymer coating” approach to introduce cohesive force. The effect of cohesive force on bubbles can be differentiated into two regimes: (i) by increasing the cohesive force within a low level, the bubble number increases, while the bubble fraction and bubble diameter decrease; (ii) when the force is large enough to cause the particles to adhere to the side walls of the bed, the bubble numbers and the bed expansion sharply decrease. With the increasing cohesive force, the bubble shape changes from roughly circular shape, to oblong shape, leading to the “short pass” of fluidizing gas through the bed. Finally, we analyzed the switching frequency and standard deviation of local pixel values to characterize the bubble dynamics.

What is the Shape of an Air Bubble on a Liquid Surface?

We have calculated the equilibrium shape of the axially symmetric meniscus along which a spherical bubble contacts a flat liquid surface by analytically integrating the Young-Laplace equation in the presence of gravity, in the limit of large Bond numbers. This method has the advantage that it provides semianalytical expressions for key geometrical properties of the bubble in terms of the Bond number. Results are in good overall agreement with experimental data and are consistent with fully numerical (Surface Evolver) calculations. In particular, we are able to describe how the bubble shape changes from hemispherical, with a flat, shallow bottom, to lenticular, with a deeper, curved bottom, as the Bond number is decreased.

Dynamics of dewetting and bubble attachment to rough hydrophobic surfaces – Measurements and modelling

The influence of solid surface roughness (hydrophobic Teflon®) on the timescale of the ascending air bubble (Rb=0.74mm) attachment and the kinetics of the spreading of the three-phase contact (TPC – gas/liquid/solid) line was studied. The moment of the rising bubble collision with a horizontal Teflon® plate immersed in ultrapure water was monitored using fast video recordings (4000fps). It was shown that, depending on the solid surface roughness, the time of the TPC formation was significantly different. Similarly to our previous studies, it was shorter for higher roughnesses. Using high-frequency video acquisition, an additional factor, kinetics of the spreading of the TPC line associated with various bubble shape changes during TPC formation, could be determined. The registered attachment kinetics and bubble shape variations were very reproducible for smooth and very rough Teflon® surfaces, whereas for Teflon® of intermediate roughness, up to five different attachment scenarios were observed, with a relatively large standard deviation of time of TPC formation. Numerical calculations used for simulation of the bubble collisions with a horizontal solid wall with precisely controlled hydrodynamic boundary conditions revealed that the experimentally observed timescales of the bubble attachment and spectacular bubble shape variations can be accurately (qualitatively) reproduced for each roughness of the Teflon® plate studied. Good agreement between experimental and numerical data is, in our opinion, rather strong evidence for air-induced rupture of the liquid film formed between the colliding bubble and the hydrophobic solid plate. This supports the hypothesis that depending on the solid surface roughness, different amounts of air entrapped in solid surface irregularities could drastically change the solid surface hydrodynamic boundary conditions and, consequently, the kinetics of spreading and formation of the TPC.

Numerical study of co‐axial bubble coalescence characteristics

AbstractBubble coalescences have a significant role in petroleum and chemical industries for proper mixing, good heat and mass transfer between the gas and liquid phases. In this study, the volume of fluid method has been used to model for investigation of co‐axial bubble coalescence dynamic evolution in stagnant fluid. The CSF method was incorporated as a source term in the momentum equation to track the motion of the bubble and liquid interface. The effects of Bond number ranges, Bo ~ 4.09–50 (variation of the surface tension) and Reynolds number range, Re ~ 0.98–120 (variation of the liquid viscosity) were investigated. Significant effects were observed in the process of bubble coalescence on the shape of bubbles when the viscosity ratio (μr), and density ratio (ρr) were kept constant. The changes in bubble shape with progressing time were validated by the experimental observation of available literature. For low Bo and Re, the late coalescence of the bubbles was found, whereas the shape of the succeeding bubbles had changed significantly. The same ratio of Bo/μr and Bo/Re does not affect the bubble coalescence process, if Bo, μr and Re is reduced to 82%. By the study of co‐axial coalescence of three bubbles, it is found that the liquid‐film rupture and bubble breakup between the middle and third bubble occurs faster for both low and high Re. © 2015 Curtin University of Technology and John Wiley & Sons, Ltd.

Clogging behavior of an elongated bubble in uniform and diverging microchannel

A numerical investigation has been performed to make a comparative study of clogging and detachment behavior of an elongated bubble during adiabatic flow through a uniform and diverging cross-sectional microchannel. The adiabatic gas–liquid two-phase flow has been numerically solved using volume of fluid approach in a finite volume code. Simulations have been made to investigate the effects of wall wettability, surface tension, and flow velocity on the bubble dynamics for both the configurations of microchannel. The net pressure drop across the bubble in the diverging channel has been calculated analytically and the results have been compared with numerical results. In both types of channels, the bubble shape goes through a transient stage followed by a steady and stable shape. The change in bubble shape during transient phase depends on the surface wettability, flow velocity, and the confinement of the channels. The bubble attains the symmetrical shape earlier in diverging channel. The formation of liquid film between the bubble and the wall plays an important role in the clogging behavior and movement of the bubble. Wall wettability has less impact on the pressure drop characteristics in the diverging channel.

Hydrodynamic study on magnetized fluidized beds with Geldart-B magnetizable particles

When Geldart-B magnetizable particles are fluidized under the simultaneous influence of the gravitational field and the magnetic field, there appear several flow regimes with different hydrodynamic behaviors. The present study focused on the hydrodynamics in the magnetized fluidized bed (MFB) flow regime. The applied magnetic field was axial, uniform, and steady; with Geldart-B iron particles as bed material experiments were conducted in both 2D and 3D beds. According to the gas–solid flow hydrodynamics, the MFB flow regime could be further divided into two distinguishable sub-regimes: magnetized-bubbling regime and channel-bubbling regime. In the channel-bubbling sub-regime, fluidization quality deteriorated due to the channel formation. In the magnetized-bubbling sub-regime, bubble sizes decreased while bubble frequency increased with increasing magnetic field intensities; fluidization quality was gradually improved. Meanwhile, bubbles were severely suppressed in the lateral direction, causing the change in bubble shape from nearly round to elliptic. Several reasons for both bubble size reduction and bubble shape change were analyzed. In addition, a new operational phase diagram was drawn to provide a panoramic view of the fluidization behaviors of Geldart-B magnetizable particles under the action of uniform magnetic fields.

Computational analysis of microbubble flows in bifurcating airways: role of gravity, inertia, and surface tension.

Although mechanical ventilation is a life-saving therapy for patients with severe lung disorders, the microbubble flows generated during ventilation generate hydrodynamic stresses, including pressure and shear stress gradients, which damage the pulmonary epithelium. In this study, we used computational fluid dynamics to investigate how gravity, inertia, and surface tension influence both microbubble flow patterns in bifurcating airways and the magnitude/distribution of hydrodynamic stresses on the airway wall. Direct interface tracking and finite element techniques were used to simulate bubble propagation in a two-dimensional (2D) liquid-filled bifurcating airway. Computational solutions of the full incompressible Navier-Stokes equation were used to investigate how inertia, gravity, and surface tension forces as characterized by the Reynolds (Re), Bond (Bo), and Capillary (Ca) numbers influence pressure and shear stress gradients at the airway wall. Gravity had a significant impact on flow patterns and hydrodynamic stress magnitudes where Bo > 1 led to dramatic changes in bubble shape and increased pressure and shear stress gradients in the upper daughter airway. Interestingly, increased pressure gradients near the bifurcation point (i.e., carina) were only elevated during asymmetric bubble splitting. Although changes in pressure gradient magnitudes were generally more sensitive to Ca, under large Re conditions, both Re and Ca significantly altered the pressure gradient magnitude. We conclude that inertia, gravity, and surface tension can all have a significant impact on microbubble flow patterns and hydrodynamic stresses in bifurcating airways.

Probing timescales for colloidal particle adsorption using slug bubbles in rectangular microchannels

The adsorption of particles to fluid–fluid interfaces is a key step in the generation of colloidosomes and particle-stabilized emulsions. Microfluidic channels are a promising tool for generating particle-stabilized drops and bubbles with independent control over the bubble size and the concentration of particles adsorbed at the fluid interface. In this paper, we present experimental observations of the adsorption of a nanoparticle-surfactant suspension to confined bubbles translating along a microchannel. Long bubbles exhibit a unique two-lobed shape that is linked to the adsorption of surface-active particles to the interface at a timescale comparable to the residence time in the channel. An accompanying decrease in the bubble velocity results from the added viscous drag at the bubble interface. We develop a transport model to describe the rate of particle adsorption to the interface and find good agreement between the model estimates of bubble shape changes and experimental observation. The formation of the two-lobed shape is due to a difference in the velocity of the front and rear of the bubble, which can promote bubble break-up.

Bubble core field modification by residual electrons inside the bubble

Bubble core field modification due to the nondepleted electrons present inside the bubble is investigated theoretically. These residual electrons induce charge and current densities that can induce the bubble core field modification as well as the bubble shape change. It is found that the electrons entering into the bubble move backward at almost light speed and would weaken the transverse bubble fields. This reduces the ratio of longitudinal to transverse radius of the bubble. For the longitudinal bubble field, two effects compensate with each other because of their competition between the enhancement by the shortening of bubble shape and the reduction by the residual electrons. Therefore the longitudinal field is hardly changeable. As a comparison we perform particle-in-cell simulations and it is found that the results from theoretical consideration are consistent with simulation results. Implication of the modification of fields on bubble electron acceleration is also discussed briefly.

Photographic Study on Bubble Motion in Subcooled Pool Boiling

Using a static contact angle of a vertical heated wall as a main experimental parameter, a photographic study was carried out to elucidate the mechanisms to determine the vapor bubble dynamics during subcooled pool boiling. The test fluid was distilled water and the experiments were performed under the atmospheric pressure; liquid subcooling was set to around 5 K. To enable clear observation of bubble behavior with a high speed camera, the experiments were conducted in an isolated bubble regime near the onset of nucleate boiling. Distinctly different bubble behaviors were observed on hydrophobic and hydrophilic surfaces: the bubbles were adhered to the surface for a long period of time when the contact angle was large while lifted-off the surface within a short period of time after the nucleation when the contact angle was small. Since buoyancy does not remove the bubble from the vertical surface, the mechanisms of bubble lift-off were investigated. It was indicated that the change in bubble shape induced by the surface tension force, unsteady growth force, and local liquid flow induced by heterogeneous condensation around the bubble are considered to promote the bubble lift-off while the surface tension force acting on the three-phase common line prevented the lift-off. Effects of the surface wettability on the lift-off bubble diameter, the elapsed time from the nucleation at the lift-off, and the condensation rate after the lift-off were also investigated.

Numerical verification of a simplified model for vapor bubble lift-off from a hydrophilic heated flat-wall

Three-dimensional interface tracking simulations were carried out to investigate the role of surface tension force in the process of vapor bubble lift-off from a hydrophilic heated surface in nucleate boiling. Since bubbles are frequently flattened along the heated surface in photographic experiments reported in literature, a bubble was assumed to be spheroidal in shape in the initial condition. The effect of phase change at the bubble interface was not taken into consideration for the sake of simplicity. In the present numerical simulations, the initially spheroidal bubble approached the spherical shape due to the surface tension force and was eventually lifted off the surface. The change in bubble shape induced local liquid flow directing toward the bubble base, that was the direct cause of the occurrence of the bubble lift-off. The dependence of the bubble migration velocity on several important parameters including the bubble size, surface tension coefficient and the density of surrounding liquid was also investigated. The change in bubble shape from flattened to more rounded causes the reduction of the surface energy, while the formation of local liquid flow leads to an increase in the kinetic energy. It was demonstrated that the bubble migration velocity after the lift-off can successfully be interpreted from the standpoint of energy conservation during the lift-off process.

MULTIPLE SPARK-GENERATED BUBBLE INTERACTIONS

The complex interactions of two and three spark-generated bubbles are studied using high speed photography. The corresponding simulations are performed using a 3D Boundary Element Method (BEM) code. The bubbles generated are between 3 to 5 mm in radius, and they are either in-phase or out-of-phase with one another. The possible interaction phenomena between two identically sized bubbles are summarized. Depending on their relative distances and phase differences, they can coalesce, jet towards or away from one another, split into smaller bubbles, or 'catapult' away from one another. The 'catapult' effect can be utilized to generated high speed jet in the absence of a solid boundary or shockwave. Also three bubble interactions are highlighted. Complicated phenomena such as bubble forming an elliptical shape and bubble splitting are observed. The BEM simulations provide insight into the physics of the phenomena by providing details such as detailed bubble shape changes (experimental observations are limited by the temporal and spatial resolution), and jet velocity. It is noted that the well-tested BEM code [1,2] utilized here is computationally very efficient as compared to other full-domain methods since only the bubble surface is meshed.