Number Of Active Bubbles Research Articles

A semi‐empirical approach for determining the number density and void fraction of acoustic cavitation bubbles in sono‐reactors

AbstractA new semi‐empirical technic has been established relying on the linkage between the chemistry in the bulk liquid and that taking place in the acoustic cavitation bubble. The open‐source COPASI software has been used for the optimization of number density according to the total yield of a single bubble and the fitting of the experimental yield of hydrogen peroxide in the sonicated solution. It was observed that the number density is increased with the rise of ultrasound frequency from 200 to 1140 kHz, independently of the saturating gas nature (O2, Ar or air). Within this range of wave frequencies, i.e. from 200 to 1140 kHz, the number of active bubbles goes up from 9.35 × 107 to 3.65 × 1015 L−1 s−1. On the other side, it has been demonstrated that the number density obtained under air atmosphere is greater than that resulting either under argon or oxygen‐saturating gas. Interestingly, with respect to the saturating gas nature (O2, Ar, air) and the range of ultrasound frequency (200–1140 kHz), it was observed that the increase of number density was not necessarily accompanied by a proportional increase of void fraction (total volume of bubbles).

Combined effects of flow, surface stabilisation and salt concentration in aqueous solution to control and enhance sonoluminescence.

Sonoluminescence (SL) intensity can be increased with potassium iodide (KI) concentration, attributed to a reduction in the gas concentration of solution. However, bubble properties and active bubble distributions at different frequencies and powers also influence SL intensities. Hence, to elucidate how salt concentration affects SL intensity, a systematic study with parametric variation was undertaken. SL from KI solutions of 0.1, 1 and 2 M concentration, without flow and in the presence of flow at 24, 228 and 626 mL/min was investigated at 44, 300 and 1000 kHz. At all frequencies an increase in KI concentration caused a change in the active SL distributions. For 44 kHz, localised and standing wave field SL activity could be expanded. Flow at this frequency augmented SL and SL was maximised at the lowest power setting under stabilisation at the highest KI concentration. At 300 and 1000 kHz, attenuation of the sound field was reduced, allowing expansion of activity throughout solution. In this instance, augmentation of SL intensity was only observed under flow conditions at concentrations of 1 M (300 kHz) and 2 M (1000 kHz) under stabilisation. It was theorised that a combination of smaller bubbles at higher KI concentrations and flow effects could reduce bubble clustering and enhance field formations. This was most prevalent where the standing wave was reinforced under stabilised (44 and 300 kHz) or flow (1000 kHz) conditions, here the number of active bubbles in high pressure regions likely increases. Lastly, it was found that in KI solutions flow could localise SL activity beneath and at the flow inlet via reflection and aeration mechanisms.

Impact of seawater salinity on the sonochemical removal of emerging organic pollutants

ABSTRACT The results presented in this study illustrate the multiple roles of seawater salinity toward the sonochemical degradation, at variable frequencies (300–1700 kHz), of several hazardous substances, i.e. propylparaben (PPR) endocrine disruptor and several synthetic dyes: naphthol blue black (NBB), malachite green (MG), basic red 29 (BR29), acid orange 7 (AO7), Rhodamine B (RhB) and basic fuchsin (BF). Sonochemical treatment degraded all pollutants in seawater at faster rates than in deionized water. The seawater-salts through increasing the ionic strength of the solution act as a potential pusher of hydrophilic pollutants toward the reactive interfacial area of cavitation bubbles. Additionally, the salts reduce the bubble coalescence, which yields higher number of active bubbles in the irradiating media. Analysing the degradation rate of PPR and NBB with two heterogeneous models based on Langmuir kinetics mechanism indicated that the bubble interfacial area was the preferred reaction zone for the ultrasonic degradation of PPR and NBB in seawater.

Characterization and application of a 1700-kHz acoustic cavitation field for water decontamination: a case study with toluidine blue

This work aimed at the characterization and application of a cavitation field induced in water by an ultrasonic reactor operating at 1700 kHz and 15 W. It was found that the size of active bubbles varied from 0.23 to 3 µm. The number of active bubbles increased from 6.1142 × 108 s−1 L−1 at 25 °C to 4.4684 × 109 s−1 L−1 at 55 °C. The most active bubbles were those achieving temperature of 4000 K and pressure of 1000 atm at the collapse. The characterized cavitation field removed efficiently toluidine blue (TB), an emerging organic contaminant, through reaction with hydroxyl radical. The best TB-removal rate was obtained under argon saturation, but CO2 completely suppressed the process. TB degradation rate sensitively enhanced with increasing initial substrate concentration and solution pH, whereas the liquid temperature did not affect the degradation rate. Formic acid, as an organic competitor, reduced considerably the degradation of the pollutant.

Experimental and numerical investigation of the effect of liquid temperature on the sonolytic degradation of some organic dyes in water

This paper presents a comprehensive experimental and numerical investigation of the effects of liquid temperature on the sonochemical degradation of three organic dyes, Rhodamine B (RhB), Acid orange 7 (AO7) and Malachite green (MG), largely used in the textile industry. The experiments have been carried out for an ultrasonic frequency of 300kHz. The obtained experimental results were discussed using a new approach combining the results of single-bubble event and the number of active bubbles. The single-bubble event was predicted using a model that combines the bubble dynamics with chemical kinetics occurring inside a bubble during the strong collapse. The number of active bubbles was predicted using a method developed in our previous work. The experiments showed that the degradation rate of the three dyes increased significantly with increasing liquid temperature in the range 25–55°C. It was predicted that the main pathway of pollutants degradation is the attack by OH radicals. The simulations showed that there exists an optimum liquid temperature of about 35°C for the production of OH inside a bubble whereas the number of active bubbles increased sharply with the rise of the liquid temperature. It was predicted that the overall production rate of OH increased with increasing liquid temperature in the range 25–55°C. Finally, it was concluded that the effect of liquid temperature on the sonochemical degradation of the three dyes in aqueous phase was controlled by the number of active bubbles in the range 35–55°C and by both the number of bubbles and the single bubble yield in the range 25–35°C.

Comprehensive experimental and numerical investigations of the effect of frequency and acoustic intensity on the sonolytic degradation of naphthol blue black in water

In the present work, comprehensive experimental and numerical investigations of the effects of frequency and acoustic intensity on the sonochemical degradation of naphthol blue black (NBB) in water have been carried out. The experiments have been examined at three frequencies (585, 860 and 1140kHz) and over a wide range of acoustic intensities. The observed experimental results have been discussed using a more realistic approach that combines the single bubble sonochemistry and the number of active bubbles. The single bubble yield has been predicted using a model that combines the bubble dynamics with chemical kinetics consisting of series of chemical reactions (73 reversible reactions) occurring inside an air bubble during the strong collapse. The experimental results showed that the sonochemical degradation rate of NBB increased substantially with increasing acoustic intensity and decreased with increasing ultrasound frequency. The numerical simulations revealed that NBB degraded mainly through the reaction with hydroxyl radical (OH), which is the dominant oxidant detected in the bubble during collapse. The production rate of OH radical inside a single bubble followed the same trend as that of NBB degradation rate. It increased with increasing acoustic intensity and decreased with increasing frequency. The enhancing effect of acoustic intensity toward the degradation of NBB was attributed to the rise of both the individual chemical bubble yield and the number of active bubbles with increasing acoustic intensity. The reducing effect of frequency was attributed to the sharp decrease in the chemical bubble yield with increasing frequency, which would not compensated by the rise of the number of active bubbles with the increase in ultrasound frequency.

Nucleate pool boiling of aqueous polymer solutions on a cylindrical heater

Heat transfer and ebullience behavior in saturated, nucleate pool boiling in aqueous dilute to semi-dilute solutions of a surface-active (hydroxyethyl cellulose or HEC-QP300) and a shear-thinning (polyacrylic acid or Carbopol 934) polymer on a horizontal, cylindrical heater are experimentally characterized. The influence of fluid rheology and interfacial properties (dynamic surface tension and wettability) on the heat transfer coefficient is delineated. In HEC solutions, nucleate boiling is enhanced with increasing concentration c till an optimum near the critical polymer concentration ( c * ∼600 wppm); at higher concentrations, the enhancement decreases considerably. Contrarily, there is continuous deterioration in the boiling heat transfer, relative to water, in Carbopol solutions with increasing concentrations. Adsorption of macromolecules, or agglomerates of smaller monomers onto a heating surface that may favor the formation of new nucleation sites, together with a decreased dynamic surface tension, are responsible for the general growth in the number of active bubbles in surface-active HEC solutions. In HEC solutions with c > c * as well as in all Carbopol solutions, on the other hand, higher viscosity tends to suppress the micro-convection near the wall and the bubble growth, thereby weakening the boiling process.