Gas Bubble Size Research Articles

Linear energy transfer of fission fragments of 235U and nucleation of gas bubbles in aqueous solutions of uranyl nitrate

Fission fragments emitted in a fissile solution create tiny gas bubbles, the size of which is determined by the linear energy transfer (LET) of the particles. The LET of fission fragments of 235U in aqueous solutions of uranyl nitrate has been determined, and using methods adapted from the literature, the size of gas bubbles generated along the tracks of these particles has been estimated, revealing important variations with respect to particle LET and solution properties. Empirical correlations are presented for the maximum radius of radiolytic gas bubbles in unsaturated solutions of uranyl nitrate as a function of solution temperature and concentration. These can be used to predict the critical concentration of dissolved hydrogen necessary for the appearance of gas voids during nuclear criticality transients. The findings are intended for use in a future model of nuclear criticality transients in aqueous fissile solutions for the purposes of nuclear criticality safety assessment.

Generality of the CFD-PBM coupled model for bubble column simulation

Computational fluid dynamics coupled with a population balance model (CFD-PBM) combines the use of CFD to calculate the flow field and phase holdup and that of PBM to calculate the local bubble size distribution. However, the generality of the CFD-PBM coupled model for different operating conditions and liquid properties has not been systematically studied. The CFD-PBM coupled model where the PBM used our bubble breakup and coalescence models was shown to give accurate results in wide operating conditions (P = 0.1–3.5 MPa, Ug = 0.02–0.16 m/s), liquid properties (0.02–0.07 N/m, 1–185 mPa·s) with the same set of model parameters. Overall, the coupled model showed good generality when the superficial gas velocity, operating pressure, liquid viscosity and surface tension are the factors in bubble coalescence and breakup and the appropriate mechanism models are used to account for them, and forces at the gas-liquid interface are calculated using the local gas holdup and bubble size distribution.

In-situ observations of melt degassing and hydrogen removal enhanced by ultrasonics in underwater wet welding

In this study, the phenomenon of gas removal behaviors in the melt pool and the flow behaviors of melt pool were observed using an in-situ X-ray imaging method. Based on the ultrasonic effects, a novel processing method was developed to reduce porosity and decrease diffusible hydrogen content in the deposited metal. The effects of ultrasonic output power on the size and frequency of gas bubble collapsing were studied. Results showed that with the assistance of ultrasonic, the gas bubble size were smaller and the collapsing frequency was significantly increased. The porosity decreased from 1.4% to 0.5% and the hydrogen diffusible content decreased from 24.5 to 18.6 ml/100 g when the ultrasonic power increased to 720 W. The possible hydrogen removal mechanism was proposed by two aspects including the microcosmic and macrocosmic scale.

Study of radiolytic gas bubbles formation and behaviour in an aqueous uranyl sulphate solution using ImageJ

This paper delves into the study of the radiolytic gas bubbles formation in uranium salt solutions using the ImageJ software for the determination of parameters of interest of the gas bubbles. In this way the gas bubbles diameter, gas bubbles velocity and local gas volume fraction for three electron bean power levels, 6, 12 and 15 kW were determined. It was determined that the radiolytic gas bubbles average diameter varies between 200 and 300 μm, increasing with the increase in power. A similar behaviour was observed for the bubble velocity, which increases accordingly with the increase in power density. The study of the gas volume fraction indicated that although the radiolytic gas bubble size does not increase significantly with increasing power, the number of bubbles and gas produced increases appreciably. All of these results were consistent with those reported by other authors in the literature.

Modeling of mass transfer and hydrodynamic investigation of H2S removal from molten sulfur using porous Sparger

Empirical and theoretical aspects of non-catalytic molten sulfur degassing as one of the vital actions in Claus Unit have been investigated in this article. A laboratory bubble column has been set up to study the hydrodynamic behavior of gas-liquid system. Values of gas holdup were also compared with predictions of some correlations in the case of application of porous spargers. The mathematical model of degassing was also developed considering reaction of H2S with sulfur molecules in the molten sulfur and generation of H2Sx species. The experimentally-measured parameters of gas holdup and bubble size were used in the mathematical model. Other parameters including Henry’s law constant of H2S-liquid sulfur system and reaction rate constants were obtained from published formulas presented by Marriott and Ji, respectively. The obtained results were compared to the empirical data of non-catalytic degassing performed in the laboratory setup. Reasonable compatibility was observed between the model-derived and experimental results. The results showed a fast removal of dissolved H2S within few minutes, followed by very slow removal of H2Sx through its chemical conversion to H2S and its purging by sweep gas. The novel gas holdup profile and images presented in this article show interesting features of hydrodynamic behavior of molten sulfur.

Passive acoustic monitoring of a natural CO2 seep site – Implications for carbon capture and storage

Estimating the range at which an acoustic receiver can detect greenhouse gas (e.g., CO2) leakage from the sub-seabed is essential for determining whether passive acoustic techniques can be an effective environmental monitoring tool above marine carbon storage sites. Here we report results from a shallow water experiment completed offshore the island of Panarea, Sicily, at a natural CO2 vent site, where the ability of passive acoustics to detect and quantify gas flux was determined at different distances. Cross-correlation methods determined the time of arrival for different travel paths which were confirmed by acoustic modelling. We develop an approach to quantify vent bubble size and gas flux. Inversion of the acoustic data was completed using the modelled impulse response to provide equivalent propagation ranges rather than physical ranges. The results show that our approach is capable of detecting a CO2 bubble plume with a gas flux rate of 2.3 L/min at ranges of up to 8 m, and determining gas flux and bubble size accurately at ranges of up to 4 m in shallow water, where the bubble sound pressure is 10 dB above that of the ambient noise.

Features of the formation of an electric field in a non-uniform conducting medium

The paper considers the influence of the electric field of the current on the behaviour of a gas bubble in an electrically conductive melt. We propose a model which makes it possible to predict a change in the size of gas bubbles in a metal, provided that its crystallization occurs in the presence of an electric current. The model takes into account the interaction of the charge induced on the surface of the gas bubble — metal-gas interface — with the electric field of the current. The force that deforms the gas bubble is introduced. The magnitude of the force is determined by the charges induced on the surface of the bubble due to the action of the electric field of the current. The surface density of the charges is proportional to the square of the current density and depends on the geometry of the object that is affected, as well as on the material parameters of the medium, such as electrical resistivity and magnetic permeability. Additional forces of electrical nature acting on the surface of the gas bubble contribute to its deformation in the electric field of the current. In the presence of such forces, the interfaces between the media — liquid metal and gaseous — can be deformed. Under the conditions of an orderly motion of free electrons and their collisions with ions of the crystal lattice, the size of the interfaces between the media can be changed. The gas bubble under the action of additional forces is deformed and can subsequently be fragmented.

ФЛОТОЕКСТРАКЦІЯ БАРВНИКА АКТИВНОГО ЯСКРАВО-ЧЕРВОНОГО 5 СХ З ВОДНИХ РОЗЧИНІВ

Presented work has researched the removal of Reactive Red 2 from water solution by solvent sublation. Solvent sublation is an innovative technology, which is an improved version of ion flotation, the use of which does not form foam. The implementation of this method of wastewater treatment provides for the simultaneous separation of the aqueous phase and the pollutant, followed by concentration of the pollutant. This process allows to remove both inorganic and organic pollutants from the aquatic environment. Reactive Red 2 is an anionic dye and widely used for printing and dyeing cellulose fibers, silk and wool. The dye is toxic and carcinogenic. Wastewater contaminated with this dye must be cleaned to eliminate the negative impact on the environment. The process of removing Reactive Red 2 is carried out with the following operating parameters: column diameter – 3.5 cm; gas – nitrogen; gas consumption – 60 ÷ 75 cm3/min; the volume of the aqueous phase – 200 cm3; the volume of the organic phase – 10 cm3; initial dye concentration – 10 mg/dm3. Hexadecyltrimethylammonium bromide is selected as a surfactant to form a poorly soluble hydrophobic compound. The organic phase is isoamyl alcohol. The effect of some parameters on the degree of dye removal has been experimentally studied: process duration, pH of the aqueous phase, molar ratio dye:surfactant, gas bubble size. According to the results, the highest degree of Reactive Red 2 removal is 96 % with a process duration of 10 minutes, pH – 6, molar ratio dye:surfactant = 1:2, and gas bubble size – 40 μm. The experimental results confirm that the proposed solvent sublation technology guarantees a sufficient level of wastewater treatment from anionic dyes. The selected method can be used in local wastewater treatment systems with subsequent return of water to the process. This solution is economically viable and will significantly reduce the level of pollution of natural reservoirs with dyes.

Experimental study of the liquid velocity and turbulence in a large-scale air-water counter-current bubble column

Measuring the local liquid velocity and turbulence in large-scale bubble columns with optical methods is complex and usually limited to low gas holdups in thin geometries. Comprehensive datasets in large bubble columns are therefore seldom published. Since the importance of Computational Fluid Dynamics (CFD) is increasing for multiphase applications, such data is also important for validating models for dispersed bubbly flows. In the present work the liquid velocity and turbulence in a pilot-scale bubble column is studied and a CFD validation database is generated by completing previous measurements of the gas void fractions and bubble sizes. For this purpose, we used a Particle Shadowgraph Velocimetry (PSV) technique that was intentionally designed to study the fluid dynamics in large-scale facilities. The measurements were realized in the 5.3 m high and 0.24 m diameter counter-current bubble column at Politecnico di Milano. The superficial gas velocity ranged from 0.37 to 1.85 cm/s, the counter current superficial liquid velocity from 0 to 9.2 cm/s. All operation points are in the so-called pseudo-homogeneous flow regime, in which the integral gas holdup (ranging from 1.02 to 7.55%) increases linear with the superficial gas velocity but an inhomogeneous flow is present. The dominant frequencies of the bubbly flow, shear rates, and turbulence levels are increasing with increasing superficial gas velocity. With increasing superficial liquid velocities, the dominant frequencies are decreasing, the averaged liquid velocities are shifted downwards, but the overall turbulence levels remain constant. In order to investigate the smaller scales at which the bubble-induced turbulence is expected, a filtering process is proposed. As a result, the filtered turbulence levels of all operation points fall on a linear trend line when plotted over the local void fraction, which is the same result obtained in other studies in small, homogenous tabletop columns. The now available database for CFD validation contains the averaged liquid velocities, basic turbulence information, local void fractions, and the bubble sizes at two different heights. The data will be in particular useful to validate the capabilities of models to upscale bubbly flows from tabletop to the pilot-scale bubble columns.

Microbubble generation with shear flow on large-area membrane for fine particle flotation

Fine particle flotation is carried out in a flotation column with a 19-channel ceramic membrane sparger. The microbubble generation is controlled by the shear flow in the membrane channels. Bubbles in the flotation column are photographed via a high speed video camera. Gas holdup and quartz recovery is investigated under different operating conditions. Gas holdup in the flotation column firstly increases and then decreases with the increase in superficial gas velocity. Similar trend in gas holdup is also found by increasing cross flow velocity. Larger particle concentration in the feed results in smaller gas holdup, while the gas holdup positively changes with collector concentration. The quartz recovery is strongly influenced by gas holdup and bubble size. Similarly, the recovery firstly increases and then decreases when increasing superficial gas velocity and cross flow velocity. The collector concentration plays a more obvious role in flotation recovery at high superficial gas velocity.

On evolving size and shape of gas bubble in marine clay under multi-stage loadings: microcomputed tomography (μCT) characterization and cavity contraction analysis

Discrete bio-gas bubbles commonly form in fine-grained marine sediments and have modified many aspects of the behavior of these sediments, including strength, stiffness, and permeability. Although the level of such modifications is known to govern by bubble shape and size, limited studies have been undertaken, mainly due to difficulty in nondestructively characterizing bubbles within a soil under in situ stresses. In this study, a mini-loading device was developed to perform one-dimensional loading tests on gassy marine clay and gassy silt in a microcomputed tomography (μCT). The evolving bubble shape, size, and pressure during loading were quantified, and the resulting stress fields around the bubble cavities were evaluated via elliptical cavity contraction analysis considering stress anisotropy. As the vertical load increased, bubble cavities were found to compress predominantly along the vertical loading direction, with little horizontal compression, because localized soil failure (LSF) and thus cavity collapse occurred mainly near the roof of the at-rest lateral earth pressure coefficient (K0)-stressed elliptical bubble cavities. The evolution of bubble shape and size under loading is significantly affected by stress anisotropy, which governs the extent and location of the LSF. A Gaussian mixture model is adopted to quantify the evolving distributions of bubble structure parameters, which are essential for developing more physically rigorous gassy soil models.

Foams generated from viscous non-Newtonian shear-thinning liquids in a continuous multi rotor-stator device

Whilst aeration is ubiquitous in the food industry, little work has been done on foams generated from viscous non-Newtonian liquids. We study the production of foams from viscous shear-thinning liquids containing a non-ionic food grade surfactant (PGE 55), Xanthan gum and caster sugar, using a continuous pilot-scale device having twelve rotor-stator pairs. The effects of process parameters (rotor speed, gas-liquid volumetric flowrate ratio (G/L)) and liquid composition (surfactant concentration, Xanthan gum concentration) on foam gas volume fraction and bubble size distribution are elucidated. X-ray micro-Computed Tomography is employed to characterise the 3D microstructure of the foams. Rotor speed and G/L ratio are the dominant factors in determining the gas volume fraction and bubble size distribution. The foams produced exhibit a rich fine texture with high static stability. For a given energy input in turbulent flow, a higher G/L ratio results in a higher gas fraction and a smaller bubble size. Aeration is ubiquitous in the food industry and innovative ways to generate stable foams with fine texture from viscous non-Newtonian media are needed to satisfy the increasing demand for better quality, healthier and cheaper products. The use of continuous multi rotor-stator devices operating in turbulent flow and under atmospheric pressure achieves optimum aeration efficiency and prevents foam expansion. The use of an effective surfactant such as PGE 55 and a thickener such as Xanthan gum help provide the desired fine texture and stability for longer shelf-life. Operating at high gas-liquid ratios in turbulent flow achieves the production of finer more uniform foams with less energy input.

Mechanism and morphology control of underwater femtosecond laser microgrooving of silicon carbide ceramics.

Silicon carbide (SiC) ceramics have been widely used for microelectronics, aerospace, and other industrial fields due to their excellent chemical stability and thermal tolerance. However, hard machinability and low machining precision of SiC ceramics are the key limitations for their further applications. To address this issue, a novel method of underwater femtosecond laser machining was introduced in this study to obtain high precision and smooth surface of the microgrooves of SiC ceramics. The removal profiles were characterized in terms of width, depth, and surface morphology, which exhibited high dependence on the femtosecond laser processing parameters. The instability during the underwater processing affected by laser-induced gas bubbles and material deposition, however, limits the high surface accuracy of microgrooves and processing efficiency. The process condition transformation from a bubble-disturbed circumstance to a disturbance-free model was carefully investigated through a high speed camera for the femtosecond laser processing of SiC ceramics in water. The experiment results indicated that degree of disturbed effect was heavily dependent on size, distribution, and motion of laser-induced gas bubble. Furthermore, some typical evolution mechanisms of gas bubble and their influence on the removal profiles of microgrooves were discussed in detail. Bubble evolution has been proven to be mainly responsible for the behavior of laser propagation (focus model, total reflection, etc.), which notably affects microstructural characteristic of the microgrooves.

Structure of the Turbulent Flow in a Submerged Axisymmetric Gas-Saturated Jet

The structure of a submerged vertically ascending round bubbly jet is numerically simulated by using the Eulerian approach. The influence of the concentration and diameter of air bubbles on the averaged parameters and fluctuating characteristics of the submerged turbulent two-phase jet is analyzed An increase in the concentration and size of gas bubbles leads to jet expansion (by 35% as compared to the one-phase jet), which testifies to intensification of turbulent mixing with the ambient space. Addition of air bubbles enhances the flow turbulence (by 20% as compared to the one-phase jet).

Surface modification of Ni foam by the dendrite Ni-Cu electrode for hydrogen evolution reaction in an alkaline solution

Hydrogen is a renewable energy source that is considered among the candidate alternatives to fossil fuels. In this research, dendrite NiCu coating is the electrodeposited and catalytic performance of this electrode is investigated toward hydrogen evolution reaction (HER) in 1.0 M KOH. The response surface methodology (RSM) as the design of experimental (DOE) method is used for optimizing the intrinsic electrocatalytic HER activity (NiCu A) and active surface area (NiCu B) of the dendrite NiCu coating. The DOE results showed that the active surface area had a stronger effect on the electrocatalyst HER activity than the intrinsic parameter. The η10 (overpotential at 10 mA cm−2) and Tafel slope of the NiCu B electrode were 202 mV vs. RHE and 82 mV dec−1, respectively. According to the Tafel slope of the optimized NiCu B electrode, the Volmer-Heyrovsky mechanism predominated the HER mechanism. The NiCu B electrode showed high electrocatalytic activity and electrochemical stability compared to recently proposed catalysts. The electrocatalyst stability of the NiCu B electrode led to the small size of the H2 bubble on the surface of the electrode. The low size and rapid detachment of the H2 gas bubbles resulted in the dendrite NiCu B structure.

Study of particle residence time in a pressurized fluidized bed with in-bed heat exchanger tubes

Anthropogenic climate change is amongst the greatest of challenges to human civilization. A key area that will play a large role in mitigating its effects are clean fossil fuel applications. Clean coal combustion can be achieved with an oxygen-fired pressurized fluidized bed combustor incorporating carbon capture and storage. In relation to pressurized fluidization processes, understanding the influence of pressure on fluidized bed hydrodynamics and, in turn, their effect on parameters including fuel residence time is essential. For the combustor under consideration here, a fraction of the heat exchanger boiler tubes are submerged in the fluidized bed such that the effect of the horizontal tube bundle on the fuel residence time is of great importance. The main focus of the present work was to evaluate the impact of gas velocity, pressure, presence or absence of a horizontal tube bundle and fuel feed rate on the average fuel residence time in a dense gas-solid fluidized bed. Experiments were conducted under cold flow conditions in a pressurized fluidized bed with an inner diameter of 0.15 m. The fluidization material was large glass beads (1.0 mm in diameter) while fuel particles were simulated with smaller glass beads (64 and 83 μm in Sauter mean diameter) that were susceptible to entrainment. Operating pressures and superficial gas velocities were maintained between 101.3 and 1200 kPa and 1.5 and 3.2 Umf, respectively. To simulate continuous fuel injection, experiments were conducted with the fuel surrogate particles being continuously fed to the fluidized bed of large particles over a desired period of time. Downstream, entrained particles were captured to determine the average entrainment rate and average mass of fuel particles inside the fluidized bed at steady state, which yielded the average fuel residence time. The combination of elevated pressure with the tube bundle present was found to have the most influential impact when compared to base conditions of atmospheric pressure and with no tube bundle present. It was found to enhance gas bubble break up and reduce the average gas bubble size substantially. In turn, this increased the average residence time of 83 μm particles by nearly three-fold in comparison to the case of atmospheric pressure with no tube bundle present. The effect of gas velocity on particle residence time was not found to be statistically significant under the range tested. Similarly, the effect of increasing fuel feed rate by 50% had no statistically significant impact.

Scale-up of a downflow bubble column: Experimental investigations

Co-current downflow bubble columns provide advantages like high gas phase residence time and small bubble diameters, which result in high gas-liquid interfacial area and high mass transfer rates. The precise knowledge of gas fraction, bubble size distribution, interfacial area (a_) and volumetric mass transfer coefficient (kLa_) data is essential for the design of such downflow bubble columns. The present work investigates the effect of superficial gas velocity, liquid velocity and gas sparging location on the volume averaged gas fraction, axial and chordal gas fraction profiles, Sauter-mean bubble size, kLa_, and a_ in a downflow bubble column modified to operate via an array of downward pointing micro-jets. Experiments were performed for air-water system with 20 PPM SDS in two-different scale downflow bubble columns: (1) volume of 5 L (Height (H) = 0.6 m, Diameter (D) = 0.1 m, aspect ratio (H/D) = 6) and (2) volume of 120 L (H = 1.5 m, D = 0.3 m, H/D = 5). The experimental results of axial and chordal gas fraction profiles and bubble size distributions are presented. Further, the empirical correlations have been proposed to predict the gas fraction (∈G), Sauter-mean bubble diameter (dB) and effective gas-liquid interfacial area a_ for better understanding the scale-up process.

Operando flow regime diagnosis using acoustic emission in a polymer electrolyte membrane water electrolyser

Polymer electrolyte membrane water electrolysers (PEMWE) are a key technology for producing clean (‘green’) hydrogen for decarbonisation of the transport sector and grid stabilisation utilising increasing levels of renewable energy. In this work, acoustic emission analysis is used as a non-destructive, operando diagnostic tool to provide information about the relative number and size of gas bubbles generated locally within a PEMWE, providing effective characterisation of the local flow conditions. An optically transparent single-channel PEMWE is used to investigate the relationship between the acoustic signals obtained and the two-phase flow conditions inside the cell. The number of acoustic hits, their frequency, and average peak amplitude is reported for several flow rates and current densities. Using high-speed imaging, the average bubble number and size in the flow cannels is compared to the acoustic signal. Results show good correlation between the number of acoustic ‘hits’ and the number of bubbles passing through the flow channel. The size of bubbles is also shown to affect the average frequency of the hits. Consequently, the transition between bubbly and slug flow regime can be identified by acoustic emission analysis, paving the way for a simple, low-cost, non-destructive means of mapping flow inside commercial-scale PEMWEs.

Effect of microchannel junction angle on two-phase liquid-gas Taylor flow

Two-phase liquid-gas Taylor flow triggered by blocking-squeezing mechanism was studied with different junction angle θ microchannels, i.e. 20°, 45°, 90°, 135° and 160°, at various liquid (ethanol) and gas (He) flow rates. We experimentally investigated the effects of flow rates and θ on the gas bubble VB and liquid slug VS volumes. A theoretical model was formulated for the quantitative predictions of bubble and slug sizes for different θ and flow rates. Good agreements were obtained between theoretical predictions and experimental observations. The unit cell volume VU (VB + VS) decreased pronouncedly for the 20° channel with decreasing liquid or increasing gas flow rate, due to the slight increase in VB and large decrease in VS. In comparison, for the 45°, 90°, 135° and 160° channels with increasing liquid or decreasing gas flow rate, VU were less sensitive to fluid flow rate changes, due to the approximate cancellation between VB decrease and VS increase. For the 20° and 45° channels, it produced larger VU, due to larger VB and VS, when compared to the 90° channel. This is caused by the larger gas bubble throat width DN at the junction when θ < 90°. As for the 135° and 160° channels (θ > 90°), DN is approximately equal to the gas channel width, with VB, VS and VU approximately the same as the 90° channel. With θ ≥ 90° (i.e. 90°, 135° and 160° channels), as evident from the smaller VU, higher gas bubble density can be obtained when compared to θ < 90° (i.e. 20° and 45° channels). Hitherto, this observation has not been realized, and the mechanics is first investigated here with the employment of extreme θ (i.e. 20° and 160°). A thorough understanding of the underlying mechanics affecting Taylor flow can facilitate its exploitation for controlled gas bubble and liquid slug generation. Our theoretical model facilitates the tuning of the channel designs and fluid flow rates to achieve the desired gas bubble and liquid slug sizes for specific applications.

Metal Phase Formation in Bubbling Reduction of Nickel and Iron from Oxide Melt

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