Role Of Bubbles Research Articles

Hydrodynamics and mass transport in an anodic oxidation reactor: Influence of gas evolution and flow path through mesh electrodes

Removal of organic compounds by anodic oxidation requires a sufficiently high rate of mass transport of target compounds to the electrode surface. Since anodic oxidation is often accelerated by the formation of hydroxyl radicals from water oxidation at high anodic potential, the role of oxygen/hydrogen evolution reactions was investigated. Residence time distributions showed that formation of bubbles in zones where fluid velocity was minimal, allowed for strongly improving global hydrodynamics in the reactor. The role of bubbles was also discussed by comparing mass transport coefficients obtained from the limiting current technique to values obtained from kinetics of COD removal during treatment of a model pollutant (phenol). The use of mesh electrodes (with liquid flowing through the electrode) further promoted convection and mass transport of organic compounds compared to plates. However, large bubbles generated at the surface of the plate electrodes also enhanced convection in their vicinity. In both cases, an adverse effect from bubble adhesion was observed because of the decrease in the active surface of electrodes. The mass transport coefficient obtained with the limiting current technique using mesh electrodes was 0.69 × 10−3 cm s-1 at 10.8 L h-1 with a mean velocity of electrolyte past the mesh surface (u¯mesh) of 0.37 cm s-1. It decreased to 0.58 × 10−3 cm s-1 during the treatment of phenol at the same flow rate and reached 0.75 × 10−3 cm s-1 when increasing the flow rate to 32.4 L h-1. By taking into consideration gas evolution at the surface of electrodes, this study allowed for accurate calibration of an advective – dispersive model with reaction that could be used for sizing such reactors according to the intended application.Abbreviations: Ae, the volumetric surface area (ratio between the geometric volume and the active surface area of the mesh electrodes) (cm−1); CA, aeration by a floor diffuser of fine bubbles and no current at the electrodes; CEP, polarization of the electrodes with a current intensity and no aeration; CW, no electrode polarization and no aeration; COD, chemical oxygen demand (mgO2 L−1); Di, diffusion coefficient of specie i (m2 s-1); D, dispersion coefficient (cm2 min−1); E(θ), residence time distribution function; EC, electrical energy consumption (kWh gCOD−1); F, Faraday constant (96485 C mol−1); HER, hydrogen evolution reaction; IOA, index of agreement; km, mass transport coefficient (m s-1); MCE, mineralization current efficiency; ME, model efficiency; OER, oxygen evolution reaction; OH, hydroxyl radical; Ql, liquid flow rate (L h-1); rAO: anodic oxidation rate (mol m−3 s−1); RTD: residence time distribution; ts¯: mean residence time or first moment order; u¯: mean flow velocity (m s-1); VT: total volume (L); σ2: variance or second moment order; γ: skewness factor or third moment order; τ: residence time; μ: order moment.

Role of bubble and impurity dynamics in electrical breakdown of dielectric liquids

Electrical discharges in liquids with and without gas injection was experimentally studied with two circuits (RC and single spark gap). Electrical breakdown could be initiated from the following mechanisms: electrode initiation; impurity initiation; and Taylor cone driven initiation. Discharges initiated by electrodes were observed only with the single spark gap circuit. Impurities and Taylor cones can initiate discharges when using the RC circuit. With the single spark gap circuit, bubbles were nearly stationary in the gap before and during discharges because of the limited time to respond to the electric field growth (dV/dt = 1 kV ns−1). Bubbles and impurities gain significant energy and undergo significant dynamic changes with the RC circuit under a long rising time. Charge relaxation time on bubbles or impurities in the fluids were close to 100 ms, which is comparable with the circuit rising time. Bubble dynamics observed include reduced bubble size, increased bubble speed and shape change. Taylor cones were observed on bubble surface using a high-speed camera during multiple discharge events, and usually led to breakdown. Taylor cones develop on a bubble surface through multiple states over a period of 10–500 μs. State one was typically seen on a bubble surface with a sharp tip caused by the residual charge. A single streamer was initiated on the tip of the cone at state two. State three developed multiple bush-like streamers on the cone and usually triggered a breakdown in the gap. Discharge behaviors in liquids were also investigated with the RC circuit at different pulsing frequencies. At a low pulsing frequency (1 Hz), discharge events are independent from each other. When pulsed above 10 Hz, bubbles created in the gap accumulate and participate in the next discharge event. Bubble accumulation results in a cloud where local secondary discharges are observed with a much higher frequency up to a few kHz for several milliseconds.

The role of bubbles in twinkling on pathological mineralizations

The Doppler ultrasound twinkling artifact has been attributed to scattering off microbubbles on and within kidney stones; however, it is not clear if microbubbles are also present on other pathological mineralizations. Uric acid, cholesterol, calcium oxalate, and calcium phosphate crystals (n = 5/type) were grown in the lab and imaged with Doppler ultrasound in ambient and hyperbaric conditions to show that microbubbles are present and causetwinkling on mineralizations other than kidney stones. [Work supported by NSF CAREER Grant No. 1943937 and NSF GRFP Grant No. DGE1255832.]

Dynamic behaviors of minor droplets and the role of bubbles in phase-separating Al[sbnd]Bi immiscible alloy

Minor droplets growth and solute segregation have their peculiarities in phase-separating immiscible alloys and can take complex pathways during solidification. The dynamic behaviors of liquid phase separation and surface enrichment in solidifying Al-10 wt% Bi immiscible alloys were revealed by synchrotron X-ray imaging technique. The large-sized Bi droplets would precipitate from Al-matrix melt directly and almost all of the droplets possess different existing states. The distribution of Bi droplets, resulting from the heterogeneous nucleation and repeatedly nucleation, varies with the growth time during cooling. And the growth dynamics of Bi droplets is revealed to be inconsistent with the Gaussian model. We also found that hydrogen bubbles can prevent the occurrence of the liquid phase separation process.

Photon Domains in Generators of Electromagnetic Energy

The role photonic domains play in natural generators of electromagnetic energy in definitions of the electrodynamics of giant energies is considered. Special attention is given to stars; specifically, to yellow dwarfs of class G2, to which our Sun belongs. A new model is proposed in which the nuclei of class G2 yellow dwarfs are thermonuclear boilers with bubble boiling, in which not only the proton–proton chain reaction is synthesized, but others as well. The role of bubbles in these boilers is played by photon domains.

Bubbles in Insulating Liquids: A Short Review

This paper deals with the influence of bubbles in insulating liquids and their role in liquid breakdown. Bubbles play a deleterious role in the dielectric strength of insulating liquids. Depending on their shape and localized electric field, bubbles may significantly lower the dielectric strength of insulating liquids. The present paper offers a short review – albeit incomplete – on the role of bubbles and tries to elucidate their relation to the total breakdown of dielectric liquids. It also proposes some further fields of research.

Bubble Dynamics in Turbulent Duct Flows: Lattice-Boltzmann Simulations and Drop Tower Experiments

Lattice-Boltzmann simulations of a turbulent duct flow have been carried out to obtain trajectories of passive tracers in the conditions of a series of microgravity experiments of turbulent bubble suspensions. The statistics of these passive tracers are compared to the corresponding measurements for single-bubble and bubble-pair statistics obtained from particle tracking techniques after the high-speed camera recordings from drop-towers experiments. In the conditions of the present experiments, comparisons indicate that experimental results on bubble velocity fluctuations are not consistent with simulations of passive tracers, which points in the direction of an active role of bubbles. The present analysis illustrates the utility of a recently introduced experimental setup to generate controlled turbulent bubble suspensions in microgravity.

Flotation of coarse coal particles in a three-phase gas-solid-liquid fluidised bed

The role of bubble and flotation reagents in the recovery of coal particles in three-phase gas-solid-liquid fluidised beds is investigated in this study. The flow rate and flotation yield in the presence or absence of bubbles were determined. The effect of bubbles attached to lower density and larger coal particles experiencing fluidised bed flotation is also discussed and validated by captured images. It is found that upward bubbly flows can significantly decrease the superficial velocity of liquid required to keep particles suspended and facilitates the capture of particles by bubble. The presence of a flotation collector improves this process and a decreased superficial flow velocity creates a relative quiescent hydrodynamic condition that enhances coarse particle-bubble aggregation and weakens the detachment of coarse particles from bubbles. The results indicate that flotation in a three-phase fluidised bed is a promising method to improve coarse coal particle separation. [Received: May 23, 2016; Accepted: February 13, 2017]

Bubbles in a Secular Stagnation Economy

The global fall in interest rates in recent decades has raised the possibility of a negative natural rate of interest. The secular stagnation hypothesis argues that a negative natural rate of interest is the underlying cause of the Great Recession. In this paper I demonstrate the role of bubbles in secular stagnation. Bubbles increase the natural rate of interest, hence they alleviate the cause of secular stagnation. This suggests a positive role for bubbles, however bubbles are intrinsically unstable. Larger bubbles are shown to be more unstable, and upon collapse the natural rate of interest falls, potentially triggering secular stagnation.

Adhesion of Steel and PMMA by Means of Laser Radiation

As the utilization of plastics is growing in our devices, their joining with other structural materials, like metals is more and more necessary. A novel method for joining polymeric materials and metals is the laser assisted metal plastic joining. The method is in focus of several researches. However, the mechanism of joint formation is not described sufficiently yet. In this study, the adhesion between structural steel and PMMA plastic and the phenomena of bubble formation is investigated. Scanning electron microscopy (SEM), thermogravimetry and mass spectrometry (TG-MS) were used to analyze joining interfaces and changes in the plastic material. Results show good adhesion between the mentioned materials and the important role of bubbles in the evolution of joining force.

Flotation of coarse coal particles in a three-phase gas-solid-liquid fluidized bed

The role of bubble and flotation reagents in the recovery of coal particles in three-phase gas-solid-liquid fluidised beds is investigated in this study. The flow rate and flotation yield in the presence or absence of bubbles were determined. The effect of bubbles attached to lower density and larger coal particles experiencing fluidised bed flotation is also discussed and validated by captured images. It is found that upward bubbly flows can significantly decrease the superficial velocity of liquid required to keep particles suspended and facilitates the capture of particles by bubble. The presence of a flotation collector improves this process and a decreased superficial flow velocity creates a relative quiescent hydrodynamic condition that enhances coarse particle-bubble aggregation and weakens the detachment of coarse particles from bubbles. The results indicate that flotation in a three-phase fluidised bed is a promising method to improve coarse coal particle separation. [Received: May 23, 2016; Accepted: February 13, 2017]

The role of bubbles during air‐sea gas exchange

AbstractThe potential for using the air‐sea exchange rate of oxygen as a tracer for net community biological production in the ocean is greatly enhanced by recent accuracy improvements for in situ measurements of oxygen on unmanned platforms. A limiting factor for determining the exchange process is evaluating the air‐sea flux contributed by bubble processes produced by breaking waves, particularly during winter months under high winds. Highly accurate measurements of noble gases (Ne, Ar & Kr) and nitrogen, N2, in seawater are tracers of the importance of bubble process in the surface mixed layer. We use measured distributions of these gases in the ventilated thermocline of the North Pacific and an annual time series of N2 in the surface ocean of the NE Subarctic Pacific to evaluate four different air‐water exchange models chosen to represent the range of model interpretation of bubble processes. We find that models must have an explicit bubble mechanism to reproduce concentrations of insoluble atmospheric gases, but there are periods when they all depart from observations. The recent model of Liang et al. (2013) stems from a highly resolved model of bubble plumes and categorizes bubble mechanisms into those that are small enough to collapse and larger ones that exchange gases before they resurface, both of which are necessary to explain the data.

The role of bubbles in generating fine ash during hydromagmatic eruptions

The abundant fine ash produced in the 2011 subglacial eruption of Grimsvotn, Iceland, highlights the fragmentation efficiency of mafic hydromagmatic eruptions, which is considerably higher than for comparable “dry” eruptions. Ash from the 2011 eruption can be divided into three morphological components—vesicular particles, shards, and dense fragments—distinguished by the size and abundance of constituent vesicles. We use the vesicle characteristics to define a new shape factor, the concavity index, which provides an unbiased way to classify individual ash particles as either bubbly (vesicular particles and shards) or dense. The relative proportion of bubbly and dense particles varies systematically with grain size, with the proportion of bubbly grains decreasing as the particle size approaches the modal bubble diameter. Measured bubble volume distributions are similar to those of rapidly quenched pyroclasts from Hawaiian fountains and suggest a comparable degassing history during magma ascent. Yet concordance between the size distributions of ash and of bubbles in the Grimsvotn samples stands in contrast to the size distributions in Hawaiian fountains, where pyroclasts are orders of magnitude larger than individual bubbles. We propose that the Grimsvotn ash formed by brittle disintegration of vesicular pyroclasts and that fragmentation efficiency was amplified by residual thermal stresses in glass quenched by glacial water. The strong control of resulting particle sizes and morphologies by the size and spatial distribution of bubbles demonstrates that the bubble population cannot be ignored when modeling hydromagmatic fragmentation.

Research Activities Related to Flotation Process

Flotation initially originated from the field of mineral processing. The importance of this process to the economy of the whole industrial world is considered to be enormous. Since for many years various particulate solids besides minerals have been extracted by using this effective gravity separation method with many interesting applications in the treatment of wastewater, looking for sustainability. Certain significant activities in flotation research were reviewed in the present, with main focus in the contribution of physical chemistry to flotation; i.e. zeta-potential measurements, contact angle etc., including examining the role of bubble and particle size played with regard to the process. Alternative bubble generation methods were examined, such as electro flotation and dissolved-air flotation-the latter technique is mainly intended for water treatment applications.

Simulating a bubbly ocean

Bubbles, which form on the ocean surface when waves break, are an important part of air‐sea gas transfer. They also affect the optical and acoustical properties of the near‐surface ocean. Observations of oceanic bubbles are limited, and scientists do not yet fully understand subsurface bubble evolution and the role of bubbles in ocean dynamics as well as air‐sea gas transfer. Liang et al., building on their previous study of bubbles in the ocean, used a multisize, multicomponent bubble model embedded in a large‐eddy simulation model for turbulent oceanic boundary flows to study how bubbles evolve in the ocean after they form, what determines the subsurface bubble distribution, how bubbles affect boundary layer turbulence, and what the subsurface bubble distribution means for bubble‐mediated gas transfer. They found that bubbles dissolve while being carried by turbulent flows, and the mean subsurface bubble distribution can be quantified using wind speed and sea state. The model could help improve understanding of bubbles' role in air‐sea gas exchange. (Journal of Geophysical Research‐Oceans, doi:10.1029/2011JC007766, 2012)

Risk assessment for uncertain cash flows: model ambiguity, discounting ambiguity, and the role of bubbles

We study the risk assessment of uncertain cash flows in terms of dynamic convex risk measures for processes as introduced in Cheridito et al. (Electron. J. Probab. 11(3):57–106, 2006). These risk measures take into account not only the amounts but also the timing of a cash flow. We discuss their robust representation in terms of suitably penalised probability measures on the optional σ-field. This yields an explicit analysis both of model and discounting ambiguity. We focus on supermartingale criteria for time consistency. In particular, we show how “bubbles” may appear in the dynamic penalisation, and how they cause a breakdown of asymptotic safety of the risk assessment procedure.

Study of ultrahigh Atwood-number Rayleigh–Taylor mixing dynamics using the nonlinear large-eddy simulation method

The Nonlinear Large-Eddy Simulation (nLES) method [G. C. Burton, “The nonlinear large-eddy simulation method (nLES) applied to Sc≈1 and Sc⪢1 passive-scalar mixing,” Phys. Fluids 20, 035103 (2008)] is employed in the first numerical study of multimode miscible Rayleigh–Taylor instability (RTI) in the ultrahigh Atwood-number regime above A≥0.90. The present work focuses on the dynamics of turbulent mixing at the large density ratios that may be encountered in certain astrophysical contexts and engineering applications. Using the initial condition from the landmark (N=30723) direct numerical study of Cabot and Cook [W. Cabot and A. W. Cook, “Reynolds number effects on Rayleigh-Taylor instability with possible implications for type-Ia supernovae,” Nat. Phys. 2, 562 (2006)], the nLES method is first validated in simulations of A=0.5 RTI mixing and is shown to recover important statistical measures of the mixing process, such as bubble and spike growth rates and mixing efficiency reported in that study, but at the significantly coarser resolutions typical of most large-eddy simulations. The first simulations of RTI at Atwood numbers A>0.90 are then used to explore the effects of varying the density ratio on mixing dynamics at very high Atwood numbers 0.75≤A≤0.96. Spike heights and mixing layer growth rates are shown to be strongly affected by the initial density ratio. An empirical power-law scaling relationship is shown to predict nearly exactly the variation in the ratio of spike to bubble heights as a function of Atwood number. Mixing efficiency is shown to be influenced by the initial density difference but the competition between increased molecular mixing and entrainment largely cancel, producing a relatively modest variation in these flow parameters when compared with the intermediate Atwood-number case. Late-time power spectra show the appearance of an inertial range, indicating that the mixing layer has transitioned to a fully turbulent state. The role of bubble and spike structures in the interscale transfer of kinetic energy is explored for the first time for high Atwood-number RTI flows. Bubble heads (stems) are shown to produce forward (reverse) transfer due to compressive (extensional) straining at intermediate Atwood number. At high Atwood number, however, spike stems are shown to produce forward transfer due to compressive straining generated from the larger spike penetration velocity and the differing spike morphology produced at the higher Atwood number. The study indicates that stable and accurate simulations of ultrahigh Atwood-number mixing may be conducted using the nLES method on grids significantly coarser than used previously to examine RTI flows.

A New Approach to Classification of Flotation Collectors

A collector is needed at the point of particle to bubble attachment and can be brought to this point either on the surface of particles, bubbles or both. In order for this to happen the collector must first be introduced into the flotation pulp. The water-soluble collectors are utilized as aqueous solutions, water-insoluble oily hydrocarbons must be dispersed in water and are used as emulsions, while waterinsoluble amphipathic compounds can spread at the gas/water interface. The rational classification of flotation collectors based on such observations is presented in this paper. The analysis of the accompanying phenomena leads to the conclusion that in all these mechanisms a very important and not always recognized part is played by the surface of bubbles. The role of bubbles in transportation of collectors to the point of particle-to-bubble attachment is especially important for the two types of water-insoluble collectors: oily hydrocarbons and long-chain amphipathic compounds.RésuméOn a besoin d'un collecteur au point d'attachement entre la particule et la bulle et on peut l'amener à ce point soit à la surface des particules, des bulles ou des deux. Afin que cela se produise, on doit d'abord introduire le collecteur dans la pulpe de flottation. Les collecteurs hydrosolubles sont utilisès en solutions aqueuses, alors que l'on doit disperser dans l'eau les hydrocarbures huileux insolubles, utilisès comme èmulsions, et que les composès bipolaires insolubles peuvent se rèpandre à l'interface gaz-eau. Dans cet article, on prèsente la classification rationnelle des collecteurs de flottation en se basant sur de telles observations. Grâce à l'analyse des phènomènes d'accompagnement, on peut conclure que, dans tous ces mècanismes, une partie très importante et pas toujours reconnue se joue à la surface des bulles. Le rôle des bulles dans le transport des collecteurs vers le point d'attachement particule à bulle est particulièrement important pour les deux types de collecteurs insolubles dans l'eau: les hydrocarbures huileux et les composès bipolaires à longue chaîne.

On the Role of Bubbles in Metallic Splat Nanopores and Adhesion

Highly nanoporous surfaces were observed on the underside of Ni splats. Experiments varying process parameters and substrate treatments were performed to determine the mechanism of pore formation. A theory of impact-induced bubble nucleation and freezing into pores is presented, and calculations are compared with experimental results. Pore formation and morphology is strongly dependent on substrate (a) thermal properties as they affect time for bubble growth before solidification into pores and (b) roughness as submicron scratches enhance nucleation by providing heterogeneous sites and several micron grooves reduce the driving force for nucleation. Splat pull-off experiments are shown that suggest bubble nucleation and pore formation strongly affect adhesion, and represent a strong contribution to the effectiveness of surface roughening. Finally, this observation shows the potential for the manufacturing of high-surface area materials using thermal spray.

Flotation Thermodynamics: Can We Learn anything from It?

The role of bubbles in the transportation of hydrophobic particles to a froth phase in flotation is well established. However, analysis of available data reveals that the bubbles are important most of all because they are the primary means of transportation of flotation agents to the points of bubble-to-particle attachment. The common view is that bubbles attach and lift hydrophobic particles to a froth layer. The thermodynamic analysis presented in this paper also leads to the conclusion that also the particles which become hydrophobic during the particle-bubble collision and attachment can be captivated by bubbles.