Bubble Generation Mechanism Research Articles

Temperature impacts on the growth of hydrogen bubbles during ultrasonic vibration-enhanced hydrogen generation

To improve the hydrogen precipitation performance on the surface of the catalytic layer of the proton exchange membrane (PEM) hydrogen cathode, ultrasonic vibration was employed to accelerate the detachment of hydrogen bubbles on the surface of the catalytic layer. Based on the energy and mechanical analyses of nano and microbubbles, the hydrogen bubble generation mechanism and the effect of temperature on bubble parameters during the evolution process when the ultrasonic field is coupled with the electric field are investigated. The nucleation frequency of the hydrogen bubbles, the relationship between the pressure and temperature and the operating temperature during the generation and detachment of bubbles as well as the detachment radius of bubbles under the action of the ultrasonic field are obtained. The effects of ultrasound and temperature on hydrogen production were verified by visual experiments. The results show that the operating temperature affects the nucleation, growth, and detachment processes of hydrogen bubbles. The effect of temperature on the nucleation frequency of bubbles mainly comes from the Gibbs free energy required for the electrolysis reaction. The bubble radius and growth rate are both related to the temperature to the power of one-third. Ultrasonic waves enhance the separation of hydrogen bubbles from the catalyst surface by acoustic cavitation and impact effects. An increase in the working temperature reduces the activation energy barriers to be overcome for the electrolysis reaction of water, which together with a decrease in the Gibbs free energy and the surface tension coefficient, leads to an increase in the nucleation frequency of the catalytic layer and a decrease in the radius of bubble detachment, and thus improves the hydrogen precipitation performance. Visualization experiments show that in actual PEM hydrogen production, ultrasonic intensification can promote the formation of nucleation sites. The ultrasonic induced fine bubble flow not only has a drag effect on the bubble, but also intensifies the polymerization growth of the bubble due to the impact of the fine bubble flow, thus speeding up the detachment of the bubble, shortening the covering time of the hydrogen bubble on the surface of the catalytic electrode, reducing the activation voltage loss and improve the hydrogen production efficiency of PEM. The experimental results show that when the electrolyte is 60°C, the maximum hydrogen production efficiency of ultrasound is increased by 7.34%, and the average hydrogen production efficiency is increased by 5.83%.

Bubble Effects on Manufacturing of Silicon Nanowires by Metal-Assisted Chemical Etching

Abstract Micro/nano-textured Si wafers manufactured using metal-assisted chemical etching (MACE) have been the focus of several studies, but the mechanism of bubble generation during the MACE process affecting textured surfaces has rarely been reported. This study investigated the bubble effect due to the different placement patterns of the Si wafer (face-up, stirred face-down, and face-down). The results indicated that the placement pattern of the Si wafer notably influences the uniformity of outward appearance. At 2 h of etching, the outward appearance uniformity of face-up etching was more homogeneous than that of stirred face-down and face-down patterns, and the Si nanowires (SiNWs) processed through face-up etching were longer (41 μm) than those subjected to stirred face-down etching (36 μm) and face-down etching (32 μm). Therefore, the placement pattern of Si wafer can affect the uniformity and properties of SiNWs because of bubbles trapped inside cavities or between SiNWs.

Machining Characteristics of Diamond Grit-Coated Tools in Grinding-Aided ECDM

ABSTRACT This study investigated the machining characteristics of two shapes of diamond grit – coated tools used in grinding-aided electrochemical discharge machining for optical glass drilling. The effects of roundness, machining accuracy, surface roughness, and tool wear under various machining depths and electrode rotating speeds were explored, and the bubble generation mechanism and its effects on the discharge current waveform and electrochemical discharge reaction were observed. This study revealed that the bubbles generated in the gaps between diamond particles combine with each other to form a stable bubble film. Owing to differences in bubble generation mechanisms, the discharging waveform of the diamond grit – coated tool was more stable than that of tools without diamond grit – coated. It is known from the experimental results that the diamond grit – coated ball end mill in this method minimized shape roundness errors, machining time and the fracture of the exit edge.

Mechanism and modeling of Taylor bubble generation in viscous liquids via the vertical squeezing route

Bubble formation routes are vital to the application of microreactors, but a systematic understanding of the vertical squeezing remains lacking. Accordingly, this study focuses on the vertical squeezing bubble generation mechanism in viscous liquids in a T-junction microchannel. The results show there are the retraction, expansion, shrinking, and necking formation stages in the vertical squeezing route. A new formation stage, the retraction stage, is seen compared with horizontal squeezing. The influence of operating conditions on the generation mechanism was investigated from the gas–liquid interface behavior and force analysis, and the different formation mechanisms of satellite bubbles in the necking stage for two different squeezing routes were revealed. In addition, the complex bubble generation frequency rules were unified using a dimensionless frequency parameter. Finally, models for the generation frequency and bubble size were proposed. This work could provide some choice and design for the fundamentals of microreactors.

Bubble generation and growth mechanism in PMMA microfluidic chip

Bubbles often impede microfluidic chip functionality, especially in areas where heating is required. In this article, the mechanism of bubble generation and growth in polymethyl methacrylate microchannels was analyzed. The sources of bubbles were taken into consideration: (i) dissolved gas in the liquid, (ii) insoluble gas in cavities of the channel, and (iii) the gas produced by the phase change of the liquid. The factors that affect bubbles in microchannels were analyzed, including the liquid flow rate, surface modification of channels, and pressure on the fluid in the microchannel. Three sets of experiments were designed and carried out, and the results demonstrated that the bubbles in the microchannel can be shrunk and even eliminated by improving the flow rate, modifying channel surface properties, and increasing the liquid pressure.

Small bubbles formation and contribution to the overall gas holdup in large diameter columns of very high viscosity oil

A limited number of studies are available in literature on the small bubbles which create from gas-very high viscosity oils interaction and its contribution to the gas holdup in the system. The rate of small bubble formation has an important impact on heat and mass transfer in many chemical and industrial processes. The work presented in the current paper provides unique information on the formation of bubbles of millimetre diameter in high viscosity oil. A column of 290 mm diameter and Silicon oil of 330 Pa.s viscosity, were employed besides Electrical Capacitance Tomography and a high-resolution camera to investigate the characteristics of the small bubbles. Mechanism of bubble generation, effect of gas injection time and flowrate were studied. The average void fraction, total gas-liquid height, overall Probability Density Function (PDF) profile, small bubbles volume fractions and diameter were measured. Small bubbles generate from the eruption of large bubbles, at gas injection nozzles, coalescence of large bubbles, and at liquid bridges at transition to churn flow regime. Properties and concentration of the small bubbles are controlled by the location of the bubble generation, gas flowrate, and gas injection time. Small bubbles contribute by 6.6- 30% to the total gas holdup. Bubble diameter increased from 0.68 mm to 0.75 mm and decreased from 1.1 mm to 0.75 mm at the top and the bottom sections of the column respectively after 60 minof gas injection.

수상 함정 기포 마스크 벨트에서 생성된 기포 유동에 관한 수치해석 연구

This paper provides the results of the basic numerical study on the bubble generation mechanism of an air bubble mask belt device to reduce engine noise by creating bubble curtain around the hull of a surface ship. To analyze two-phase flows, a large eddy simulation (LES) approach coupled with a volume of fluid (VOF) method was adopted. This numerical approach was verified by comparison with the experimental and other numerical results for a single bubble rising problem in a stationary flow field. Due to laminar and turbulent break-up phenomena, the bubble mask belt produced bubbles of various sizes ranging from 100μm to 3000μm depending on the flow conditions. The present numerical results will be used to determine the air flow rate of the mask belt and other flow conditions required to generate a desired size bubble at a given ship speed.

Bubble Collapse Phenomenon of Multiphase Flow Field in a Venturi Tube

Microbubbles have various properties and are expected to be used in a wide range of fields. In this research, we focus on one of microbubble generators with a converging- diverging nozzle, i.e. a Venturi tube. In order to control bubble distribution of generated bubble diameter, knowledge of bubble generation mechanism in the tube is required. The purpose of this research is to clarify the effect of complex flow in a Venturi tube on bubble collapse. In this research, we focus on the change of pressure loss coefficient calculated from the pressure distribution in the tube and the sound pressure generated at the time of bubble collapse. For results of the experiment, the pressure loss coefficient increased due to the cavitation in the liquid single-phase flow. The same tendency was further obtained in the bubbly flow. It was also confirmed that the collapse behavior of bubbles differed depending on the inlet conditions, and sound pressure increased under conditions where rapid bubble collapse occurred.

Bubble formation in a step-emulsification microdevice: hydrodynamic effects in the cavity

This study focuses on the generation of bubbles in a step-emulsification microdevice via a two-angle photography method. It's found that the bubble generation mechanism is controlled by the interfacial tension, below a critical capillary number; while controlled by viscous force, inertial force and the disturbance induced by the bubble swarm, above the critical capillary number. From the two-angle photography method, a model is established for predicting the bubble size, by taking into account of the dynamic contact angle between gas-liquid interface and wall, and the hydrodynamic feedback of the cavity on bubble formation via the quantification of resistance by the volume fraction of gas in the cavity.

Bubble formation in a step-emulsification microdevice with parallel microchannels

In this paper, the mechanism of bubble generation in a step-type microdevice with 10 parallel microchannels is studied by using a high-speed camera. The evolution of the gas–liquid interface during bubble formation is explored. It is found that the evolution of the gas–liquid interface is controlled by the inertial force, viscous shear stress, liquid pressure difference, and the Laplace pressure. The second expansion stage of the head distance of the gaseous thread is the key to determine the bubble size. The size of bubbles depends on the physical properties of the fluids, which is different from the results for droplets. A semi-empirical model for predicting bubble’s volume is proposed. Finally, a pressure drop model is proposed to understand the mechanism of the multi-microchannel alternating generation mode, and the feedback effect of the bubble behavior in the confined cavity on the generation mode is analyzed.

Basic research of water jet cavitation toward the development of super high temperature and pressure bubble generation mechanism

Basic research of water jet cavitation toward the development of super high temperature and pressure bubble generation mechanism

New fabrication method and properties of porous metals produced by ultrasonic control of microbubble size

Porous metals with a cellular structure have attracted attention because of their excellent characteristics. These include a higher stiffness to weight ratio and improved impact energy absorption. Porous metals are formed in the vapor phase, liquid phase, or solid phase. However, conventional processing methods pose some difficulties, such as large pore growth and process complexity. A novel fabrication method for closed-cell porous metals is presented herein. In the new method, a bubble generation mechanism creates small bubbles in molten metal with a cylindrical hollow ultrasonic step horn. The fabrication system consists of a gas supply system, a temperature regulator, and a microbubble generator, which can break down supplied gas into microbubbles by using ultrasound waves. This fabrication method can be used to readily produce closed-cell porous metals and to create lightweight alternatives for many industrial materials. Microbubbles have low buoyancy and oxide films are formed in molten metal. With the new method, the generated microbubbles remain within the molten metal until it solidifies by cooling, resulting in a closed-cell porous metal. The porosity of the fabricated porous metal is greater than 70%.

Effect of Orifice Diameter on Bubble Generation Process in Melt Gas Injection to Prepare Aluminum Foams

The bubble generation process in conditioned A356 alloy melt through submerged spiry orifices with a wide diameter range (from 0.07 to 1.0 mm) is investigated in order to prepare aluminum foams with fine pores. The gas flow rate and chamber pressure relationship for each orifice is first determined when blowing gas in atmospheric environment. The effects of chamber pressure (Pc) and orifice diameter (Do) on bubble size are then analyzed separately when blowing gas in melt. A three-dimensional fitting curve is obtained illustrating both the influences of orifice diameter and chamber pressure on bubble size based on the experimental data. It is found that the bubble size has a V-shaped relationship with orifice diameter and chamber pressure neighboring the optimized parameter (Do = 0.25 mm, Pc = 0.4 MPa). The bubble generation mechanism is proposed based on the Rayleigh–Plesset equation. It is found that the bubbles will not be generated until a threshold pressure difference is reached. The threshold pressure difference is dependent on the orifice diameter, which determines the time span of pre-formation stage and bubble growth stage.

The mechanism of bubble generation using a slit elastic tube and an acoustic pressure wave in the gas phase

A bubble generation method that uses a slit elastic tube and an acoustic pressure wave in the gas phase can produce single bubbles of various sizes. In this study, we experimentally investigated the mechanism of bubble generation in a slit elastic tube. We used high-speed photography to observe the bubble generation process and slit motion in different liquids with different surface tensions. For the small bubble generation process, the results indicated that there was no significant difference in the slit opening time, even when the amplitude of the acoustic pressure wave was changed, and that the radius of the bubble generated was determined by the opening displacement of the slit, which was governed by the surface tension. In addition, the shape oscillation of a bubble due to surface tension was found to promote its detachment from an elastic tube with poor wettability.

Bubble generation and mechanism in polydimethylsiloxane based polymerase chain reaction chip

In order to explain the mechanism of bubble generation in polydimethylsiloxane (PDMS), we investigated the crucial factors: the surface wettability and permeability of PDMS. Two microfluidic chips were designed and fabricated: a PDMS/glass chip and a glass/PDMS/glass sandwich chip (about 1 μm in thickness of PDMS). Then, two sets of experiments were carried out: a comparison between the PDMS/glass chips untreated and treated with O2 plasma, and another comparison between a PDMS/glass chip and a glass/PDMS/glass sandwich chip. The bubble in the PDMS/glass chip was avoided by treating with O2 plasma. After the treatment, the residual gas between the PDMS surface and water was eliminated in that the PDMS surface became hydrophilic. In addition, the gas molecules required higher energy to enter the chambers due to the reduced contact angle of PDMS and water. The glass/PDMS/glass sandwich chip was treated with the vacuum processing to eliminate the residual gas. And the gas outside of the chip did not enter the chambers through the thin layer of PDMS easily. This further verified that the surface wettability and the permeability of PDMS can result in the bubble generation.

Investigation of Anodic Gas Film Behavior in Hall–Heroult Cell Using Low Temperature Electrolyte

A novel one-fourth scale low temperature electrolytic model of the Hall–Heroult cell was constructed to investigate the electrolytic bubble formation, coalescence, and movement under the horizontal anode surface. The principles of geometric and dynamic similarities were applied in developing this model electrolytic cell for the first time. A 0.28 M aqueous CuSO4 +2 0 pct H2SO4 solution was used as the electrolyte. Analogous to the Hall–Heroult cell, copper (Cu) was deposited at the cathode and oxygen (O2) bubbles were generated underneath the anode. The bubble generation mechanism, development, coalescence, and detachment underneath the anode were observed using a high speed camera. It was found that electrolytic bubbles were generated uniformly under the entire anode surface and grew through gas diffusion and coalescence. With increasing current density (CD) and anode inclination angle to the horizontal, bubble velocity increased underneath the anode surface. Moreover, the bubble layer thickness and bubble sizes decreased with an increase in anode inclination angle. Bubble coverage under the anode also decreased with increasing anode inclination angle, but was found to be insensitive to the change in CD. Finally, the calculated bubble resistance was found to decrease with increasing anode angle.

Electron temperature enhancements in nighttime equatorial ionosphere under the occurrence of plasma bubbles

Simultaneous in-situ measurements of electron density and temperature in the nighttime equatorial region were performed by a rocket experiment launched under solar minimum and geomagnetic quiet conditions from the station of Alcântara (2.24°S; 44.24°W), Brazil, on Dec. 18, 1995, 21:17 LT. These measurements detected during the upleg flight a large overheated area around the base of density profile. The presence of plasma bubbles was revealed during the downleg phase, as well as temperature enhancements detected preferentially at altitudes where plasma depletions are found. It was assumed that the region traversed by the rocket during the downleg was preceded before the bubble's onset by a large overheating as observed during the upleg flight. Analyzing this framework under the light of the Global Self-Consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP), as well as a 2D numerical code that simulate the growth of an instability and the evolution of thermal energy inside a bubble, we found that despite an overheated area in the F-region bottomside can disturb the electron density profile around the altitude interval where such heat is deposited, it seems not have a direct influence over parameters responsible for the bubble onset. Additionally, the phenomenon of the intra-bubble thermal enhancement could be formed due to the convection of hot-electron fluid transported from the overheated region surrounding the base of the F-region to upper altitudes by the underlying mechanism of bubble generation.

Generating microbubbles in a co-flowing microfluidic device

This paper introduces an experimental study on the microbubble generation mechanism in a co-flowing microfluidic device. Using propane, butane and air as the dispersed phases and different SDS–PEG solutions as the continuous phases, two kinds of bubble generating phenomena, named “single-bubble generation” flow and “dual-bubble generation” flow, were observed in experiment and uniform bubbles with average diameters ranging from 391μm to 713μm and polydispersity less than 2.9% were successfully prepared. The pressure fluctuation of gas phase is carefully analyzed with the Young–Laplace equation and the bubble generation mechanism is summarized from the dynamic movement of gas–liquid interface. The main factors impacting the bubble size variation are illustrated for both the organic and inorganic gases. A universal correlation is established to predict the average bubble diameters.

Control of electrolysis-generated microbubbles for sensor surface passivation

This letter outlines our work in generating and controlling microbubbles as protective “lids” for samples collected from the environment. The fabrication method uses “strain architecture” to construct three-dimensional cages with high surface area. These structures confine the bubbles and perform as electrodes for electrochemical sample collection and electrolysis-based gas bubble generation. The focus of this article is on the interaction between the microcages and generated bubbles, including the bubble generation mechanism, bubble growth rate, response to hydrostatic pressure, effect of interfacial-tension modifying coatings, and long-term stability.

Improvements in Predicting Void Fraction in Subcooled Boiling

A simple two-phase thermal-hydraulic tool with the drift-flux model has been used to develop a subcooled boiling model. The tool is composed of four governing equations: mixture mass, vapor mass, mixture momentum, and mixture enthalpy. Using the developed tool, various subcooled boiling models were investigated through the published experimental data. In the process of evaluation, two models were developed associated with the subcooled boiling. First, the Saha and Zuber correlation predicting the point of the net vapor generation was modified to consider the thermal and dynamic effects at the high-velocity region. Second, the pumping factor model was developed using the pi-theorem based on parameters related to the bubble generation mechanism, and it produced an additional parameter: the boiling number. The proposed models and several other models were evaluated against a series of subcooled flow boiling experiments at the pressure range of 1 to 146.8 bars. From the root-mean-square analysis for the predicted void fraction in the subcooled boiling region, the results of the proposed model presented the best predictions for the whole-pressure ranges. Also, the implementation of the developed models into RELAP5/MOD3.3 brought about improved results compared to those of the default model of the code.