Bubble Concentration Research Articles

Small bubbles formation and contribution to the overall gas holdup in large diameter columns of very high viscosity oil

A limited number of studies are available in literature on the small bubbles which create from gas-very high viscosity oils interaction and its contribution to the gas holdup in the system. The rate of small bubble formation has an important impact on heat and mass transfer in many chemical and industrial processes. The work presented in the current paper provides unique information on the formation of bubbles of millimetre diameter in high viscosity oil. A column of 290 mm diameter and Silicon oil of 330 Pa.s viscosity, were employed besides Electrical Capacitance Tomography and a high-resolution camera to investigate the characteristics of the small bubbles. Mechanism of bubble generation, effect of gas injection time and flowrate were studied. The average void fraction, total gas-liquid height, overall Probability Density Function (PDF) profile, small bubbles volume fractions and diameter were measured. Small bubbles generate from the eruption of large bubbles, at gas injection nozzles, coalescence of large bubbles, and at liquid bridges at transition to churn flow regime. Properties and concentration of the small bubbles are controlled by the location of the bubble generation, gas flowrate, and gas injection time. Small bubbles contribute by 6.6- 30% to the total gas holdup. Bubble diameter increased from 0.68 mm to 0.75 mm and decreased from 1.1 mm to 0.75 mm at the top and the bottom sections of the column respectively after 60 minof gas injection.

Numerical Modeling of Transient Two-Phase Flow and the Coalescence and Breakup of Bubbles in a Continuous Casting Mold.

The multiphase flow and spatial distribution of bubbles inside a continuous casting (CC) mold is a popular research issue due to its direct impact on the quality of the CC slab. The behavior of bubbles in the mold, and how they coalesce and break apart, have an important influence on the flow pattern and entrapment of bubbles. However, due to the limitations of experiments and measurement methods, it is impossible to directly observe the multiphase flow and bubble distribution during the CC process. Thus, a three-dimensional mathematical model which combined the large eddy simulation (LES) turbulent model, VOF multiphase model, and discrete phase model (DPM) was developed to study the transient two-phase flow and spatial distribution of bubbles in a continuous casting mold. The interaction between the liquid and bubbles and the coalescence, bounce, and breakup of bubbles were considered. The measured meniscus speed and bubble diameter were in good agreement with the measured results. The meniscus speed increased first and then decreased from the nozzle to the narrow face, with a maximum value of 0.07 m/s, and appeared at 1/4 the width of the mold. The current mathematical model successfully predicted the transient asymmetric two-phase flow and completely reproduced the coalescence, bounce, and breakup of bubbles in the mold. The breakup mainly occurred near the bottom of the submerged entry nozzle (SEN) due to the strong turbulent motion of the molten steel after hitting the bottom of the SEN. The average bubble diameter was about 0.6 mm near the nozzle and gradually decreased to 0.05 mm from the nozzle to the narrow face. The larger bubbles floated up near the SEN due to the effect of their greater buoyancy, while the small bubbles were distributed discretely in the entire mold with the action of the molten steel jet. Overall, the bubbles were distributed in a fan shape. The largest concentration of bubbles was in the lower part of the SEN and the upper edge of the SEN outlet.

Numerical Simulation of Micro-Bubbles Dispersion by Surface Waves

This paper presents an algorithm for numerical modeling of bubble dispersion occurring in the near-surface layer of the upper ocean under the action of non-breaking two-dimensional (2D) surface waves. The algorithm is based on a Eulerian-Lagrangian approach where full, 3D Navier-Stokes equations for the carrier flow induced by a waved water surface are solved in a Eulerian frame, and the trajectories of individual bubbles are simultaneously tracked in a Lagrangian frame, taking into account the impact of the bubbles on the carrier flow. The bubbles diameters are considered in the range from 200 to 400 microns (thus, micro-bubbles), and the effects related to the bubbles deformation and dissolution in water are neglected. The algorithm allows evaluation of the instantaneous as well as statistically stationary, phase-averaged profiles of the carrier-flow turbulence, bubble concentration (void fraction) and void-fraction fluxes for different flow regimes, both with and without wind-induced surface drift. The simulations results show that bubbles are capable of enhancing the carrier-flow turbulence, as compared to the bubble-free flow, and that the vertical water velocity fluctuations are mostly augmented, and increasingly so by larger bubbles. The results also show that the bubbles dynamics are governed by buoyancy, the surrounding fluid acceleration force and the drag force whereas the impact of the lift force remains negligible.

超音波照射によるウルトラファインバブルの発生と消滅

Ultrasound was irradiated to ultrapure water. Ultrafine bubbles with a diameter of 90 - 100 nm were generated. Number concentration of ultrafine bubbles increased with time and approached asymptotically to an equilibrium value. Ultrafine bubble concentration increased with increasing ultrasonic power and decreasing frequency. For comparison, highly concentrated ultrafine bubble water was prepared by pressurized dissolution method, and ultrasound was also irradiated to the ultrafine bubble water. Reduction of ultrafine bubbles was observed and ultrafine bubble concentration decreased to the equilibrium value. Ultrafine bubble concentration decreased with increasing ultrasonic power and frequency. Generation and reduction of ultrafine bubbles by ultrasound was modeled to analyze experimental data. The results calculated by our model were in good agreement with the experimental data.

Application of micro- and nano-bubbles in spray drying of milk protein concentrates

Micro- and nano-bubbles (MNB) have unique properties and have attracted great attention in the past 2 decades, offering prospective applications in various disciplines. The first objective of this study was to investigate whether venturi-style MNB generation is capable of producing sufficient bulk MNB. A nanoparticle tracking system was used to measure the bubble concentration and particle size of MNB-treated deionized water. The MNB-treated deionized water had a bubble concentration of 3.76 × 108 particles/mL (∼350 million bubbles/mL more compared with control) and a mean particle size of 249.8 nm. The second objective of this study was to investigate the effects of MNB treatment on the microstructure and functional properties of milk protein concentrate (MPC) dispersions. Reconstituted MPC dispersions (21%, wt/wt) without air injection were considered as control (C-MPC), and MPC dispersions passed through the MNB system were considered as MNB-treated (MNB-MPC) dispersions. Control and MNB-MPC dispersions were evaluated in terms of rheological behavior and microstructure. The microscopic observations of MNB-MPC dispersions showed less aggregated microstructures and greater structural differences compared with C-MPC dispersions, therefore lowering the viscosity. The viscosity of MNB-MPC at a shear rate of 100 s-1 significantly decreased to 57.58 mPa·s (C-MPC: 162.40 mPa·s), a net decrease in viscosity by ∼65% after MNB treatment. Additionally, MPC dispersions were spray dried after the MNB treatment, and the resultant MNB-MPC powders were characterized and compared with the control MPC in terms of rehydration characteristics and microstructure. Focused beam reflectance measurement of the MNB-MPC powders indicated lower counts of large particles (150-300 μm) during dissolution, signifying that MNB-MPC powders exhibited better rehydration properties than the C-MPC powders. This study, therefore, recommends the possibility of using MNB treatment for more efficient drying while improving the functional properties of the resultant MPC powders.

Effect of grid-screen on bubble characteristics of vertically discharged bubble plumes

A series of laboratory experiments was conducted to improve the entrainment and mixing capacity of vertically discharged bubble plumes by employing a grid-screed with different openings and at different distances from the nozzle. Bubble characteristics such as bubble size, bubble size distribution, bubble concentration, and bubble velocity were measured using an accurate Refractive Bubble Index (RBI) probe along the plume, before, and after the grid-screen. The effects of grid-screen openings and airflow discharge on variations of bubble characteristics were examined. Experimental results showed that the size of grid-screen and its distance from the nozzle decreased the vertical velocity of bubbles by an average of 38%. Dynamics of bubbles before and after the grid-screen was analyzed and a regime classification was proposed based on variations of the normalized bubble velocity with the distance from the nozzle. It was found that increasing the distance between the grid-screen and nozzle increased bubble concentration by approximately 9% and reduced bubble size by 31%. At a certain distance from the nozzle, the effect of the grid-screen became negligible and bubble velocity reached its initial value. This specific distance was extracted for all tests and the results showed that such critical distance was correlated with the nozzle Reynolds number. Empirical correlations were developed to estimate the effective mixing length after the grid-screen. The proposed equations were found to be correlated with the bubble Reynolds number.

The relationship between bubble concentration and the acoustic emission energy of separate frequency bands.

This letter presents the relationship between bubble concentration and the energy ratio of low to high frequency bands of their acoustic emissions. Two sensors, placed perpendicular and concentric to a transmitter, captured the emissions from sonicated microbubbles. Emissions from different bubbles arrived at the perpendicular sensor with small time differences. Low frequencies with periods longer than the time differences interfered constructively, while higher frequencies interfered both constructively and destructively. The low-frequency (2nd-3rd harmonics) to high-frequency (7th-12th harmonics) energy ratio increased with the bubble concentration. The relationship was not observed with the concentric sensor, where the time differences were larger.

Dopant binding with vacancies and helium in metal hydrides

Metal hydrides are crucial for the long-term storage of tritium but suffer degradation due to the buildup and release of helium decay products. Therefore, it is of interest to explore how dopants in these metal hydrides may impact helium bubble nucleation and distributions, as well as associated fracture and helium retention. Prior studies have focused on helium behavior in pure metal hydrides or with one or two types of impurities. Analytical models have also shown that the concentration of nucleated bubbles can impact the time to fracture of materials. This study utilizes high-throughput density functional theory calculations to identify the impact of transition metal substitutional dopants on helium binding energies in face-centered cubic metal hydrides, such as erbium hydride, holmium hydride, scandium hydride, titanium hydride, yttrium hydride, and zirconium hydride. This study also explores the impact of hydrogen vacancies on the binding energy of helium near the substitutional defects. Finally, this study presents an initial assessment of dopant stability and solubility limits at many temperatures and pressures. Several metals strongly bind to helium in these metal hydrides, making them promising for influencing helium nucleation and subsequent bubble growth. However, many of these potentially beneficial substitutional defects have low solubility limits. The calculations show that some strong binding dopants may be soluble in quantities that affect the bubble concentration and impact material performance.

Sonoluminescence emission spectra of a 3.6 MHz HIFU in sweeping mode

Use of sweeping mode with a 3.6 MHz High Intensity Focused Ultrasound (HIFU) allows cavitation activity to be controlled. This is especially true in the pre-focal zone where the high concentration of bubbles acts as an acoustic reflector and quenches cavitation above this area. Previous studies attributed the enhancement of cavitation activity under negative sweep to the activation of more bubble nuclei, requiring deeper investigations. After mapping this activity with SCL measurements, cavitation noise spectra were recorded. The behavior of the acoustic broadband noise follows the sonochemical one i.e., showing the same attenuation (positive scan) or intensification (negative scan) of cavitational activity. In 1 M NaCl 3.7 mM 2-propanol solution saturated by a mixture of Ar-15.5%O2-2.2%N2, intensities of SL spectra are high enough to allow detection of several molecular emissions (OH, NH, C2, Na) under negative frequency sweeps. This is the first report of molecular emissions at such high frequency. Their intensities are low, and they are very broad, following the trend obtained at fixed frequency up to 1 MHz. Under optimized conditions, CN emission chosen as a spectroscopic probe is strong enough to be simulated, which is reported for the first time at such high frequency. The resulting characteristics of the plasma do not show any spectral difference, so bubble nature is the same in the pre-and post-focal zone under different sweeping parameters. Consequently, SL and SCL intensification was not related to a change in plasma nature inside the bubbles but to the number of cavitation bubbles.

Analysis of Flow State in Venturi Tube for Fine Bubble Generation

Bubbles with a diameter of 100 μm or less are called fine bubbles, which have been applied widely in the industrial fields due to its strong detergent, bactericidal effect, and bioactivity. Ballast water is the seawater to stabilize the center of gravity of a ship. In water purification, a technology has been developed to reduce the dissolved oxygen (D.O.) concentration and pH by mixing water with gas to deaerate D.O. and dissolving CO2, but gas is mixed with such water. When purifying water quality, it is important to reduce the bubble diameter when gas is mixed with water, and to improve the efficiency of dissolving CO2 by degassing D.O.. In this research, a method to improve the mixing efficiency of ballast water treatment equipment by improving the fine bubble generation efficiency of the venturi tube is investigated by experiment and simulation analysis. We design venturi tubes with different divergent angles and investigate the optimized geometric combination by experiment and numerical simulation for improving the fine bubble size and volume. From the experimental results, we find that in the nozzles with small divergent angles the low pressure region becomes longer and the pressure recovery slows down, which promote the breakup of bubbles. The simulation results show that in the nozzles with the divergent angles of 15°and 20°, the affection region of gas-liquid two-phase flow becomes larger, and the concentration of bubbles near the wall or near the center of the nozzle will promote the breakup process of bubbles.

Utility of low-cost recreational-grade echosounders in imaging and characterizing bubbly coastal submarine groundwater discharge

Despite the growing knowledge on the significance of submarine groundwater discharge (SGD), mapping its occurrence is a continuing challenge. This study explores the capability and applicability of low-cost, off-the-shelf, recreational-grade echosounders (RGESs) to image different types and locate point sources of bubbly coastal SGD. Standard and systematic methodologies for efficient imaging and processing were established. The use of RGES was validated using a research-grade side scan sonar (RGSSS), continuous resistivity profiling, conductivity-temperature-depth casting, and MantaCam and SCUBA diving surveys. Lower frequencies (77/83 kHz) of RGESs showed more distinct acoustic signatures of bubbly SGD, as these were nearly the same as the effective resonance frequency of the bubbles. The clusters of bubbly discharges have higher backscatter strength than the water column noise, resulting in the definitive and convenient manual detection of SGD features. Hence, showing more accurate point sources of SGD. Three types of known SGD occurrence were identified and characterized based on acoustic behavior and spatial distribution: 1) sparse, discrete and sporadic discharge over wide area, 2) curtain, high and continuous bubble concentrations from widespread discharge, and 3) spring, direct bubble discharge from intense seafloor degassing at a single point source. These results showed that RGES provides a good alternative for more efficient and cost-effective preliminary coastal SGD works. Additional research on areas with water-dominated discharge but no bubbling is recommended.

Genetic Identification Of Ground Ice By Petrographic Method

The advantages and limitations of the petrography method and the relevance of its use for the study of natural ice are reviewed in the present work. The petrographic method of ground ice study is often used for solving paleogeographic issues. The petrofabric analysis of ground ice is not only useful for descriptive purposes but, like the study of cryostructures, helps to infer growth processes and conditions. Different types of natural ice have specific features that can help us to determine ice genesis. Surface ice, such as glacier ice is often presented by foliation formed by large crystals (50-60 mm); lake ice is characterised by the upper zone of small (6 mm x 3 mm) dendritic and equigranular crystals, which change with increasing depth to large (may exceed 200 mm) columnar and prismatic crystals; segregated ice is composed by crystals forming foliation. Ground ice, such as ice wedge is presented by vertical-band appearance and small crystals (2-2.5 mm); closed-cavity ice is often distinguished by radial-ray appearance produced by elongated ice crystals; injection ice is composed by anhedral crystals, showing the movement of water; snowbank ice is presented by a high concentration of circular bubbles and small (0.1-1 mm) equigranular crystals; icing is described by foliation and mostly columnar crystals. Identification of the origin of ground ice is a complicated task for geocryology because it is difficult to distinguish different types of ground ice based on only visual explorations. The simplest way to get an ice texture pattern is by using polarized light. Distinctions between genetic types of ground ice are not always made in studies, and that can produce erroneous inferences. Petrography studies of an ice object are helpful to clarify the data interpretation, e.g., of isotopic analyses. It is particularly relevant for heterogeneous ice wedges’ study.

Stability of Oxygen Nanobubbles under Freshwater Conditions

The use of nanobubbles (NBs) has gained significant attention in various applications (e.g., aeration in biological water treatment, water disinfection, membrane defouling, and ground water and sediment remediation) in recent decades because of their superior characteristics such as the improved mass transfer at the gas-liquid interfaces, their lifetime up to a couple of weeks, the formation of reactive oxygen species (ROS) with high oxidative potential. However, there is a lack of information about the effect of various factors on the stability of NBs for a long storage period under freshwater conditions. In this study, a comprehensive investigation was conducted to systematically examine the stability of oxygen NBs in water under various conditions which are closely related to a typical freshwater or the drinking water treatment. The oxygen NB stability in water was evaluated by monitoring the change in the bubble concentrations, size distribution, average diameter, and zeta potential for 60 days of storage time under different pH, hardness, ionic strength, natural organic matter (NOM), chlorine, and temperature conditions. In addition, the formation of hydroxyl radical (•OH) was investigated using disodium terephthalate which form fluorescent adducts with •OH in the presence of oxygen NBs. Among the parameters investigated, the impacts of cations, low pH, and high SUVA254 NOM on the stability of oxygen NBs were more significant than other conditions. The half-lives of oxygen NBs under various conditions follow the order Ca2+ < Na+ < pH 3 < high SUVA254 NOM < pH 5 < 30 °C. Oxygen NBs were more stable in softwater than hardwater. Oxygen NBs were relatively stable for 3 days regardless of pH. For a longer storage period, oxygen NBs disappeared faster at pH 3 than at high pH. High SUVA254 NOM destabilized NBs more than low SUVA254 NOM, indicating the impact of hydrophobicity on the NB stability. The temperature effect on the NB stability was negligible for a short storage time, while higher temperature destabilized oxygen NBs for a longer storage time. One of the main disappearance pathway of oxygen NBs in water was found to be coalescing, rising, and leaving the container, which would be promoted greatly by cations, low pH and NOM with high aromaticity. The formation of hydroxyl radical in NB solutions was detected at pH 3 by a florescent probe molecule. When oxygen NBs are released in water bodies, high calcium, high SUVA254 NOM, and low pH would significantly reduce the availability of NBs and their residence time in freshwater.

Estimation of gassy sediment parameters in shallow water using shipping noise signals in frequency domain near cut-off

Modal structure of shipping noise in a shallow water waveguide is studied experimentally and theoretically. Experiments were carried out in the Sea of Galilee (Lake Kinneret, Israel) having maximal depth of 40.5 m and remarkable concentration of methane bubbles in the upper sedimentary layer. Receiving system consisted of 27-m vertical line arrays (VLAs) with ten hydrophones at each. They spanned a lower part of the waveguide with interval ∼3 m. The VLAs were fixed in the centre of the lake with the distance ∼40 m between them. R/V “Hermona” was moving along a straight line connecting VLAs at the range of up to 1 km from the arrays. Modal filtration technique on two synchronized VLAs with a phase correction was proposed to extract complex eigenvalues of each mode, including modal attenuation, which strongly increase in the proximity of cut-off frequency for all normal modes. Based on frequency dependences of attenuation coefficients of lower modes, sound speed in gas-saturated bottom layer was estimated. For application of this technique, there is no need to know exact coordinate and speed of noise source, which seems to us to be an advantage of the method. [Work was supported by RFBR, Grant No. 20-05-00119.]

Reflection of sound pulses from an inhomogeneous bubble medium

The effect of changes in the volume concentration of bubbles in the boundary zone of the bubble medium on the nature of reflection and radiation of the excited bubble medium is studied. The spectral characteristics of the radiation of a bubble medium are obtained at the initial stage of transition radiation and at large times when the radiation is stationary. It is shown that in the initial phase the emission spectrum is broadband and is located in the absorption band of the bubble medium, and at large times the emission spectrum is located outside this band.

Observations of high-frequency acoustic attenuation due to bubble entrainment at estuarine fronts

Frontal boundary features with strong gradients in water properties and velocity are common features in coastal environments where fresh discharge meets ambient ocean waters. Convergence at these fronts results in two features that support the entrainment of bubbles. First, horizontal velocity gradients causes surface waves to steepen and break at these boundaries. Second, localized downwelling can further entrain and transport these bubbles. In experiments using broadband echosounders (45–420 kHz) at the Connecticut River ebb plume front and James River tidal intrusion front, bubble entrainment was observed to cause frequency-dependent excess attenuation. Seabed backscattering near fronts is used as a baseline, whereas a decrease in observed backscattering from the seabed under bubble plumes was attributed to excess attenuation due to bubble concentrations. Despite observing significant (>10 dB in some cases) excess attenuation in lower-frequency channels, we hypothesize that these fronts are unlikely to significantly affect propagation in the horizontal at these frequencies and that factors, such as refraction and three-dimension structure, are more operationally significant. [This work was funded by ONR.]

All-regime two-phase flow modeling using a novel four-field large interface simulation approach

Two-phase flows may occur in many regimes, ranging from dispersed flow, slug or plug flow to stratified flow. To capture all those regimes with a monolithic approach, a so-called all-regime two-phase model should be developed. This paper presents such an all-regime two-phase modeling approach called Four-Field Large Interface Simulation (FF-LIS), in which we consistently consider two phases (gas and liquid) which can both be either resolved or sub-grid, resulting in four fields. The efficacy of this approach hinges on the accuracy with which the inter-scale mass, momentum and energy transfer terms between dispersed and resolved regimes are captured. In this paper, we introduce three phenomenological mechanisms to control the inter-scale transfer: absorption, break-up and coalescence. The separation of multiphase scales allows for the explicit modeling of transfer between them, whereas other methods often rely on an algebraic regime map in which transfer between scales is accounted for implicitly. We apply FF-LIS to the simulation of both bubbly and fully resolved single bubble flow, as a way of validating FF-LIS in those two two-phase limits. Next, the transfer terms are illustrated by simulation of bubbles rising in a column. As the bubble concentration increases, it is shown that a transfer from sub-grid to resolved occurs. Finally, we apply FF-LIS to the simulation of two-phase horizontal pipe flow in which the several flow regimes, depending on the superficial velocities, are recovered, and to the simulation of a plunging jet in which FF-LIS is shown to produce an entrained bubble plume. FF-LIS presents a computational platform for faithful two-phase resolved and sub-grid simulation of relevant real-world two-phase problems, and allows two-phase modelers for the explicit control on what can be resolved, and what should be sub-grid.

Experiments and modelling of three-phase vertical pipe flow

A set of two- and three-phase experiments were conducted in a 50 m long 4″ vertical pipe using nitrogen, Exxsol D60 and water at 45 bara pressure. The results show that the liquid content and pressure drop are highly sensitive to the injected water cut. The proposed explanation for this surprising result is that the presence of liquid droplets constrains the gas bubble capacity of the liquid so that the concentration of small bubbles inside the liquid becomes smaller in three-phase flows than in two-phase flows.To test this hypothesis, a simple flow model was implemented using closure laws from the public literature, combined with the assumption that the concentration of gas bubbles is reduced by the presence of liquid droplets. The model shows that the observed three-phase effects can be reproduced very accurately using this assumption.

The role of pre-existing defects in shock-generated ejecta in copper

The interaction of shock waves with non-planar free surfaces can cause atoms to eject from the surface, leading to the formation of ejecta. These non-planarities in the free surface can occur due to machining of the free surface or can be induced in the shock wave itself due to the presence of heterogeneities in the material. Both cases lead to the formation of ejecta. While the effect of machining on ejecta has been well studied, the latter has not been a topic of significant investigations. In this work, molecular dynamics simulations are used to systematically investigate the effect of size and concentration of He bubbles in Cu with planar free surfaces on ejecta production. It is shown that the presence of defects leads to the formation of non-planarity in the shock wave itself producing ejecta as the front reaches the flat free surface. The cluster size and velocity of ejected particles greatly exceeds that of pure Cu; the radius, density, and nature of the helium-filled voids alter the mass, velocity, and size distribution of the ejected matter.

Influence of bulk nanobubble concentration on the intensity of sonoluminescence

The present study mainly examined the effects of the volumetric concentration of nanobubbles (ultrafine bubbles) on the intensity of sonoluminescence (SL). The addition of nanobubbles at high acoustic amplitude enhanced the SL intensity for various bubble concentrations in comparison with that in pure water. This probably means that the resulting high amplitude is over the Blake threshold, and accordingly nanobubbles expand to some extent, leading to higher SL intensity. Therefore, nanobubbles have the potential to provide nucleation sites for sonochemistry. The influence of bubble size on the intensity of SL was also evaluated.