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Electrochemical gas evolution reactions are common but essential in many electrochemical processes including water electrolysis. During these processes, gas bubbles are constantly nucleating on reaction interfaces in electrolyte a...
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Electrochemical gas evolution reactions are common but essential in many electrochemical processes including water electrolysis. During these processes, gas bubbles are constantly nucleating on reaction interfaces in electrolyte and consequently exert an impact on catalysts and the performance. In the past few decades, extensive studies have been conducted to characterize bubbles with emerging advanced technologies, manage behaviors of bubbles, and apply bubbles to various domains. In this review, we summarize representative discoveries as well as recent advancements in electrochemical gas evolution reactions from the perspective of gas bubbles. Finally, we end up this review with a profound outlook on future research topics from the combination of experiments and theoretical techniques, non-negligible bubble effects, gravity-free situation, and reactions under practical industrial conditions.
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The bubble formation and coalescence phenomena above submerged nozzles having diameters between 1 and 7 mm were studied visually in a stagnant water column. Air was supplied at the nozzles under constant flow rate conditions and t...
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The bubble formation and coalescence phenomena above submerged nozzles having diameters between 1 and 7 mm were studied visually in a stagnant water column. Air was supplied at the nozzles under constant flow rate conditions and the field of view extended from the nozzle exit up to 14 cm above it. The air Reynolds number based on the nozzle diameter covered a range between 50 and 40000. The study was focused on a unified description of the bubbling phe- nomena, beginning from the single bubble formation up to the jet formation, and the transitions between the various bub- bling regions.
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The effects of liquid velocity, nozzle diameter, gas chamber volume and gas flow rate on volumes, shapes and growth curves of bubbles formed at a nozzle submerged in a cocurrently upward flowing liquid in a bubble column were expe...
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The effects of liquid velocity, nozzle diameter, gas chamber volume and gas flow rate on volumes, shapes and growth curves of bubbles formed at a nozzle submerged in a cocurrently upward flowing liquid in a bubble column were exper- imentally investigated. The bubble volume decreases with increasing liquid flow velocity. The effect of liquid flow velocity on the volume of bubble increase with an increase in the gas flow rate. To simulate bubble formation at a nozzle submerged in cocurrently upward flowing liquid, a revised non-spherical bubble ormation model was proposed.
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The dynamics of bubble departures (at a frequency of ?=3 Hz) from a glass nozzle submerged in a tank filled with distilled water has been experimentally and theoretically studied. The volume of the system that supplies air to the ...
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The dynamics of bubble departures (at a frequency of ?=3 Hz) from a glass nozzle submerged in a tank filled with distilled water has been experimentally and theoretically studied. The volume of the system that supplies air to the nozzle (plenum chamber volume) and the air volume flow rate were changed in the experiment. The air pressure, bubble paths and liquid flow inside the nozzle were simultaneously recorded using a data acquisition system and a high-speed camera. It was shown that an increase in the plenum chamber volume leads to an increase in the intensity of the occurrences of chaotic changes in the subsequent waiting times. The analysis of the mechanism of the stability loss of the periodic bubble departures was based on changes in the time of the air pressure, the depth of the liquid penetration into the nozzle, the time of the bubble growth, the waiting time, and the bubble paths and their sizes, which is presented in this paper. The results of the analysis are compared with simulations that are based on the models of bubble growth and liquid flow inside the nozzle during the waiting time. It was shown that the air pressure rise, Δ_(Pl) during the waiting time is a non-linear function of the gas pressure after the bubble departure and the liquid velocity around the nozzle outlet. The nonlinearity of Δ_(Pl) increases when the plenum chamber volume increases, and it decreases when the air volume flow rate increases.
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Full scale bubbly flow experiments were performed on a 6 m flat bottom survey boat, measuring the void fraction, bubble velocity and size distributions as the bubbles naturally entrained at the bow of the boat interact with the bo...
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Full scale bubbly flow experiments were performed on a 6 m flat bottom survey boat, measuring the void fraction, bubble velocity and size distributions as the bubbles naturally entrained at the bow of the boat interact with the boat's boundary layer. Double-tip sapphire optical probes capable of measuring bubbles down to 50 mu m in diameter were specifically designed and built for this experiment. The probes were positioned under the hull at the bow near the bubble entrainment region and at the stern at the exit of the bottom flat plate. Motorized positioners were used to vary the probe distance to the wall from 0 to 50 mm. The experiments were performed in fresh water (Coralville Lake, IA) and salt water (Panama City Beach, FL), at varying velocities with most data analysis performed at 10, 14 and 18 knots. The results indicate that the bubbles interact significantly with the boundary layer. At low velocity in fresh water, bubble accumulation under the hull and coalescence are evident by the presence of large bubbles at the stern. At high speeds bubble breakup dominates and very small bubbles are produced near the wall. It is also observed that salt water inhibits coalescence, even at low boat speeds. The void fraction increases with speed beyond 10 knots and peaks near the wall. Bubble velocities show slip with the wall at all speeds and exhibit large RMS fluctuations, increasing near the wall. (C) 2015 Elsevier Ltd. All rights reserved.
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Two perpendicular projections of rising bubbles were observed in counter-current downstream diverging flow. Evidently, the bubbles did not enter the boundary layer at the channel wall and a plug liquid flow assumption was acceptab...
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Two perpendicular projections of rising bubbles were observed in counter-current downstream diverging flow. Evidently, the bubbles did not enter the boundary layer at the channel wall and a plug liquid flow assumption was acceptable in our experimental equipment. This confirmed that the experiment was appropriate for simulation of bubble rises in a quiescent liquid column. Recent data obtained by a high-speed camera permitted recording over a period of 60 s. Image analysis by a tailor-made program provided a time-series of quantities related to the position, size, and shape of bubbles. In addition to determination of the aspect ratio of the equivalent oblate ellipsoid, deviation from this shape was investigated in respect of the difference between the bubble’s centre of mass and the geometrical centre of bubble projection. Autocorrelation of the data indicated that the bubble inclination oscillated harmonically with a frequency of 5–10 Hz; cross correlation showed that the horizontal shift of the centre of mass, as well as the horizontal velocity, increased with increasing bubble inclination, and the vertical shift of the centre of mass increased with an increases in the absolute value of the bubble inclination. There is no significant phase shift in the oscillation of these quantities. The bulky bottom side of the bubbles is in accordance with the model of bubble oscillation induced by instability of the equilibrium of gravity and surface tension forces. The oscillation frequency dependence on surface forces (E?tv?s number) is evident, while viscosity does not play a significant role in low-viscosity liquids. Therefore, vortex-shedding is more likely to be an effect of the oscillation and not its cause.
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We report a detailed experimental characterization of the periodic bubbling regimes that take place in an axisymmetric air-water jet when the inner air stream is forced by periodic modulations of the pressure at the upstream air f...
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We report a detailed experimental characterization of the periodic bubbling regimes that take place in an axisymmetric air-water jet when the inner air stream is forced by periodic modulations of the pressure at the upstream air feeding chamber. When the forcing pressure amplitude is larger than a critical value, the bubble formation process is controlled by the forcing frequency, leading to the formation of nearly monodisperse bubbles whose volume is reduced as the forcing rate increases. We reveal the existence of two different breakup modes, M1 and M2, under effective forcing conditions. The bubble formation in mode M1 resembles the natural bubbling process, featuring an initial radial expansion of an air ligament attached to the injector, whose initial length is smaller than the wavelength of a small interfacial perturbation induced by the oscillating air flow rate. The expansion stage is followed by a ligament collapse stage, which begins with the formation of an incipient neck that propagates downstream while collapsing radially inwards, leading to the pinch-off of a new bubble. These two stages take place faster than in the unforced case due to the air flow modulation induced by the forcing system. The breakup mode M2 takes place with an intact ligament longer than one disturbance wavelength, whereby the interface already presents a local necking region at pinch-off, and leads to the formation of bubbles from the tip of an elongated air filament without an expansion stage. Scaling laws that provide closed expressions for the bubble volume, the intact ligament length, and the transition from the M1 breakup mode to the M2, as functions of the relevant governing parameters, are deduced from the experimental data. In particular, it has been found that the transition from mode M1 to mode M2 occurs at (St(f) Lambda We)(c) = 0.25 and that the intact ligament scales as l(i)/r(0) proportional to st(f)(-1) Lambda(1/5) We(1/4) within the breakup mode M1. Here r(0) is the radius of the gas stream, A the water-to-air velocity ratio, We the Weber number and St(f) the dimensionless forcing frequency. (C) 2020 Elsevier Ltd. All rights reserved.
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We analyse the controlled generation of bubbles of a given size at a determined bubbling rate in a co-flowing water stream forcing the gas flow. The temporal evolution of the bubble size, R(t), the air flow rate, Q(a)(t), and the ...
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We analyse the controlled generation of bubbles of a given size at a determined bubbling rate in a co-flowing water stream forcing the gas flow. The temporal evolution of the bubble size, R(t), the air flow rate, Q(a)(t), and the pressure evolution inside the bubble, p(b)(t), during the bubbling process are reported. To that aim, the temporal evolution of the bubble shape and the pressure inside the air feeding chamber, p(c)(t), where a harmonic perturbation is induced using a loudspeaker, are obtained from high-speed images synchronized with pressure measurements. A model is developed to describe the unsteady motion of the gas stream along the injection needle, coupled with the Rayleigh-Plesset equation for the growing bubble, allowing us to obtain p(b)(t). Thus, the minimum pressure amplitudes required inside the forming bubble to control their size and bubbling frequency are provided as a function of the gas flow rate, the liquid velocity, u(w), and the forcing frequency, f(f). Two different behaviors have been observed, depending on the liquid-to-gas velocity ratio, Lambda = u(w)/u(a). For small enough values of Lambda, the critical pressure amplitude is given by p(s) similar to rho(a) cu(a) St(f)(3), associated to a rapid pressure increase taking place during an interval of time of the order of the acoustic time. However, for larger values of Lambda, p(s) similar to rho u(w)(2) St(f)(3 )Lambda(-1/5)We(-1/4). Here rho and rho(a) are the liquid and gas densities respectively, c the speed of sound in air and St(f) = f(f)r(0)/u(w) and We = rho u(w)(2)r(o)/sigma the Strouhal and Weber numbers, where tau(o) denotes the outer radius of the injector. (C) 2020 Elsevier Ltd. All rights reserved.
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The dynamics of a bubble near a corner formed by two flat rigid boundaries (walls), is studied experimentally using a spark-generated bubble. The expansion, collapse, rebound, re-collapse and migration of the bubble, along with je...
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The dynamics of a bubble near a corner formed by two flat rigid boundaries (walls), is studied experimentally using a spark-generated bubble. The expansion, collapse, rebound, re-collapse and migration of the bubble, along with jetting and protrusion, are captured using a high-speed camera. Our experimental observations reveal the behaviour of the bubble in terms of the corner angle and the dimensionless standoff distances to the near and far walls in terms of the maximum bubble radius. The bubble remains approximately spherical during expansion except for its surface becoming flattened when in close proximity to a wall. When a bubble is initiated at the bisector of the two walls, the bubble becomes oblate along the bisector during the late stages of collapse. A jet forms towards the end of collapse, pointing to the corner. The closer the bubble to the two walls, the more oblate along the bisector the bubble becomes, and the wider the jet. A bubble initiated near one of the two walls is mainly influenced by the nearer wall. The jet formed is pointing to the near wall but inclined towards the corner. After the jet penetrates through the bubble surface, the bubble becomes a bubble ring, and a bubble protrusion forms following the jet. The bubble ring collapses and subsequently disappears, while the protrusion firstly expands, and then collapses and migrates to the corner.
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In this study, a theoretical model for prediction of bubbling regimes in bubble formation from a single submerged orifice has been developed. The model takes into account both chamber pressure fluctuations, and bubble-bubble and b...
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In this study, a theoretical model for prediction of bubbling regimes in bubble formation from a single submerged orifice has been developed. The model takes into account both chamber pressure fluctuations, and bubble-bubble and bubble-wall interactions on bubbling regimes. In order to investigate the bubbling regimes experimentally, high-speed photographic, and bubble frequency measurements were used. The bubbling regimes included single bubbling, pairing and multiple bubbling. Predictions of the bubbling regimes using the model were in excellent agreement with the experimental results.
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