Behavior Of Gas Bubbles Research Articles

Radiation re-solution of fission gas in non-oxide nuclear fuel

Renewed interest in fast nuclear reactors is creating a need for better understanding of fission gas bubble behavior in non-oxide fuels to support very long fuel lifetimes. Collisions between fission fragments and their subsequent cascades can knock fission gas atoms out of bubbles and back into the fuel lattice. We showed that these collisions can be treated as using the so-called “homogenous” atom-by-atom re-solution theory and calculated using the Binary Collision Approximation code 3DOT. The calculations showed that there is a decrease in the re-solution parameter as bubble radius increases until about 50nm, at which the re-solution parameter stays nearly constant. Furthermore, our model shows ion cascades created in the fuel result in many more implanted fission gas atoms than collisions directly with fission fragments. This calculated re-solution parameter can be used to find a re-solution rate for future bubble simulations.

Bubble formation and Kr distribution in Kr-irradiated UO2

In situ and ex situ transmission electron microscopy observation of small Kr bubbles in both single-crystal and polycrystalline UO2 were conducted to understand the inert gas bubble behavior in oxide nuclear fuel. The bubble size and volume swelling are shown as weak functions of ion dose but strongly depend on the temperature. The Kr bubble formation at room temperature was observed for the first time. The depth profiles of implanted Kr determined by atom probe tomography are in good agreement with the calculated profiles by SRIM, but the measured concentration of Kr is about 1/4 of the calculated concentration. This difference is mainly due to low solubility of Kr in UO2 matrix and high release of Kr from sample surface under irradiation.

Gas bubble behaviors in a gas–solid fluidized bed with an arch agitator

The performance of gas–solid fluidized bed reactors (FBRs) strongly depends on the behavior of gas bubbles. Mechanical agitation is widely used in industrial FBRs to enhance gas–solid mixing; however, its influence on bubble behaviors has been poorly studied, especially in experimental study. Pressure fluctuations in a 300mm inner diameter plexiglas column fluidized bed of Geldart D particles with an arch agitator were measured under various gas velocities and agitation speeds. The bubble sizes were evaluated by incoherent analysis of pressure fluctuations. The effects of mechanical agitation on axial bubble growth, bubble sizes, regime transition velocity and fines elutriation were comprehensively investigated. Results show that mechanical agitation promotes bubble growth at 1.7 Umf but reduces the bubble sizes with increasing gas velocities. The axial maximum bubble size with increasing gas velocity occurs at 1.2m/s without agitation and 1.0m/s with agitation speeds of 14 and 21rpm. Interestingly, mechanical agitation leads to a small increase in minimum fluidizing velocity and a slight decrease in critical turbulent velocity. Furthermore, the entrainment rate is reduced with mechanical agitation, due to the decreasing bed height and small fines ejection at the bed surface. The arch agitator helps to reduce the channel flow, enhance the gas–solid contact efficiency and improve the fluidization quality.

Post-breakdown dielectric recovery characteristics of high-pressure liquid co<sub>2</sub> including supercritical phase

The paper describes results of experimental studies to evaluate the effects of phasechange and/or density variation at the post-breakdown on the dielectric recovery characteristics of liquid and supercritical CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The study aims to explore the possibility of using supercritical CO2 to develop high-repetition-rate pulsed-power switches. The experimental setup consisted of a sphere-to-sphere 10 mm radius electrodes configuration with 50 μm gap distance. Three phases of pressurized CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ; subcritical liquid state (SLS), liquid with pressure higher than critical point (LHCP), and supercritical phase (SC), were used for the experiments. Experimental results revealed that SC CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> has a very short recovery time making it an appropriate fluid for MHz range highly-repetitive switches. Post-breakdown stage in SLS consisted of gas phase and liquid phase at the plasma channel between the electrodes, with the alternative expansion-contraction motion behavior of gas bubble determining dielectric recovery characteristics. Post-breakdown stages in SC were similar to those in LHCP, generating a low density region at the post-breakdown, where the state of density variation determined dielectric recovery characteristic. The low density region could be compressed by the surrounding medium, resulting in a considerably shorter recovery time.

Propagation of Underwater Shock Wave and Gas Bubble Behavior Induced by Electrical Discharge in Water

The basic behavior during underwater electrical discharge up to 10 kJ stored energy is investigated through both numerical analysis and optical-observation experiments. Since the authors have been investigating the use of underwater exploding phenomena for food processing and other applications, it is necessary to know the whole process which may affect the quality of the samples recovered. In the present investigation, the propagation of underwater shock wave was measured using a high-speed camera and compared with the numerically simulated results using LS-DYNA. Also, the motion of gas bubble, which is induced quite after the propagation of the shock wave, was measured and compared with the numerical simulation using the same code. The pressure of the bubble is not such high as the shock wave, but it is known that the impulse is not possible to be ignored due to its relatively long duration of the pressurization. Through a series of optical-observation experiments, it is confirmed that the numerical simulation is potentially possible to predict the whole phenomena for processing foods and other materials.

Hydrodynamics of gas bubbles in the gap of lantern blade electrodes without forced flow of electrolyte: Experiments and CFD modelling

Gas–liquid flow is of prime importance in multiphase processes, in particular in electrochemical cells for which generated gas enhances the ohmic drop and the energy consumption. The present investigation was aimed at investigating the hydrodynamics of electrogenerated oxygen bubbles in a process still in development for iron electrodeposition in alkaline solutions; the lantern-blade configuration of the pilot anode was emulated by facing parallel anodes in a laboratory cell without forced circulation of the liquid. Distributions of bubble velocities and void fractions in the anode-to-anode space were determined through experimental observations and CFD simulations to investigate the behaviour of gas bubbles in the anode gap. The effects of the anode gap, current density and cell inclination on the hydrodynamics of the gas phase were followed. Predicted gas velocity near the anode wall was validated by experiment. However, CFD showed that a negative lift coefficient is required for simulations in order to reproduce the experimental dispersion of the bubbles curtain: wall and repulsion forces that were not taken into account in the model might play a major role in the expansion of the bubble swarm.

Cold Model Study of Submerged Peripheral Gas Bubbling from a Cylindrical Dispenser

Modeling of the emerging gas flow through the cylindrical dispenser was important and critical where magnesium vapors were generated through chemical reaction between MgO and Al at 1673 K inside the dispenser and forced out through its perforations along with a carrier gas (Ar) into the liquid iron bath for desulphurization of hot metal. At lower part of the dispenser and 5 mm above from bottom, six equidistant perforations each of 1 mm diameter were drilled along the axis of the dispenser. Cold model study was carried out to assess the flow rate of carrier gas, dispenser clearance and bath height on the rate of ascent of gas bubble through the bath influence and mixing time, when gas emerges from the periphery of a cylindrical dispenser immersed into a liquid bath. Water was used for representing the liquid bath. The diameter of the gas bubbles which were released through the perforations of immerged dispenser increased with increase in gas flow rate and was not significantly influenced by dispenser clearance. The minimum gas flow rate required to escape the gas bubble from dispenser decreased with clearance. The mixing time in the bath due to the agitation caused by the bubbles was found to be a function of the gas flow rate, clearance and the bath height. The rate of ascent and the mixing time could be modeled successfully with mathematical formulations. The data were used for predicting the behavior of gas bubbles in 5 ton liquid iron bath.

Molecular dynamics simulation of fluid containing gas in hydrophilic rough wall nanochannels

This study performed the molecular dynamic simulations to investigate the boundary behavior of liquid water with entrapped gas bubbles over various hydrophilic roughened substrates. A “liquid–gas–vapor coexistence setup” was employed to maintain a constant thermodynamic state during individual equilibrium simulations and corresponding non-equilibrium Poiseuille flow cases. The two roughened substrates (Si(100) and graphite) adopted in this study present similar contact angles and slip length with gas-free fluid. By considering the effects of argon molecules at the interface, we demonstrated that the boundary slip behavior differed dramatically between these two rough wall channels. This divergence can be attributed to differences in the morphology of argon bubble at the interface due to discrepancies in the atomic arrangement and wall–fluid interaction energy. Furthermore, the density of gas at the interface had a significant impact on the effective slip length of the roughened graphite substrate, whereas shear rate \(\dot{\gamma }\) presented no noticeable influence. On the roughened Si(100) surface, the morphology of the argon bubbles exhibited far higher meniscus curvature and unstable properties under hydrodynamic effects. Thus, this substrate exhibited no slip to slight negative slip and no remarkable influence from either the density of gas at the interface or shear rate. In the present study, we demonstrate that the morphology and behavior of interfacial gas bubbles are influenced by the parameters of wall–fluid interaction as well as the atomic arrangement of the substrate. Our results related to nanochannel flow reveal that different surfaces, such as Si(100) and graphite, may possess similar intrinsic wettability; however, properties of the interfacial gas bubbles can lead to noticeable changes in interfacial characteristics resulting in various degrees of boundary slippage.

Visualization of Gas Bubble Behavior of a Regenerative Fuel Cell in Electrolysis Mode By Soft X-Ray Radiography

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Radial Profiles of Gas Bubble Behavior in a Gas‐Liquid Bubble Column Reactor under Elevated Pressures

AbstractThe effects of operating conditions on radial variation of gas holdups, bubble swarm rising velocity, and Sauter mean diameter were investigated in a bubble column reactor under elevated pressures using a conductivity probe method. Air served as gas phase and tap water as liquid phase with varying gas velocity and pressure. All three parameters increased constantly with higher superficial gas velocity. Maximum holdup, velocity, and Sauter mean diameter were found at the center of the cross section. Two different cases for Sauter mean diameter distribution were observed. The gas holdups increase continuously with higher system pressure, but decrease for bubble swarm rising velocity and Sauter mean diameter. According to experimental results, an empirical correlation of the gas holdup profiles is presented.

Visualization of Gas Bubble Behavior of a Regenerative Fuel Cell in Electrolysis Mode by Soft X-ray Radiography

Regenerative fuel cells (RFC) are candidates for hydrogen energy to become common in daily life since it lowers the first investment cost by combining two different devices into one. In an earlier study, it is proved that the hydrogen mass transport was limited by water accumulation, hence lowers the cell efficiency. Soft X-Ray radiography experiments performed to investigate hydrogen and oxygen gas bubble behavior in a regenerative fuel cell. The cell was visualized with high spatial resolution at different current densities. The hydrogen gas bubbles appear first compared to the oxygen, since the higher stoichiometric hydrogen generation and lower solubility of hydrogen in water. The gas bubbles tend to nucleate at certain locations which are thought to be cracks on the catalyst layer.

A new model for two-dimensional numerical simulation of pseudo-2D gas–solids fluidized beds

Pseudo-two dimensional (pseudo-2D) fluidized beds, for which the thickness of the system is much smaller than the other two dimensions, are widely used to perform fundamental studies on bubble behavior, solids mixing, or clustering phenomenon in different gas–solids fluidization systems. The abundant data from such experimental systems are very useful for numerical model development and validation. However, it has been reported that two-dimensional (2D) computational fluid dynamic (CFD) simulations of pseudo-2D gas–solids fluidized beds usually predict poor quantitative agreement with the experimental data, especially for the solids velocity field. In this paper, a new model is proposed to improve the 2D numerical simulations of pseudo-2D gas–solids fluidized beds by properly accounting for the frictional effect of the front and back walls. Two previously reported pseudo-2D experimental systems were simulated with this model. Compared to the traditional 2D simulations, significant improvements in the numerical predictions have been observed and the predicted results are in better agreement with the available experimental data.

Chaotic behavior of gas bubble in non-Newtonian fluid: a numerical study

In the present paper, the nonlinear behavior of bubble growth under the excitation of an acoustic pressure pulse in non-Newtonian fluid domain has been investigated. Due to the importance of the bubble in the medical applications such as drug, protein or gene delivery, blood is assumed to be the reference fluid. Effects of viscoelasticity term, Deborah number, amplitude and frequency of the acoustic pulse are studied. We have studied the dynamic behavior of the radial response of bubble using Lyapunov exponent spectra, bifurcation diagrams, time series and phase diagram. A period-doubling bifurcation structure is predicted to occur for certain values of the effects of parameters. The results show that by increasing the elasticity of the fluid, the growth phenomenon will be unstable. On the other hand, when the frequency of the external pulse increases the bubble growth experiences more stable condition. It is shown that the results are in good agreement with the previous studies.

The Effects of Pressure on Gases in Solution: Possible Insights to Improve Microbubble Filtration for Extracorporeal Circulation

Improvements in micropore arterial line filter designs used for extracorporeal circulation are still needed because microbubbles larger than the rated pore sizes are being detected beyond the filter outlet. Linked to principles governing the function of micropore filters, fluid pressures contained in extracorporeal circuits also influence the behavior of gas bubbles and the extent to which they are carried in a fluid flow. To better understand the relationship between pressure and microbubble behavior, two ex vivo test circuits with and without inline resistance were designed to assess changes in microbubble load with changes in pressure. Ultrasound Doppler probes were used to measure and compare the quality and quantity of microbubbles generated in each test circuit. Analysis of microbubble load was separated into two distinct phases, the time periods during and immediately after bubble generation. Although microbubble number decreased similarly in both test circuits, changes in microbubble volume were significant only in the test circuit with inline resistance. The test circuit with inline resistance also showed a decrease in the rate of volume transferred across each ultrasound Doppler probe and the microbubble number and size range measured in the postbubble generation period. The present research proposes that fluid pressures contained in extracorporeal circuits may be used to affect gases in solution as a possible method to improve microbubble filtration during extracorporeal circulation.

Modeling of filling and solidification process for TiAl exhaust valves during suction casting

Investment and suction casting (ISC) represents an economic and promising process route to fabricate automotive exhaust valves of γ-TiAl based alloys, but information available on the metal flow and the temperature changes during mould filling and solidification process for the ISC process is meager. A sequentially coupled mathematical flow-thermal model, based on the commercial finite-volume/finite-difference code FLOW-3D and the finite-element code PROCAST, has been developed to investigate the ISC process. In term of calculating the flow and temperature fields during the filling and solidification stages, potential defects including the gas bubbles and the surface air entrainment occurred in the mould filling process and the shrinkage porosities formed in the solidification process are predicted and the reasons for the formation of these defects are also analyzed. The effects of filling pressure difference control methods and moulds on gas bubble and surface air entrainment behavior are presented. It is found that by changing the filling pressure difference control methods from general suction casting to “air leakage” suction casting and reducing air leakage flow rates, the gas bubbles are eliminated effectively, and the surface air entrainment attenuate dramatically. With resort to a mould with a tetragonal runner, the surface air entrainment decrease to the lowest level. Finally, the water analogue and suction casting experiments of exhaust valves are implemented for further validation of the simulation results.

Depolarized SnO2-based gas anodes for electrowinning of silver in molten chlorides

SnO2-based porous anodes were prepared and the behavior of gas bubbles on the porous anodes with different original coarse grain size, immersed in ethanol to simulate molten chlorides, was primarily investigated. SnO2-based porous anodes were used as gas anodes for the electrowinning of silver in CaCl2-NaCl-CaO-AgCl melts at 680?C. Hydrogen was introduced to the anode/electrolyte interface through the gas anode. Carbon was used as the cathode. Obvious depolarization of the anode potential was observed after the introduction of hydrogen comparing with no reducing gas introduced, indicating the involvement of hydrogen in the anode reaction. Metallic silver was deposited on the cathode.

Molecular dynamics simulation of surface energy of low miller index surfaces in UO2

Molecular dynamics simulation based on the rigid-ion potential is carried out to investigate the surface energies of low miller index crystallographic faces such as (100), (110) and (111) in UO2 in a temperature range of 300 K-1500 K. The results indicate that the surface energies of the three low miller index crystallographic faces decline gradually with temperature rising, and the variation of the surface energy with temperature is confirmed to be consistent with the experimental data. The (111) crystallographic face which is the closest surface has the lowest surface energy; the (100) crystallographic face has the biggest surface energy; the (100) crystallographic face has a surface energy in between them. The surface atoms have compressed towards the vertical line of surface with respect to the inside atoms layer obviously. The symmetry of surface atoms declines. Surface phenomena such as relaxation and reconstruction occur on the surface atoms and the relaxation effect extends to the five layers. The results presented in the study are useful for understanding the behaviors of fission gas bubbles growing up and cracking up due to the swelling in fuels under the irradiation.

Evaluating the Behavior of Electrolytic Gas Bubbles and Their Effect on the Cell Voltage in Alkaline Water Electrolysis

The behavior of electrolytic gas bubbles and their effect on the cell voltage in water electrolysis were studied theoretically and experimentally. A fundamental force analysis was employed to predict the critical diameter for the departure of the electrolytic gas bubbles. Good agreement between the predictions and observations was obtained. It was found that increasing the electrode potential strengthened the force due to the interfacial tension and increased the critical diameter, while increasing the electrolyte concentration led to a reduction. This was explained by the changes in both the contact angle and surface tension. Many more fine gas bubbles were observed at high current density, which was explained by that the enhanced natural convection forced bubbles to depart prematurely. The cell voltage was only slightly reduced by the electrolyte circulation, which reduced the critical diameter for bubble departure. This confirmed that the layer of fine bubbles represented a significant energy barrier, ...

Experimental study on gas holdup and bubble behavior in carbon capture systems with ionic liquid

Ionic liquids (ILs) have been widely considered as potential green solvents for CO2 capture, and many promising ionic liquids have been synthesized and reported. However, knowledge related to the hydrodynamics of CO2 absorption with ionic liquids especially the bubble behavior is still scarce compared with the conventional molecular solvents. In this study, it is the first time to discuss the multi-bubble behavior in a carbon capture system with ionic liquid by using a high speed image pick-up system. The bubble behavior, such as bubble size distribution, total gas holdup, Sauter diameter and interfacial area at different operating conditions are investigated. The experimental results show that the bubble diameter of CO2–[bmim][BF4] system is smaller than that of N2–[bmim][BF4] system. The bubble diameter increases as the gas superficial velocity increases and the height along the axis of the column rises, but decreases with the increase of the temperature. The total gas holdup in [bmim][BF4] increases with the increase of the system temperature due to the decrease of the ionic liquid viscosity. In addition, the gas superficial velocity improves the total gas holdup due to the higher gas momentum favors the formation of small bubbles. A new correlation based on the experimental data is proposed for the prediction of Sauter diameter in gas–ionic liquid systems. The prediction results calculated by the new correlation are in good agreement with the experimental data with a relative error less than 6%.

A prediction of defluidization time in biomass fired fluidized bed combustion

Two mathematical models to describe the complete defluidization time in biomass FBC were presented in this study; one was newly developed and another was a modified model. They considered the effects of the chemical properties of fuel ash and bed operating conditions such as air velocity, bed particle size and static bed height. The model formulations employed the approach of force balance, defining the viscous flow sintering and the gas bubble behaviors to evaluate the adhesive and segregated forces, respectively. The models were achieved by the statistical regression analyzes with the early laboratory scale experimental results and then verified by the pilot scale experimental results and previous published data. The regression analyzes revealed that the inorganic composition of ash in the fuel and temperature were most significant. In addition, the fluidizing velocity showed a strong effect among the observed operating variables. The model predictive results followed the lab scale data with the error range of ±22% and ±35% and provided fairly well predictability with the pilot scale and published data of previous bed agglomeration tests, carried out with several kinds of biomass fuels.