Formation Of Air Bubbles Research Articles

Using Field-Effect Gas Sensors for Monitoring H2 in Transformer Oil

Introduction Hydrogen can be released during the thermal decomposition of organic materials; therefore, monitoring its level in the working industrial high-voltage transformer oil allows you to identify the development of degenerative processes in advance, because these processes can lead to an accident in the future. In experiments has shown that highly sensitive and small-sized field effect gas sensor based on the metal-insulator-semiconductor (MIS) structure can be used for measuring of H2 in oil with direct contact of its structure with transformer oil. Hydrogen Diffusion in Transformer Oil Numerical estimates show that with the diffusion coefficient D = 4*10-9 m2/s [1] (at room temperature), the time required for H2 saturation in a transformer oil with 10 cm depth to a level 0,9C0 is more than 1 year. In reality, there are convective flows occurred due to the temperature gradient which are combined with diffusion. Nevertheless, such a large diffusion time must be taken into account when designing the experimental setup and the obtained results interpreting. Field-Effect Gas Sensor A schematic representation of the field-effect gas sensor is shown on Figure 1 and described in work [2]. The measuring H2 concentration with the field-effect gas sensor is as follows. When hydrogen molecules from the external environment interact with the metal electrode, H2 molecules decompose into atoms and diffuse to the metal-insulator interface. It is known that on the surface of palladium hydrogen atoms have a dipole moment. The electric field of dipoles leads to a change in the electric field in the dielectric and in the surface layer of the semiconductor. As a result, the capacitance of the MIS-structure changes, the value of which is fixed by the electronic unit while maintaining a constant bias voltage on the MIS-structure. Results and Conclusions The setup used in experiment (Fig. 2) provides similar to a transformer conditions of H2 diffusion. Both field-effect sensors were preliminarily calibrated for H2 in air (in the concentration range 5-100 ppm) at the sensing element temperatures of 23 and 100°С. The optimal temperature of the sensor structure is 100-150°C. However, in experiments, the main sensor could not be maintained at this temperature due to intense heat exchange with oil. For this reason, its heating element was turned off and equal to the temperature of the oil and was 23°C. The reference sensor was maintained at a temperature of 100°C. Relative humidity above oil was 25%. The experiment was carried out as follows. Through drainage tubes, fixed H2 concentrations in the range from 5 ppm to 100 ppm were fed into the container. After the mixture was supplied, the tubes were blocked and, subsequently, the H2 concentration in the vessel was monitored by sensor (Fig. 2 (3)). To accelerate the process of oil saturation with H2, a mixing device was used, the rotation frequency of which was selected to exclude the formation of air bubbles in the oil. Figure 3 shows the dependence of the response of the sensors (main and reference) on the concentration of H2. Table 1 shows the measurement results.The experiment showed that the temperature of the sensor has a significant effect on the detection time of H2. Immersion of the sensor in oil leads to an increase in the noise level of the signal and a decrease in the sensitivity of the sensor in the low concentrations range. Also, the presence of transformer oil does not change the functional dependence of the sensor capacity on the H2 concentration. The response time to H2 is significantly less than the time of its diffusion in oil, which makes it possible to use the sensor as a sensitive element in the control systems for the level of dissolved H2 in the oil of a working transformer. Acknowledgment This work was funded by the Russian Science Foundation under Grant no 18-79-10230 from 08.08.2018.

Suppression of liquid surface instability induced by finite-amplitude oscillation in liquid piston Stirling engine

This study addresses a problem introduced by finite-amplitude oscillations in a liquid piston thermoacoustic Stirling engine and proposes methods to overcome it. After the engine was operated by external heat, the liquid piston oscillated, keeping a flat surface when the amplitude was sufficiently low. However, it became unstable to cause ejection of droplets and formation of air bubbles because the acceleration amplitude exceeded gravitational acceleration. Two methods were proposed to recover the stable liquid surface: one method was the enlargement of the cross-sectional area of the tube around the liquid surface and the other method was the use of submerged polyethylene floats. These methods successfully suppressed the instability of the liquid piston Stirling engine and increased the pressure amplitude by almost twice of the original engine. In addition, adjusting the liquid filling level resulted in the ratio of pressure amplitude over the mean pressure reaching 18% when the acceleration amplitude was elevated to 36m/s2 by the installation of floats. The high-pressure amplitude is an advantage of a liquid piston thermoacoustic Stirling engine over thermoacoustic engines that use only gas as the working fluid.

A Review on Bubble Stability in Fresh Concrete: Mechanisms and Main Factors.

In order to improve the stability of air bubbles in fresh concrete, it is of great significance to have a better understanding of the mechanisms and main influencing factors of bubble stability. In the present review, the formation and collapse process of air bubbles in fresh concrete are essentially detailed; and the advances of major influencing factors of bubble stability are summarized. The results show that the surface tension of air–liquid interface exerts a huge impact on bubble stability by reducing surface free energy and Plateau drainage, as well as increasing the Gibbs surface elasticity. However, surface tension may not be the only determinant of bubble stability. Both the strength of bubble film and the diffusion rate of air through the membrane may also dominate bubble stability. The application of nano-silica is a current trend and plays a key role in ameliorating bubble stability. The foam stability could be increased by 6 times when the mass fraction of nano-particle reached 1.5%.

Numerical simulation of underwater air bubble formation under rolling conditions

Underwater gas bubbles are universal phenomenon in various industries. These bubbles appear under ocean conditions especially rolling conditions, that affect the mass transfer and produce different acoustic signals. The bubble formation and oscillation frequency characteristics under rolling conditions were investigated in this study by using a numerical method. Results showed that the bubbles followed two basic steps under rolling conditions, namely, expansion and detachment. The bubble detachment surface area and shape exhibited periodic changes because of the periodical motion of the surrounding water. A correlation was established to predict the bubble detachment surface area under rolling conditions. Similar to the bubble detachment surface area, the bubble oscillation frequency periodically changed. The oscillation mode of the bubbles under rolling conditions was the same as those under motionless condition, that is zeroth mode. A modified Minnaert formula was established to predict the bubble oscillation frequency under rolling conditions.

Experimental Investigation of Various Regimes of Bubble Formation and Growth—A Theoretical View of Double Coalescence Regime

Abstract Bubble formation and growth in a liquid is an important process in many industries. Bubble formation regime determines the essential characteristics of the bubble formation process such as bubble volume, shape, and formation time. The formation regime changes from single to double and multiple by increasing the volumetric gas flowrate. In the present research, various regimes of air bubble formation in pure water and glycerin solutions (glycerin 92%, glycerin 96%, and glycerin 100%) were observed by conducting experiments, and they have been defined in terms of the dimensionless Froude (Fr) and Bond (Bo) numbers. The liquids that have been used in the experiments provide a wide range of viscosity from 0.001 to 1.07 Pa·s. However, the surface tension remains approximately constant. In these experiments, relatively small needle sizes (lower than 0.6 mm), Bond numbers smaller than 0.05, and Froude numbers smaller than 70,000 were used, and the boundaries between various regimes were determined. The results indicate that the Froude number associated with the boundaries between various regimes decreases by increasing Bond number. In addition, for a given needle diameter, the air flowrate at which the regime changes from single to double is lower in glycerin solutions than in water. The mentioned flowrate decreases by increasing the liquid viscosity. Finally, based on the governing equations and experimental results of this study, a new correlation has been obtained to estimate the volume of the first bubble at the moment of the detachment in the double coalescence regime.

Mechanisms of inter‐ply void formation during vacuum bag‐only cure of woven prepregs

AbstractThe removal of inter‐ply air is critical for limiting porosity in laminates. In this study, an in situ monitoring technique was employed to observe inter‐ply air evolution during vacuum bag‐only cure. Observations showed that reduced vacuum resulted in inefficient inter‐ply air evacuation, a more rapid bubble expansion rate, and formation of new air bubbles. A modified tow impregnation model showed that resin infiltration was impeded at reduced vacuum conditions due to the presence of intra‐tow air. However, the cured laminates showed that tows were fully impregnated in all cases, indicating that the entrapped intra‐tow air migrated to inter‐ply regions during cure. The interactions between intra‐tow and inter‐tow air at deficient vacuum conditions were revealed. Findings led to the conclusion that air remaining in intra‐tow regions contributed more to the increase of inter‐ply voids than the reduction in consolidation pressure difference associated with reduced vacuum.

Formation of air bubbles during blood warming with Astotherm plus™.

Formation of air bubbles during blood warming with Astotherm plus™.

Bonding of thermoplastic microfluidics by using dry adhesive tape.

In this study, we investigated the effects of adhesive tape structure, adhesive tape thickness (30, 60, and 80 μm), and bonding time (5 and 15 seconds) on the bonding of inflexible and flexible substrates. We performed microchannel bonding by using a manual scraper press or a hot press machine. Rapid prototyping and mass production capabilities were achieved in the dry adhesive tape bonding of polymer microfluidic systems with both the aforementioned approaches. With process control, 95.16% and 99.53% bonding coverage could be achieved for the inflexible and flexible substrates, respectively, by using a manual scraper press. When using a press machine, the bonding coverage could be further enhanced to 99.24% for the inflexible substrates and 99.81% for the flexible substrates. Due to the viscoelastic nature of the adhesive layer in the adhesive tapes, we observed Saffman–Taylor finger and air bubble formation around the microchannel under high pumping pressure. The results indicated that the probability of Saffman–Taylor finger formation was lower and the bonding pressure was higher when using the thinner adhesive tape than when using thicker tape. Moreover, due to their rigidity, the inflexible substrates exhibited a higher bonding strength than the flexible substrates did. Bonding stability tests indicated that the bonded substrates had high bonding quality and bonding strength under long-term storage of up to 60 days.

Orange peel waste from juicing as raw material for plastic composites intended for use in food packaging

ABSTRACTUtilization of orange peel (OP) from the juicing industry as filler material within a polymer matrix can add value to this agricultural waste and reduce dependence on nonrenewable resources. This study aims to investigate the impact of OP filler's physical characteristics including size, loading, and retention/removal of zest on plastic properties of interest for food packaging. Linear low‐density polyethylene (LLDPE) is selected to produce plastic composites due to its widespread use in food packaging. Eight different LLDPE/OP sheets from combinations of different OP characteristics are examined. Results show that OP particle sizes of 75–177 μm without zest at 0.4‐g loading improve thermal stability, color, dispersion, tensile strength, modulus of elasticity, and elongation at break as well as reduce air bubble formation and OP agglomerate sizes in LLDPE/OP composites. Results show a promise in utilization of OP fillers to produce a composite comparable to neat LLDPE and composites with UV‐blocking properties. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48841.

A numerical study of single air bubble formation comparison between in viscous liquid and in water

AbstractThe performance of bubble column reactors is dependent on bubble size that is formed through a sparger. Here, a single air bubble formation (BF) using an orifice in the viscous liquid is modeled by using the volume of fluid method. The interface between the air and the liquid phase is calculated by using the continuum surface force equation. Detailed characteristics of BF such as the instantaneous contact angle, bubble departure volume, bond number due to orifice size and surface tension, and capillary number are examined for different orifice sizes at constant air flow rate boundary condition. The numerical results are validated with available correlation and experimental data for orifice bond number reported in the literature. A comparison of BF characteristics between in the viscous liquid and in the water is examined. The instantaneous contact angle decreases steeply for the orifice with small size than a large orifice. The bubble volume increases with increasing contact angle and decreasing the orifice bond number. In comparison with water, a decreasing detachment time and dome width of bubble and an increasing bubble neck elongation are found in viscous liquid and decrease with increasing bond number. Moreover, there is a minimum capillary number, where the bubble neck elongation height and breakup processes are relatively fast. This study can helpful in designing a coarse gas sparger to develop pseudo‐homogeneous bubbly flow regime at lower superficial gas velocity conditions.

Parylene based thin-film microfluidic lens array fabricated by iCVD nano-adhesive bonding

Microfluidic lenses that have been fabricated using PDMS have serious limitations due to the high gas permeability of the material, which leads to evaporation of the filling liquid and air bubble formation. We report a fast and reliable method of fabricating flexible, thin-film parylene microfluidic devices, and their application in tunable microfluidic lens arrays. A parylene microfluidic lens was built by molding and bonding 1.4-μm-thick parylene layers using nano-adhesivelayers deposited by initiated chemical vapor deposition (iCVD). The focal length of the lens could be tuned by changing the radius of curvature of the lens and/or changing the refractive indices of the filling liquids. The parylene microfluidic lens showed a large range of focal length tuning, and high reproducibility over hundreds of cycles.

Internal-Flow-Mediated, Tunable One-dimensional Cassie-to-Wenzel Wetting Transition on Superhydrophobic Microcavity Surfaces during Evaporation

ABSTRACTSuperhydrophobic textured surfaces are known to maintain a nonwetted state unless external stimuli are applied since they can withstand high wetting pressure. Herein, we report a new category of tunable, one-dimensional (1D) Cassie-to-Wenzel wetting transitions during evaporation, even on superhydrophobic surfaces. The transition initiates at the periphery of the evaporating drop, and the wetting transition propagates toward the center of the drop. The transitions are observed for surfaces with wetting pressures as high as ~ 7,568 Pa, which is much higher than the Laplace pressure, i.e., ~200 Pa. In situ high-contrast fluorescence microscopy images of the evaporating drop show that the transition is induced by preferential depinning of the air-water interface and subsequent formation of air bubbles in the cavities near the three-phase contact line. The evaporation-induced internal flow enhances the pressure within the water droplet and subsequently causes a Cassie-to-Wenzel wetting transition.

The effect of water’s presence around the Phase Change Material

As part of the research in the field of thermal control of electronic components, a phase change material is confined in a liquid and is heated vertically on one side by a hot plate. The presence of the liquid around the phase change material prevents the formation of air bubbles produced in case of direct contact between the hotplate and the phase change material (extends the lifetime of the phase change material by reducing overheating zones). It improves heat transfer by increasing the thermal conductivity around the phase change material (raising the thermal exchange surface) and by accelerating the convective transfer. This work examines experimentally and numerically the effect of the water on the phase change material and on the heating plate. The water is used around the phase change material and a comparative study of the comportment of some important parameters like the melt front form, melting time, flow direction, temperature, and operating time is realized. It is found that the presences of the liquid around the phase change material seems to be more interesting for a thermal protection role than the standard case of the phase change material directly heated by the hotplate.

Influence of animal glue on mineralogy, strength and weathering resistance of lime plasters

In order to optimize lime plasters for conservation purposes, the influence of animal glue on the mineralogical evolution, mechanical properties, and weathering resistance was studied. The organic additive caused a decrease in carbonation rate and affected mechanical properties negatively, facilitating the formation of large air bubbles in the plaster matrix. However, the weathering resistance (water spraying/SO2 exposure) improved significantly. The organic additive also reduced crack formation and formed a physical barrier around the calcite crystals, which delayed sulfation. Lime-based composite materials are a valuable alternative to conventional lime plasters, showing improved adherence to the substrate and no significant color changes.

Optical experiment and Large Eddy Simulation on effects of in-nozzle stagnant air bubbles and diesel on near-nozzle spray structure variation in diesel injector

This study aims to improve the understanding of sources of diesel spray variation, especially employing split injection strategies. The formation and evolution of residual air bubbles and diesel inside the optical real-size diesel nozzle at start and end of injection are successfully captured by a high-speed camera coupled with a micro lens. The corresponding shot-to-shot variations of near-nozzle spray structures are simultaneously obtained by the optical experiments. Furthermore, the numerical models are validated against the experimental results of various near-nozzle spray patterns. Then, the validated numerical models are employed to further investigate the effects of the distributions of the residual air bubbles and initial diesel inside the nozzle on near-nozzle spray patterns. The results indicate that various spray structures are attributed to the interaction between the residual air bubbles and initial diesel, and the radial propagation of the initial jets caused by the air drags. In addition, different positions and sizes of the stagnant air bubbles and diesel exert significant effect on the near-nozzle spray patterns through the mechanisms of the mushroom-like structure and tail.

BD-BARRICOR® tubes in biochemistry emergency: A new window opening on blood sample processing and analysis

Transport of blood samples through pneumatic tube systems (PTSs) generates air bubbles in transported blood samples and, with increasing duration of transport, the appearance of hemolysis. We investigated the role of air-bubble formation in PTS-induced hemolysis.Air was introduced into blood samples for 0, 1, 3 or 5 min to form air bubbles. Hemolysis in the blood was assessed by (H)-index, lactate dehydrogenase (LD) and potassium in plasma. In an effort to prevent PTS-induced hemolysis, blood sample tubes were completely filled, to prevent air bubble formation, and compared with partially filled samples after PTS transport. We also compared hemolysis in anticoagulated vs clotted blood subjected to PTS transport.As with transport through PTSs, the duration of air bubble formation in blood by a gentle stream of air predicted the extent of hemolysis as measured by H-index (p < 0.01), LD (p < 0.01), and potassium (p < 0.02) in plasma. Removing air space in a blood sample prevented bubble formation and fully protected the blood from PTS-induced hemolysis (p < 0.02 vs conventionally filled collection tube). Clotted blood developed less foaming during PTS transport and was partially protected from hemolysis vs anticoagulated blood as indicated by lower LD (p < 0.03) in serum than in plasma after PTS sample transport.Prevention of air bubble formation in blood samples during PTS transport protects samples from hemolysis.

Optimal Timing of Simethicone Addition for Bowel Preparation Using Polyethylene Glycol Plus Ascorbic Acid.

Colonic bubbles obscure the colonic mucosa during colonoscopy following bowel preparation with polyethylene glycol plus ascorbic acid (PEG-Asc). Simethicone is used to enhance visualization during colonoscopy. We aimed to determine the optimal timing of simethicone addition to improve bowel preparation using PEG-Asc. This prospective, randomized study enrolled patients undergoing elective colonoscopy from April 2017 to January 2018. They were randomly assigned to one of the following three groups: PEG-Asc only (control) or simethicone addition in the morning on the day of colonoscopy (PEG-S1) or in the evening of the day prior to colonoscopy (PEG-S2). The primary outcome was the quality of colon cleansing, and the secondary outcomes were the adenoma detection rate (ADR), polyp detection rate (PDR), and diminutive (≤ 5mm) ADR. In total, 240 patients were randomly allocated to the three groups; six patients were withdrawn. Of the 234 patients evaluated, 78, 79, and 77 were allocated to the control, PEG-S1, and PEG-S2 groups, respectively. The bubble scores of all colonic segments were lowest in the PEG-S2 group. There was no significant difference in ADR or PDR among the three groups. However, the diminutive ADR was significantly higher in the PEG-S2 group compared to the other two groups (control 5.1% vs. PEG-S1 8.9% vs. PEG-S2 20.8%; P = 0.009). Addition of simethicone to PEG-Asc at the optimal time prevents the formation of air bubbles and so improves the quality of bowel preparation, especially enhancing diminutive ADR.

Preferential adsorption to air-water interfaces: a novel cryoprotective mechanism for LEA proteins.

Late embryogenesis abundant (LEA) proteins comprise a diverse family whose members play a key role in abiotic stress tolerance. As intrinsically disordered proteins, LEA proteins are highly hydrophilic and inherently stress tolerant. They have been shown to stabilise multiple client proteins under a variety of stresses, but current hypotheses do not fully explain how such broad range stabilisation is achieved. Here, using neutron reflection and surface tension experiments, we examine in detail the mechanism by which model LEA proteins, AavLEA1 and ERD10, protect the enzyme citrate synthase (CS) from aggregation during freeze–thaw. We find that a major contributing factor to CS aggregation is the formation of air bubbles during the freeze–thaw process. This greatly increases the air–water interfacial area, which is known to be detrimental to folded protein stability. Both model LEA proteins preferentially adsorb to this interface and compete with CS, thereby reducing surface-induced aggregation. This novel surface activity provides a general mechanism by which diverse members of the LEA protein family might function to provide aggregation protection that is not specific to the client protein.

Bubble formation on submerged micrometer-sized nozzles in polymer solutions: An experimental investigation

This research presents detailed formation of air bubbles on different submerged micrometer-sized nozzles in Newtonian and non-Newtonian fluids. An experimental study is performed using the high-speed camera on three nozzles with diameters of 150, 450, and 600 μm under relatively low gas flow rate conditions (0.1∼1 ml/min). Distilled water and various concentrations of Carboxymethyl cellulose (CMC) aqueous solutions are utilized as the continuous phase for each type of fluid. Different characteristics of the bubble formation such as volume change, frequency, height, maximum width, and instantaneous contact angle are obtained using an image processing technique and the Young-Laplace equation. Effect of different parameters including gas flow rate, nozzle diameter, concentration, and rheological properties of the continuous phases are investigated. Also, a detailed dynamic force analysis is performed to survey the role of fluid properties on the effective forces. The force balance reveals that the surface tension force is dominant in the smallest orifice, while its influence decreases as the orifice diameter increases. Moreover, it was observed that the drag and hydrostatic forces have relatively noticeable contributions in this low gas flow rates. Furthermore, results show that under the conditions of this research, the characteristics of the bubble are weakly dependent on the gas flow rate. On the other hand, the concentration of the CMC and the rheological behavior of the non-Newtonian fluids so-obtained have a significant influence on bubble formation process. Also, observations made and predictions done as to the bubble’s curvature (say, based on Young-Laplace equation) suggest that the necking phenomenon does not exhibit itself at the top of the nozzle in non-Newtonian liquids flowing through micron-sized nozzles.

Experimental study on characteristics of air entrainment and distribution of void fraction in surf zone

The study of breaking wave-induced air bubble flows near the seashore is a great challenge. The process of air entrainment is highly complex and air bubbles evolution takes place over a wide range of spatial and temporal scales. Surf wave breaking can generate strong turbulence and entrain a large amount of air in the water and produce many air bubbles with different lifetimes. It is a typical two-phase flow involving water and air. The evolution and transport of wave-induced air bubbles can significantly affect coastal structures with respect to impact intensity, seawater-air exchange, sediment transport and wave energy dissipation. Studying the characteristics of wave-induced air bubbles in the nearshore wave breaking zone has important theoretical value and scientific significance. A better understanding of this process is also important to improving engineering practice and environmental protection in the nearshore region. Past studies have been focused on the transport law of the wave-induced air bubbles and the related statistics of air bubbles involved in the wave breaking process. Due to the complexity of water and gas mixing processes and the difficulty of the corresponding measurements, the prior research mainly focused on the formation and distribution of wave-induced air bubbles and their relationship with wave energy dissipation. However, quantitative research on how air bubble characteristics vary with different wave parameters still lacks in-depth discussion. This work presents a wave tank experiment to fully understand the void fraction distribution and its relationship with prominent wave parameters during the air entrainment process during surf wave breaking. Spatial and temporal variations of air entrainment and void fraction evolutions are recorded by an air bubble measuring system and high speed camera, respectively. The main factors affecting the distribution of void fraction are determined. The experimental data demonstrates that the distribution of void fraction is closely related to the water depth, prominent wave parameters and distance from the wave breaking location. We discuss the spatial and temporal evolution of the void fraction and derive an empirical formula to predict the void fraction by introducing two factors named C 0 and k 1. The factor k 1 is inversely proportional to wave height and C 0 varies with the wave steepness, relative water depth and dimensionless horizontal position. The empirical formula is used to analyze the relationship between each wave parameter and the distribution of void fraction. Experimental results show that the maximum void faction was close to 20% at the water surface and that the maximum penetration depth of air bubbles was close to 5 cm below the still water surface. As the water depth increased, void faction varied with e-index. Results of this work can be used to analyze and predict the spatiotemporal distribution of the void fraction in wave breaking zone and provide a good reference for further study of air bubble transport during surf wave breaking.