Abstract: The growth kinetics of electrogenerated hydrogen, oxygen and chlorine gas bubbles formed at microelectrodes, were determined photographically and fitted by regression analysis to the equation;r(t)=βt x , wherer(t) is the bubble radius at timet after nucleation,β the ‘growth coefficient”, andx the ‘time coefficient’. The coefficientx was found to decrease from a short time ( 100 ms). The current efficiency for bubble growth increased with bubble lifetime, reflecting the decrease in local dissolved gas supersaturation. The pH dependency of the bubble departure diameter indicated that, in surfactant-free electrolytes, double layer interaction forces between the negatively charged hydrogen evolving cathode or positively charged oxygen/chlorine evolving anode and positively (pH \s 3) charged bubbles, were the determining factor. The effect of addition of an increasing concentration of cationic (DoTAB) or anionic (SDoS) surfactant was to progressively reduce the pH effect on departure diameter, due to surfactant adsorption on the bubble and, to a lesser extent, on the electrode.

Abstract: Wave breaking is believed to be important in air–sea interaction. Laboratory measurements1 suggest that the momentum flux from the atmosphere to the ocean may be significantly enhanced by breaking and recent field measurements2–4 have demonstrated the important role of breaking in bubble generation and gas transfer. It has long been speculated that the loss of momentum flux from the wave field due to breaking could act as a source of momentum for current generation5 and Mitsuyasu6 has drawn attention to the large discrepancy between the momentum flux from the wind and that carried by the waves, suggesting that the loss may be due to wave breaking. Here we present what we believe are the first well-controlled laboratory measurements of the momentum flux lost by wave breaking. These measurements are consistent with Mitsuyasu's hypothesis and recent measurements of wave growth, and support the conclusion that wave breaking plays an important role in momentum transfer across the air–sea interface.

Abstract: Measurements of the properties of shallow underwater explosions have been obtained in recent experiments with 0.82‐kg SUS (signal underwater sound) charges at depths from 23.5 to 194.5 m. The data were recorded in the farfield with a high‐resolution digital recording system to ensure good reproduction of both the high‐ and low‐frequency components of the waveform. The measurements provide new results for the pressures, impulses, durations, and time constants of the shock wave and the first bubble pulse, and in addition, extend the available information to include the properties of the subsequent bubble pulses and the negative phases. Comparison of the data with previously published semiempirical relationships based on experiments with deeper charges indicates that those expressions generally do not provide adequate descriptions of the shallow charge data. In particular, there is significant disagreement with the measurements of the shock wave decay constant, and with the measurements of all of the bubble phase properties. The effect of vertical migration is appreciable for shallow charges and was observed in the measurements of bubble pulse periods, pressures, and impulses. Semiempirical relationships were derived from the data for the properties of both the shock wave and the bubble pulse series. These expressions account for the depth dependence due to migration observed in the bubble phase properties and consequently provide parameter values appropriate for modeling the pressure waveform of shallow charges.

Abstract: Radial and axial profiles of gas concentration and bubble frequency have been measured for vertical gas bubble jets in the systems air/water, helium/water, and nitrogen/mercury using an electroresistivity probe. Gas velocities have been determined in the air/water system. All radial profiles were close to Gaussian. An integral model was applied to calculate axial distributions theoretically. Entrainment coefficients were determined for the experimental conditions. Axial profiles were correlated also in nondimensional representations. The bubble frequencies were used to compute local and average values of bubble size. Jet expansion and bubble size were found to depend considerably on the physical properties of the system.

Abstract: The effects of surfactants and antifoam agent on the gas holdup e, the liquid-phase mass transfer coefficient kL and the volumetric liquid-phase mass transfer coefficient kLa in a bubble column were studied experimentally. An addition of surfactants such as n-alcohols to water increases e and decreases kL, and an addition of antifoam agent to water decreases e, kL and kLa. However, the degree of reduction of kL value by an addition of surfactant to water is lower for bubble swarms in a bubble column than that for a single bubble in stagnant liquid. Based on these observations, the previous model for estimating kL of a single bubble in aqueous solutions of surface-active substance is modified so as to be applicable to the estimation of kL for bubble swarms in a bubble column.

Abstract: : The topic of this chapter, acoustic cavitation, is but one of several possible mechanisms through which ultrasound can interact with a liquid medium Acoustic cavitation can affect a liquid through two possible avenues The first is the bubble itself The liquid is disrupted by the inhomogeneous presence of the bubble The second avenue through which bubbles affect a fluid is through bubble dynamics The bubble's interior and the liquid immediately surrounding the bubble are regions which undergo continual change The bubble-liquid interface continually changes shape and size; liquid molecules diffuse into and out of the bubble; the concentration of gas in the surrounding liquid varies; acoustic streaming occurs in the liquid in the vicinity of the bubble often resulting in severe shear stresses; the interior pressure and temperature fluctuate rapidly; the bubble radiates acoustic energy as it oscillates; thermal and viscous damping hinder the bubble oscillations

Abstract: A theoretical approach to gas transfer by bubbles created by breaking waves at the air-waterinterface is undertaken. Based on a simple model, this study affords a basic understanding of thephysical mechanisms. The behaviour of a single bubble is examined. Similarities and differencesbetween the formulation of gas fluxes across a “flat” air-water interface and through bubbles areshown. The gas flux through bubbles is not strictly proportional to the solubility and to thedifference of concentrations between the bulk of the water and the surface. Nevertheless, fortrace gases, a linear relationship with this difference of concentrations, increasing with solubilityis predicted. Overpressure in the bubbles and dissolution favour invasion into the water andimply a water supersaturation at equilibrium. This supersaturation is more important when thegas solubility is low. The r61e of surfactants is studied and it is found that they can reduce the gasexchange by a factor of 5. The consequences of the model for the transfer velocity are brieflypresented. A better definition of the bubble lifetime, i t . , a better representation of waterdynamics under a breaking wave, is necessary to make more reliable model predictions, mostimportantly for gases with low solubility. Theoretical studies on the bubble source are necessaryin order to avoid, as far as possible, the use of the bubble distribution. DOI: 10.1111/j.1600-0889.1985.tb00075.x

Abstract: Dense phase voidage, epsilon /SUB D/ , dense phase superficial gas velocity, u /SUB Do/ , and absolute bubble rise velocity, u /SUB B/ , were measured at pressures up to 8300 kPa in a pilot-scale, fluidized bed of Group A and boundary Group A/B powders. The mean equivalent bubble diameter, d /SUB b/ , near the top of the bed was inferred from u /SUB B/ and known bed operating conditions. Increased pressure at fixed superficial gas velocity, u /SUB o/ , increased epsilon /SUB D/ and u /SUB Do/ and decreased d /SUB b/ for Group A powders. The marked decrease in inferred maximum bubble size, d /SUB bmax/ , with increased pressure could not be explained by a decrease in gas contributing to bubble flow, u /SUB Bo/ , but rather appeared to be the result of a bubble instability phenomenon limiting bubble growth.

Abstract: Dispersion of gas bubbles in water has a wide application in aeration and ozonation processes. The rate of mass transfer in such systems depends strongly on the wastewater quality as well as on the design and operational characteristics of gas liquid contact units. Small amounts of surface active compounds in water were shown to reduce the overall mass transfer coefficient, kLa, in single bubble systems1,2,3 and in swarms of bubbles.4 This effect was attributed to the decrease in the liquid-phase mass transfer coefficient, kL.l~5 The adsorption of the large molecules of surfactants reduced the surface tension of water,1,467 reduced the bubble size,1,3,4,8 lowered the terminal velocity of bub bles,9-11 and increased the drag coefficient.5,10 Accordingly, surfactants were believed to reduce the kL by depressing the hydrodynamic activity, and by offering additional barriers for the passage of gas molecules at the gas-liquid interface. Higher concentrations of the surfactants improved kLa\ this was attributed to the increase in the interfacial area caused by the formation of smaller bubbles.1,4 On the other hand, Zieminski and his co-workers8,12"14 reported a definite improvement in kLa in the presence of various alcohols and carboxylic acids. The authors stated the principle action of such substances to be prevention of bubble coalescence. This study was undertaken to investigate the effects of various organic substances on the characteristics of oxygen transfer from air bubbles to water. The compounds listed in Table 1 were chosen to represent various groups of water contaminants. To elucidate the effective mechanisms, experi ments were conducted by using either a fritted glass diffuser or a capillary for gas dispersion. The effects of gas flow rate, pH, and ionic strength were also examined.

Abstract: The injection of powder into liquids has been investigated by physical modeling and by multi-phase fluid dynamic modeling. The transition from gas-particle jets which penetrate deeply into the liquid and a gas bubbling regime was found to depend on the coupling between gas and particle phases in the conveying line; fine particles at high loading couple well and form jets, whereas coarse particles separate from the gas during bubble formation. The measured penetration depths of submerged jets in water and lead and top jets in water were very well described by equations balancing the momentum of the jet and its buoyancy. A regime of particle-liquid jets that forms in conjunction with bubbling also appears to depend on coupling, between the particle and liquid phases. The effect of surface tension on the particle penetration through a bubble interface was modeled for the single particle and multi-phase cases and compared with the work of others. On the basis of this modeling, the expected regime of flow for many powder injection conditions can be predicted. The flow regimes of existing processes are discussed, and guidelines for the design of processes employing various types of reactions are presented.

Abstract: Method and apparatus are disclosed for determining one or more physical characteristics of individual bubbles in a gas-liquid system and a gas-liquid-solid system at high temperatures and pressures. An in situ probe device is inserted into the system over which individual bubbles flow. The probe device has a plurality of independent probes. Each has a rounded fibre optic end portion projecting into the system. A source of incident light is directed onto each of the probes. The rounded end portion of each probe is formed with a radius of curvature sufficiently large whereby the angle of incidence of the source light at the rounded portion is greater than the angle of total reflection for the fibre optic when in contact with the gas. The angle of incidence is less than the angle of total reflection for the fibre optic when in contact with the liquid. The plurality of probes are spatially arranged to detect one or more of the bubble physical characteristics as a bubble flows over the probe device. The change in light intensity of reflected light emerging from the probe is measured. The change in light intensities of each of the probes over time is evaluated to determine the one or more bubble characteristics. Each probe is formed of sufficiently thin fibre optic and spaced from the other probes of the device to enable detection of the bubble characteristics for individual bubbles flowing over the probe device. This probe system enables the monitoring of physical characteristics of bubbles in two and three phase systems in an efficient, reliable, economical manner. The system also provides for a measure of solid, gas and liquid hold-ups in a three-phase system.

Abstract: A simplified model to study the mechanism controlling the growth of vapor bubbles in superheated pure and multicomponent liquids is formulated. The model is used to analyze the effect of ambient pressure and of the presence of solid particles or gas pockets on the nature and character of the liquid-phase disruption which may occur during the vaporization and burnng of emulsified and multicomponent fuel droplets. The analysis is in good qualitative agreement with existing atmospheric pressure experimental results. The “micro-explosive” buring of water-in-fuel emulsion droplets is caused by the very fast growth of superheated water-vapor bubbles. It is shown that the growth rate of these bubbles is primarily governed by the pressure difference between the superheated vapor and the liquid and by the inertia imparted to the liquid by the motion of the bubble surface (“Inertia controlled growth”). For the multicomponent fuel cases the model shows that the disruption of the droplets results from a much slower vapor-bubble growth which is governed by heat diffusion from the liquid to the bubble rather than by inertial and pressure effects (“Diffusion controlled growth”). Furthermore, it is predicted that any emulsion or multicomponent fuel droplet for which liquid-phase disruption is observed at atmospheric pressure will also exhibit disruption at higher pressures. However, as the ambient-pressure is increased the bubble growth rate will decrease in both solution and emulsion cases, resulting in a less effective and slower disruption. Finally, it is shown that through a reduction in the super-heat limit temperature, the presence of solid particles or dissolved gases in the liquid may also result in a less effective and slower disruption.

Abstract: Four aspects of liquid-phase mixing in semi-batch bubble columns operating with viscous and non-Newtonian liquids were studied: the influence of the central plume on mixing rate, the velocity profiles, the gas hold-up, and the back-mixing. Three regimes of the bubble-induced mixing were identified and associated with the mode of the central plume. One of the modes, due to a moderate gas rate, was found to lead to optimal mixing. Also the gas hold-up and the degree of back-mixing were associated with the central plume mode. A model of the velocity profile proposed earlier was now modified and extended to apply also to the turbulent regime.

Abstract: In order to investigate the mechanism of breakdown of the liquid-filled, triggered spark gap, a flash-illuminated, shadowgraph optical system has been used to photograph the pre-breakdown events in a triggered gap. Photographs indicate that in all cases the trigger spark is followed by the growth of a hemispherical bubble, or vapor cavity, and this bubble appears to be the precursor of the main gap breakdown. A theoretical investigation shows that the expansion and collapse of the cavity in the low-viscosity limit, follows a simple hydrodynamic model. We find that only about 1% of the circuit energy is transferred to kinetic energy of the liquid surrounding the expanding cavity. The time required for an expanding bubble to fill a 1 mm gap is of the same order as the breakdown time lag for the triggered gap. It is concluded that the bubble generated by the trigger spark clears the gap of liquid, leaving a low-density gas or vapor between the electrodes, so that the actual process of electrical breakdown takes place through the low strength gas, not through the liquid. In the case of longer gaps, of 2 mm and above, the bubble may have time to expand across the entire gap and it is suggested that an electrohydrodynamic instability may cause the bubble surface to breakup into streamers, which cross the gap and cause breakdown.

Abstract: A technique has been developed which allows the radius of a bubble in a fluid to be accurately measured without disturbing the bubble. Results are presented which show that radius measurements of single air bubbles in water can be achieved by shining a He–Ne laser on the bubble and measuring the light scattered at 80°. Light intensity measurements at a scattering angle of 55° were used to calibrate a photodiode detection system and to demonstrate the correlation of data with Mie scattering theory. The photodiode was then moved to 80° for actual radius measurements. This technique was accurate to within 3% for bubbles with radii of <80 μm.

Abstract: The evolution regime of electrolytically generated hydrogen bubbles in acidic medium has been studied by means of spectral analysis. A stochastic model of the time series displayed by the electrolysis current during bubble evolution has been devised. The identification of the characteristic parameters of the evolution regime (nucleation rate, life time etc.) has been performed through the measured power spectral density. This analysis has been applied to the determination of the dissolution current of an iron electrode in sulphuric acid medium.

Abstract: The bubble growth in polymeric fluids under nonisothermal conditions has been molded using the viscoelastic constitutive equation of Leonov incorporating the diffusion of gas from the melt to the bubbles. The gas diffusion seems to be the primary mechanism of bubble growth. The theoretical predictions for Han and Yoo's isothermal experiment compare well with the experimental data. The bubble size distribution in a foam molded specimen of NORYL resin has also been determined by measuring the bubble size from micrographs. Theoretical predictions for this specimen are in reasonable agreement with the data. A parametric study indicates that the blowing agent concentration has the strongest influence on the final bubble size, followed by melt and the mold temperature.