Single Air Bubble Research Articles

Water Model Experiments on the Effect of an Argon Bubble on the Meniscus Near the Immersion Nozzle

Water model experiments have been carried out to understand the behavior of the meniscus of molten steel near the immersion nozzle. The meniscus is disturbed by a large argon bubble rising along the immersion nozzle. Water, silicone oil, and air are used as models for molten steel, mold powder, and argon, respectively. A cylindrical rod is used as a model for the immersion nozzle. The contact angle of a water droplet on the rod is adjusted to become nearly the same as that of a molten steel droplet on the immersion nozzle by coating repellent on the rod surface. A single air bubble of a predetermined volume is released from a cap-shaped container to attach to the bottom of the rod. The behavior of the bubble passing across the interface between water and silicone oil is observed with a high-speed video camera to make clear the entrapment of silicone oil droplets into the water bath.

Direct numerical simulation of single gas bubbles in pure and contaminated liquids

AbstractDisperse gas bubbles play an important role in many industrial applications. Knowing the rising velocity, the interfacial area, or the critical size for break–up or coalescence in different systems can be crucial for the process design. Hence, knowing the fundamental behaviour of a single bubble appears mandatory for the examination of bubble swarms and for the Euler–Lagrange or Euler–Euler modelling of disperse systems. In the present work a level–set–based volume–tracking method is implemented into the CFD–code OpenFOAM to follow the free interface of a single bubble. The volume–tracking method is coupled with a transport model for surfactants on the interface, including adsorption and desorption processes. The dependency of surface tension on the local surfactant concentration on the interface is modelled by a non–linear (Langmuir) equation of state. Marangoni forces, resulting from surface tension gradients, are included. The rise of a single air bubble (i) in pure water and (ii) in the presence of surfactants of different strengths is simulated. The results show good agreement with available correlations from literature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical Simulation of a Single Air Bubble Rising in Water with Various Models of Surface Tension Force

Different numerical methods are employed and developed for simulating interfacial flows. A large range of applications belong to this group, e.g. two-phase flows of air bubbles in water or water drops in air. In such problems surface tension effects often play a dominant role. In this paper, various models of surface tension force for interfacial flows, the CSF, CSS, PCIL and SGIP models have been applied to simulate the motion of small air bubbles in water and the results were compared and reviewed. It has been pointed out that by using SGIP or PCIL models, we are able to simulate bubble rise and obtain results in close agreement with the experimental data. the advection equation, by a geometrically based calculation technique of the void fraction fluxes at the cell faces based on the reconstructed interface. A significant improvement of the interface representation was achieved by Youngs (1) by introducing a piecewise-linear method. The PLIC method approximate the interface is by a straight line of arbitrary orientation in each cell. Its orientation is found the distribution of one of the fluids in the neighbor cell. Given the volume fraction of one of the two fluids in each computational cell and an estimate of the normal vector to the interface, a plane surface is constructed within the cell having the same normal and dividing the cell into two parts each of which contains the proper volume of one of the two fluids. This has several advantages: the fluxes of F with which the phase field updated, can be determined more accurately, and essentially free of numerical diffusion. Fluid properties can be calculated accurately. In contrast to the interface representation methods, the methods for introducing surface tension effects at the interface into the physical model remain a problem. Generally, the influence of surface tension is incorporated into the momentum equation following the continuum surface force (CSF) model of Brackbill et al. (2). For doing so, the local curvature and the interface normal vector have to be calculated. This task is difficult, since the discontinuous void fraction function disallows the application of ordinary discretization schemes. An inconsistent calculation of the surface tension force can then result in the well-known phenomena of so-called spurious (11). Usually problems with parasitic currents arise when flows with high density ratios are considered. Unfortunately, a large range of applications belong to this group, e.g. two-phase flows of air bubbles in water. In such problems surface tension effects often play a dominant role. The behavior of each algorithm under surface tension-dominant problems is discussed in the next section.

513 単一気泡の上昇挙動に関する実験的研究(流体工学VI)

513 単一気泡の上昇挙動に関する実験的研究(流体工学VI)

Formation of Single Bubbles from a Submerged Orifice Using Pulsed Ultrasound Waves

A new experimental method is presented in which single small gas bubbles are generated in a liquid from a submerged orifice using pulsed ultrasound waves. Pulsed ultrasound waves having a frequency of 15 kHz and a maximum pressure amplitude of approximately 10 kPa are irradiated to a bubble growing from an orifice. Single air bubbles ranging from approximately 0.05 to 0.2 mm in radius are obtained in silicone oil (kinematic viscosity: 1 mm2/s) by using two orifices (0.02 and 0.04 mm in diameter) and by shifting the onset of the detachment-assistance pressure wave. The bubble deformation and detaching processes were visualized and analyzed using high-speed video imaging and direct numerical simulation. Consequently, it is revealed that the bubbles are forced to elongate upward due to the fast oscillatory flow of gas through the orifice, and the elongation causes the bubbles to detach from the orifice. The size of the bubbles at detachment is well estimated by employing a common spherical bubble formation model.

Pressure-oscillation defoaming for viscoelastic fluid

The enhancement of bubble rising velocity was experimentally investigated by mechanically applying an oscillating pressure to a single small air bubble (e.g., 1 mm 3) in a viscoelastic fluid. For shear-thinning fluids, the cyclic change in bubble diameter induced by the oscillating pressure generates a continuous strong local flow near the bubble surface. Consequently, the apparent liquid viscosity is reduced and the bubble rising velocity increases by 400 times or more compared to the case without oscillating pressure. However, for a Newtonian fluid, almost no effect was observed with oscillating pressure. Time-series data of the longitudinal and horizontal bubble diameters were obtained experimentally using a stroboscope and a video system, and these data were used to estimate the local shear rate and the local shear viscosity. The increase in bubble rising velocity estimated from the shear viscosity behavior agreed well with the experimental data. Additionally, a periodic change in the bubble shape from a sphere at the maximum bubble size to a cusped shape at the minimum bubble size was observed under strong oscillatory pressure.

Surface breakup and air bubble formation by drop impact in the irregular entrainment region

Drop impact on a water surface can be followed by underwater sounds originating not at the drop impact but when the entrained bubbles oscillate. Although the sound mechanism in the regular bubble entrainment region is well-known, there is less knowledge on the impact phenomena in the irregular bubble entrainment region where various situations can exist, such as many types of bubble formation or even no bubble generation under some conditions. In the present study, the aim is to clarify the dynamics of the water surface after the impact of a primary drop, mainly with diameter 5.2, 5.7 and 6.2mm, each of which is accompanied by a single satellite drop. Special attention was paid to the breakup behaviour of the water surface for Froude number Fr < 300. It was found that three underwater sounds were generated for a single drop impact, besides the sound due to impact itself. The first two were audible to the human ear, but the third one was almost inaudible. The first underwater sound resulted from the oscillation of a single air bubble formed as a result of the satellite drop impact on the bottom of the contracting cavity, and the second sound was due to the oscillation of air bubbles generated during the collapse of the water column. The formation of these air bubbles strongly depends on the Froude number, Weber number (or Bond number) and the aspect ratio of the drop at impact, although involving probability characteristics. Furthermore it is suggested that an air bubble entrapped in a water column plays an important role in increasing the probability of contact between the column surface and the curved free surface. A Japanese Suikinkutsu was introduced as an application of drop-impact-induced sounds. Using an open-type Suikinkutsu an additional experiment was carried out with larger drops with average diameters of 6.2, 7.2 and 7.8, mm.

Levitation of air bubbles and slugs in liquids under low-frequency vibration excitement

This experimental study reports the influence of low-frequency vibrations, in the range of 60–400 Hz, on the rise of single air bubbles and slugs injected into two columns (of diameters 0.014 and 0.05 m), filled with liquids of varying densities (in the range 889– 1381 kg m - 3 ) and viscosities (in the range 0.48–1.4 Pa s). For a specified set of operating conditions the bubbles or slugs can be made to levitate, i.e. held stationary in the column. The height of the liquid, h, above the position at which the gas bubble is levitated was determined for a wide range of operating conditions (vibration frequency and amplitude, operating pressure, column diameter, liquid density and viscosity). The experimentally determined values of h are in good agreement with the theoretical model of Baird [1963a. Resonant bubbles in a vertically vibrating column. Canadian Journal of Chemical Engineering 41, 52–55].

On the critical bubble volume at the rise velocity jump discontinuity in viscoelastic liquids

Bubbles rising in viscoelastic liquids may exhibit a jump discontinuity of the rise velocity as a critical bubble volume is exceeded. We carried out detailed experiments to investigate the occurrence of this discontinuity with single air bubble rising in various polymer solutions without influence of surfactants. The polymer solutions were characterized thoroughly by means of shear and elongational rheometry, as well as tensiometry. The experiments showed that a jump discontinuity can exist only if the non-dimensional group λ E( g 3 ρ 1/ σ) 1/4, found by dimensional analysis, exceeds a critical value. A universal correlation of non-dimensional numbers for the non-dimensional critical bubble volume at the jump discontinuity was found. The non-dimensional numbers represent the relevant rheological and dynamic liquid properties. This is the first time that the prediction of the critical bubble volume as well as the potential of the solution to exhibit a bubble rise velocity discontinuity becomes possible based on liquid properties only. In the correlation found, the relaxation time of the polymer solutions in elongational flow of the viscoelastic liquid was found to play a key role.

Schlieren and “focused” shadowgraphy visualization of the shape and wake of single air bubbles freely rising in quiescent water

Schlieren and “focused” shadowgraphy visualization of the shape and wake of single air bubbles freely rising in quiescent water

Behavior of Water Jet Accompanied with Air Suction

In order to atomize a liquid, the authors have investigated the behavior of air-water jets. In a series of experiments, we have discovered a strange phenomenon that the water jet accompanied with air suction from the free surface has made a periodic radial splash of water drop. The purpose of the present paper is to clear out the origin of this phenomenon and the behavior of water jet accompanied with air suction. The behavior of water jet has been photographed by a digital camera aided with a flashlight and high-speed video camera. Those experiments enable us to find the origin of a periodic radial splash due to a formation of single air bubble at the flow separation region inside the nozzle and due to explosive expansion of the bubble after injected in the free space. In order to analyze the radial splash of water, we have conducted the equation of spherical liquid membrane. The numerical results obtained have been compared with the experimental results and good agreement has been obtained in radial expansion velocity.

A study of velocity discontinuity for single air bubbles rising in an associative polymer

The motion of air bubbles in aqueous solutions of a hydrophobic alkali-swellable associative polymer is studied in this work. The associative nature of these polymer systems dictates their rheological properties: for moderate values of the shear rate, the formation of structure can lead to a shear-thickening behavior and to the appearance of first normal stress difference. For larger shear rates, the polymer associations can be broken, leading to shear thinning. In general, these fluids show a Newtonian behavior for small values of the shear rate, but behave as viscoelastic liquids for large shear rates. Experimental results show the appearance of a critical bubble volume at which a discontinuity in the relation velocity-volume occurs; however, the velocity increase found in this case is not as large as that previously reported for the case of shear-thinning viscoelastic fluids. The discontinuity is associated with a significant change of the bubble shape: before the critical volume, the bubbles are convex spheroids, while past the critical volume a sharp cusped end appears. The appearance of the tail is also associated with the appearance of an inflection point (change of curvature) on the bubble surface. Moreover, since the rheology of the liquids is measured it was found that the discontinuity, and hence the change of shape, occurs when the elastic nature of the liquid first manifests itself (appearance of a first normal stress difference). A comparison of the measured velocities for small bubbles with predictions from a Stokes-Hadamard law shows a discrepancy. The Newtonian viscosity measured in a viscometric flow was smaller than that determined from a falling-ball arrangement. Considering the viscosity measured under this nonviscometric flow, the comparison between theory and experiments was very good for bubbles having volumes lower than the critical one. Moreover, due to the importance of the elasticity, and due to the change of the shape of the bubble, a dimensionless number formed as the ratio of elastic to surface tension forces clearly defines the change of the behavior for the bubbles rising in these fluids. Finally, a photographic study of the peculiar shapes of the bubble tails, tip-, and edge-streaming phenomena is presented. To our knowledge, experiments in this class of fluids have not been reported to date.

Microbubble shape oscillations excited through an ultrasound‐driven parametric instability

An ultrasonically driven air bubble can become shape‐unstable through a parametric instability. Here, we report time‐resolved optical observations of shape oscillations (mode n=2 to 6) of micron‐sized single air bubbles for a range of acoustic pressures. The observed mode number n was found to be linearly related to the resting radius of the bubble. Above the critical driving pressure threshold for shape oscillations, which as expected is minimum at the resonance of the volumetric radial mode, the observed mode number n is independent of the forcing pressure amplitude. The microbubble shape oscillations were also analyzed numerically by introducing a small, nonspherical linear perturbation into a Rayleigh‐Plesset‐type equation model which includes a physical thermal damping mechanism describing heat and mass transport in the thin boundary layer at the bubble‐to‐water interface. Indeed, a parametric instability is responsible for the shape oscillations, and the Rayleigh‐Plesset‐type equation captures the experimental observations in great detail.

An experimental investigation of the motion of single bubbles under a slightly inclined surface

The movement of single air bubbles under a downward facing inclined solid surface has been investigated in motionless distilled water body. The effects of surface inclination θ and bubble volume V on the terminal Froude number are studied in details. The bubble volume was varied in the 0.3–9 cm 3 range and the inclination angles up to 10° in a gravity driven system. For a typical correlation curve of the terminal velocity u T versus the volume at a given inclination it was found that the increase of u T with V is not monotonous and that four distinct sub-regimes, each characterized by a specific bubble shape, exist. The effect of inclination on u T is more sensitive at both low V and low θ. The different sub-regimes are explained qualitatively. In addition, the drag coefficients for moving bubble underneath a solid surface are presented.

Formation of Single Bubbles from a Submerged Orifice Using Pulsed Ultrasound Wave

A new experimental method to generate single small gas bubbles from a submerged orifice using pulsed ultrasound wave in a liquid is presented. Pulsed ultrasound wave having frequency of 15 kHz and maximum pressure amplitude of about 10 kPa is irradiated to a bubble growing from an orifice. Single air bubbles ranging from about 0.05 to 0.2 mm in radius are obtainable in silicone oil (kinematic viscosity : 1 mm2/s) using two orifices (0.02 and 0.04 mm in diameter), and by varying the timing to apply the pulse. Bubble deformation and detaching process was visualized and analyzed using both high-speed photography and direct numerical simulation. Consequently, it is revealed that bubbles are forced to elongate upward due to the oscillatory flow of gas through an orifice with high velocity, and the elongation causes the bubble to detach from an orifice. The sizes of bubbles at detachment can be well estimated with a common spherical bubble formation model.

Effect of Surfactants on the Rate of Growth of an Air Bubble by Rectified Diffusion

The rectified diffusion growth of a single air bubble levitated in an acoustic field (frequency = 22.35 kHz) in water and in aqueous solutions containing surfactants (sodium dodecyl sulfate and sodium dodecylbenzene sulfonate) was investigated. As reported by Crum (J. Acoust. Soc. Am. 1980, 68, 203), the presence of surfactants at the bubble/liquid interface enhanced the growth rate of the bubble by rectified diffusion. It is suggested in this paper that in addition to the effect of surfactants on the surface tension and interfacial resistance to mass transfer, the effect of surface rheological properties may also contribute to the cause of the enhancement observed in the bubble growth rate.

Numerical investigation of closures for interface forces acting on single air-bubbles in water using Volume of Fluid and Front Tracking models

Closures for the drag and virtual mass forces acting on a single air bubble rising in initially quiescent pure water have been numerically investigated using direct numerical simulation techniques. A 3D Front Tracking model was used and the results were compared with simulation results obtained with a 2D Volume of Fluid model to assess the influence of the third dimension. In the simulations realistic values were taken for the physical properties, i.e., a density ratio of 800. The computed time-averaged terminal rise velocity and mean aspect ratio for individual air bubbles ranging in equivalent diameter from 1 to 10 mm rising in pure water compare well with available experimental data.

Experimental study of the onset of the 3D oscillatory thermocapillary convection around a single air or vapor bubble.: Influence on heat transfer

In a first section, Marangoni convection induced by a temperature gradient is studied around a single air bubble. The bubble is introduced under a downward facing heating element into a liquid layer heated from upper and cooled from below (silicone oils Pr = 16.7 and Pr = 228 or FC-72, Pr = 12.3). In this configuration stationary thermocapillary convection can be followed by the oscillatory convection. Firstly the 3D oscillatory thermocapillary convection is presented for a silicone oil ( Pr = 16.7). Then, the critical threshold between the stationary state and the first oscillatory mode is determined. The influence of the liquid kind on this threshold is presented for one layer height. Moreover, other parameters (horizontal wall temperature, layer height) are tested for one liquid. At least, we consider heat transfer induced by this convection, and we discuss the results versus the convection state. The second section of this paper concerns preliminary results on Marangoni convection with boiling around a single vapor bubble growing under a downward facing heating element into a subcooled liquid (FC-72). The oscillatory thermocapillary convection is observed. It occurs during the growth from a critical bubble radius. The influence of the subcooling level on its occurrence threshold is shown.

Behavior of Water Jet Accompanied with Air Suction

In order to atomize a liquid, the authors have investigated the behavior of air-water jets. In a series of experiments, we have discovered a strange phenomenon that the water jet accompanied with an air suction from the free surface has made a periodic radial splash of water drop. The purpose of the present paper is clear out the origin of this phenomenon and the behavior of water jet accompanied with an air suction. The behavior of water jet has been photographed by a digital camera aided with a flash light and high-speed video camera. Those experiments enable us to find the origin of a periodic radial splash due to a formation of single air bubble at the flow separation region inside the nozzle and due to explosive expansion of the bubble after injected in the free space. In order to analyze the radial splash of water, we have conducted the equation of spherical liquid membrane. The numerical results obtained have been compared with the experimental results and good agreement has been obtained in radial expansion velocity.

Rise velocity of an air bubble in porous media: Theoretical studies

The rise velocity of injected air phase from the injection point toward the vadose zone is a critical factor in in‐situ air sparging operations. It has been reported in the literature that air injected into saturated gravel rises as discrete air bubbles in bubbly flow of air phase. The objective of this study is to develop a quantitative technique to estimate the rise velocity of an air bubble in coarse porous media. The model is based on the macroscopic balance equation for forces acting on a bubble rising in a porous medium. The governing equation incorporates inertial force, added mass force, buoyant force, surface tension and drag force that results from the momentum transfer between the phases. The momentum transfer terms take into account the viscous as well as the kinetic energy losses at high velocities. Analytical solutions are obtained for steady, quasi‐steady, and accelerated bubble rise velocities. Results show that air bubbles moving up through a porous medium equilibrate after a short travel time and very short distances of rise. It is determined that the terminal rise velocity of a single air bubble in an otherwise water saturated porous medium cannot exceed 18.5 cm/s. The theoretical model results compared favorably with the experimental data reported in the literature. A dimensional analysis conducted to study the effect of individual forces indicates that the buoyant force is largely balanced by the drag force for bubbles with an equivalent radius of 0.2–0.5 cm. With increasing bubble radius, the dimensionless number representing the effect of the surface tension force decreases rapidly. Since the total inertial force is quite small, the accelerated bubble rise velocity can be approximated by the terminal velocity.