Shallow Bubble Research Articles

Determining patterns in the formation of a polymer shell by powder paint on wood surface

An issue related to using wood and wood products for building structures is to ensure their stability and durability during operation within wide limits. Therefore, the object of this study was a change in the properties of the polymer shell of powder paint on wood during thermal modification. It is proved that in the process of thermal modification of wood, its structure changes, and accordingly, during the polymerization of powder paint, degassing occurs, which affects the polymer shell. Namely, during the thermal polymerization of powder paint at a temperature of 180 °C for untreated wood, shallow bubbles and craters are characteristic of the formed polymer shell. Instead, a smooth surface is marked for a sample of thermally modified wood. Thermogravimetric analysis data show thermogravimetric curves characterized by the loss of mass of the sample of the original wood with increasing temperature due to the processes of dehydration, destruction of hemicellulose and lignin. This is dehydration, accompanied by the destruction of the pyranose cycle, and carbonization to form a carbon residue and a complex mixture of volatile products. Due to this, bubbles and craters are formed in the polymer shell of the coating. Based on the obtained results of adhesion of the polymer shell on wood, which is treated with a mixture of epoxy polyester system with functional additives and a polymerization temperature of 180 °C, the adhesion level is 2.1 MPa. Reducing the polymerization temperature of a mixture of the epoxy polyester system with functional additives to 130 °C increases adhesion by 1.75 times, and the nature of the destruction passes through the polymer shell. For thermally modified wood, the level of adhesion is within 2.1 MPa, and the destruction takes place through the wood. This is due to the increased fragility of the surface after thermal modification of wood

Non-Richtmyer–Meshkov instability ejecta production based on shallow bubble collapse

The study of shock-driven ejecta production has focused on Richtmyer–Meshkov instability (RMI) growth from geometric features of the material surface. Extensive study of this mechanism under both single- and multiple-shock conditions has found that the ejected mass tends to be closely associated with the shocked surface phase, and its temperature is not dramatically greater than the hydrodynamic shock temperature of the bulk. In this work, we propose and demonstrate a new ejecta production mechanism that can occur under multiple-shock conditions based on the collapse of bubbles near the free surface of the material. This mechanism produces ejected mass that is much greater in quantity than observed in the RMI case. The particles are much hotter than predicted by the shock Hugoniot state, and the ejected mass does not appear to be strongly dependent upon initial surface finish. The ejecta source extends into the material with no clear remaining free surface. We name this mechanism Shallow Bubble Collapse (SBC) and discuss the conditions under which it activates. We demonstrate resolved modeling methods that enable the calculation, design, and study of SBC as a mechanism and perform a series of experiments to compare with the models. Under some multiple-shock conditions, SBC ejection produces ten times more ejected mass than RMI growth.

Constraining the release of Sn to the ambient melting point following shock loading using time-resolved x-ray diffraction

The formation of liquid following release from a shocked state governs the transition from spall to cavitation and the formation of ejecta in metals. In order to build physics-based models of these processes, it is necessary to critically evaluate the relative importance of kinetics and entropy generation during the release along with the accuracy of multiphase equations of state. Tin (Sn) has served as a testbed for a variety of experiments examining strength and ejecta due to its accessible melt boundary and solid–solid phase transitions. This work presents experiments examining the phase evolution of high purity Sn following the shock and release to ambient stress near the melting point. Sn is found to release to states between its ambient solidus and liquidus from approximately 19 to 33 GPa under uniaxial loading, with the two-phase region being characterized by a reduction in the intensity of the (220), (211) β-Sn doublet. Jetting experiments performed at 27–28 GPa exhibit comparable diffraction patterns with what is observed following the uniaxial release. The solid fractions of β-Sn in the ambient mixed phase region are found to decrease linearly with increasing shock stress as increasing liquid Sn is formed. The results provide much needed information for interpreting measurements of dynamic strength at a high strain rate and experiments examining cavitation and shallow bubble collapse in Sn.

Molecular dynamics simulation of bubble growth under surface of tungsten under helium irradiation

Understanding the fuzz growth mechanism on tungsten (W) surface under the helium (He) particles irradiation is an important guide for structural design of the first wall materials. In this work, molecular dynamics simulation was used to explore the process of He bubble formation, growth and rupture under irradiation to understand the initial stage of fuzz nanostructure formation. It was found that He clusters had the most movable ability at the surface temperature around 800∼1800 K and thus big bubbles were favorable to be aggregated. We simulated the growth, rupture and merging of He bubbles to reproduce the process of protrusion's formation on the surface which induced the formation of fuzz nanostructure. It was found that the shallow bubble burst led to the pinhole in surface layer. The dislocation slip in the growth process of deep bubbles was the main factor for the configurations of protrusion on surface. It mainly pushes the W above He bubble along the <111> directions, while the different surfaces have different relation with the <111> directions. Thus, the different morphology of protrusion induced by the bubble appeared on different surfaces as observed in experiments.

Ocean bubbles under high wind conditions – Part 2: Bubble size distributions and implications for models of bubble dynamics

Abstract. Bubbles formed by breaking waves in the open ocean influence many surface processes but are poorly understood. We report here on detailed bubble size distributions measured during the High Wind Speed Gas Exchange Study (HiWinGS) in the North Atlantic, during four separate storms with hourly averaged wind speeds from 10–27 m s−1. The measurements focus on the deeper plumes formed by advection downwards (at 2 m depth and below), rather than the initial surface distributions. Our results suggest that bubbles reaching a depth of 2 m have already evolved to form a heterogeneous but statistically stable population in the top 1–2 m of the ocean. These shallow bubble populations are carried downwards by coherent near-surface circulations; bubble evolution at greater depths is consistent with control by local gas saturation, surfactant coatings and pressure. We find that at 2 m the maximum bubble radius observed has a very weak wind speed dependence and is too small to be explained by simple buoyancy arguments. For void fractions greater than 10−6, bubble size distributions at 2 m can be fitted by a two-slope power law (with slopes of −0.3 for bubbles of radius &lt;80 µm and −4.4 for larger sizes). If normalised by void fraction, these distributions collapse to a very narrow range, implying that the bubble population is relatively stable and the void fraction is determined by bubbles spreading out in space rather than changing their size over time. In regions with these relatively high void fractions we see no evidence for slow bubble dissolution. When void fractions are below 10−6, the peak volume of the bubble size distribution is more variable and can change systematically across a plume at lower wind speeds, tracking the void fraction. Relatively large bubbles (80 µm in radius) are observed to persist for several hours in some cases, following periods of very high wind. Our results suggest that local gas supersaturation around the bubble plume may have a strong influence on bubble lifetime, but significantly, the gas in the bubbles contained in the deep plumes cannot be responsible for this supersaturation. We propose that the supersaturation is predominately controlled by the dissolution of bubbles in the top metre of the ocean, and that this bulk water is then drawn downwards, surrounding the deep bubble plume and influencing its lifetime. In this scenario, oxygen uptake is associated with deep bubble plumes but is not driven directly by them. We suggest that as bubbles move to depths greater than 2 m, sudden collapse may be more significant as a bubble termination mechanism than slow dissolution, especially in regions of high void fraction. Finally, we present a proposal for the processes and timescales which form and control these deeper bubble plumes.

Hydrodynamics of gas phase in a shallow bubble column from in-line photography

In shallow bubble columns (SBC), the under-developing hydrodynamics have not been well understood. The spatially-resolved measurements are conducted to probe bubble shape, size and gas holdup in SBC using a telecentric vision probe in tap water and CMC solution. More regular bubble shape and more serious polarization of bubble size are found in CMC. Bubble shape distributions agree well with the Wellek correlation and Bozzano-Dente correlation because of weaker interaction between bubbles. Different from the literature, relationship between aspect ratio and Eotvos number does depend on sampling locations. Too many tiny bubbles detected interfere with the observation of the conventional bubble size distribution. Assisted by the bubble area distributions, the actual effects of small and large bubbles on gas-liquid mass transfer are clarified. Higher volume fraction of the plume zone in SBC leads to the parabolic BHD in contrast to the almost flat profiles in the developed flow in TBC.

Simulation of CO2 capture by aqueous solution of ammonia in shallow bubble column reactor

In this paper, CO2 capture by an aqueous solution of ammonia in a shallow bubble column reactor is investigated. A two-dimensional CFD model was used to predict liquid axial velocity, gas holds up, turbulent kinetic energy and conversion of reaction with single orifice sparger. The standard k-e turbulence model along with two-fluid model was used for investigating the gas sparging effects on conversion of reaction in bubble column reactor with height to diameter ratio of 2. It is observed that the conversion of reaction significantly depends on the local gas hold-up. Uniform gas hold-up distribution increases the conversion of reaction, whereas more increase in the gas hold-up do not provide a uniform conversion in the radial direction of the column. For single orifice gas distributor, gas hold-up distribution is high in the column center and low near the walls; therefore, in the center of column lack of liquid phase decreases the conversion; while near the walls, the absence of gas phase reduces local conversion.

Characteristics of puffing activity revealed by ground-based, thermal infrared imaging: the example of Stromboli Volcano (Italy)

Puffing, i.e., the frequent (1 s ca.) release of small (0.1–10 m3), over-pressurized pockets of magmatic gases, is a typical feature of open-conduit basaltic volcanoes worldwide. Despite its non-trivial contribution to the degassing budget of these volcanoes and its recognized role in volcano monitoring, detection and metering tools for puffing are still limited. Taking advantage of the recent developments in high-speed thermal infrared imaging, we developed a specific processing algorithm to detect the emission of individual puffs and measure their duration, size, volume, and apparent temperature at the vent. As a test case, we applied our method at Stromboli Volcano (Italy), studying “snapshots” of 1 min collected in the years 2012, 2013, and 2014 at several vents. In all 3 years, puffing occurred simultaneously at three or more vents with variable features. At the scale of the single vent, a direct relationship links puff temperature and radius, suggesting that the apparent temperature is mostly a function of puff thickness, while the real gas temperature is constant for all puffs. Once released in the atmosphere, puffs dissipate in less than 20 m. On a broader scale, puffing activity is highly variable from vent to vent and year to year, with a link between average frequency, temperature, and volume from 136 puffs per minute, 600 K above ambient temperature, 0.1 m3, and the occasional ejection of pyroclasts to 20 puffs per minute, 3 K above ambient, 20 m3, and no pyroclasts. Frequent, small, hot puffs occur at random intervals, while as the frequency decreases and size increases, an increasingly longer minimum interval between puffs, up to 0.5 s, appears. These less frequent and smaller puffs also display a positive correlation between puff volume and the delay from the previous puff. Our results suggest an important role of shallow bubble coalescence in controlling puffing activity. The smaller and more frequent puffing at “hotter” vents is in agreement with the rapid rise of small gas pockets with only limited coalescence, while slower rise, allowing more time for coalescence, leads to larger but less frequent puffing at “colder” vents. This link between puffing style and vent thermal state points to a feedback between gas flux and magma temperature (and viscosity), where higher gas flux stirs and heats the magma, which, by getting less viscous, becomes a preferential way for bubble rise. Such a link has implications for the monitoring of the state of the shallow conduit at open-vent volcanoes as well as for determination of their total gas budget, relevant for hazard forecast and environmental studies.

Fluid Motions Associated with Engineered Area Bubble Plumes

AbstractCurrent bubble oil booms (BOBs) create bubble curtains to protect fixed installations in calm waters such as harbors, but fail in more energetic environments. Field and lab studies reveal that area bubble plumes enable BOB applications in such environments. Area bubble plumes exhibit dynamics distinct from point- or line-source plumes, including thicker and more robust outwelling flows and relatively smaller turbulence scales. Area bubble plume studies broaden our understanding of natural systems, where currents and waves are ubiquitous, leading to a variety of marine applications. Laboratory studies in a large flume have investigated the fluid dynamics of shallow line and area bubble plumes. These studies focused on area-plume application to oil spill response, which, based on preliminary tests, featured improved outflows with a downstream barrier. A range of ambient currents, current–plume angles, and airflows were tested. Key to understanding current–plume interactions was the upstream rotor or...

Heat transfer to a horizontal cylinder in a shallow bubble column

Abstract Heat transfer coefficient correlations for tall bubble columns are unable to predict heat transfer in shallow bubble columns, which have unique geometry and fluid dynamics. In this work, the heat transfer coefficient is measured on the surface of a horizontal cylinder immersed in a shallow air–water bubble column. Superficial velocity, liquid depth, and cylinder height and horizontal position with respect to the sparger orifices are varied. The heat transfer coefficient is found to increase with height until reaching a critical height, and a dimensionless, semi-theoretical correlation is developed that incorporates superficial velocity, liquid properties, and height. Additionally, the more minor effects of flow regime, column region, and bubble impact are discussed. Notably, the heat transfer coefficient can be as high in the region of bubble coalescence as in the bulk of the column, but only if bubbles impact the cylinder. The correlation and discussion provide a framework for modeling and designing shallow, coil-cooled bubble columns.

Mass transfer studies in shallow bubble column reactors

Mass transfer studies are carried out in a bubble column with an internal diameter of 14cm and various static liquid heights. The mass transfer coefficient is evaluated by using an oxygen sorption method. A model considering the gas holdup flushing and the sensor response is used. The interfacial mass transfer area is determined according to the measured bubble size distribution. The liquid-side mass transfer coefficient is also estimated from the volumetric mass transfer coefficient and the interfacial mass transfer area found. Results show that the effect of static liquid height on gas–liquid mass transfer is primarily on the interfacial mass transfer area. The mass transfer process is also governed by the type of gas distributor used. A single nozzle distributor is not suitable for shallow bubble column operations due to the large initial bubbles and the large volume of dead zone generated. It is also found that the different dependence of the liquid-side mass transfer coefficient on the superficial gas velocity observed in the literatures is due to the different bubble rising regimes.

Discovery of a natural CO2seep in the German North Sea: Implications for shallow dissolved gas and seep detection

A natural carbon dioxide (CO2) seep was discovered during an expedition to the southern German North Sea (October 2008). Elevated CO2 levels of ∼10–20 times above background were detected in seawater above a natural salt dome ∼30 km north of the East-Frisian Island Juist. A single elevated value 53 times higher than background was measured, indicating a possible CO2 point source from the seafloor. Measured pH values of around 6.8 support modeled pH values for the observed high CO2 concentration. These results are presented in the context of CO2 seepage detection, in light of proposed subsurface CO2 sequestering and growing concern of ocean acidification. We explore the boundary conditions of CO2 bubble and plume seepage and potential flux paths to the atmosphere. Shallow bubble release experiments conducted in a lake combined with discrete-bubble modeling suggest that shallow CO2 outgassing will be difficult to detect as bubbles dissolve very rapidly (within meters). Bubble-plume modeling further shows that a CO2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly CO2-enriched water tends to sink toward the seabed. Results suggest that released CO2 will tend to stay near the bottom in shallow systems (<200 m) and will vent to the atmosphere only during deep water convection (water column turnover). While isotope signatures point to a biogenic source, the exact origin is inconclusive because of dilution. This site could serve as a natural laboratory to further study the effects of carbon sequestration below the seafloor.

Effect of gas distributor on hydrodynamics in shallow bubble column reactors

AbstractThe effects of gas distributor on hydrodynamics in an air–water shallow bubble column reactor are investigated. Three types of distributors, namely, single nozzle, perforated plate and porous plate are being studied. The overall gas holdup, bubble size distributions and bubble rise velocity distributions are studied over a range of superficial gas velocities. The results show that single nozzle is not suitable for shallow bed operation. While perforated plate and porous plate distributors have comparable behaviour in the absence of solids, the addition of solids particles causes the two distributors to behaviour differently. The presence of solids promotes bubble coalescence for perforated plate distributor while the same inhibits bubble coalescence for porous plate distributor.

Bubble characteristics in shallow bubble column reactors

The bubble characteristics have been investigated in an air–water bubble column with shallow bed heights. The effect of bed height, location and the presence of solids on the bubble size, bubble rise velocity and overall and sectional gas holdup are studied over a range of superficial gas velocities. Optimal shallow bed operation relies on the combined entrance and exit effects at the distributor and the liquid bed surface. The gas holdup is found to decrease with an increase in H/ D ratio but the effect is diminishing at high H/ D ratios. A H/ D ratio of 2–4 is found to be suitable for shallow bed operation. The presence of solids causes the formation of larger bubbles at the distributor and the effect is diminishing as the gas velocity is increased.

Numerical simulations of fluid dynamics and mixing in a large shallow bubble column: effects of the sparging pipe allocations for multiple-pipe gas distributors

Numerical simulations of gas-liquid flow in a bubble column of 800 mm in diameter using two-fluid model and multi-size-group (MUSIG) model were conducted to investigate the effect of allocation of sparging pipes in gas distributors on hydrodynamic flow, gas hold-up, mixing characteristics, turbulence and bubble size. In the simulation, six different arrangements were assessed, which four-sparging pipes are mounted to a gas distributor with the allocation ring to bubble column ratios (d/DT) being 0.3, 0.37, 0.4, 0.5, 0.6 and 0.7, respectively. The simulation results have clearly demonstrated that the allocation of sparging pipes to the gas distributor has an important impact on gas hold-up and mixing characteristics. It was found from the simulations that the mixing time and overall gas hold-up increase initially with the increase of d/DT but decrease with further increase of d/DT. As a result, it was revealed that the optimum value of d/DT is around 0.4 for the highest overall gas hold-up and is confirmed by the experiment. It was also indicated from the simulation that d/DT of 0.5 yields the maximum mixing time.

Simulations of Gas Distributors in the Design of Shallow Bubble Column Reactors

AbstractComputational fluid dynamics (CFD) was used to simulate the effect of sparger construction in gas holdup and liquid axial velocity in a shallow bubble column reactor for the air‐water system. Model parameters were evaluated in 2‐ and 3‐D simulations by using a two‐fluid model and the standard k‐ϵ turbulence model. The Eulerian‐Eulerian approach was employed to predict the height of column that is affected by the sparger. It was found that increasing the number of orifices in the sparger increases the total gas holdup. Moreover, each orifice causes an increase in the circulation and mixing of liquid in the column. The results of the simulations follow the trends observed in the findings of Dhotre and Joshi [1].

Gas Absorption during the Burst of an Individual Bubble on the Water Surface

The rate of absorption of carbon dioxide from a bubble to a liquid were investigated using a shallow bubble column with a single nozzle. The overall mass transfer rate was classified into three categories depending on the bubble behavior: bubble formation at the nozzle, bubble rising in the bulk liquid and a bubble bursting (often followed by drifting) on the surface of water in the column. Flow properties including bubble diameter, the frequency of bubble formation and the velocity of bubble when rising were measured to determine the interfacial surface area of the bubbles for each region. As a result, it was possible to accurately measure the respective mass transfer coefficients. The contributions of bubble formation and bubble burst were found to be significant for the mass transfer rate in a shallow bubble column. The mass transfer coefficient of bubble formation was much higher than those for bubble burst and bubble rising.

Modelling of fluid dynamics and mixing in shallow bubble column reactors: influence of sparger design

Fluid dynamics and mixing in shallow bubble column reactors is controlled by sparger configuration. In this paper, we evaluate the applicability of using computational fluid dynamics (CFD)-based models to simulate transient fluid dynamics and mixing in shallow bubble columns. A two-fluid model along with the standard k– ε turbulence model was used to simulate flow generated by gas sparged through two different spargers in a bubble column with height to diameter ratio of 2. Different mixing time definitions were evaluated using the CFD-based simulations of mixing of passive tracer. The role of unsteady fluid dynamics and implications of possible simplifications in the geometry of bubble columns for simulating mixing in bubble columns were studied. The simulated results were analysed with reference to the experimental data of Haque et.al. (1986. Chemical Engineering Journal, 33, 63–69). The presented results and analysis will be useful to extend the application of CFD models for screening alternative sparger designs.

Tomographic reconstruction of shallow water bubble fields

In March of 1997, a shallow water experiment was conducted near the Scripps Pier in La Jolla, CA, USA. The goal was to determine the dynamics, distribution, and acoustic effects of bubbles just offshore from active surf. A major component of the experiment was the "Delta Frame", an apparatus that supported two acoustic sources and eight receivers. Acoustic intensity was measured at frequencies between 39 and 244 kHz over the resulting 16 horizontal ray paths. Paths ranged in length from 2.5 to 8.6 m. In the present paper, a tomography algorithm is developed and implemented using Delta Frame data. Measurements are combined to produce quantitative cross-sectional images of the attenuation associated with the bubble cloud. Numerical simulations predict that the Frame ran resolve details of the field down to about 2 m. Images constructed at different acoustic frequencies are scaled and compared. A 5-min sequence of images is studied in detail. Swell waves are shown to cause rapid fluctuations in the images.

Transient three-dimensional numerical analyses of shallow bubble column

Two-phase flow in a shallow bubble column (radius equal to the height) was studied. The primary objectives were to study both the effect of the free surface on the whole flow mode and the three-dimensionality in the whole domain. Air bubbles were injected through a central porous bottom wall of a cylindrical water tank. The flow pattern was visualized with dispersed aluminium particles. The velocity of the liquid phase was measured with a laser-Doppler velocimeter. The system was numerically analysed with a dispersed flow model. The inertial terms in the momentum equations were approximated by the QUICK scheme. Two models for the top surface were employed, namely either slip or non-slip conditions. The computed result with non-slip conditions agreed fairly well with the experimental result. A three-dimensional calculation was carried out with non-slip conditions for the top water surface. The calculated flow pattern agreed better with the experimental pattern than the two-dimensional calculation. In the results, two spiral vortices moving vertically were found outside the central upward flow. The flow was strongly three-dimensional, even though the vessel was shallow.