Distribution Of Gas Bubbles Research Articles

In Situ Two-Phase Flow Investigation of Proton Exchange Membrane (PEM) Electrolyzer by Simultaneous Optical and Neutron Imaging

In PEM electrolyzers, oxygen evolution in the anode and flooding due to water cross-over results in two distinct two-phase transport conditions, and these two phenomena were found to strongly affect the performance. A comprehensive understanding of two-phase flow in PEM electrolyzer is required to increase efficiency and aid in material selection and flow field design. In this study, two-phase transport in an electrolyzer cell is visualized by simultaneous neutron radiography and optical imaging. Optical and neutron data are used as complementary to aid in understanding of the two-phase behavior. Behavior of gas bubbles is investigated and two different gas bubble evolution and departure mechanism is found. It is also found that there is a strong non-uniformity in the gas bubble distribution across the active area, suggesting the importance of the flow field design.

Gas Bubble Distribution in Liquid Slug of Horizontal Air-Water Flow

The slug flow regime may be appeared in subsea gas-liquid pipeline of the offshore petroleum industry. The gas entrainment process and the gas bubble distribution in liquid slug are crucial for the model of slug flow. An experimental facility was constructed and the gas bubble distributions in the liquid slug were measured by the dual-tip conductivity probe. It is found that the entrained gas is broken up into small bubbles by the high turbulent shear stress in the turbulent shear layer in the mixing zone. The small bubbles are dispersed completely and the profiles of void fraction and bubble frequency have peaks in this layer. The mechanism of gas entrainment is also presented.

Behavior and Shape of Gas Kicks in Wellbores

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 128276, ’Behavior and Shape of Gas Kicks in Well Bores,’ by H.F. Spoerker, SPE, OMV E&P, and T. Tuschl, SPE, Leoben University, originally prepared for the 2010 IADC/SPE Drilling Conference and Exhibition, New Orleans, 2-4 February. The paper has not been peer reviewed. The full-length paper describes the driving mechanisms of gas entering into the wellbore and the methodology used for simulating and visualizing the percolation of gas after the well has been shut in. It is part of a multi-year research project with the goal of understanding the wellbore/gas-influx system in more detail. Introduction While of lesser importance for the volumetric and hydraulic calculations required in well-control operations, understanding of shape and distribution of gas bubbles in the annulus during a kick bridges the gap between the hydraulic and the chemical processes happening in the wellbore. Until now, representation of the total gas influx volume as one single-phase, dry-gas “bubble” slowly moving up the shut-in wellbore (i.e., annulus) can be found in many well-control models. Starting in the late 1960s, and especially in the late 1980s and early 1990s, efforts were made to better simulate and measure the behavior of gas kicks in wellbores. These projects could be divided into two groups: (1) the theoretical approach using mathematical concepts and material-balance equations to simulate gas behavior and (2) the practical approach of building several test facilities and flow loops to measure gas behavior under “real” conditions All of these projects attest to the assumption that gas entering the well-bore will not accumulate as one major void fraction. Depending on different factors such as geology, mud rheology, and differential pressure between formation and wellbore, it can be assumed that gas—in contrast to prior assumptions—once having entered the wellbore remains dispersed in the form of multiple small bubbles slowly percolating upward. Formation/Wellbore Interaction Common reservoir-engineering theory explains the movement of gas through a porous medium, in its simplest form adequately represented by the Darcy equation and relating the volumetric flow through a porous medium to the differential pressure applied, the cross-sectional area and length of the rock volume, the dynamic viscosity of the fluid, and the permeability of the rock. Applying Darcy’s law for radial flow, the flow rate for a reservoir/wellbore system at a given differential pressure can be calculated, assuming conventional sandstone reservoirs. While permeabilities for such reservoir rocks are typically in the range of 0.01 to 1 darcy, secondary-porosity fracture systems in limestones and dolomites will behave in a radically different way and can exhibit effective permeabilities of the fracture systems several orders of magnitude higher than those of conventional sandstones.

Quantitative characterization of xenon bubbles in silicon: Correlation of bubble size with the damage generated during implantation

Making use of Fresnel fringe contrast under different focusing conditions in transmission electron microscopy (TEM), we present a detailed evaluation of the depth dependent size distribution of gas bubbles contained in a stationary profile of 40 keV Xe implanted in Si. Voids generated during sample preparation by ion milling were also characterized carefully. The largest bubbles, with mean and maximum sizes of 5 and 7 nm, respectively, were observed at depths <22 nm. However, the first 2 nm of the sample did not contain any bubbles. Towards the end of range the bubble size decreased rapidly. No bubbles were found beyond 45 nm (the minimum size of detectable bubbles was estimated to be about 1.8 nm). Some observations suggest that the bubbles were over-pressurized. The derived data could be converted to a depth dependence of the Xe concentration contained in bubbles, n Xe,b. Comparison with the previously reported depth distribution of Xe measured by Rutherford backscattering spectrometry (RBS), n Xe,b turned out to be depth dependent, with a maximum of ∼28% in the region of maximum bubble size. n Xe,b is shown to correlate closely with the damage density generated during Xe implantation. The findings lead to a model of bubble formation which involves the idea that the redistribution and transport processes initiated by ion impact take place mostly during the lifetime of the collision cascade.

Effect of Bubble Size and Angle of Tapering Upriser Pipe on the Performance of Airlift Pumps

Airlift pumps are devices with the ability to lift liquid phase by injecting the gas phase. Parameters that affect the performance of these pumps are divided into two groups. The first group contains design parameters such as diameter of the pipe, tapering angle of the upriser pipe, and the submergence ratio, which is the ratio of immersed length to the total length of the upriser. The second group includes operating parameters such as the gas flow rate, bubble diameter, bubble distribution, and inlet gas pressure. In this research, the performance of an airlift pump is investigated numerically for different submergence ratios and different diameter of the upriser pipe. For this purpose, an airlift pump with a riser length of 914 mm and different diameters (6, 8, and 10 mm) and seven tapering angles (0°, 0.25°, 0.5°, 1°, 1.5°, 2°, and 3°) is numerically modeled and analyzed. Different submergence ratios are used: 0.4, 0.6, and 0.8. The numerical results are compared with the existing experimental data in the literature and show reasonable agreement. The results indicate that decrease in size of the bubble diameter increases mass flow rate of liquid at constant submergence ratios. The present study reports the improved performance of this pump with decrease in bubble size and increase in angle of tapering upriser pipe. Moreover, the results show that the tapering upriser pipe with 3° tapering angle gives the highest efficiency at nearly all submergence ratios. Further, the highest efficiency of the pump is shown to be at the largest submergence ratio.

Application of the phase-field method in predicting gas bubble microstructure evolution in nuclear fuels

Abstract Fission product accumulation and gas bubble microstructure evolution in nuclear fuels strongly influence their thermo-mechanical properties such as thermal conductivity, gas release, volume swelling and cracking, and hence fuel performance. In this paper, a general phase-field model is developed to predict gas bubble formation and evolution. Important materials processes and thermodynamic properties including the generation of gas atoms and vacancies, sinks for vacancies and gas atoms, elastic interaction among defects, gas re-solution, and inhomogeneity of elasticity and diffusivity are accounted for in the model. The results demonstrate the potential applications of the phase-field method in investigating: 1) heterogeneous nucleation of gas bubbles at defects; 2) effect of elastic interaction, inhomogeneity of material properties, and gas re-solution on gas bubble microstructures; and 3) effective properties from the output of phase-field simulations such as distribution of defects, gas bubbles, and stress fields.

Numerical Modeling of Time-dependent Fluid Dynamics and Differentiation of a Shallow Basaltic Magma Chamber

This contribution presents a numerical model of time-dependent physical processes and differentiation in a cooling magma chamber to obtain the magma and liquid composition and model the distribution of minerals and H2O gas bubbles at any moment and place. The numerical experiments were mainly carried out simulating 10 kyr of differentiation of a 50 km cylindrical stock-like reservoir as one of the end-members of a wide spectrum of magma body geometries. A detailed space^time distribution of compositional varieties and exsolved phases in a chamber of initially superheated basaltic composition (corresponding to a lava of the HudsonVolcano, Chile) was obtained through numerical simulation using the finite-element method.The roof of the chamber is located at 2 km depth in a crust with a geothermal gradient of 308C/km. The model considers two spatial directions (z and r) and assumes the magma to be an incompressible non-Newtonian liquid, following the Navier^Stokes formulation, where the density and viscosity depend on the temperature and exsolved solid and volatile phase content, but viscosity also depends on the shear rate. The temperature transfer equation used includes conduction, convection and latent heat. The setting of the model considers that: (1) an extension of 1km into the country rocks around the chamber imposed on the heat flow across the margins is appropriate; (2) the reaction rates for phase (solid and gas) exsolution correspond to a Gaussian probability function; (3) the involved phases, calibrated from MELTS, are olivine, clinopyroxene, magnetite, plagioclase, orthopyroxene and H2O gas; (4) the velocity of the exsolved phase movements with respect to the hosting liquid is given by the Stokes’ velocity; (5) the sizes of crystals and bubbles in the melt vary in space and time up to a maximum given by the modal crystal size observed in the Hudson Volcano. Our results indicate that the convection dynamics of the reservoir are characterized by three distinct flows of decreasing velocity with time: convective cells, plumes and layer flows along the walls. The along-wall magma flow, which persists during most of the 10 kyr of magma cooling, contributes to the thermal insulation of the chamber interior, giving rise to a nearly permanent coexistence of liquids of contrasting composition. A strong compositional stratification is generated in the upper half of the chamber, with a downward-increasing thickness of layers. Such stratification is mainly the result of continuous upward flow and storage on top of the sidewall residual melts. The lower half of the chamber exhibits an independent convective pattern dominated by ascending plumes. A less pronounced stratification is generated here as a consequence of mixing between the residual liquid in the ascending plumes and the surrounding melt. Crystal accumulations of olivine, clinopyroxene and magnetite at the bottom generate a significant volume of ultrabasic magma at the end of 10 kyr. A plagioclase-rich solidification front along the walls is formed during the last stages of crystallization, when the along-wall magma flow diminishes in velocity. An application of the numerical modeling to a cylindrical sill-like chamber is presented to emphasize the role of the aspect ratio in the magma fluid dynamics, compositional gradients and exsolved phase distribution. The stock-like chamber favors the formation of steep compositional gradients and H2O gas concentration at the roof. Concentration of the exsolved phases along the margins is favored in a sill-like chamber. From the simulation results, it is possible to infer that stock-like chambers would be more eruptible and would exhibit a wider compositional spectrum of eruptive materials than sill-like chambers. Because solid and liquid dispersion follow different patterns in a convective chamber, the record of crystal^liquid equilibrium would be an exception.

Voids engulfment in shaped sapphire crystals

In shaped crystals besides the usual structural defects, such as dislocations, faceting, and grain boundaries, so-called “voids” have been observed almost always; this defect affects especially the optical quality of the crystals. The influence of growth conditions on size and distribution of trapped gas bubbles in shaped sapphire crystals has been studied both experimentally and by numerical simulation of the fluid flow in the meniscus. The influence of the pulling rate, of the ambient temperature and of the shaper design has been taken into account.

Upper- and Lower-Bound Estimates of Flux for Gas-Sparged Ultrafiltration with Hollow Fiber Membranes

Cross-flow ultrafiltration is an important industrial process, which is inherently limited by concentration polarization and fouling. The introduction of gas bubbles into the filtration feed improves flux. However, for design purposes, accurate models of the process are required. In the first part of this paper a mechanism of flux enhancement is proposed for gas-sparged hollow fiber membrane ultrafiltration. This mechanism is determined from previously published experimental and computational fluid dynamics studies of the capillary slug flow process, as well as from dimensional analysis of the process. A physicochemical model for flux prediction is designed around the postulated enhancement mechanism. The flux-prediction model enables estimation of upper and lower bounds. The model comprises an approximate solution of the flow problem, assuming controlled gas bubble distribution, coupled with a one-dimensional, integral method boundary layer analysis and flux models. The boundary layer analysis is adapted...

Mathematical modeling and computer simulation of molten metal cleansing by the rotating impeller degasser: Part I. Fluid flow

The removal of dissolved hydrogen and solid impurity particles from molten alloys by rotary degassing is a widely used foundry practice. Rotary degassing involves purging a gas into the molten alloy through holes in a rotating impeller. Monatomic dissolved hydrogen either diffuses into these gas bubbles or it forms diatomic hydrogen gas at the bubbles’ surface; in any case, hydrogen is removed from the melt with the rising bubbles. Simultaneously, solid particles in the melt collide with one another due to turbulence created by the impeller rotation and the gas flow and form aggregates. These aggregates either settle to the furnace floor, or are captured by the rising gas bubbles and are also removed from the melt. A mathematical model has been developed to simulate the turbulent multiphase flow field that develops in the melt during rotary degassing. The mathematical model allows calculation of the mean turbulence dissipation energy and the distribution of gas bubbles in the melt. Both these quantities are input into other mathematical models that simulate the removal of dissolved hydrogen and impurity solid particles from the melt.

In situ accumulation of methane bubbles in a natural wetland soil

SummaryNatural wetlands are a significant source of atmospheric methane, an important greenhouse gas. Compared with numerous papers on measurements of methane emission from natural wetland surfaces, there are few reports on methane configuration and distribution within wetland soil profiles. By using a newly designed gas sampler, we succeeded in collecting free‐phase gas from beneath the water table down to 120 cm in a peat. The volumetric percentage of methane in the gas phase increased with depth and was generally more than 50% beneath the zone within which the water table fluctuates. The volume of the gas phase in the peat beneath the water table was estimated to be from 0 to 19% with significant variation with depth, suggesting uneven distribution of gas bubbles. Using the volume ratio of the gas and liquid phases and methane concentration data in the gas phase, as well as assuming that methane was in equilibrium (based on Henry's Law between the two phases), we calculated that ∼60% of the methane accumulates in the form of bubbles. These results suggest the importance of ebullition in methane emission, which might be a major cause for the reportedly large variation of methane emission in both space and time. Most importantly, our results show the need to consider gaseous‐phase methane for understanding the production, transport and emission mechanisms of methane in wetlands, which has been overlooked to date.

A temperature‐dependent, structural‐optical model of first‐year sea ice

A model has been developed that relates the structural properties of first‐year sea ice to its inherent optical properties, quantities needed by detailed radiative transfer models. The structural‐optical model makes it possible to calculate absorption coefficients, scattering coefficients, and phase functions for the ice from information about its physical properties. The model takes into account scattering by brine inclusions in the ice, gas bubbles in both brine and ice, and precipitated salt crystals. The model was developed using concurrent laboratory measurements of the microstructure and apparent optical properties of first‐year, interior sea ice between temperatures of −33°C and −1°C. Results show that the structural‐optical properties of sea ice can be divided into three distinct thermal regimes: cold (T &lt; −23°C), moderate (−23°C &lt; T &lt; −8°C), and warm (T &gt; −8°C). Relationships between structural and optical properties in each regime involve different sets of physical processes, of which most are strongly tied to freezing equilibrium of the brine and ice. Volume scattering in cold ice is dominated by the size and number distribution of precipitated hydrohalite crystals. Scattering at intermediate temperatures is controlled by changes in the distribution of brine inclusions, gas bubbles, and mirabilite crystals. Total volume scattering in this regime is approximately independent of temperature because of a balance between increasing and decreasing scattering related to the thermal evolution of these inclusions and scattering by drained inclusions. In warm ice, scattering is controlled principally by temperature‐dependent changes in the real refractive index of brine and by the escape of gas bubbles from the ice. Model predictions indicate that scattering coefficients can exceed 3000 m−1 for cold ice, averaging ∼450 m−1 for moderate and warm ice and reaching a minimum of ∼340 m−1 at −8°C. Scattering in all three regimes is very strongly forward peaked, with values of the asymmetry parameter g generally falling between 0.975 (T = −8°C) and 0.995 (T = −33°C).

Mass-Transfer Enhancement in Large-Diameter Pipeline under Water/Gas Stationary Slug Flow

In multiphase flow, gas bubbles are produced that collapse at the liquid/solid interface. In order to examine such effects, mass-transfer studies have been performed in an electrochemical cell and in a large-diameter flow loop under water/gas two-phase stationary slug flow. A limiting current density technique was used for oxidation of or reduction of in concentrated NaOH solutions. High-frequency and high-magnitude, instantaneous mass-transfer coefficients were found by potentiostatic measurements and may account for the corrosion rate enhancement in water/gas slug flow. A physical model of the stationary slug flow is proposed to explain the mechanism of gas bubble formation and bubble distribution. The effect of the gas bubbles on the mass-transfer enhancement is reported and a mechanism for the mass-transfer coefficient fluctuations is proposed. This study demonstrates that gas bubbles produced in stationary slug flow have a direct enhancement impact on the mass-transfer and corrosion rate of carbon steel pipeline. © 2004 The Electrochemical Society. All rights reserved.

Bubbling Behavior in Gas-Liquid Countercurrent Bubble Column Bioreactors

Characteristics of bubbling behavior and bubble properties were investigated and diagnosed in a gas–liquid countercurrent bubble column bioreactor which is 0.152 m in inside diameter and 3.5 m in height. The effects of gas and liquid velocities and bubble distribution mode (even, wall-side, central or asymmetric distribution) on the bubble properties such as chord length, frequency, rising velocity, holdup and bubbling behavior were examined. For the analysis of resultant bubbling behavior, pressure fluctuation signals were measured and analyzed by adopting the concept of the chaos theory; the signals were interpreted by means of an attractor in the phase space portraits and correlation-dimension. It was found that the resultant bubbling behavior could be detected effectively and quantitatively in terms of the phase space portraits and correlation dimension of pressure fluctuations in the bubble column bioreactor. The bubble size, frequency and holdup increased with increasing gas (UG) or liquid velocity (UL). The rising velocity of bubbles increased with increasing UG, whereas decreased with increasing UL. The uniformity of bubble size distribution and bubble holdup decreased when the distribution mode of bubbles at the gas distributor was changed from even to wall-side, central or asymmetric. The central distribution of bubbles was better than the asymmetric mode but worse than wall-side distribution, on considering the bubble holdup and uniformity of distribution. The bubble holdup and size were well correlated in terms of correlation dimension of pressure fluctuations elucidating the bubbling phenomena in the gas–liquid countercurrent bubble column bioreactor.

The effects of a nickel oxide precoat on the gas bubble structures and fish-scaling resistance in vitreous enamels

The effects of a NiO precoat on the interfacial microchemistry and the structure of gas bubbles at the steel–enamel interface were investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), thermogravimetric analysis (TGA) and image analysis techniques. The experimental evidence demonstrates that nickel oxide applied to the steel substrate as a precoat accelerates the diffusion of Fe from the steel into the enamel and promotes decarburisation of the steel substrate, this latter reaction generating CO and/or CO 2 as gases. The resulting increase in FeO concentration reduces the viscosity of enamel. The apparent decrease in the viscosity of liquid enamel, along with formation of substantial quantities of CO and/or CO 2 gases control the distribution of gas bubbles in the enamel layer. The investigation also explains the reason for the association of large gas bubbles with Fe–Ni metal-rich particles with a dendritic appearance (termed ‘dendrites in the following text) at the enamel–steel interface. The role of these Fe–Ni metal-rich ‘dendrites’ in reducing the tendency for hydrogen flaking or cracking of the enamel layer, generally referred to as fish-scaling, is also elucidated.

Gas-assisted injection molding: the effects of process variables and gas channel geometry

Polystyrene parts with different rib geometries but having the same aspect ratio were molded using gas-assisted injection molding (GAIM). The process variables that have an influence on the gas bubble distribution, residual wall thickness (RWT), fingering formation and mechanical properties were explored. The test results revealed that there is an inherent relationship between gas fingering and gas bubble penetration that has consequences on part strength. Shot size and delay time are the most dominant factors affecting the gas penetration, fingering formation, RWT and mechanical properties of the GAIM parts. The effects of rib geometries are also discussed. Computer simulation of the GAIM process was carried out using Moldflow, a commercial software. The outcomes predicted by simulation are compared with the experimental results. Based on the results, some useful design guidelines are suggested.

連鋳鋳型内メニスカスにおけるArガス気泡の挙動

The behavior of argon gas bubbles at meniscus in the continuous casting mold was studied using a Wood's metal model. Argon gas was injected into Wood's metal flow through a porous plate or an orifice set in the immersion nozzle. Both the diameter and the number of argon gas bubbles generated at meniscus regions were measured with a color high-speed video camera. Generation rate of argon gas at each region of meniscus was measured with a mass flow meter. The distribution of argon gas bubbles at meniscus was dependent on the flow rate of argon gas injected into the immersion nozzle, but was independent on the flow velocity of Wood's metal in the nozzle. The number of argon gas bubbles at meniscus was dependent on the diameter of bubble. The ratio of the number of argon gas bubbles at meniscus in a unit time was estimated as the following function of bubble diameter.n=Ωd-5.5where n is the ratio of the number of argon gas bubbles at meniscus in a unit time, Ω is the constant and d is the diameter of argon gas bubble.The flow velocity of Wood's metal at meniscus in mold was influenced by the injection rate of argon gas into the immersion nozzle.

Diagnosis of bubble distribution and mass transfer in pressurized bubble columns with viscous liquid medium

Bubble distribution and its effects on the gas–liquid mass transfer have been investigated in a pressurized bubble column whose diameter and height are 0.152 and 2.0 m, respectively. Effects of gas velocity (0.02–0.20 m/s), pressure (0.1–0.6 MPa), liquid viscosity (1.0–38.0 MPa s) and bubble distribution mode (even, wall-side, central and asymmetric distribution) on the volumetric gas–liquid mass transfer coefficient and bubble flow behavior have been examined. Pressure fluctuations have been measured and analyzed by adopting the deterministic chaos theory as well as spectral analysis. It has been found that the even distribution of bubbles is the best mode for mass transfer. The volumetric gas–liquid mass transfer coefficients when the bubbles are distributed in a mode of wall-side distribution have been relatively higher than those when the bubble distribution mode has been central or asymmetric. The bubble flow regime and its transition have been detected by analyzing the pressure fluctuations in the column. The gas holdup and volumetric gas–liquid mass transfer coefficients have been well correlated with the correlation dimension of pressure fluctuations elucidating the bubble distribution in the pressurized bubble columns.

Instability of the two-phase flow in vertical interelectrode gaps

In vertical gaps between electrode and membrane, the smooth upward flow of the gas–liquid dispersion near the electrode in the form of a bubble street may be upset under certain conditions. At a certain channel height, a downward flow develops near the membrane resulting in a more or less uniform distribution of gas bubbles over the whole cross-sectional area. The controlling conditions of the conversion of the regular bubble street flow into irregular downward flow were analysed quantitatively in a previous theoretical model, the results of which are now compared with experimental data confirming the theory. The experimental data are further used to derive a relationship for the distribution of the volume fraction of gas.

Flow structure and phase distributions in a slurry bubble column

In this paper, the axial and radial distributions of the gas and solid concentrations in a slurry bubble column are measured by an ultrasonic penetration technique, which provides a method capable of monitoring simultaneously the concentrations of the gas bubbles and the solid particles in the system. The macroscopic flow structure of the slurry bubble column is analyzed based on the structures of the phase distributions. The flow structure became important in determining the particle distribution in the column. The relationships among the gas bubble distribution, the flow structure and the particle concentration distribution are discussed. It is found that the solid particles are partly concentrated in the region near but not adjacent to the column wall and in the region between the central bubble stream and the region where the vortical-spiral liquid flow is established.