Bubble Path Research Articles

Mixing of thermally stratified water layer by a free rising wobbling air bubble

The purpose of this research was to provide further insights on bubble-induced agitation of heated bulk liquid. Fundamental studies on the bubble disturbance of a stratified thermal layer were carried out for a 6mm sphere-volume equivalent diameter air bubble suspended in water (Eo=1.2). A video digital image system and thermochromic liquid crystals were used to reproduce the bubble movement as well as the wake drift of the liquid. A three-dimensional interface tracking simulation was used as a numerical tool. The results have revealed a long open wake region that is formed along the fluctuating bubble path. The amplitude of the bubble wake is much larger than that of the bubble path. In addition to longitudinal mixing, strong lateral mixing is also caused by the movement of vortices in the transverse direction. Details of numerical simulations have revealed a wake that tends to form a chain of Omega shaped vortex rings. These “rings” are connected to each other in the near wake region. The coherent effects further downstream lead to more complex vortex patterns in the far wake. The disintegration of the coherent chain of vortices due to bubble surface deformability is highly complex and not clear at this stage. A comparison with liquid crystal temperature response has revealed that the time scale of the mixing is much longer than the ascending bubble residence, approximately 8s as obtained by several experimental runs. A transverse propagation of entrained cold water has given an estimate of the bubble-induced diffusion to be about 170 times larger than the diffusivity of momentum. The bulk liquid agitation cannot be correctly modeled without taking into account the bubble size dependent wake structure.

Bubble Enhanced Heat Transfer from a Vertical Heated Surface

A rising bubble in a liquid can greatly enhance heat transfer from heated surfaces by acting like a bluff body, displacing fluid as it moves and via the wake generated by the bubble, increasing the mixing of the liquid. The current research quantifies the effect a single free rising ellipsoidal air bubble has on heat transfer from a vertical heated block immersed in water. By measuring the time varying heat transfer and tracking the bubble dynamics, further understanding of the heat transfer mechanism is achieved. Both the plane in which the bubble oscillates and the proximity of the bubble path are shown to influence the surface heat transfer.

Dynamical Model of Bubble Path Instability

Millimeter-sized air bubbles rising through still water are known to exhibit zigzag and spiral oscillatory trajectories. We present a system of four ordinary differential equations which effectively model these dynamics. The model is based on Kirchhoff's equations and several physical arguments derived from our experimental observations. In the framework of this model, the zigzag and the spiral motions result from the same underlying bifurcation to wake instability.

Effects of probe numbers and arrangement on the measurement of charge distributions around a rising bubble in a two-dimensional fluidized bed

The charge density distribution around a rising bubble in a two-dimensional fluidized bed was measured using eight induction probes flush with the outside wall of the column and connected to charge amplifiers for recording induced charge signals as bubbles passed by. The charge distribution surrounding the bubble was reconstructed using an iterative linear back projection (LBP) algorithm by assuming that the bubble is symmetrical and the charge around the bubble remains constant as the bubble rises. The results showed that the emulsion phase far from the bubble in a two-dimensional fluidized bed of glass beads was charged negatively, and the charge density decreased gradually toward the bubble-dense phase interface, with a nearly zero charge density inside the bubble. The effects of the number of probes are also investigated. The reconstructed results using both simulated charge distribution and experimental data for different numbers of probes of the same probe diameter of 3 mm showed that increasing the number of probes can improve the reconstruction resolution. The addition of one or two more probes outside the bubble path also increased the reconstruction resolution in the region outside the bubble.

Shape oscillations and path transition of bubbles rising in a model bubble column

We investigate experimentally the occurrence of shape oscillations accompanied by path transition of periodically produced air bubbles rising in water. Within the period of bubble formation, the induced velocity is measured to examine bubble–liquid and bubble–bubble interactions. The flow is produced in a small-scale bubble column with square-shaped cross section. A capillary aerator produces bubbles of size 3.4 mm at a frequency of 5 Hz. Measuring techniques employed are high-speed imaging to capture bubble shape oscillations and path geometry, and laser-Doppler anemometry (LDA) to measure the velocity in the liquid near the rising bubbles. The experimentally obtained bubble shape data are expanded in Legendre polynomials. The results show the occurrence of oscillations by the periodicity of the expansion coefficients in space. Significant shape oscillations accompanied by path transition are observed as the second-mode oscillation frequency converges to the frequency of the initial shape oscillations. The mean velocity field in the water obtained by LDA agrees well with potential theory. An analysis of the decay of the induced flow shows that there is no interaction between the flow fields of two succeeding 3.4 mm bubbles in the rectilinear path when the bubble production frequency is lower than 7.4 Hz.

Application of the double optic probe technique to distorted tumbling bubbles in aqueous or organic liquid

The optic probe technique is widely used to investigate bubble reactors. To derive values of bubble local velocities and bubble local sizes, a specific signal treatment is usually applied under severe assumptions for bubble path and shape. However, in most industrial reactors, bubble motion is chaotic and no common shape can be assumed. In this work, the reliability of the signal treatment associated with the optic probe technique is examined for distorted and tumbling bubbles. A double-tip optic probe is settled in a glass tank and the rise of bubbles is filmed simultaneously. Several trains of bubbles are studied, interactions between bubbles being gradually increased. Referring to image analysis, several ways to derive mean bubble velocities from optic probe data have been compared. Crenels from front tip and rear tip raw signals are associated and individual bubble velocities are derived. Nevertheless, complete velocity distributions are difficult to obtain, as they depend on the choice of the time within which the bubble is searched on the second tip. Using a simpler approach it is shown that the most probable velocity, calculated through the raw signals inter-correlation, is a correct estimation of the average bubble velocity. Concerning bubble size, bubble chord distributions show too high values due to bubble distortion and deviation. A simplified estimation of bubble mean Sauter diameter, using the most reliable measurements only (i.e., local gas hold-up, local mean bubbling frequency, and most probable bubble velocity), was tested for highly distorted bubbles; this method was validated both in water and cyclohexane.

The structure of bubble non-equilibrium movement in free-rise and agitated-rise conditions

Experimental data on the air bubble rise speed ν z, the bubble mean lateral displacement s x and the mean frequency f z of bubble periodic movement are presented as funtions of bubble size for pure and contaminated water, in free-rise and agitated-rise conditions. The entire scope of bubble non-rectilinear movement is considered. The interdependence of the ν z, s z and f z values, as functions of bubble equivalent sphere diameter d e, is is shown to be very consistent. There are two peaks in s, vs. d e, the first characteristic for periodic bubble movement and the second for bubble random rocking. In contrast with the work in the literature, where attempts were made to evaluate the drag on the bubble in a way which is consistent only for the firstpeak of s z, the philosophy presented originates from the second peak of s z. Postulating that a visible bubble path is a unit reflection of several microscales which dissipate bubble rectilinear motion, a new definition of bubble drag coefficient as a function of fractal-like dimension is proposed.

Influence of pressure and fluidization velocity on the hydrodynamics of a fluidized bed containing horizontal tubes

An experimental investigation of the influence of horizontal tubes on the bubble behaviour and through-flow velocity of gas inside bubbles was carried out and the influence of pressure and fluidization velocity was studied. The visible bubble flow rate and the gas velocity through the bubbles were measured simultaneously in a pressurized fluidized bed, using capacitance and Pitot-static pressures probes respectively. Silica sand with a mean particle diameter of 0.7 mm was fluidized at pressures between 01. and 1.6 MPa, in a bed of 0.2 × 0.3 m cross-section. Six different horizontal tube configurations were used and comparisons were made with results obtained in the same bed operated without tubes. The results from the bed with tubes show that the influence of pressure and fluidization velocity is basically the same as in the bed without tubes; an increase in pressure and fluidization velocity increases the bubble activity. At low excess gas velocities, the influence of immersed tubes on the bubble behaviour is less pronounced than at high excess gas velocities. Measurements and visual observations show that a small tube pitch gives a more uniform distribution of the visible bubble flow over the bed cross-section. At a combination of high pressure, high excess gas velocity and a large tube pitch, a preferred bubble path was observed in the centre of the bed cross-section. The pressure probe measurements show that the through-flow velocity of gas inside the bubbles is in most cases higher when tubes are present in the bed. The through-flow velocity consistently decreases with increasing pressure in the bed without tubes. The tendency is largely the same at a low excess gas velocity when tubes are present, but at a high excess gas velocity the through-flow velocity first decreases and then increases as the pressure is increased.

Ammonia Volatilization from Liquid Hog Manure: Influence of Aeration and Trapping Systems

AbstractMeasurements of N loss from manures may reflect differences in the types of manures and methodologies used. This study's objective was to test the suitability of 0.32 M H3BO3 and 0.9 M H2SO4 for trapping NH3 volatilized from liquid hog manure (LHM) under various experimental conditions. Samples of LHM were incubated for 15 d in containers with aeration outlets positioned above, at the middle, or at the bottom of the LHM. Lowering the position of the aeration outlet increased both the pH of the LHM and the amount of NH3 volatilized. More NH3 was trapped in H2SO4 than H3BO3, and the difference in trapping efficiency of the two acids increased with their NH3 concentrations. Neither the amount of NH3 trapped nor the exponential relationship between the NH+4 concentrations of two H3BO3 traps in series was influenced by changing the bubble path length through the acid. Regression analysis indicated that >95% trapping efficiency was obtained only when the NH3 concentration of H3BO3 was below 0.42 mg N mL−1, much lower than the 0.9 mg N mL−1 reported to be the limit for using H3BO3 in the Kjeldahl method. Even when using two traps in series, H3BO3 appeared to trap less NH3 than H2SO4. Amending LHM with sucrose lowered the pH of H3BO3 used to trap volatilized NH3, thereby interfering with NH3 determination and rendering H3BO3 unsuitable for determining NH3 volatilization. Investigators who use H3BO3 to measure NH3 volatilization in other systems must ensure that similar interferences do not occur and that NH3 concentrations do not exceed 0.42 mg N mL−1.

On the stability of gas bubbles rising in an inviscid fluid

The stability to three dimensional disturbances of bubbles rising rectilinearly in an inviscid fluid is studied numerically. It is found, in contrast with earlier work, that the interaction of hydrodynamic pressure forces and surface tension does not lead to linear instability of the bubble path.

Flow of bubbles through nozzles

One of the fundamental objectives in the mathematical modeling of two-phase flow is to understand and to formulate the interaction forces between two phases. For this purpose a well-defined two-phase flow situation, flow of bubbles through nozzles, was set up in the laboratory. A range of liquid flow fields was set up by flowing water through nozzles. Individual bubbles were injected into the water stream and their trajectories were recorded to provide data for evaluation and comparison with theories. Because of the limitations of the conventional photographic method of recording the bubble path, a computer-based optical system was designed for fast data acquisition. The optimal system works on the principle of the interruption of a light beam by a bubble passing between a sheet of light and a row of phototransistors. The bubble position (horizontal as well as vertical) along the nozzle is determined by its crossing through the light path to several rows of phototransistors attached to the nozzle. The performance and accuracy of the optical system were tested under various known physical situations. All the tests showed that the optical system is competent and effective in studying the motion of bubbles flowing through nozzles. A mathematical model was set up to predict the bubble motion. The various forces included in the model are the drag force, the apparent mass force, the buoyancy force, the bubble expansion force and the history forces. Comparisons between experiments and theories were made through a “point-to-point” numerical testing procedure. Results show that the bubble trajectory can be reasonably predicted (within 10% accuracy) by an equation of motion which includes a suitable drag force (dependent on relative velocity) and an apparent mass force (proportional to relative acceleration). Suggestions regarding the further improvement of the theory are also made.

Bubble path in the wake of a cavitating propeller

Propellers that operate underwater at high rpm's cavitate at the tip. The tip cavitation creates air bubbles that are then swept downstream by the motion of the surrounding fluid. In this paper, a theory is presented to predict the local velocity and the path of the bubble. The bubble motion is assumed to be governed by a group of terms due to the acceleration of the displaced fluid, the convective term, and the drag due to the cross‐sectional area of the bubble. At very low and very high Reynolds numbers, the equations have been solved in closed form. Results are presented for the bubble velocity and path for the following flow fields: (a) uniform axial flow field and (b) uniform flow field with an axially decaying swirling component. In all cases presented the bubble axial velocity component asymptotes to the free stream velocity; the manner in which it asymptotes is exponential at very low Reynolds numbers and algebraic at high Reynolds numbers. Bubble helical paths and velocity patterns are shown for different bubble sizes.

Condensation of an Accelerating-Decelerating Bubble: Experimental and Phenomenological Analysis

An experimental and theoretical phenomenological study of accelerating–decelerating bubbles, condensing in an immiscible liquid, was conducted. The system consisted of a column of water, and bubbles of freon-113, 4–5×10−3 m in diameter. Shadowgraphing of the process has illustrated the wake formation behind the bubble, wake shedding, forward movement of vortices, and envelopment of the decelerating bubble in its wake. The bubble size, shape, and path were videotaped and analyzed for the collapse rate, and the instantaneous position. The visualized hydrodynamic phenomena provided a phenomenological basis for the theoretical formulations. The theoretical model postulated an eccentrically positioned vapor sphere in the collapsing bubble, a boundary layer and wake over the accelerating bubble, and a concentric vorticular envelope around the decelerating bubble. The theoretical–phenomenological predictions compared well with experiment.

Characteristics of laser-doppler signals from bubbles

The present paper concerns the application of laser-Doppler anemometry to two-phase bubbly flows. A summary of available LDA-measurements in particulate, droplet and bubble flow systems is given which shows detailed measurements of signal properties and their dependence on optical parameters were not previously available. However, such information is necessary for optimizing the optical and electronic schemes for size measurements. Consequently, the signal characteristics of light scattered from single bubbles rising in quiescent water were obtained for a forward-scatter arrangement and for three different beam-intersection angles. In addition to the dependence of the signal properties of the bubble path through the beam-intersection region, the effects of aperture diameter and aperture position in the optical system are discussed. The resultant data were used to determine an optimum forward-scatter LDA-system which was then employed to measure the dependence of bubble velocity on bubble diameter over the range 0.2–1.0 mm. Finally, a theoretical model proposed to describe the dependence of the signal properties on the location of the centreline bubble path through the beam-intersection region is shown to be in agreement with the measurements. The treble amplitude peaks observed in the signals from bubbles in this size range are readily explained by applying the laws of geometrical optics.

Alkali and alkaline earth metal chemistry of marine aerosols generated in the laboratory with natural sea waters

Atmospheric sea salt particles, produced in the laboratory by bubbles bursting at the surface of natural seawater solutions, were examined for their alkali and alkaline earth metal content in several particle size categories. No enrichments of K, Ca and Mg relative to the seawater source were found in these atmospheric particles using 1. (1) Sargasso seawater or Narragansett Bay seawater. 2. (2) filtered seawater or unaltered seawater. 3. (3) two different bubble path lengths. Only with the addition of natural surfactants was any enrichment over the seawater ratio found in these aerosols. In this case, K and Mg may have been enriched on the smallest particles. Calculations, indicate that the organic carbon content of aerosols produced from even highly productive natural seawaters cannot account for apparent enrichments of K observed in the environment. These experiments suggest that enrichment of alkali and alkaline earth metals in sea salt particles produced from most natural seawaters is environmentally insignificant.

Viscosity Effects on the Motion of Gas Bubbles Rising through Stagnant Liquids

In this investigation, viscosity effects on bubble path, rise velocity, drag coefficient, and deformation are studied. The liquid viscosity is varied throughout a wide range using glycerin and glyc...