Stream Of Bubbles Research Articles

Continuous Photo-Oxidation in a Vortex Reactor: Efficient Operations Using Air Drawn from the Laboratory.

We report the construction and use of a vortex reactor which uses a rapidly rotating cylinder to generate Taylor vortices for continuous flow thermal and photochemical reactions. The reactor is designed to operate under conditions required for vortex generation. The flow pattern of the vortices has been represented using computational fluid dynamics, and the presence of the vortices can be easily visualized by observing streams of bubbles within the reactor. This approach presents certain advantages for reactions with added gases. For reactions with oxygen, the reactor offers an alternative to traditional setups as it efficiently draws in air from the lab without the need specifically to pressurize with oxygen. The rapid mixing generated by the vortices enables rapid mass transfer between the gas and the liquid phases allowing for a high efficiency dissolution of gases. The reactor has been applied to several photochemical reactions involving singlet oxygen (1O2) including the photo-oxidations of α-terpinene and furfuryl alcohol and the photodeborylation of phenyl boronic acid. The rotation speed of the cylinder proved to be key for reaction efficiency, and in the operation we found that the uptake of air was highest at 4000 rpm. The reactor has also been successfully applied to the synthesis of artemisinin, a potent antimalarial compound; and this three-step synthesis involving a Schenk-ene reaction with 1O2, Hock cleavage with H+, and an oxidative cyclization cascade with triplet oxygen (3O2), from dihydroartemisinic acid was carried out as a single process in the vortex reactor.

Corrosion Behavior of Magnesium Based Foam Structure in Hank’s Solution

Metal foam is a new class of materials with promising applications and a unique combination of physical, chemical, and mechanical properties. The purpose of biodegradable implants is to support tissue regeneration and healing in a particular application by material degradation and implant replacement through the surrounding tissue. Magnesium alloys are expected to be degraded in the body and its corrosion products not deleterious to the surrounding tissue. In the present study, the foam metal was manufactured via powder metallurgy with a different variation of sintering temperature and TiH2 used as a foaming agent which are added to Mg-1Ca-3Zn alloy as much 3% wt TiH2. The sintering temperatures were 500, 550 and 600 °C with a constant holding time of 5 hours. It’s critical that the sintering temperature is carefully selected in consideration of their corrosion behavior. This paper reports the study of the behavior of the Mg-Ca-Zn alloy metal foam which evaluated by SEM, EDX, and electrochemical corrosion test in Hank’s solutions. After exposure, the SEM result of Mg-Ca-Zn-3TiH2 to Hank’s solution, a volcano-like structure is formed. The streams of H2 bubbles form at local sites on the Mg alloy surface where electrochemical reactions are taking place, leading to the particular structure with around shape and often with a hole in the center. The corresponding EDS result maps reveal enrichment of O, Ca, P and Mg as corrosion product. Potentiodynamic polarization experiments conducted at 37 °C and pH 7.4 indicated the increased biodegradation rates resulted from porous structure of foam samples. Corrosion rate in 500oC sintering temperature were 1.99 millimiles per year (mmpy) with corrosion current density (Icorr) 87.3.10-6 A/cm2, corrosion rate in 550 °C sintering temperature were 2,16 mmpy with Icorr 94.4.10-6 A/cm2 and rate in 600 °C sintering temperature were 2.41 mmpy with Icorr 105.10-6 A/cm2. The results showed that the increasing of sintering temperature could influence the corrosion resistance of Mg alloy.

Application of Side-View Imaging and Real-Time Hydrogen Measurement to the Investigation of Magnesium Corrosion

This work illustrates how side-view optical imaging, anodic potentiodynamic polarization, and real-time hydrogen evolution measurements can be applied and complementarily used to study the processes occurring on a corroding magnesium surface. Side-view imaging reveals that hydrogen evolution takes three different forms: (i) large and stable bubbles on the uncorroded regions, (ii) a stream of fine hydrogen bubbles at the corrosion front, and (iii) medium-sized bubbles behind the corrosion front. The observation suggests that the relatively large bubbles ahead and behind the corrosion front are associated with a purely cathodic reaction that provides the “remote current.” This current, combined with the depassivating role of chloride ions, maintains the corrosion front active, generating regions where either the metal is locally directly exposed to the electrolyte or is only covered by a poorly protective chloride-rich film. Regardless of the precise nature or morphology of the poorly protective (or absent)...

Bubble streams in Titan’s seas as a product of liquid N2 + CH4 + C2H6 cryogenic mixture

Titan, Saturn's largest moon, is the only extraterrestrial body known to support stable liquid on its surface, in the form of seas and lakes that dot the polar regions. Many indications suggest that the liquid should be composed of a mixture of N2, CH4 and C2H6. Recent RADAR observations of Titan's large sea, called ``Ligeia Mare'', have shown unexplained and ephemeral bright features, possibly due to rising bubbles. Here we report that our numerical model, when combined with experimental data found in the literature, shows that Ligeia Mare's bed is a favourable place for nitrogen exsolution. This process could produce centimeter-sized and radar-detectable bubbles.

Capacitive gas-phase detection in liquid nitrogen

Abstract. The main and upper stages of heavy lift launchers for space applications are often fuelled by cryogenic liquids. In order to enable the re-ignition of a cryogenic upper stage for orbital changes, it is crucial to study the behaviour of these fluids in microgravity. As gaseous bubbles entering the fuel lines of the engine can cause the destruction of the engine, these bubbles are a risk for the functionality of the re-ignition mode. To measure an evolving gaseous phase and its volume, a capacitive measurement system for two-phase mixtures was realised. Its electrodes are arranged in such a way that phase changes inside a vessel can be detected without parasitic heating under cryogenic conditions. Two cases have been investigated: a fill-level measurement involving a large gas bubble above a homogenous liquid on the one hand, and the identification of a bubble stream inside a liquid on the other hand. The system concept was tested in a cryogenic environment allowing the controlled generation of bubble streams inside liquid nitrogen and of a contiguous gaseous volume above the liquid. The characteristics of the measurable capacitances of different pairs of electrodes were experimentally determined and compared with finite-element simulations (Ansys). In addition, the electrical flux density was computed to corroborate the simulated capacitance curves with theoretical statements. The experimental findings closely agree with the simulated results if possible disturbances due to the characteristics of the capacitance measurement hardware are properly taken into account. Thus, by measuring various capacitances, it was possible to determine the level up to which a receptacle inside a liquid-nitrogen tank was filled with liquid (the space above the liquid being taken up by gaseous nitrogen), to identify the existence of a bubble stream in the liquid nitrogen and to demonstrate that the capacitance measurement results enable one to differentiate between the two cases.

Time series video analysis of bubble release processes at natural hydrocarbon seeps in the Northern Gulf of Mexico

This research quantifies the rate and volume of oil and gas released from two natural seep sites in the Gulf of Mexico: lease blocks GC600 (1200 m depth) and MC118 (850 m depth). Our objectives were to determine variability in release rates and bubble size at five individual vents and to investigate the effects of tidal fluctuations on bubble release. Observations with autonomous video cameras captured the formation of individual bubbles as they were released through partially exposed deposits of gas hydrate. Image processing techniques determined bubble type (oily, gaseous, and mixed: oily and gaseous), size distribution, release rate, and temporal variations (observation intervals ranged from 3 h to 26 d). A semi-automatic bubble counting algorithm was developed to analyze bubble count and release rates from video data. This method is suitable for discrete vents with small bubble streams commonly seen at seeps and is adaptable to multiple in situ set-ups. Two vents at GC600 (Birthday Candles 1 and Birthday Candles 2) were analyzed. They released oily bubbles with an average diameter of 5.0 mm at a rate of 4.7 bubbles s−1, and 1.3 bubbles s−1, respectively. Approximately 1 km away, within the GC600 seep site, two more vents (Mega Plume 1 and Mega Plume 2) were analyzed. These vents released a mixture of oily and gaseous bubbles with an average diameter of 3.9 mm at a rate of 49 bubbles s−1, and 81 bubbles s−1, respectively. The fifth vent at MC118 (Rudyville) released gaseous bubbles with an average diameter of 3.0 mm at a rate of 127 bubbles s−1. Pressure records at Mega Plume and Rudyville showed a diurnal tidal cycle (24.5 h). Rudyville was the only vent that demonstrated any positive correlation (ρ = 0.60) to the 24.5 h diurnal tidal cycle. However, these observations were not conclusive regarding tidal effects on bubble release.

La Mélancholie Des Dragons by Phillipe Quesne, and: Gardens Speak by Tania El Khoury, and: Top Secret International (State 1) by Rimini Protokoll

Reviewed by: La Mélancholie Des Dragonsby Phillipe Quesne, and: Gardens Speakby Tania El Khoury, and: Top Secret International (State 1)by Rimini Protokoll Matthew Robert Moore LA MÉLANCHOLIE DES DRAGONS. By Phillipe Quesne. Under the Radar Festival, The Kitchen, New York City. 01 11, 2016. GARDENS SPEAK. By Tania El Khoury. Under the Radar Festival, NYU Tisch Abe Burrows Theatre, New York City. 01 6, 2016. TOP SECRET INTERNATIONAL (STATE 1). By Rimini Protokoll. Under the Radar Festival, Brooklyn Museum, Brooklyn, New York. 01 6, 2016. Each year, the Public Theater's Under the Radar Festival introduces New York audiences to cuttingedge international theatre-makers. Although several of the 2017 festival's headliners were anything but obscure, their work powerfully challenged mainstream (American) definitions of theatre, suggesting an expanded performance frame and questioning the efficacy of live theatre engagement. Variously pointing to the audience's role as co-creators of the event, the potential of hybrid embodied/digital technologies to facilitate intersubjective experience, and the inherent theatricality of seemingly objective cultural spaces, the three shows I attended injected a decidedly postdramatic and anti-consumerist aesthetic into the city's season. Touring since 2008, Phillipe Quesne's La Mélancholie des Dragonsinterrogated the limits and processes of theatrical spectacle, asking viewers to marvel at the mundane and to notice their own role in the processes of creating meaning. Situational rather than plot-driven, the piece centered on seven metalheads, broken down in a snowy parking lot, where they inventoried the low-tech attractions of their mobile theme park for a mechanic friend, Isabelle. Largely devoid of spoken language, her name became a beckoning refrain evocative of childhood wonder as the "artists" eagerly rehearsed their eventual park opening. While it seemed that we were to accept the realistic circumstances of their predicament, it was not long before the stranded rockers were peeling back cotton batting (snow) to get at electrical cables and dragging props on from offstage, thereby undermining any sense of realism and highlighting the multiple frames of theatre at work in this performance about a deferred performance. The simplicity of their amateur "theatre"—a bubble machine, wigs "dancing" in the wind, a fountain made from a tiny pump and a Rubbermaid container—initially inspired chuckles. However, by the end of the performance/demonstration (if one accepted their invitation to imagine, to play), there was no denying the force of their aesthetic as they recreated the rhythms and scale of their production research (showcased in their park "library") through increasingly grand theatrical gestures. Romantic paintings, Artaud's theatre theory, and an Appian vision of mass and light took form against a backdrop of ordinary people playing with simplistic techniques. Seduced by the performers' earnestness and led by Isabelle's wonder, viewers entered a park of the mind in which their own associations operated creatively, placing their active gazes at the center of the theatrical event. In this potent state of disarmed curiosity and mutual effort, simple acts like a Scorpions' song on acoustic guitar and recorder or a Chinese New Year-style parade with inflated tarp-dragons, became incredibly moving performances while also remaining rather funny. Both actor and audience herself, Isabelle functioned throughout as our surrogate in the process of immersion and participation that the park hoped to stage. Her sincerity challenged us to suspend judgment as she modeled a new ideal of spectation. And when she climbed a ladder amid a stream of smoke and bubbles to reproduce Wanderer above the Sea of Fogby Caspar David Friedrich, she appeared as co-(re)creator and primary spectator of the work, provoking our spectatorial/imaginative experience and drawing audience and performer together in perceptual solidarity. Her simultaneous insider/outsider status overturned easy assumptions about audience passivity and our role in creating spectacle; her easy naiveté encouraged us to follow her into the possibility of profundity as we watched her being swept away by the world she had both created and imagined. Putting his audience onstage, Quesne deconstructed illusion and conditioned expectations, finally evoking theatre in its simplest and most beautiful terms. Stemming from a desire to overcome a kind of existential melancholy (embodied in the...

Prediction and Monitoring of Underwater Sound Levels from the Implosion of a Reinforced Concrete Bridge Pier

After the completion of the new east span of the San Francisco–Oakland Bay Bridge in California, large concrete piers of the old span needed to be demolished. To consider using controlled blasting for this action, hydroacoustic levels were predicted and monitored with regard to specified fish and marine mammal criteria. The metrics included peak pressure and sound exposure levels at distances from 25 to more than 4,000 ft from the pier. For peak pressure, the measured levels were slightly higher than estimated, although for sound exposure levels, the measured levels were somewhat lower than estimated because of the effect of surface reflection. A blast attenuation system consisting of a wide bubble stream was used to minimize the hydroacoustic levels in the water surrounding the pier. The implosion event consisted of 588 individual charge detonations ranging from 35 to 21 lb (15.9 to 9.5 kg/delay) spaced 9 ms apart. Although there were 135 individual detonations of the larger 35-lb charges, the highest peak pressures varied in level by 10 to 15 dB during the course of the implosion. The methods for predicting the levels, measured results, analysis of data, and performance of the blast attenuation system are reviewed.

Numerical Modeling of Flow Dynamics in The Aluminum Smelting Process: Comparison Between Air–Water and CO2–Cryolite Systems

Air–water models have been widely applied as substitutes for CO2–cryolite systems in the study of the complex bubble dynamics and bubble-driven flow that occurs in the molten electrolyte phase in the aluminum electrolytic process, but the detailed difference between the two systems has not been studied. This paper makes a numerical comparison between the bubble dynamics for the two systems. Simulations of both single bubble and continuous bubbling were conducted using a three-dimensional computational fluid dynamics (3D CFD) modeling approach with a volume of fluid (VOF) method to capture the phase interfaces. In the single bubble simulations, it was found that bubbles sliding under an anode in a CO2–cryolite system have a smaller bubble thickness and a higher sliding velocity than those in the air–water system for bubbles of the same volume. Dimensionless analysis and numerical simulation show that contact angle is the dominant factor producing these differences; the effects of kinematic viscosity, surface tension, and density are very small. In the continuous bubbling simulations, the continuous stream of air bubbles detaches from the anode sidewall after a period of climbing, just as it does in the single bubble simulation, but bubbles have less tendency to migrate away from the wall. Quasi-stable state flow characteristics, i.e., time-averaged bath flow pattern, turbulence kinetic energy, turbulence dissipation rate, and gas volume fraction, show a remarkable agreement between the two systems in terms of distribution and magnitude. From the current numerical comparisons, it is believed that the air–water model is a close substitutive model for studying bubble-driven bath flow in aluminum smelting processes. However, because of the difference in bubble morphologies between the two systems, and also the reactive generation and growth of bubbles in the real system, there will likely be some differences in bubble coverage of the anode in the anode–cathode gap.

Centimeter-sized epitaxial h-BN films

We report the growth and transfer of centimeter-sized, epitaxial hexagonal boron nitride (h-BN) few-layer films using Ni(111) single-crystal substrates. The h-BN films were heteroepitaxially grown on 10 × 10 mm2 or 20 × 20 mm2 Ni(111) substrates using atmospheric pressure chemical vapor deposition with a single ammonia-borane precursor. The grown films were transferred to arbitrary substrates via an electrochemical delamination technique, and the remaining Ni(111) substrates were repeatedly re-used. A careful analysis of the growth parameters revealed that the crystallinity and area coverage of the h-BN films were mostly sensitive to the sublimation temperature of the ammonia-borane source. Moreover, various physical characterizations confirmed that the grown films exhibited the typical characteristics of hexagonal boron nitride layers over the entire area. Furthermore, the heteroepitaxial relationship between h-BN and Ni(111) and the overall crystallinity of the film were thoroughly investigated using a synchrotron radiation X-ray diffraction analysis including θ–2θ scans, grazing incident diffraction, and reciprocal space mapping. The crystallinity at the microscopic scale was further investigated using transmission electron microscopy (TEM)-based techniques, including selective area electron diffraction pattern mapping, electron back-scattered diffraction, and high-resolution TEM. Transparent and flexible ceramic films for electronic devices can now be made at centimetre scales by a low-cost technique. Hexagonal boron nitride (h-BN) crystals are attractive platforms for building next-generation circuits because their flat, graphite-like surfaces resist mechanical and chemical damage while offering excellent signal isolation. Gyu-Chul Yi from Seoul National University and colleagues have developed a way to improve large-scale manufacturing of these films using a reusable nickel template. Nickel has a surface structure that helps h-BN films grow with high crystallinity following chemical vapor deposition, but detaching the resulting ceramic layer has always proved troublesome. The Korean team solved this problem through an electrochemical delamination process that seamlessly separates h-BN films with dimensions up to 20 × 20 square millimetres using a stream of hydrogen bubbles. Centimeter-sized, epitaxial hexagonal boron nitride (h-BN) few-layer films were heteroepitaxially grown on Ni(111) single-crystal substrates using atmospheric pressure chemical vapor deposition with ammonia-borane single precursor. The grown films were transferred to arbitrary substrates via an electrochemical delamination technique, and the remaining Ni(111) substrates were repeatedly re-used. Over repeated growth and transfer after the initial annealing, no significant degradation of Ni(111) substrates was observed and the crystallinity of h-BN layers was reproduced reliably. The grown h-BN films showed typical physical characteristics of h-BN, with a high uniformity over a wide area. The large-area synthesis and transfer of atomically thin uniform epitaxial h-BN layers can be applied in various fields where high quality two-dimensional insulating layers are required.

Processing of multibeam water column image data for automated bubble/seep detection and repeated mapping

AbstractWater Column Imaging Multibeam Echosounder Systems (MBES) are effective and sensitive tools for investigating free gas (bubble) release and its rise through the water column. The main advantages of MBES are the detection range and lateral coverage in the water column and at the seafloor; furthermore, they are becoming increasingly available on research vessels worldwide. However, high noise levels and systematic artefacts due to side‐lobe induced signal interference degrade MBES Water Column Images (WCIs) and hampered automated bubble detection and related gas seepage investigations. We present a new technique advancing automated detection of bubble streams and moving toward a quantitative gas‐release assessment. It is shown that bubble streams can be detected reliably by their spatio‐temporal behavior even when they are discontinuous in WCI data. Using assumptions about the bubble rising trajectories, bubble release spots at the seafloor can be traced even if the source location is obscured by acoustic noise or unwanted acoustic targets. A map with acoustic response and source locations of bubbles being released can be produced and serves as a starting point for more detailed quantitative analyses. The efficiency of the method has been assessed at a methane seep site in the Dutch North Sea. Multiple survey lines are merged to a detailed acoustic map of the area. Processed results are in good agreement with manual investigations of the WCI data as well as ROV‐based video analysis.

Thermocapillary Flow and Coalescences of Heterogeneous Bubble Size Diameter in a Rotating Cylinder: 3D Study

Two dimensional axisymmetric and three-dimensional VOF simulations of gas/liquid transient flow were performed using a multiphase flow algorithm based on the finite-volume method. The results for motion of a multiple bubbles of a heterogeneous sizes aligned horizontally and perpendicular to a hot surface incorporating thermocapillary forces in a rotating liquid in a zero-gravity environment have been presented for the first time. No bubbles broke in any of the cases observed. The results also show that collision and agglomeration of bubbles of unequal sizes diameter are different from those of similar size diameters presented from earlier research work of Alhendal et al. Acta Astronaut. 117, 484–496 (2015). Different flow patterns such as thermocapillary bubble migration, collision, and stream function were observed and presented for the 2-D and 3-D models.

Determining the flux of methane into Hudson Canyon at the edge of methane clathrate hydrate stability

AbstractMethane seeps were investigated in Hudson Canyon, the largest shelf‐break canyon on the northern U.S. Atlantic Margin. The seeps investigated are located at or updip of the nominal limit of methane clathrate hydrate stability. The acoustic identification of bubble streams was used to guide water column sampling in a 32 km2 region within the canyon's thalweg. By incorporating measurements of dissolved methane concentration with methane oxidation rates and current velocity into a steady state box model, the total emission of methane to the water column in this region was estimated to be 12 kmol methane per day (range: 6–24 kmol methane per day). These analyses suggest that the emitted methane is largely retained inside the canyon walls below 300 m water depth, and that it is aerobically oxidized to near completion within the larger extent of Hudson Canyon. Based on estimated methane emissions and measured oxidation rates, the oxidation of this methane to dissolved CO2 is expected to have minimal influences on seawater pH.

Doppler spectral moments of multibeam echosounder data to infer the ascent velocity of methane bubbles emanating from natural seafloor seeps

A multibeam echosounder operating at 500 kHz was deployed on a remotely operated vehicle to detect methane bubble streams emanating from natural seafloor seeps in 1400-m water depths. The sonar transmitted 10-ms tone bursts every 100 ms over a 7°×120° beam, and formed 128 receive beams regularly spaced 0.94° apart over the 120° sector. The sonar head faced upward and signals backscattered from ascending bubbles were sampled at about 6 kHz between 10 m and 50 m from the head. Doppler spectral moments were computed for each ping with the conventional data autocovariance method. For unimodal and symmetric Doppler spectra, the mean power is related to the backscattering strength of the sampled volume, the mean Doppler frequency yields the mean radial velocity of the bubbles, and the spectral width measures the mean velocity uncertainty with some indication of bubble oscillation. Mean ascent velocities of 0.2 m/s matched those obtained from concurrent optical measurements. However, beams free of bubbles showed mean descent velocities of about 1.5 cm/s, most likely due to marine snow, which could bias mean ascent velocity estimates for bubbles at greater distance from the sonar because the corresponding Doppler spectra might no longer be unimodal.

Development of a capillary plasma pump with vapour bubble for water purification: experimental and theoretical investigation

This paper describes the development of a small-sized reactive plasma pump driven by capillary bubble discharge for the purification of treated water. The apparatus we developed decomposes the pollutants in the water by using chemical species generated by the plasma discharge. The resulting stream of bubbles obviates the need for an external gas supply or pump to transport the water. A high-speed camera was used to investigate the bubble dynamics responsible for the pumping effect, which is achieved by selecting the shape of the capillary such that the bubble ejections within enhance the ‘self-repetition’ action required for the pumping motion. Our experiments showed that optimal bubble generation requires a consumed power of 17.8 W. A theoretical model was developed to investigate the pumping mechanism. We solve the problems associated with liquid oscillations in the U-shaped water reservoir by employing a non-uniform cross-sectional area in our model. The chemical reactivity of the device was confirmed by using emission spectroscopy of OH radical and by measuring the decomposition of methylene blue.

Analysis of thulium fiber laser induced bubble dynamics for ablation of kidney stones.

The Thulium fiber laser (TFL) is being explored as an alternative to the Holmium : YAG laser for lithotripsy. TFL parameters differ in several fundamental ways from Holmium laser, including smaller fiber delivery, more strongly absorbed wavelength, low pulse energy/high pulse rate operation, and more uniform temporal pulse structure. High speed imaging of laser induced bubbles was performed at 105,000 frames per second and 10 μm spatial resolution to determine influence of these laser parameters on bubble formation and needle hydrophone data was also used to measure pressure transients. The TFL was operated at 1908 nm with pulse energies of 5-65mJ, and pulse durations of 200-1000μs, delivered through 105-μm-core and 270-μm-core silica optical fibers. Bubble dynamics using Holmium laser at a wavelength of 2100 nm with pulse energies of 200-1000 mJ and pulse duration of 350 μs was studied, for comparison. A single, 500 μs TFL pulse produced a bubble stream extending 1200 ± 90 μm and 1070 ± 50 μm from fiber tip, with maximum bubble widths averaging 650 ± 20 μm and 870 ± 40μm (n = 4), for 105 μm and 270 μm fibers, respectively. These observations are consistent with previous studies which reported TFL ablation stallout at working distances beyond 1.0 mm. TFL bubble dimensions were four times smaller than for Holmium laser due to lower peak power and smaller fiber diameter used. The maximum pressure transients measured 0.6 bars at 35 mJ pulse energy for TFL and 7.5 bars at 600 mJ pulse energy for Holmium laser. These fundamental studies of bubble dynamics as a function of specific laser and fiber parameters may assist with optimization of the TFL parameters for safe and efficient lithotripsy in the clinic. Image of bubble formation during fiber optic delivery of Thulium fiber laser energy in saline (35 mJ, 500 μs).

A CFD study of the parameters influencing heat transfer in microchannel slug flow boiling

Boiling heat transfer in microchannels has been investigated extensively in the last two decades. However, most results from independent experimental studies display a substantial disagreement on the influence of the flow conditions and mechanisms on the heat transfer performance. The objective of the present paper is to clarify by means of CFD simulations the effect of the primary flow parameters on the saturated flow boiling heat transfer performance of a slug flow within a horizontal, circular microchannel. The vapor quality, heat flux, mass flux, channel diameter, bubble frequency, and saturation temperature are the parameters under analysis. The numerical model is based on the volume of fluid method included within ANSYS Fluent v. 14.5, which is here augmented by implementing ad-hoc models to calculate the surface tension force, the mass and energy transfer at the interface due to phase change, and to generate a continuous stream of bubbles with an arbitrary frequency. The high fidelity of the numerical framework provides unprecedented insight on the effect of these flow parameters on the bubble dynamics and heat transfer performance, which are tightly interrelated in microchannel slug flow boiling. In principle, the heat transfer performance is enhanced by flow conditions promoting a thin liquid film surrounding the elongated vapor bubbles and by the flow within short liquid slugs between the bubbles. Furthermore, numerous interesting secondary effects driven by the different flow conditions are observed and described according to the underlying flow dynamics: time-dependent interfacial waves on the bubble surface which significantly reduce the local film thickness; the response time of the channel wall temperature to changes of the flow boundary conditions at different channel sizes; the liquid film thickness dependence on the bubble length and on the liquid slug length; the heat transfer dependence on heat flux via the bubble generation frequency. A dimensional analysis is finally conducted in order to identify the nondimensional groups relevant to the microchannel bubble dynamics and heat transfer in the slug flow regime.

Inverse Leidenfrost Effect: Levitating Drops on Liquid Nitrogen.

We explore the interaction between a liquid drop (initially at room temperature) and a bath of liquid nitrogen. In this scenario, heat transfer occurs through film-boiling: a nitrogen vapor layer develops that may cause the drop to levitate at the bath surface. We report the phenomenology of this inverse Leidenfrost effect, investigating the effect of the drop size and density by using an aqueous solution of a tungsten salt to vary the drop density. We find that (depending on its size and density) a drop either levitates or instantaneously sinks into the bulk nitrogen. We begin by measuring the duration of the levitation as a function of the radius R and density ρd of the liquid drop. We find that the levitation time increases roughly linearly with drop radius but depends weakly on the drop density. However, for sufficiently large drops, R ≥ Rc(ρd), the drop sinks instantaneously; levitation does not occur. This sinking of a (relatively) hot droplet induces film-boiling, releasing a stream of vapor bubbles for a well-defined length of time. We study the duration of this immersed-drop bubbling finding similar scalings (but with different prefactors) to the levitating drop case. With these observations, we study the physical factors limiting the levitation and immersed-film-boiling times, proposing a simple model that explains the scalings observed for the duration of these phenomena, as well as the boundary of (R,ρd) parameter space that separates them.

Characterising gas behaviour during gas–liquid co-current up-flow in packed beds using magnetic resonance imaging

Magnetic resonance (MR) imaging techniques have been used to study gas phase dynamics during co-current up-flow in a column of inner diameter 43mm, packed with spherical non-porous elements of diameters of 1.8, 3 and 5mm. MR measurements of gas hold-up, bubble-size distribution, and bubble-rise velocities were made as a function of flow rate and packing size. Gas and liquid flow rates were studied in the range of 20–250cm3s−1 and 0–200cm3min−1, respectively. The gas hold-up within the beds was found to increase with gas flow rate, while decreasing with increasing packing size and to a lesser extent with increasing liquid flow rate. The gas hold-up can be separated into a dynamic gas hold-up, only weakly dependent on packing size and associated with bubbles rising up the bed, and a ‘static’ hold-up which refers to locations within the bed associated with temporally-invariant gas hold-up, over the measurement times of 512s, associated either with gas trapped within the void structure of the bed or with gas channels within the bed. This ‘static’ gas hold-up is strongly dependent on packing size, showing an increase with decreasing packing size. The dynamic gas hold-up is comprised of small bubbles – of order of the packing size – which have rise velocities of 10–40mms−1 and which move between the packing elements within the bed, along with much larger bubbles, or agglomerates of bubbles, which move with higher rise velocities (100–300mms−1). These ‘larger’ bubbles, which may exist as streams of smaller bubbles or ‘amoeboid’ bubbles, behave as a single large bubble in terms of the observed high rise velocity. Elongation of the bubbles in the direction of flow was observed for all packings.

A study on magnesium corrosion by real-time imaging and electrochemical methods: relationship between local processes and hydrogen evolution

This work investigates the hydrogen evolution process on magnesium by real time imaging of the corroding surface at the free corrosion potential and during potentiodynamic polarization in alkaline and neutral environments, in the presence and absence of chloride ions. High-magnification imaging of the corrosion process from side view discloses that three different types of hydrogen bubbles are generated during corrosion: i) large and stable bubbles on the uncorroded regions, ii) a stream of fine bubbles at the corrosion front and iii) medium size bubbles behind the corrosion front. It is proposed that the generation of streams of bubbles at the corrosion front is due to the disruption of the oxide/hydroxide film that is possible when an anodic current is available and environmental conditions can induce depassivation. This results in direct exposure of the underlying magnesium to the electrolyte and, due to the large overpotential available, in hydrogen evolution. Thus, the corrosion front act as a current amplifier where the rate of magnesium oxidation is proportional to the sum of the current produced remotely, that maintains the depassivation, and the current produced at the corrosion front by hydrogen evolution.