Bubble Properties Research Articles

Impact of plenum conditions and gas-liquid distributor on bubble column hydrodynamics

In an LC-FiningTM hydroprocessor, the orientation of the free surface in the plenum chamber and the efficiency of the internal gas-liquid separator impact the amounts of the gas and the liquid passing through individual bubble cap risers in the grid. Various gas and liquid flowrates were thus passed through a single bubble cap riser to determine resulting bubble properties. Bubble sizes correlated well with energy dissipation rate and gas-to-liquid velocity ratio to the power of −0.4 and 0.6, respectively, in agreement with theory on the turbulent breakup of dense dispersions. Bubbles were also influenced by the gas-liquid distributor geometry, where the bubble cap produced larger bubbles than a perforated plate. Finally, the size of bubbles sparged in the plenum marginally affected overall gas holdup, suggesting that a bubble cap distributor redesign should be prioritized over upstream gas introduction equipment to optimize the performance of the hydroprocessor.

Three-Dimensional Shock-Wave/Boundary-Layer Interaction in Supersonic Flow Past a Finite-Span Sharp Wedge

Shock-wave/boundary-layer interactions (SWBLI) are of great importance in supersonic transport vehicles. The shock-induced separation and its unsteadiness may lead to harmful influences on the aerodynamic performance and fatigue life of supersonic air-intakes, turbo-machine cascades and supersonic nozzles. We particularly focus on a three-dimensional SWBLI in supersonic flow past a finite-span sharp wedge. Implicit large-eddy simulation is performed to investigate the flow features in the three-dimensional SWBLI. Results show that a bow-type side-edge shock wave is generated from the leading edge of the finite-span sharp wedge. The shock impinges on the turbulent boundary layer and causes additional turbulence fluctuations in the spanwise direction. Three-dimensional features dominate the shock impingement and reflection. A large-scale separation bubble is induced by the bow-type side-edge shock wave. Properties of this separation bubble are examined and qualitatively compared with a two-dimensional SWBLI case.

Effects of Ultrafine Bubbles on Gram-Negative Bacteria: Inhibition or Selection?

Ultrafine bubbles exist in all liquids and are naturally stable. As their properties are not entirely known, it is unclear how they impact the surrounding solution and comparable-sized particles within it. It is essential to further investigate the properties of ultrafine bubbles in order to expand their industrial application. In this regard, the effect of ultrafine bubbles on bacterial development is of particular interest. Our current study, using optical density measurements and fluorescence microscopic images has demonstrated that ultrafine gas bubbles impact the morphology and phenotype of Escherichia coli and Pseudomonas aeruginosa. Specifically, Fourier transform infrared spectroscopic measurements indicated a thickening of bacterial membranes in samples exposed to ultrafine bubbles. The study also confirmed that ultrafine bubbles can inhibit bacterial cell growth. This study signifies the role of surface phenomena in bacterial culture, which is crucial in the upstream processes of recombinant DNA technology applications.

APPLICATION OF LASER-INDUCED BREAKDOWN CAVITATION BUBBLES FOR CELL LYSIS IN VITRO

Objective: Understanding the basic mechanism of the cavitation bubble action on living cells as a crucial step of development and application ofsophisticated methods based on controlled cavitation in cell behaviour manipulation. Optimisation of parameters in order to expand cell lysis regioncreated by a single bubble.Methods: The cavitation bubbles are generated by the laser-induced breakdown method. The impact of controlled cavitation bubble on thebiological system is synchronously monitored under a microscope and recorded. Visualization of the cavitation bubble course is monitored by a highspeedcamera. The impact of technology on the healthy confluent cell layer is verified. Evaluation of the cavitation bubbles´ effect on cells in real timeand by subsequent analysis of the cell lysis region and impact of the cavitation bubble on cell viability is carried out by optical visualization and life/dead fluorescence staining.Results: Cavitation bubble induced in distance of 1.5 mm from the cell surface overcomes properties of sessile bubble and enables to create cell lysisregion over 1000 μm in diameter due to transient shear stress produced by liquid displaced by the bubble expansion.Conclusion: Cell lysis region is strongly dependent on the spot laser energy (SLE) and the bubble induction distance from cells. This knowledge iscrucial for application in chemical free cell lysis in vitro, wound induction for experimental purposes and cell layers patterning in desired scale.

Temperature‐Driven Bubble Migration as Proxy for Internal Bubble Pressures and Bubble Trapping Function in Ice Cores

AbstractIce core data record significant and abrupt past climate changes that are associated with large and rapid changes in atmospheric greenhouse gases, such as methane. Due to the gradual close‐off of gas bubbles and the relatively fast diffusion of gases within the firn column, even a discrete or quick step increase in air composition may be smoothed or integrated in the data; current laboratory analyses of gases consider the mean gas content value across all bubbles in a sample, rather than the content of individual bubbles. The convolution of the distribution of trapping ages with the history of atmospheric composition thus smears the measured gas record in each sample. We developed a nondestructive method to determine pressure distribution in all bubbles in a sample and estimate the shape of the trapping function derived from that bubble pressure distribution and site characteristics. Our method works not only for present conditions but also through varying paleo‐atmospheric conditions, while providing accurate measurements of morphological bubble properties. The method is based on using temperature‐driven air bubble migration as a proxy for the pressure of individual bubbles, which we combine with a model for bubbly ice densification to obtain the gas trapping functions and constrain the age distribution of air bubbles for past conditions, which are preserved at different depths. The trapping functions will help us to obtain a more accurate gas signal in the future that is less attenuated through the age distribution of the gas during the close‐off process.

Micromechanical analysis of bubbles formed in fluidized beds operated with a continuous single jet

Advancement in the knowledge of bubble dynamics and properties can significantly aid to improve the design and operation of gas-solid fluidized beds. In this work, bubble properties in a single jet fluidized bed were studied by the combined approach of CFD and DEM. The results illustrated that the process of the continuous injection of a central air jet to the bed was successfully reproduced, with the formation of a series of bubbles which rise through the bed and burst at the bed top. During this process, the series of bubbles align vertically which show coalescence in three different levels of the bed. However, the coalescence patterns depend upon jet velocity and are fairly regular. The bubbles are categorized into two types: primary/leading bubbles and trailing bubbles. The size of primary bubbles keeps increasing with bed height while the size of trailing bubbles decreases since trailing bubbles coalesce with primary bubbles. Primary bubbles, although formed at different times, are of similar shape at different heights of the bed while trailing bubbles are generally elongated in shape. Trailing bubbles have a higher velocity than primary bubbles. The spatial analysis of pressure gradient, drag, and particle-fluid interaction force shows that the presence of bubbles leads to different particle flow zones where forces vary significantly. Moreover, the bubble passage can be delineated by the temporal variation of the particle-fluid interaction force.

Effects of 20 kHz ultrasound on coal flotation: The roles of cavitation and acoustic radiation force

Cavitation and acoustic radiation force are two main effects of ultrasound. In this study, ultrasonic pretreatment (UPT) and simultaneous treatment (UST) methods were applied to coal flotation. The ultrasonic effects were analyzed by measuring changes in ash content and yield of concentrate with or without ultrasound treatment. The decrease in ash content after UPT or UST was due to the cleaning effect of ultrasound. This effect is generated by the collapse of tiny bubbles trapped on the rough coal surfaces, as seen in images obtained by scanning electron microscopy (SEM). An improvement in recovery was only seen with the UST method. Properties of large bubbles, specifically their aggregation and shape, were influenced by ultrasound through the oscillation behavior and acoustic radiation force in the sound field, respectively. This work is expected to provide a novel perspective to the study of ultrasound-assisted flotation.

Using a bio-inspired copper complex to investigate reactive mass transfer around an oxygen bubble rising freely in a thin-gap cell

The present study describes an original colorimetric method to visualize and quantify the local oxygen mass transfer around a rising bubble in reactive media. This method is based on the use of a colorless bio-inspired copper complex, Cu(btmgp)I, specially tailored for the study, which, dissolved in acetonitrile, oxidizes into an orange copper-complex [Cu2O2(btmgp)2]I2. The latter complex, unstable at ambient temperature, decays quite fast into two Cu(II) complexes, leaving a permanent pale-green color as final products. The flow investigated consists in a pure oxygen single bubble rising freely in a confined thin-gap cell (400 × 200 × 1 mm). A wide range of motion regimes for the bubbles are observed as the Archimedes number ranges from 860<Ar<18,800 and the Reynolds number from 580<Re<8200. A high-resolution 16-bit sCMOS camera, combined with specific filters, is used to capture images from a region-of-interest of the cell, illuminated by a white LED backlight panel. An ad hoc calibration protocol is developed to correlate the grey-levels from the colored signal to the equivalent oxygen concentrations. This procedure then allows to measure indirectly the amount of oxygen transferred to the liquid phase. The series of images are also treated to identify the bubble motion and properties. Thanks to this method, equivalent oxygen concentration fields, gap-averaged and time-averaged, can be reached with high precision in the far-field wake of the bubbles, enabling thus to deeply characterize the mass transfer mechanisms under reactive conditions in such confined configuration, and to establish a dimensionless representation in terms of Sherwood number versus Peclet number. At last, thanks to the knowledge of the kinetic rate of the reaction and of the diffusion coefficients, the Hatta number and the enhancement factor are estimated, and thus the intrinsic Sherwood numbers; these results demonstrate that the enhancement of the mass transfer by the reactions involved with the copper-complexes is not negligible (almost 12–15%).

Combined effects of flow, surface stabilisation and salt concentration in aqueous solution to control and enhance sonoluminescence.

Sonoluminescence (SL) intensity can be increased with potassium iodide (KI) concentration, attributed to a reduction in the gas concentration of solution. However, bubble properties and active bubble distributions at different frequencies and powers also influence SL intensities. Hence, to elucidate how salt concentration affects SL intensity, a systematic study with parametric variation was undertaken. SL from KI solutions of 0.1, 1 and 2 M concentration, without flow and in the presence of flow at 24, 228 and 626 mL/min was investigated at 44, 300 and 1000 kHz. At all frequencies an increase in KI concentration caused a change in the active SL distributions. For 44 kHz, localised and standing wave field SL activity could be expanded. Flow at this frequency augmented SL and SL was maximised at the lowest power setting under stabilisation at the highest KI concentration. At 300 and 1000 kHz, attenuation of the sound field was reduced, allowing expansion of activity throughout solution. In this instance, augmentation of SL intensity was only observed under flow conditions at concentrations of 1 M (300 kHz) and 2 M (1000 kHz) under stabilisation. It was theorised that a combination of smaller bubbles at higher KI concentrations and flow effects could reduce bubble clustering and enhance field formations. This was most prevalent where the standing wave was reinforced under stabilised (44 and 300 kHz) or flow (1000 kHz) conditions, here the number of active bubbles in high pressure regions likely increases. Lastly, it was found that in KI solutions flow could localise SL activity beneath and at the flow inlet via reflection and aeration mechanisms.

Investigation of a bubbling fluidized bed methanation reactor by using CFD-DEM and approximate image processing method

Computational fluid dynamics and discrete element method (CFD-DEM) integrated with a new local-structure-dependent (LSD) drag model and modified reaction kinetics are used to simulate the bubbling fluidized bed methanation process. The nonlinear Hertzian model is employed to describe the particle-particle collision process. An approximate image processing method (AIPM) is developed to investigate the bubble properties in the reactive gas-solid fluidized bed, by which the bubbles can be located, measured, and tracked from the CFD-DEM results. The results of the bubble properties and reaction behavior are in good agreement with the correlations in the literature and the experimental data, respectively. The sensitivity study of particles Young’s modulus indicates that this parameter has a minor influence on the simulation results but the simulation efficiency can be significantly improved by reducing this parameter. The feasibility of the isothermal flow assumption is also validated for simulations of the fluidized bed methanation process by the CFD-DEM method. In addition, the comparison between CFD-DEM and the two-fluid model (TFM) results shows that both models gave similar results for the reactor performance but the TFM provides smaller bubble size.

WITHDRAWN: Investigation of a Bubbling Fluidized Bed Methanation Reactor by using CFD-DEM and Approximate Image Processing Method

Abstract Computational fluid dynamics and discrete element method (CFD-DEM) integrated with a new local-structure-dependent (LSD) drag model and modified reaction kinetics are used to simulate the bubbling fluidized bed methanation process. The nonlinear Hertzian model is employed to describe the particle-particle collision process. An approximate image processing method (AIPM) is developed to investigate the bubble properties in the reactive gas-solid fluidized bed, by which the bubbles can be located, measured, and tracked from the CFD-DEM results. The results of the bubble properties and reaction behavior are in good agreement with the correlations in the literature and the experimental data, respectively. The sensitivity study of particles Young’s modulus indicates that this parameter has a minor influence on the simulation results but the simulation efficiency can be significantly improved by reducing this parameter. The feasibility of the isothermal flow assumption is also validated for simulations of the fluidized bed methanation process by the CFD-DEM method. In addition, the comparison between CFD-DEM and the two-fluid model (TFM) results shows that both models gave similar results for the reactor performance but the TFM provides smaller bubble size.

Experimental investigation of local bubble properties: Comparison to the sectional quadrature method of moments

AbstractIn recent years, population balance models coupled to computational fluid dynamics have been used as a tool to model bubbly flows. In this work, we established a setup to measure the local size distribution, the velocity and orientation of bubbles in different zones of a bubble column. As a population balance model, we used the sectional quadrature method of moments for the first time in OpenFOAM to simulate the local change in the bubble size distribution and compared its results to the quadrature method of moments as well as to the experimental results. A satisfactory agreement between local experimental values of three investigated flow rates and simulations were found and enables the characterization of heterogeneous bubbly regimes as found in industrial reactive bubble columns.

A molecular dynamics study of the behavior of Xe in U3Si2

Uranium-silicide (U-Si) fuels are being pursued as a possible accident tolerant fuel (ATF). This uranium alloy fuel benefits from higher thermal conductivity and higher fissile density compared to uranium dioxide (UO2). The role of fission gas swelling on the operational performance of U-Si fuels remains an open question, however, fission gas swelling is a critical phenomenon in UO2, U-Zr and U-Mo nuclear fuels. Given the lack of experimental data, in order to study the fundamentals of bubble formation and evolution in U-Si, it is critical that there be an atomistic description of Xe within the U-Si system. In this work, a recently developed U-Si MEAM interatomic potential is leveraged to generate a description of the U-Si-Xe system fit to density functional theory data. The point defect energies of Xe in U3Si2 are determined, in addition to the point defect segregation energy for Xe with respect to two grain boundary orientations. Finally, the properties of small Xe bubbles are analyzed and an equation of state is developed.

Mechanisms of Nucleation and Stationary States of Electrochemically Generated Nanobubbles.

Gas evolving reactions are ubiquitous in the operation of electrochemical devices. Recent studies of individual gas bubbles on nanoelectrodes have resulted in unprecedented control and insights on their formation. The experiments, however, lack the spatial resolution to elucidate the molecular pathway of nucleation of nanobubbles and their stationary size and shape. Here we use molecular simulations with an algorithm that mimics the electrochemical formation of gas, to investigate the mechanisms of nucleation of gas bubbles on nanoelectrodes, and characterize their stationary states. The simulations reproduce the experimental currents in the induction and stationary stages, and indicate that surface nanobubbles nucleate through a classical mechanism. We identify three distinct regimes for bubble nucleation, depending on the binding free energy per area of bubble to the electrode, Δγbind. If Δγbind is negative, the nucleation is heterogeneous and the nanobubble remains bound to the electrode, resulting in a low-current stationary state. For very negative Δγ, the bubble fully wets the electrode, forming a one-layer-thick micropancake that nucleates without supersaturation. On the other hand, when Δγbind > 0 the nanobubble nucleates homogeneously close to the electrode, but never attaches to it. We conclude that all surface nanobubbles must nucleate heterogeneously. The simulations reveal that the size and contact angle of stationary nanobubbles increase with the reaction driving force, although their residual current is invariant. The myriad of driven nonequilibrium stationary states with the same rate of production of gas, but distinct bubble properties, suggests that these dissipative systems have attractors that control the stationary current.

Acoustic Characteristics of Biosynthetic Bubbles for Ultrasound Contrast Imaging.

Biosynthetic bubbles produced by floating microorganisms, such as bacteria and algae, have recently attracted wide attention as novel ultrasound contrast agents owing to their significant potential in ultrasound imaging and acoustic reporter gene-based imaging. However, the acoustics properties of these bubbles are unclear. In this study, we developed a finite-element model to describe the oscillation of nonspherical biosynthetic bubbles composed of a gas core encapsulated in a protein shell. In this model, the elastic properties of the bubble shells were characterized in terms of the density, thickness, Young's modulus, and Poisson's ratio. Theoretical calculations were performed for a single bubble and an assembly of randomly oriented bubbles. Our results demonstrate that (1) there are many types of surface oscillation modes for nonspherical biosynthetic bubbles, and a systematic relationship exists between the surface modes and the resonance frequencies; (2) the bubble shell shape has a significant effect on the acoustic behavior; (3) the resonance frequency of an ellipsoidal bubble decreases with the decrease in its polar radius-to-equatorial axis ratio; and (4) the acoustic scattering of a randomly oriented suspension is isotropic at and below the first resonance frequency. Our findings provide physical insight into the biomedical applications of biosynthetic bubbles and can be used to optimize the acoustics properties of such bubbles.

Enhancement of CO2 dissolution and sweep efficiency in saline aquifer by micro bubble CO2 injection

Due to the favorable physical properties of micro bubbles (MBs), MB CO2 injection into saline aquifers was proposed as a novel, economic, and leak-free underground injection method in the purpose of sequestration. In this study, supercritical CO2 injection into a Berea sandstone sample at 0.05 mL/min under reservoir conditions was investigated using X-ray CT. Experiments with micro bubble (MB) and normal bubble (NB) CO2 injection were conducted. The dependence of CO2 migration on pore space shows macroscopic bypassing via preferential flow paths, as the distribution and saturation of CO2 were largely influenced by the heterogeneous pore structure. Most CO2 migrated forwards via three flow paths that formed along the bedding layers with high porosity. The CO2 distribution was discontinuous, and isolated CO2 clusters appeared due to the heterogeneity of the pore structure and tortuosity of the flow paths. MB CO2 migrated into the areas adjacent to major flow paths, forming irregular channels, while NB CO2 migrated through smooth channels. MB CO2 breakthrough occurred at 0.18 pore volume (PV), which was larger than NB CO2. A long breakthrough time reflects high pore space utilization. A significant differences of MB and NB CO2 in the existing duration of flow paths in the whole displacement period was observed, which reflected higher stability of MB CO2 in the sample. Additionally, little MB CO2 was observed near the inlet, suggesting that MBs accelerated dissolution in the early stage of injection (0.02 PV). Smaller saturation and larger volume differences were observed for MB CO2 compared to NB CO2 in the early stage because of dissolution. The enhanced dissolution efficiency of MB CO2 reached 18.7% at 0.02 PV. Moreover, pore space utilization improved because MB CO2 permeated into regions of low porosity and low flow ability. The improvement in the utilization efficiency reached 4.9% in low-porosity regions. And the storage efficiency was increased up to 23.6%.The results of this study suggest that MB CO2 injection has higher dissolution and improved pore space utilization, making it an efficient method of CO2 sequestration in low-porosity regions.

BubGAN: Bubble generative adversarial networks for synthesizing realistic bubbly flow images

Bubble segmentation and size detection algorithms have been developed in recent years for their high efficiency and accuracy in measuring bubbly two-phase flows. In this work, we proposed an architecture called bubble generative adversarial networks (BubGAN) for the generation of realistic synthetic images which could be further used as training or benchmarking data for the development of advanced image processing algorithms. The BubGAN is trained initially on a labeled bubble dataset consisting of ten thousand images. By learning the distribution of these bubbles, the BubGAN can generate more realistic bubbles compared to the conventional models used in the literature. The trained BubGAN is conditioned on bubble feature parameters and has full control of bubble properties in terms of aspect ratio, rotation angle, circularity and edge ratio. A million bubble dataset is pre-generated using the trained BubGAN. One can then assemble realistic bubbly flow images using this dataset and associated image processing tool. These images contain detailed bubble information, therefore do not require additional manual labeling. This is more useful compared with the conventional GAN which generates images without labeling information. The tool could be used to provide benchmarking and training data for existing image processing algorithms and to guide the future development of bubble detecting algorithms.

A quantitative comparison of phase-averaged models for bubbly, cavitating flows

We compare the computational performance of two modeling approaches for the flow of dilute cavitation bubbles in a liquid. The first approach is a deterministic model, for which bubbles are represented in a Lagrangian framework as advected features, each sampled from a distribution of equilibrium bubble sizes. The dynamic coupling to the liquid phase is modeled through local volume averaging. The second approach is stochastic; ensemble-phase averaging is used to derive mixture-averaged equations and field equations for the associated bubble properties are evolved in an Eulerian reference frame. For polydisperse mixtures, the probability density function of the equilibrium bubble radii is discretized and bubble properties are solved for each representative bin. In both cases, the equations are closed by solving Rayleigh–Plesset-like equations for the bubble dynamics as forced by the local or mixture-averaged pressure, respectively. An acoustically excited dilute bubble screen is used as a case study for comparisons. We show that observables of ensemble- and volume-averaged simulations match closely and that their convergence is first order under grid refinement. Guidelines are established for phase-averaged simulations by comparing the computational costs of methods. The primary costs are shown to be associated with stochastic closure; polydisperse ensemble-averaging requires many samples of the underlying PDF and volume-averaging requires repeated, randomized simulations to accurately represent a homogeneous bubble population. The relative sensitivities of these costs to spatial resolution and bubble void fraction are presented.

Microstructure evolution and Young's modulus of He-implanted nanocrystalline tungsten film

Nanocrystalline tungsten has been widely investigated due to its excellent irradiation resistance. In this study, the microstructure and related Young's modulus of nanocrystalline tungsten film prepared by dual ion beam sputtering deposition and subsequently irradiated by 30 keV He+ of varying fluences at room temperature have been studied by experimental and simulation methods. The mean He bubble size is about 0.7 nm, and the implanted He is preferred to be trapped by the grain boundaries. Few He bubbles are formed nearby the high density defect regions due to more trapping sites for He atoms. The lattice swelling increases with the increase of He fluence. The Young's modulus of the films was measured by LAwave, which indicates the Young's modulus decreases with the increase of He fulence. Molecular dynamics simulations are performed to explore the dependence of Young's modulus on the swelling and the properties of He bubbles. The results indicate that the lattice swelling has a limited influence on Young's modulus. The concentration of He is confirmed to seriously affect the Young's modulus after He implantation. With the same concentration of He atoms, the distribution of He bubbles is a key factor on tensile strength. The He bubbles at grain boundary have a stronger influence on the tensile strength than that in grain interior. He bubbles with higher He/V ratios affect more significantly on the mechanical properties. All these results provide new understandings for radiation damages of nano-materials used in reactors.

Particle shape effect on bubble dynamics in central air jet pseudo-2D fluidized beds: A CFD-DEM study

Bubbles considerably influence the characteristics of gas-solid fluidized bed, hence play an important role in determining process performance. This paper presents a CFD-DEM study on the effect of particle shape on the bed microstructure and bubble properties in a pseudo-2D bubbling gas-solid fluidized bed, operated with a continuous central jet. The bubble formation process is successfully generated, where bubbles rise through the bed and burst at the top of the bed. The numerical results show that ellipsoids have slightly different flow patterns from those of spheres. However, the mechanisms of bubble splitting and coalescence are found strongly dependent on particle shape. For spheres, the bubble trajectories mainly follow the bed centreline, whereas the bubbles for ellipsoids are widely distributed on both sides of the bed centreline. This result suggests that the lateral drift of bubbles is high for ellipsoids because such particles prefer to orient their longest major axis in the direction of the fluid flow. At the lower part of the bed, both gas and particle velocity profiles are found axially similar to a Gaussian distribution. In contrast, at the upper part of the bed, their peaks become flatter and broader for ellipsoids. Additionally, the bubble equivalent diameters are higher for ellipsoids while bubbles become more circular for spheres. Both bubble frequency and bubble velocity for ellipsoids are lower than spheres. The results obtained from this study can improve the understanding of bubble dynamics in the fluidization of non-spherical particles.