Small Gas Bubbles Research Articles

Eulerian–Lagrangian 3-D simulations of unsteady two-phase gas–liquid flow in a rectangular column by considering bubble interactions

This work discusses the development of a three-dimensional Eulerian–Lagrangian CFD model for a gas–liquid flow in a rectangular column. The model resolves the time-dependent, three-dimensional motion of small gas bubbles in a liquid to simulate the dynamic characteristics of the oscillating bubble plume. Our model incorporates drag, gravity, buoyancy, lift, pressure gradient and virtual mass forces acting on a bubble rising in a liquid, and accounts for two-way momentum coupling between the phases. We use MUSIG model that provides a framework in which the population balance method together with the break up and coalescence models can be incorporated into three-dimensional CFD calculations. We use turbulent flow to describe liquid flow field. The standard κ–ε of turbulence is selected for calculating the properties of turbulent flow. The effect of aspect ratio of the column on the flow pattern, liquid velocity and gas hold-up profiles is discussed.

An energy-efficient process for decomposing perfluorooctanoic and perfluorooctane sulfonic acids using dc plasmas generated within gas bubbles

Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) are environmentally harmful and persistent substances. Their decomposition was investigated using dc plasmas generated within small gas bubbles in a solution. The plasma characteristics including discharge voltage, voltage drop in the liquid, plasma shape and the emission spectrum were examined with different gases. The decomposition rate and energy efficiency were evaluated by measuring the concentration of fluoride and sulfate ions released from PFOA/PFOS molecules. The concentration of fluoride ions and energy efficiency in the treatment of a PFOS solution were 17.7 mg l−1 (54.8% of the initial amount of fluorine atoms) and 26 mg kWh−1, respectively, after 240 min of operation. The addition of scavengers of hydroxyl radicals and hydrated electrons showed little effect on the decomposition. The decomposition processes were analyzed with an assumption that positive species reacted with PFOA/PFOS molecules at the boundary of the plasma–solution surface. This type of plasma showed a much higher decomposition energy efficiency compared with energy efficiencies reported in other studies.

On the numerical study of bubbly flow created by ventilated cavity in vertical pipe

The dispersion of bubbles into a down-liquid flow in a vertical pipe is investigated. At low flow rates, the intended design of a swarm of discrete bubbles is achieved. At high flow rates, a ventilated cavity is nonetheless formed, which is attached close to the gas sparger. Behind this ventilated cavity, three different flow regimes characterize the complex bubbly flow field downstream of the down-liquid flow: vortex region with high void fraction, transitional region and pipe flow region. In this study, a numerical model that solved the entire development of the gas–liquid flow including the extended single-phase liquid region upstream to the wall-jet and recirculating-vortex zones in order to allow a more realistic determination of the boundary conditions of the down-liquid flow was adopted. Coupling with the Eulerian–Eulerian two-fluid model to solve the respective gas and liquid phases, a population balance model was also applied to predict the bubble size distribution in the wake right below the cavity base as well as further downstream in the transitional and fully-developed pipe flow regions. The numerical model was evaluated by comparing the numerical results against the data derived from theoretical, numerical and experimental approaches. Prediction of the Sauter mean bubble diameter distributions by the population balance approach at different axial locations confirmed the dominance of breakage due to the high turbulent intensity below the ventilated cavity which led to the generation of small gas bubbles at high void fraction. Further downstream, the coalescence effect dominated leading to merging of the small bubbles to form bigger bubbles.

Mechanisms of Forge-and-Welding of Japanese Steel Using “Wakibana” Sparks in Flame as a Start Signal of the Work

“Tsumiwakashi Tanren” is the forge-and-welding process to form a steel block from steel pieces that are produced by Tatara. “Orikaeshi Tanren” is the forge-and-welding process to hold up a steel block to produce a steel plate. Steel blocks were covered with straw ash and mud and heated in blacksmith's furnace. When the temperature of steel blocks increased over 1100°C, the fire flame of charcoal colored yellow followed by orange. The yellow flame was D-line spectrum of sodium with 589 nm wavelength and was a sign of production of molten fayalite slag, so called “Noro”. When temperature increased over 1190°C, small and bright sparks so called “Wakibana” started to appear in fire flame from charcoal. When temperature attained at 1290°C and many “Wakibana” sparks appeared, blacksmith took out steel blocks on a anvil and forged them to weld by hammering. At this time, temperature at the interface of steel blocks increased to about 1470°C by oxidation of iron and decarburization and the surface of steel block melted and wetted. Then, by hammering, “Noro” film was broken and steel blocks easily welded. When small bubbles of CO gas were produced at steel surface by strong force, fine steel particles were caught in CO babbles and oxidized in air to make bright sparks of “Wakibana”. “Wakibana” is a start signal of forge-and-welding process.

Contrast-enhanced and targeted ultrasound.

Ultrasonic imaging is becoming the most popular medical imaging modality, owing to the low price per examination and its safety. However, blood is a poor scatterer of ultrasound waves at clinical diagnostic transmit frequencies. For perfusion imaging, markers have been designed to enhance the contrast in B-mode imaging. These so-called ultrasound contrast agents consist of microscopically small gas bubbles encapsulated in biodegradable shells. In this review, the physical principles of ultrasound contrast agent microbubble behavior and their adjustment for drug delivery including sonoporation are described. Furthermore, an outline of clinical imaging applications of contrast-enhanced ultrasound is given. It is a challenging task to quantify and predict which bubble phenomenon occurs under which acoustic condition, and how these phenomena may be utilized in ultrasonic imaging. Aided by high-speed photography, our improved understanding of encapsulated microbubble behavior will lead to more sophisticated detection and delivery techniques. More sophisticated methods use quantitative approaches to measure the amount and the time course of bolus or reperfusion curves, and have shown great promise in revealing effective tumor responses to anti-angiogenic drugs in humans before tumor shrinkage occurs. These are beginning to be accepted into clinical practice. In the long term, targeted microbubbles for molecular imaging and eventually for directed anti-tumor therapy are expected to be tested.

The Effect of Gas on the Injectivity of Particles in Sandstone

Summary Many operations involve the injection of fluids into the formation around a well. In many cases, the fluids contain colloidal particles, either initially present or introduced during the operation through dirt or naturally occurring particles. Therefore, all injection schemes potentially suffer from injectivity decline. This injectivity decline is caused by clogging of the formation by particles, forming an external filter cake on the surface of the formation and blocking the pores inside the formation. This paper reports on the effects of gas on the injectivity of particles in sandstone. Experiments were performed in which water containing micron-sized particles (hematite) was injected into sandstone cores with or without small gas bubbles (nitrogen) present in the water. The position and amount of particle deposition could be determined both visually and by chemical analysis. It was found that the presence of gas reduces the external filter cake formed on the inlet surface of the core. Also, with gas, the particles penetrate deeper inside the core and more particles pass through the core and are detected in the effluent stream. The same effects are enhanced when the mixture of gas bubbles and water is replaced by foam. This suggests that the presence of gas/water interfaces has a major influence on the retention of particles in the sandstone. Possible mechanisms are discussed. The pressure drop across the core when gas or foam is present is initially higher than in an identical test without gas because of relative permeability effects or foam-flow resistance. However, because fewer particles are retained, ultimately the pressure drop is significantly less when gas is present. This effect may be significant in injection operations involving foam and offers ways to mitigate injectivity loss.

Opposed bubbly jets at different impact angles: Jet structure and bubble properties

The structure of two colliding water jets containing small gas bubbles is studied experimentally. The effects of the separation distance between jets, as well as the orientation angle, on the spatial distribution of bubbles have been considered. Results on the global structure of the final jet and bubble properties have been obtained using a high-speed video camera, and measurements of the positions of coalescence events are presented. Jets are introduced through inclined pipes (with a diameter of 0.7 mm) into a large water tank to avoid wall effects. Inclination angle has been changed from 0° to 45° with respect to the horizontal, resulting in a 0° up to 90° impact angle between jets. Generation of bubbles is controlled by a T-junction device where a regular slug-flow is created prior to injection. Bubble sizes have been measured, and a mean diameter of around 1 mm has been obtained using high values of the liquid flow rate. In the studied range of separation distances between the bubbly jets, a more homogeneous dispersion of bubbles is created as the distance between jets is decreased and the momentum flux of each jet is increased. Higher numbers of coalescences are observed when using smaller distance between jets, and the obtained measurements revealed that the number of bubble coalescence events is reduced significantly using high values of liquid flow rates.

Free energy wells for small gas bubbles in soft deformable materials

Thermodynamic expressions are derived for the system relative Gibbs free energy, and the relative Gibbs free energy per bubble, for all possible equilibrium bubble states that can form in a soft slightly rigid material, initially supersaturated with a dissolved inert gas (N(2)). While the thermodynamic manipulations are exact, the final expressions are approximate, due to an approximation made in deriving the expression for the elastic free energy of a soft material containing more than a single bubble. The expressions predict that provided the shear modulus of the soft material is not negligibly small, free energy wells which stabilize small gas bubbles for finite periods of time exist in such materials. This is consistent with a previous calculation, based solely on the bubble pressure equation, which resulted in the conjecture that bubbles found in soft materials with some rigidity (or shear resistance) are likely to be small. The possible relevance of this to the field of decompression sickness is outlined.

Magma vesiculation and infrasonic activity at Stromboli open conduit volcano

Explosive activity at Stromboli is explained in terms of dynamics of large gas bubbles that ascend in the magma conduit and burst at the free surface generating acoustic pressure that propagates as infrasonic signals in the atmosphere. The rate and the amplitude of the infrasonic activity is directly linked to the rate and the overpressure of the bursting gas bubbles and thus reflects the rate at which magma column degasses under non-equilibrium pressure conditions. We investigate the link between explosive degassing and magma vesiculation by comparing the rate of infrasonic activity with the bubble size distributions (BSDs) of scoria clasts collected during several days of explosive activity at Stromboli. BSDs of scoria show a characteristic power law distribution, which reflect a gas bubble concentration mainly controlled by a combined process of bubble nucleation and coalescence. The cumulative distribution of the infrasonic pressure follows two power laws, indicating a clear separation between the frequent, but weak, bursting of small gas bubbles (puffing) and the more energetic explosions of large gas slugs. The exponents of power laws derived for puffing and explosive infrasonic activity show strongly correlated (0.96) changes with time indicating that when the puffing rate is high, the number of energetic explosions is also elevated. This correlation suggests that both puffing and explosive activity are driven by the same magma degassing dynamics. In addition, changes of both infrasonic power law exponents are very well correlated (0.92 with puffing and 0.87 with explosions) with variations of the BSD exponents of the scoria clasts, providing evidence of the strong interplay between scoria vesiculation and magma explosivity. Our analysis indicates that variable magma vesiculation regimes recorded in the scoria correlate with the event number and energy of the explosive activity. We propose that monitoring infrasound on active volcanoes may be an alternative way to look at the vesiculation process in open conduit systems.

Evaluation of a glow discharge chamber coupled to time of flight mass spectrometry for the analysis of small gas volumes and bubbles in glass

Glow discharge time-of-flight mass spectrometry (GD-TOFMS) is becoming a powerful tool not only for simultaneous elemental determinations but also for possible molecular analysis of solid and gaseous samples. In this work, a modified in-house GD ion source, operated with radiofrequency (rf) power and coupled to an orthogonal TOFMS, is investigated for the analysis of small volumes of gaseous samples. Moreover, the results are compared with those obtained using a previous GD source design containing an internal flow tube. The modified in-house GD ion source has a reduced internal volume and no flow tube built in, allowing the use of lower gas flow rates. Additionally, this design allows the GD source to be placed closer to the sampler cone of the MS, resulting in improved ion transport efficiency. Limits of detection obtained for the analysis of small volumes of gas, turned out to be more than one order of magnitude better when using this modified in-house GD ion source. Furthermore, the pulsed mode of the rf-GD system (both, in millisecond and microsecond regimes) has been investigated to further improve the analytical performance of the technique for small gas volumes analysis. Calibration curves are obtained for the selected molecular compounds including nitrogen, oxygen and carbon dioxide. The results show that the limits of detection can be further improved by using a 100 μs-pulsed-rf-GD-TOFMS system, where limits of detection observed were below nanolitre for molecular nitrogen and oxygen, and below picolitre for carbon dioxide.

Topological Sensitivity Analysis for the Location of Small Cavities in Stokes Flow

The moulds' filling process may generate flaws consisting of small gas bubbles trapped inside the material, which weaken the solidity of the casted piece. We consider here the inverse problem of determining these small size flaws' locations from velocities boundary measurements. The fluid flow is described by a simplified model based on the Stokes system. A numerical algorithm based on the topological sensitivity analysis applied to an energy-like misfit functional is worked out to that end.

Ground deformation caused by magma ascent in an open conduit

This study examines magma ascent processes of three basic types prior to intermittent explosions such as Vulcanian or Strombolian types: specifically their relations to volcano ground deformation. Such repetitive explosions eject magma from an open conduit at short time intervals. Consequently, magma pressure is expected to decrease at the conduit and/or reservoir, and thereby deflate the volcano. Then, magma within the conduit rises, exerting normal and shear stresses on the conduit wall, thereby inflating the volcano. Diffusive mass transfer of water molecules from the melt to bubbles might occur subject to a sudden depressurization by eruption when magma includes numerous small gas bubbles. Such gas bubble expansion lifts the magma in the conduit. Calculations of ground deformation on a semi-infinite medium for such rising magma show that vertical and radial displacements and tilt motions recorded in the far fields are proportional to the 1.5 power of time. For low-viscosity magma, gas bubbles might rise in the melt because of buoyancy force. The gas bubbles expand rapidly with time because of decreased ambient pressure, pushing the magma upward in the conduit. Consequently, the volcano slowly inflates initially; then the rate of deformation increases over time, eventually engendering a rapid inflation immediately before eruption. These temporal changes in ground deformations contrast against cases in which magma does not include gas bubbles. In such cases, amplitudes of ground deformation increase almost linearly or even decrease with time at far fields. These differences, which are recognized as basic characteristics of temporal changes of ground deformation, enable us to know the driving forces of magma in an open conduit and to evaluate the gas bubble behavior quantitatively in magma before eruption.

Contact Angle and Stability of Interfacial Nanobubbles

Small bubbles of gas are known to exist at the interface between hydrophobic solids and water. Two features of these bubbles are unexplained: the very low contact angle and the stability. A self-consistent explanation of both of these effects is that there is a film of contaminant at the air-water interface that decreases the surface tension and thus the contact angle, and also hinders diffusion of gases from the bubble, thereby increasing the lifetime. If, during the lifetime of the bubble, the surface tension increases faster than the area of the air-water decreases, the interfacial energy can lead to a stabilization of the bubbles.

The association between central venous pressure, pneumoperitoneum, and venous carbon dioxide embolism in laparoscopic hepatectomy

Laparoscopic hepatectomy (LH) is increasingly used. However, the safety and outcomes of LH have yet to be elucidated. The risk of venous gas embolism is increased during liver parenchymal transection. This risk may be increased with positive pressure carbon dioxide (CO(2)) pneumoperitoneum (PP). This may be exacerbated further when low central venous pressure (CVP) anesthesia is used to minimize hemorrhage during liver resection. To determine the risk of CO(2) venous embolism, hand-assisted laparoscopic left hepatic lobectomy was performed for 26 domestic pigs. They were divided into three groups involving, respectively, positive gradient (normal-pressure PP of 12-14 mmHg and low CVP of 5-7 mmHg), negative gradient (low-pressure PP of 7-8 mmHg and normal CVP of 10-12 mmHg), and neutral gradient (normal-pressure PP and normal CVP or low-pressure PP and low CVP). Transesophageal echocardiography (TEE) was used intraoperatively to assess the presence of emboli in the suprahepatic vena cava and the right side of the heart. The TEE was recorded and analyzed by blinded observers. Carbon dioxide embolism also was monitored using end-tidal CO(2) and compared with TEE. Carbon dioxide embolism was demonstrated in 19 of the 26 cases. The majority of gas emboli were small gas bubbles associated with dissection of the major hepatic veins. No statistically significant difference in the occurrence of gas emboli was observed between the groups. Of the 19 animals, 18 experienced no significant hemodynamic changes. One pig in the positive gradient group experienced hypotension in relation to gas embolism. The effects were only transient and did not preclude safe completion of the operation. Carbon dioxide embolism during LH occurs frequently. Clinically, this finding appears to be nominal, but care must be taken when dissection around large veins is performed, and awareness by the surgical and anesthesiology teams of potential venous air embolism is essential. Further evaluation of this phenomenon is required.

A Simple Mechanistic Model for Void-Fraction and Pressure-Gradient Prediction in Vertical and Inclined Gas/Liquid Flow

Summary A new mechanistic model for two-phase flow in vertical and inclined pipes was proposed on the basis of the drift-flux approach. The proposed model, unlike the other mechanistic models [Ansari et al. (1994); Xiao et al. (1990); Zhang et al.(2003)] that incorporate a system of nonlinear equations to solve, uses an explicit equation for liquid-holdup prediction, thus reducing computation time significantly. Coupled with some simplified assumptions on pressure/volume/temperature (PVT), such a simple form of a liquid-holdup-prediction formula enables an analytical integration of the pressure gradient in two-phase flow along the pipe. This procedure is used usually to speed up the calculation of bottomhole pressure (BHP) for a large number of wells for oil-production-optimization purposes (Khasanov et al. 2006). The drift-flux approach can predict liquid holdup for bubbly flow quite accurately. However, for slug flow, it usually underestimates the void fraction. Because slug flow is the most common flow in producing wells, this leads to the pressure drop being overestimated significantly; this can be proved by comparing computational results to the experimental data and mechanistic models. Small gas bubbles in liquid slugs should be accounted for when predicting liquid holdup for slug flow more accurately. Gas in the slug body is considered by adding a proper term to the void-fraction expression. This term is based on the correlation for liquid holdup in the slug body. The model was evaluated with Rosneft's field data and the Tulsa University Fluid Flow Projects (TUFFP) databank. The model was evaluated by comparing it with three mechanistic models for multiphase flow.

A Case of Postcardiac Injury Syndrome Presenting as Acute Mediastinitis

A 41-year-old man sought evaluation at the emergency department for pain in the anterior chest that had been ongoing for approximately 35 hours. The electrocardiogram showed marked ST segment elevation in the precordial leads. Cardiac biomarker levels were elevated. He subsequently underwent coronary angioplasty and stenting of the left anterior descending artery using two sirolimus-eluting stents. The following day, the patient complained of severe pain in his chest and shoulders. Computed tomography (CT) of the chest showed small gas bubbles around the aortic wall and mild pericardial thickening with subtle air densities, suggesting acute mediastinitis. With an impression of postcardiac injury syndrome and acute mediastinitis, he was treated with intravenous antibiotics and oral ibuprofen. Two days later, the patient had subjective improvement and the friction rub was no longer heard.

Effect of pore geometry and dissolved C O2 on ultrasonic transmission during pore pressure changes

It is common practice to use changes in fluid transmission velocity to determine whether a fluid is in gas or liquid phase during pore pressure changes, but transmission amplitude also changes when fluid phase changes during pore pressure reduction. We used two sizes of glass beads in a low-pressure cell to simulate porous rock and conducted experiments with pore spaces filled with distilled water, and with distilled water in which [Formula: see text] of [Formula: see text] was dissolved. Ultrasonic transmission tests above and below bubble point showed that transmission frequencies and amplitudes were higher for distilled water and the smaller beads. There was a greater reduction in frequencies and amplitudes when gas was liberated by scattering from the small gas bubbles associated with the small beads. The water-[Formula: see text] mixture produced higher transmission amplitudes than distilled wateralone, which is consistent with increasing fluid density in the pores. Although Henry’s law was appropriate for predicting the onset of bubbles, the ultrasonic response sensed bubble nucleation before the pressure predicted by Henry’s law was reached. We also found that transmission amplitudes and frequencies changed more quickly than transmission velocity, which changed little by comparison. Our study suggests that for time-lapse monitoring of [Formula: see text] sequestration operations, changes of transmission amplitude and frequency may provide a quantitative assessment of the amount of dissolved [Formula: see text] in connate water. Observations of ultrasonic transmission amplitude and frequency are more important in this regard than velocity observations. This knowledge can be applied where [Formula: see text] migrates or changes phase after sequestration, be it at depth, or as a result of near-surface leakage. Walk-away VSP data can provide a suitable monitoring tool for this purpose.

Mechanism and estimation of porosity defects in ductile cast iron

Although there are many works on the formation mechanism of porosity defects due to solidification in ductile cast iron, the formation mechanism is still not clear and decreasing the porosity defects is still a main issue in the industry. This paper critically reviews conventional explanations for the porosity formation including estimation methods. Based on the discussion the authors propose a formation theory where it considers gas and oxide entrapment during mould filling, expansion of outer part of casting due to graphite formation and pressure decrease in the inner part, followed by the growth of entrapped small gas bubbles. This mechanism can explain various facts in practice and be usable to estimate the defects. It also gives a good way to design effective risers. Future challenges are also discussed including the effect of inoculation on the fluidity of the mushy region.

Self-Pulsing Regime of DC Electric Discharge in Dielectric Tube Filled With Water Containing Gas Bubble

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Effective generation of biochemically active species inside a liquid can be performed by an electric discharge in small gas bubbles admixed from outside into the liquid. In such a case, active species are produced by a gas discharge plasma inside bubbles and then transported due to diffusion into the liquid. This paper presents experimental data of electric discharges in small bubbles inserted into a thin dielectric tube filled with water. A low-frequency regime of periodic electrical breakdowns is revealed and described. </para>

The effect of solids on the dense phase gas fraction and gas–liquid mass transfer at conditions close to the heterogeneous regime in a mechanically agitated vessel

In the heterogeneous regime, there is a strong literature evidence (discussed herein) that solids can supplant small bubbles in the dense phase and reduce the gas hold-up. This work examines the effects of the addition of 205 μm glass ballotini on the gas hold-up and k L a in a 0.286 m diameter stirred tank operated under intense conditions ( P/ V ≥ 5 kW m −3) close to the heterogeneous regime and above the agitator speed corresponding to the just suspended point, N JS. The tests were carried out on two systems: air–water (coalescing) and air–0.2 M sodium sulphate (salt) solution (which resists coalescence in the bubble regime). For the air–water system it was observed that the overall gas plus solids hold-up remains approximately constant until all the small gas bubbles are supplanted and then increased in direct relation to the solids volume. The k L a mirrored the gas hold-up trend and decreased with a fall in gas hold-up. In the salt solution k L a decreased in direct relationship to the solids concentration, to 40% of the no solids value at around 19% solids by volume of dispersion. Dynamic engagement and disengagement experiments established that the salt solution behaves differently than water with the small bubbles initially growing in size rather than being coalesced directly into the large bubble population.