Decrease In Bubble Size Research Articles

Shock waves in a fluid with bubbles containing evaporating drops of a liquefied gas

The propagation and reflection of one-dimensional plane unsteady waves and pulses in a mixture of a fluid with “two-phase” bubbles containing evaporating drops is investigated. A significant effect of unsteady evaporation of the drops in the zone ahead of the shock wave on the wave propagation is demonstrated. The evaporation of the drops results in a pressure increase ahead of the wave and the shock wave as it were “climbs” to increasing pressure level. In contrast to bubbly fluids with “single-phase” bubbles, in a fluid with “two-phase” bubbles, at a fixed phase volume fraction, a decrease in bubble size results in an increase rather than a decrease of the oscillation amplitude. The wave reflection from a solid wall is essentially nonlinear and the maximum pressure attained at the wall is several times greater than the incident-wave intensity.

Experimental and modelling study of gas dispersion in a double turbine stirred tank

Gas dispersion in a double turbine stirred tank is experimentally characterised by measuring local gas holdups and local bubble size distributions throughout the tank, for three liquid media: tap water, aqueous sulphate solution and aqueous sulphate solution with PEG. For all these media, bubble coalescence generally prevails over breakage. Where average bubble size decreases, this can be attributed to the difference in slip velocity between different sized bubbles. Most of the coalescence takes place in the turbine discharge stream. A compartment model that takes into account the combined effect of bubble coalescence and breakage is used to simulate gas dispersion. The model predicts spatial distribution of gas holdup and of average bubble size, with average bubble size at the turbines as an input. Reasonable agreement between experiment and simulation is achieved with optimisation of two parameters, one affecting mainly the slip velocity, the other related mainly to the bubble coalescence/breakage balance. Different sets of parameters are required for each of the three liquid systems under study, but are independent of stirring/aeration conditions. The model only fails to simulate the smaller average bubble diameters at the bottom of the tank.

Discrete-phase simulation of single bubble rise behavior at elevated pressures in a bubble column

Numerical simulation is performed on the flow behavior of a bubble rising in a liquid medium at high pressures. In the numerical scheme, a computational fluid dynamics (CFD) method is used to account for the flow characteristics of the continuous liquid phase and the gas phase inside the bubble. The volume tracking based on the volume-of-fluid (VOF) method is used to describe the free surface motion of gas bubble, and the deformational bubble shapes. Simulations are performed simultaneously for both liquid and gas phases by considering the volume-fraction-weighted averaged properties on the interface. The behavior of a single gas bubble rising in Paratherm NF heat transfer liquid at pressures of 0.1 and 19.4 MPa is simulated and compared with the experimental results obtained in this study. The comparison is satisfactory. The results of the simulation indicate that as the pressure increases, the maximum stable bubble size decreases, which is in conformity with the experimental findings.

Heat-Transfer Characteristics in Slurry Bubble Columns at Elevated Pressures and Temperatures

The heat-transfer behavior between an immersed heating surface and the surrounding gas−liquid−solid medium in a slurry bubble column is studied experimentally and analytically. The operating pressures and temperatures vary up to 4.2 MPa and 81 °C, respectively. Nitrogen, Paratherm NF heat-transfer fluid, and 53-μm glass beads are used as the gas, liquid, and solid phases, respectively. The solids concentrations are varied up to 35 vol %, while the superficial gas velocities are varied up to 20 cm/s. The effects of gas velocity, solids concentration, pressure, and temperature on the heat-transfer coefficient are examined. It is found that pressure has a significant effect on heat-transfer characteristics. The heat-transfer coefficient in a slurry bubble column decreases appreciably with an increase in pressure. It is noted that the variation in the heat-transfer coefficient with pressure is attributed to the counteracting effects of the increased liquid viscosity, decreased bubble size, and increased gas h...

Gas dispersion properties: bubble surface area flux and gas holdup

Bubble surface area flux has been related to flotation performance and advocated as a key “machine variable”. Analysis of available data suggests that bubble surface area flux (S b) and gas holdup ( ε g) are related by S b ∼ 5.5 ε g. This was the case for flotation columns and mechanical cells, both laboratory and plant scale, over the approximate range S b < 130s −1 and ε g < 25%. A relationship is expected given their similar dependence on gas rate and bubble size (both increase as gas rate increases and bubble size decreases). Reasons that a relationship may be obscured, especially in full size units because of the difficulty in defining the overall S b and ε g, are discussed. The possibility to replace S b by ε g as the machine variable has the advantage that gas holdup is easier to measure.

A Review of Interfacial Phenomena of Mixture Boiling under Microgravity

This paper provides a mini-review for current understanding toward the boiling transport phenomena in microgravity environment, considering interfacial behavior, bubble dynamics and boiling heat transfer rate. The Marangoni effect, or the thermocapillary-driven liquid flow around bubbles, has finite impact on pool boiling in one-g environment; however, its role becomes much more significant as the bubble size and/or gravity decrease. Under micro- or zero-gravity condition the interfacial flow, replacing buoyancy and natural convection, dominates the interfacial behavior, bubble dynamics and heat transfer performance. The models and heat transfer correlations had been proposed for predicting pool boiling heat transfer of mixture of liquids in microgravity environment.

Water blown free rise polyurethane foams

AbstractA model is developed in this work for predicting the bubble size distribution in polyurethane foams generated by using water as a chemical blowing agent. The model combines equations of energy balance, kinetics of the reactions of isocyanate with water and polyol, and nucleation and growth of CO2 bubbles. It is found that as the water content of the reaction mixture is increased, the mean bubble size decreases and the bubble size distribution become narrower. Exactly the opposite occurs in polyurethanes foamed with a physical blowing agent, e.g., DuPont Freons. This suggests that a combination of physical and chemical blowing agents can be employed to control bubble size distribution.

Flow characteristics and modelling of foam generation in a continuous rotor/stator mixer

Within this study, the Newton/Reynolds characteristics of a continuous rotor/ stator mixer for foam production were determined with special geometrically adapted Newton and Reynolds numbers. A critical Reynolds number of 105 was derived. Equations for the laminar and turbulent flow field for pre-calculating the maximum bubble size in rotor/stator mixers were derived and experimentally confirmed. These equations combine process, recipe and geometry parameters and a direct scale-up is possible. An increase of the rotor speed from 600 to 2000 rpm resulted in a decrease in bubble size of about a factor of 4.5 for a low viscous liquid, whereas for a highly viscous liquid, the decrease in bubble size in the same rotor speed range was only 20%. Drainage stability improvement of about 100% and an increase in foam viscosity of about 100% were achieved in the same rpm range.

Bubble size control in a two-dimensional fluidized bed using a moving double plate distributor

Pulsating the flow in a fluidized bed by using a moving double plate distributor (MDPD) to control the bubble size and enhance the fluidization characteristics is a new approach. In this novel design, a second plate is placed under the existing distributor such that it can make a reciprocating motion generating pulses to the flow field. The frequency and amplitude of the pulsation can be adjusted to break up the large bubbles in the bed. The aim of this study is to investigate the effect of pulsation on the bubble size, bubble speed, bubble fraction and bed expansion in a fluidized bed. For this purpose, the bubbles were followed in a two-dimensional fluidized bed with turnip seed and sand as the bed material for different pulsation frequencies and operating velocities. Bubble size, speed and bed expansion were measured for pulsed flow and compared with those of continuous flow. Image processing techniques were utilized to measure the bubble fraction. Interesting bubble flow patterns were observed and a significant decrease in bubble size was achieved for the pulsed flow.

High‐pressure three‐phase fluidization: Hydrodynamics and heat transfer

AbstractThe phase holdups and the heat‐transfer behavior were studied experimentally in three‐phase fluidized beds over a pressure range of 0.1–15.6 MPa. Bubble characteristics in the bed are examined by direct flow visualization. Pressure effects on the bubble coalescence and breakup are analyzed mechanistically. The study indicates that the pressure affects the hydrodynamics and heat‐transfer properties of a three‐phase fluidized bed significantly. The average bubble size decreases and the bubble‐size distribution becomes narrower with an increase in pressure. The bubble‐size reduction leads to an increase in the transition gas velocity from the dispersed bubble regime to the coalesced bubble regime, an increase in the gas holdup, and a decrease in the liquid and solids holdups. The pressure effect is insignificant above 6 MPa. The heat‐transfer coefficient between an immersed surface and the bed increases to a maximum at pressure 6–8 MPa and then decreases with an increase in pressure at a given gas and liquid flow rate. This variation is attributed to the pressure effects on phase holdups and physical properties of the gas and liquid phases. A mechanistic analysis revealed that the major heat‐transfer resistance in high‐pressure three‐phase fluidized beds resides in a liquid film surrounding the heat‐transfer surface. An empirical correlation is proposed to predict the heat‐transfer coefficient under high‐pressure conditions.

Liquid laundry detergent formulations of zeolites and surfactants

Recently we showed that triple surfactant mixtures, comprising the anionic surfactant SLES, zwitterionic surfactant CAPB and long-chain fatty acid (FAc), possess very high surface modulus which can be used to modify the dynamic properties of foams (Golemanov et al. Langmuir 2008, 24, 9956). In the current paper we perform a systematic experimental study of the effects of several factors on the surface tension, surface dilatational modulus, viscous friction inside sheared foam, foam-wall friction, and mean bubble size in sheared foams, for such triple surfactant mixtures. The factors studied are: (1) chain length of fatty acid; (2) fatty acid concentration; (3) pH; (4) presence of glycerol in the aqueous solution. Lauric (C12Ac), myristic (C14Ac) and palmitic (C16Ac) acids are studied. The concentrations of the main surfactants were fixed, 10 mM for SLES and 5 mM for CAPB. Varying the myristic acid (C14Ac) concentration, CC14Ac, we observed two ranges: (1) At CC14Ac <0.06 mM, the acid increases the solution surface modulus up to 50 mN/m, however, without affecting significantly any of the other properties studied; (2) At CC14Ac > 0.11 mM, C14Ac leads to significant decrease of surface tension (from 29 to 22 mN/m), increase of surface modulus (from 50 to 400 mN/m), decrease of mean bubble size in sheared foams (from 300 to 150 μm), and strong increase of the inside-foam friction and foam-wall friction. Qualitatively similar results are obtained with the other FAc studied. Mass-balance estimates show that low surface tension and high surface modulus are observed, when the molar fraction of FAc in the adsorption layer approaches ca. 30%. The main role of FAc is to induce a surface phase transition, leading to formation of surface condensed phase in the mixed adsorption layer. For all systems studied, the increase of pH above a certain transitional value leads to a sharp increase of surface tension and decrease of surface dilatational modulus, which is accompanied with a decrease in the inside-foam and foam-wall viscous friction. The transitional pH value varies between 8 and 11, and increases with the fatty acid chain-length. The increase of pH causes ionization of the FAc molecules – the latter start to behave as usual anionic surfactant without forming surface condensed phase. The main effect of glycerol is to decrease the transitional pH (by 1 to 1.5 units for 40 wt% glycerol), without affecting significantly the system properties away from the transitional pH. These results clarify the compositional domain, in which the fatty acids can be used for control of surface and foam properties in such surfactant mixtures.

Stability of Aqueous Foams with Polymer Additives: III. Measurements and Calculations of Stability of Foams Generated at Different Pressures

The stability of aqueous foams with polymer additives has been studied at different generation pressures. While the stability increases significantly with the increase of generation pressure for foams without any additive, it, however, changes weakly with the change in generation pressure for foams with polymer additives. Consistent with the previously reported studies, it is found that the foam stability (drainage half-life) increases, primarily due to the decrease of bubble size, at higher generation pressures, when the foam is produced in a packed bed. Calculations of drainage half-life were conducted using a modified lamella drainage model as well as a simple network model. It is found that the network model predictions agree reasonably well with measurements.

Simulation of Argon Gas Flow Effects in a Continuous Slab Caster

Three-dimensional finite-volume-based numerical models of fluid, heat, and mass transport have been developed and applied to help explain the complex inter-related phenomena of multiphase fluid flow, superheat dissipation, and grade intermixing during the continuous casting of steel slabs. Gas bubbles are simulated using a continuum model, which calculates the volume fraction and velocities of the gas, and its effect on the liquid flow. Turbulence has been incorporated using the standardK-e turbulence model. Reasonable agreement has been achieved between predicted velocities and corresponding measurements and observations in full-scale water models, both with and without gas injection. The effects of argon gas bubble injection on flow-related phenomena are investigated with simulations of a typical steel slab caster. Argon bubbles alter the flow pattern in the upper recirculation zone, shifting the impingement point and recirculation zones upward. The effect increases with increasing gas fraction and decreasing bubble size. Argon injection also causes superheat to be removed higher in the caster, moves the hot spot upward, lowers the peak heat flux, and increases heat extraction from the wide face and meniscus regions. During a steel grade transition, argon injection slightly affects slab surface composition but has no effect on intermixing in the slab interior.

Computer simulations of the rupture of a gas bubble at a gas—liquid interface and its implications in animal cell damage

Hydrodynamic information of the flow occurring as a bubble breaks at a gas—liquid interface has been obtained from computer simulations. The transient rate of energy dissipation per unit volume in the region close to the bubble interface has been calculated. It was found that when a bubble collapses, there is a very high, localized energy dissipation which can be used to explain animal cell damage in sparged suspended animal cell cultures. The rate of energy dissipation was found to be a strong function of bubble size, increasing rapidly as the bubble size decreases.

Microbubble flotation

The first-order flotation rate is directly proportional to the superficial gas rate and varies approximately as the inverse cube of bubble size, suggesting that decreasing bubble size is a more effective means of improving flotation rate than increasing gas rate. For this reason, many new flotation machines, mostly in the form of columns, have been developed recently, in which various forms of microbubble generators are incorporated. Despite the successful demonstration in industry, there are still problems concerning scale-up and maintenance of the bubble generators. This paper provides a theoretical basis of the microbubble flotation technology and offers a discussion on matters concerning scale-up.

The effect of surface heterogeneity on pseudo-line tension and the flotation limit of fine particles

The contact angles θ for water at methylated quartz surfaces were measured using the sessile-drop technique to determine the advancing contact angle, and using a captive-bubble technique to examine the effect of bubble size on contact angle. No linearity between cos θ and 1 r where r is the drop base radius, was observed for these systems as would be expected for an ideal system. In fact the pseudo-line tension decreased with decreasing bubble size. Also, the degree of quartz methylation effected a change in the pseudo-line tension. The pseudo-line tension increased from 0.4·10 −6 N to 3.3·10 −6 N with an increase in the fractional coverage of trimethylsilyl groups from 0.14 to 0.51 for large bubbles (bubble base diameter d > 0.34 mm), whereas the pseudo-line tension decreased from 2.2·10 −7 N to 0.8·10 −7 N with an increase in fractional coverage for small bubbles (bubble base diameter d = 0.06–0.2 mm). The flotation limit of fine particles has been re-examined based on the effect of bubble size on contact angle, and a new surface chemistry-limited relationship describing the minimum particle size which can be floated is proposed: ▪ where r c is the critical bubble (drop) radius for which there is no effective attachment between solid surface and dispersed phase, 7 LV is the interfacial tension at the liquid-vapor interface, Δ p is the density difference between the particle and the liquid and V is the bubble ascent velocity.

Pressure effects on the hydrodynamic behavior of gas-liquid-solid fluidized beds

Basic hydrodynamic behavior of a gas-liquid-solid fluidized bed at elevated pressures up to 1 MPa was studied by measurement and visualization techniques in a specially constructed rectangular column. Conditions of gas and liquid velocities, particle size, distributor type, and pressure were varied in the apparatus, which was designed to allow operation in both two- and three-dimensional modes. Visualization in two-dimensional operation and measurement in three-dimensional operation were combined to provide a unique tie point between the two modes. The study indicated that bubble size decreases, bubble size distribution narrows, and gas holdup increases with increasing pressure. The type of distributor was found to have a marked impact on bed behavior, including bulk circulation, bubble coalescence, and gas holdup. Possible mechanisms for the observed pressure effects on the hydrodynamic behavior are also discussed, combined with available experimental observations. Analysis indicates that initial bubble size and maximum stable bubble size should decrease with increasing pressure due to changes in interfacial physical properties as well as in gas density

Gas induced bath circulation in aluminium reduction cells

Gas induced bath circulation in the interpolar gap of aluminium cells was studied in a room temperature physical model and by computer simulation. The circulation velocity increased with increasing gas formation rate, increasing angle of inclination and decreasing bath viscosity, while it was less affected by anode immersion depth, interpolar distance (in the normal range), and convection in the metal. A typical bath velocity near the cathode was 0.05 m s−1. The flow velocity decreased with decreasing bubble size. The results were fitted to a simple semi-empirical expression, and the velocities measured in the model experiments were in good agreement with the findings of the computer simulation.

Particle size dependence in flotation derived from a fundamental model of the capture process

A flotation model is described wherein particle collection is considered to occur by particle-bubble collision followed by the particle sliding over the bubble during which attachment may occur. Collision is quantified by a collision efficiency E C. The existing model for E C is extended by including particle inertia. Attachment is quantified by an attachment efficiency E A calculated as the fraction of particles which reside on the bubble for a time greater than the induction time. These calculations use recent data on the liquid velocity distribution over the upper half of rigid spheres. Collection efficiency E K is given by E K= E C· E A. Detachment is not considered. The model is examined and shown to agree with much experimental data. For example the maximum observed in recovery vs particle size ( d p) can be explained as E C increases with increasing d p, while E A decreases. The decrease in selectivity at fine sizes is in accord with the model result that for very small particle sizes E A is large even for long induction times. A decrease in bubble size does increase E K but does not improve selectivity. A high particle density (> 5 g cm −3) is shown to cause a sharp peak in recovery vs size and a decreased maximum size of flotability. Finally, the effects of bubble concentration are explored.

Influence of temperature and properties of solids on the size and growth of bubbles in gas fluidized beds

AbstractIn the present paper, models are developed for the description of coalescence and splitting of bubbles in gas fluidized beds. The distinction between slow and fast bubbles, which originates from the Davidson model of gas flow in and around a rising bubble, was found to be decisive for the modelling of coalescence processes. On the basis of the respective models, it is shown that a change in bed temperature does influence the mechanisms of both bubble coalescence and splitting. In cases of both slow and fast bubbles undergoing splitting, the theory predicts a decrease of bubble size with temperature whereas in the case of a non‐splitting system with fast bubbles, typically encountered with Geldart type B particles, no significant temperature effect on bubble size is expected. The theoretical predictions are shown to be in agreement with own measurements as well as with the results of other workers.