Bubble Growth Process Research Articles

Research on dynamic behavior of bubble under the action of electric field

Based on the assumption of the creeping flow with small Reynolds number, a coupled model of bubble rising and mass transfer was established. Through numerical simulation, the velocity field inside and outside the bubble and the deformation of the bubble at different time under different voltage conditions were studied. The behavior characteristics of bubbles under different electric fields were compared and analyzed. The results show that the electric field force has an effect on the flow field and the shape of bubble rising. Under the action of voltage, the bubbles are elongated and deformed along the field strength direction. The electric stress on the bubble interface is the key factor of bubble deformation. With the electric field applied, the flow rate of liquid and the volume fraction of vapor near heat source increase. The degree of inconsistency increases with the improvement of electric field intensity. Electric field enhances convection and disturbance of fluid. The thermal efficiency of fluid near heat source improves. The process of bubble growth and detachment accelerates. The thermal resistance between the heat source and the fluid decreases, and the heat transfer efficiency increases. The research results can be guided for further studying the mechanism of electric field enhanced boiling heat transfer.

The thermal Marangoni effect during photoelectrochemical conversion

Marangoni effect is a convective effect produced by surface tension gradient and has an important influence on the efficiency of hydrogen production by photoelectrochemical conversion. In this paper, COMSOL Multiphysics was used for numerical simulation to build a Marangoni effect model at the bubble interface in the photoelectrochemical conversion process, to clarify the mechanism of Marangoni effect of bubbles on the surface of photoelectrodes, and to discuss the influence of Marangoni effect on the process of bubble growth and separation. The results show that the thermal effect of light will cause the Marangoni convection around the bubbles and form a negative pressure zone at the root of the bubbles, which will attract the surrounding bubbles. Gravity has little effect on Marangoni convection, and an increase in the intensity of incoming light enhances the Marangoni effect. At the same time, Marangoni convection leads to a Marangoni force on the surface of the bubble, which prevents the bubble from leaving the surface of the photoelectrode.

Magnetic field-structuring as versatile approach to shape the anisotropic mechanical response of composite foams

The use of a high amount of reinforcing phase improves the mechanical performance of a polymeric composite structure. In composite foams this approach is associated to difficulties in developing a regular cellular structure, due to the hindering effect of such high amount of non polymeric phase on the bubble growth process. Additionally, the compressive response of foams produced with conventional forming processes is isotropic, and cannot fully exploit the reinforcing potential of the reinforcement. To overcome this limitation, the reinforcing phase should be aligned along preferential directions but available industrial methods are costly post-processing procedures that can induce damages in the cellular structure. Recently, several research proposals investigated the use of a contactless magnetic field to build reinforcing structures in polymeric foams, but they were mainly focused on the linear elastic region of the compressive response, while the large strain behaviour was not under control. In this paper, a process based on a time-variable magnetic field is exploited to manage in a simple and fully adaptable way the shape of the stress-strain response of the foam (up to at least 60% strain) during the foaming process. The resulting composite foam is characterized by mechanical anisotropy with a homogeneous and regular cellular morphology. This goal is achieved by imparting suitable geometrical features to the field-structured reinforcing aggregates by means of the proper time-profile and intensity of a magnetic field during the production process.

Dynamics of a gas bubble during fluid decompression at a constant rate

The bubble growth process in a magmatic melt during decompression at a constant rate under various conditions is numerically studied. The effect of the formation of a narrow boundary layer with a high viscosity around the bubble, which significantly changes the dynamics of the bubble, is taken into account. It is shown that, at a low initial pressure in the medium, in the bubble high pressure is maintained throughout the calculation time. In the case of long decompression time, the final bubble radius is bigger than in the case with small decompression time. This effect can be associated with different dynamics of the medium viscosity in a narrow boundary layer around the bubble.

Analytical Description of Vapor Bubble Growth in a Superheated Liquid: A New Approach

This article presents a mathematical model of vapor bubble growth in a superheated liquid, which simultaneously takes into account both dynamic and thermal effects and includes the well-known classical equations, the momentum equation and the heat equation, written to take into account the process of liquid evaporation. An approximate semi-analytical solution of the problem is found, its construction based on the existence of a quasi-stationary state for the bubble growth process. This makes it possible to reduce the original moving boundary value problem to a system of ordinary differential equations of the first order. The solution obtained is valid at all stages of the process and for a wide range of system parameters. It is shown that at large times the solution becomes self-similar and in limiting cases it agrees with the known solutions of other authors.

Experimental Study on the Gas Bubble Temperature Around an Arc Under Insulation Oil

The internal temperature of a gas bubble caused by a fault arc is a key basic parameter for investigating the bubble behavior and the pressure rise inside oil-filled power equipment. Due to the constraints imposed by the measurement methods and experimental conditions, this temperature has always been estimated as a constant value of 2000 K in past decades. In this letter, we present direct experimental evidence of the gas bubble temperature around an arc. A large oil-filled transformer was reformed and used to conduct arcing fault tests. A sapphire fiber optic infrared thermometer was established and used to detect the internal gas bubble temperature. The experimental results confirm that this internal temperature is not constant but rather varies over a wide range from 2100 K to 1500 K, plummeting with rapid bubble growth and then exhibiting small variations until arc extinction. Furthermore, the bubble growth process throughout the arc duration can be approximately regarded as first polytropic and then isothermal.

Experimental characterization of the bubble neck formation during its growth process on a superhydrophobic surface

Bubble growth is a common phenomenon in nuclear power generation, which is significantly affected by the wettability of the heating surface. In this paper, the bubble growth on the 153.15° superhydrophobic surface was observed by a high-speed camera, and the velocity field of the water surrounding the bubble was also analyzed using particle image velocimetry (PIV). The experimental results show that the special of the bubble growth on the superhydrophobic surface is that it forms a bubble neck. This is because of the superhydrophobic surface affinity for vapor, the surface is covered by a vapor film when heated. The bubble is generated from the film through the neck. Therefore, this paper focuses on the bubble neck formation stage during the bubble growth process. When the surface heating power changes, the time required for bubble growth changes accordingly. But the bubble size is not affected by the heating power. However, the effect of heating power on bubble growth time is mainly at the stage of the vapor film bulging stage, and it also has no effect on the bubble neck formation time. The reason for the above phenomenon is that the neck restricted the amount and rate of vapor entering the bubble. Besides, the velocity field of water is observed and it is found that the annular flow field strengthened the heat transfer and promotes the fracture of the neck.

The boiling crisis of water under exponentially escalating heat inputs in subcooled flow boiling at atmospheric pressure

We investigated the boiling crisis of water under exponentially escalating heat inputs using a specially-designed experimental apparatus featuring high-speed infrared thermography to measure the time-dependent temperature and heat flux distributions on the boiling surface, and high-speed video diagnostics to image the bubble growth process. We conducted flow boiling experiments with water at atmospheric pressure on a plate-type heater installed in a 3 × 1 cm2 cross section channel. We tested various values of flow velocity (corresponding to Reynolds numbers from 0 to 35,000), water subcooling (from 10 to 75°C) and rate of power rise (corresponding to exponential power escalation periods, τ, from 1.5 to 500 ms). At long periods critical heat flux (CHF) is independent of the power escalation period and the boiling processes appear to be physically similar to steady state. As the period decreases, the CHF values tend to increase following an asymptotic 1/τ trend. The mechanism that determines this trend depends on subcooling. For high subcooling, CHF monotonically increases as the period decreases, and the DNB occurs through a fully developed nucleate boiling process. By contrast, for low subcooling, CHF first increases as observed for high subcoolings, then decreases, and finally increases again as the period decreases. We observe that such non-monotonic transition is due to a change in the boiling crisis mechanism. Specifically, for very short periods and low subcooling, the boiling crisis happens during the growth of the very first generation of bubbles, which never detach from the heated surface, shortly after the onset of nucleate boiling. In such cases, fully-developed boiling is not achieved.

Vapor bubble–bubble penetration during subcooled pool boiling in a nonionic surfactant aqueous solution

Boiling is encountered in industrial processes as an effective heat transfer method. It generates vapor to remove a large amount of heat. The evolution of vapor bubbles in a boiling liquid, especially the growth of bubbles, is one of the parameters that determine the heat transfer strength of a heating surface. The growth process of bubble is complicated due to many physical or chemical factors. Here, for the first time, we report a new phenomenon of vapor bubble–bubble interaction process (a bubble interacts with its neighboring one(s) generated at the same nucleation site) during pool boiling aided by a nonionic surfactant. The different bubble interactions are defined as bubble–bubble penetration, bubble–bubble piercing, and double penetrations. It is motivated by microbubble emission from heater in subcooled pool boiling. Bubble dynamics in aqueous surfactant solution was visualized and analyzed. Much attention was paid on the vapor bubble generation, growth, and interaction in the presence of surfactant. It is worth noting that the bubble penetration frequently occurs in the current conditions. The findings are of fundamental and practical relevance, and in the hope to contribute to better understanding of surfactant-aided boiling mechanism.

Numerical Simulation on Bubble Growth and Micro-explosion Characteristics of Biodiesel/ethanol Fuel Droplets

In this paper, the biodiesel/ethanol gas-containing fuel droplet was numerically simulated at room temperature. The growth characteristics of the bubble growth process were obtained, and the influencing factors affecting the bubble growth and micro-explosion process were explored. Studies have shown that bubble growth and droplet micro-explosion at room temperature are dominated by the pressure difference between the inside and outside of the bubble. Within a certain range, reducing the initial radius ratio of droplets as much as possible can also promote the bubble growth and micro-explosion of droplets. The proportion of fuel in the droplet will also affect the bubble growth and micro-explosion characteristics.

Effects of simulation system on the phase transition behavior of liquid film: A molecular dynamics study

In the present study, the effects of liquid film thickness and simulation box geometry on the molecular dynamics simulation studies of phase transition are investigated. Results show that the simulation system has significant impacts on phase transition behaviors of the liquid film because of the applications of periodic boundary and reflecting boundary. The evaporation and bubble nucleation phenomena happen on the substrate with a temperature of 140 K. If the heated liquid film is thin or the height of the simulation box is low, the evaporated liquid atoms from the interface of vapor-liquid cause a high vapor pressure and prevent the happening of bubble nucleation in the vicinity of the substrate, and only evaporation phenomenon happens. When the substrate temperature is raised to 150 K, a bubble nucleus turns up on the substrate. If the length of the simulation box is short, the bubble nucleus converts into a vapor film quickly under the joint effects of liquid film lifting and high-temperature liquid vaporization, and the explosive boiling phenomenon happens. However, with the increase of the length of the simulation box, a distinct bubble nucleation and growth process are observed. The present study can provide a reference for researchers in the molecular dynamics simulation studies of phase transition.

Aspects of the reduction of the coating fire resistance by the use of nanosized additives

As a new generation fire-resistant material, the intumescent coating has significant advantages over conventional fire protection materials. The applications of the intumescent coating have changed widely and have dominated the fire engineering market. In this study, intumescent coatings with different nanosized additives such as fumed silica, organo-montmorillonite, aluminum oxide and titanium dioxide were investigated. It was shown that organically modified montmorillonite and hydrophobized silica reduced fire resistance properties of intumescent coatings. The reason for such decrease is the shift of the expansion processes to higher temperatures. This phenomenon increases the time of direct heating of the substrate. Moreover, it was established that the minor concentrations of organically modified montmorillonite could significantly change the structure of coatings and increase the specific heat insulation properties of the carbon foams. Such an effect was also observed in the case of the use of hydrophobic silica, but to a lesser extent. It was shown that nanosized additives have effect on the expansion factor and the foam structure. It was suggested that this effect could be explained by the complex influence of the nanoparticles on the bubble growth processes through the melt viscosity modification.

Understanding the growth mechanism of single vapor bubble on a hydrophobic surface: Experiments under nucleate pool boiling regime

Growth and departure characteristics of single vapor bubble on hydrophobic surfaces have been investigated under saturated bulk conditions. Nucleate pool boiling experiments, with water as the working fluid, have been carried out on two sets of hydrophobic surfaces; (1) black painted ITO-coated glass substrate, and (2) ITO-coated sapphire substrate subjected to two different levels of constant heat flux conditions. High speed camera was employed to characterize the bubble dynamics. In conjunction with the high speed camera, an IR thermal imaging camera was used to simultaneously map the spatio-temporal temperature distribution of the substrate underneath the growing vapor bubble. Compared to the black paint coated glass substrate, significantly higher transmissivity of sapphire substrate in the mid-IR spectral range ensured the authenticity of the spatio-temporal accuracy of temperature measurements of the bubble nucleation site using the employed IR camera. Results on the bubble growth clearly showed that, irrespective of the substrate employed, the bubble exhibits almost zero waiting time, leaving a small residue of vapor on the substrate after the bubble departure, which further acts as a nucleus for the next bubble. Moreover, the bubble growth process was observed to be extremely slow. The IR camera-based temperature field of the hydrophobic substrates revealed that the presence of vapor bubble does change the local temperature of the substrate underneath the vapor bubble. However, it was observed that the temperature field of the nucleation site does not change with bubble growth and its departure, which, in turn, indicated towards the possibility that the microlayer ceases to exist underneath the vapor bubble. This is in contrast to the microlayer evaporation-driven growth of single vapor bubble on a hydrophilic substrate. Based on these direct experimental observations, plausible bubble growth mechanism(s) on the given hydrophobic surfaces have been predicted and discussed.

Black Liquor Droplet Swelling Model during Pyrolysis

Black liquor is a spent pulping liquor from kraft pulping process. The concentrated black liquor is combusted or gasified to recover the chemical energy and to recycle the pulping chemicals. It undergoes several processes, i.e. drying, pyrolysis, and combustion and/or gasification. The droplet swells slightly during drying. Conversely, it swells heavily during pyrolysis. It can swell up to 20-30 time its original volume. The swelling stops when the pyrolysis reaction ends and/or when the solid loses its viscoelastic properties. Swelling phenomena also happens in other system, e.g. polymer foam moulding and coal pyrolysis. Two key phenomena for swelling behaviour are: gas production inside a material that forms gas bubbles and the material viscoelastic properties. The bubbles grow and whole droplet then swells accordingly. For black liquor droplet swelling, some empirical models has been developed but none of them are based on the theoretical bubble growth process. In this paper, the black liquor droplet swelling modelling based on the bubble growth process is developed. The model generally in agreement with the black liquor droplet swelling experimental data from a single droplet Thermogravimetry Analyser (TGA).

Numerical simulation of magnetic nanofluid (MNF) film boiling on cylindrical heated magnet using phase field method

In this study, the film boiling of MNF under a magnetic field generated around a constant heat flux magnetic cylinder was investigated two-dimensionally using the Phase Field method. The aim of this work is improving heat transfer rate in film boiling regime around cylinder as well as raising this rate with adding non-heated magnet in vicinity of heated cylinder. The whole process of heat transfer and bubble growth have been investigated quantitatively and qualitatively. It was observed that as we increased the magnet's remnant flux density, the increased strength of the magnetic field started to affect the bubble shape and growth more aggressively. With the increase in the magnetic force applied at the interface of phases, the thickness of the vapor layer around the cylinder decreased and the Nusselt number increased. It was found that the heat transfer rate can be significantly increased by changing the orientation of magnet poles, and that film boiling on the surface of the heated cylinder can be easily influenced by the placement of a heat-insulated magnet in its vicinity. The growth and movement of vapor bubbles and the Nusselt number on the cylinder surface in the presence of a non-heated (insulated) magnet at different angles and distances from the heated magnet were studied. When the non-heated magnet was placed at certain distances from the heated magnet, the heated magnet produced two vapor bubbles at the same time, which rose above independently and ultimately turned into two vapor columns. The major outcomes of this study is that changing the magnet's remnant flux density, orientation of magnet poles and the magnetic field interaction of heated and non-heated magnets can profoundly affect the rate of heat transfer on the cylinder surface. Besides that, this rate can be practically controlled with the help of insertion of second heat-insulated cylinder nearby the heated magnet in different distances and angles.

Multigear Bubble Propulsion of Transient Micromotors.

Transient, chemically powered micromotors are promising biocompatible engines for microrobots. We propose a framework to investigate in detail the dynamics and the underlying mechanisms of bubble propulsion for transient chemically powered micromotors. Our observations on the variations of the micromotor active material and geometry over its lifetime, from initial activation to the final inactive state, indicate different bubble growth and ejection mechanisms that occur stochastically, resulting in time-varying micromotor velocity. We identify three processes of bubble growth and ejection, and in analogy with macroscopic multigear machines, we call each process a gear. Gear 1 refers to bubbles that grow on the micromotor surface before detachment while in Gear 2 bubbles hop out of the micromotor. Gear 3 is similar in nature to Gear 2, but the bubbles are too small to contribute to micromotor motion. We study the characteristics of these gears in terms of bubble size and ejection time, and how they contribute to micromotor displacement. The ability to tailor the shell polarity and hence the bubble growth and ejection and the surrounding fluid flow is demonstrated. Such understanding of the complex multigear bubble propulsion of transient chemical micromotors should guide their future design principles and serve for fine tuning the performance of these micromotors.

1Pb4-2 Sonoluminescence in the initial bubble growth process

1Pb4-2 Sonoluminescence in the initial bubble growth process

On the surface tension role in bubble growth and detachment in a micro-tube

The role of surface tension in the mechanism of bubble growth and detachment for a co-flowing air-water two-phase flow in a micro-tube is addressed. A numerical investigation for a horizontal axisymmetric flow with the assumption of zero gravity and an upward flow accounting for gravity contribution is carried out. The continuous liquid phase is flowing in a tube of 500 μm inner diameter and the gas phase is axially injected through a nozzle of 110 μm inner diameter and 210 μm outer diameter. A single-fluid model is used to determine the flow field, solving the continuity and momentum equations associated with the volume of fluid method for interface tracking. An open source software, OpenFOAM, is utilized for solving numerically this problem. The prediction results show that the surface tension plays a double role. First, it keeps the bubble attached to the injection nozzle during bubble growth and neck formation. Then, it destabilizes the interface by pinching off the neck in the immediate vicinity of the nozzle at about a distance of 0.5 the nozzle diameter rather than right at the nozzle exit. In-depth analysis of the mechanism of bubble formation induced by the effect of surface tension is carried out. It is highlighted that this latter acts as an attachment force at the injection nozzle during the bubble growth and it acts over the entire interface of the bubble yielding the formation of a neck. Later, the capillary effects reduce the diameter of the neck until it breaks and yields the detachment of the bubble. Further investigation at the nozzle wall allows depicting the motion of the contact line during the process of bubble growth and its significant effect on the bubble formation.

Deterioration of boiling heat transfer on biphilic surfaces under very low pressures

Surface wettability engineering has attracted growing attention in recent years as an effective tool to enhance boiling heat transfer. On wettability-patterned (so-called biphilic) surfaces in particular, subatmospheric boiling has been shown to be nearly free of the severe degradation of heat transfer rate that tends otherwise to prevail on plain surfaces. The surprisingly consistent performance under reduced-pressure conditions can be attributed to the rather strong pinning of the three-phase contact line (TPCL) at the border between the hydrophobic and hydrophilic surfaces, which essentially eliminates the waiting period between bubble cycles. Only when the pressure is decreased sufficiently low does the transition to the undesired mode of intermittent boiling eventually occur on the biphilic surface. The purpose of the present study is to investigate the physical mechanism for the heat transfer deterioration on a mixed-wettability surface at very low pressures. To that end, we performed high-speed visualization experiments of the process of bubble nucleation and growth on a smooth copper surface coated with a single hydrophobic polytetrafluoroethylene (PTFE) spot, under different surface superheats and system pressures. The results show an interesting correlation between the TPCL behavior and the bubble growth dynamics. Specifically, under some certain threshold of pressure, it would become increasingly likely for the TPCL to be dislodged from its pinned position on the biphilic surface under a particularly rapid bubble expansion. As a result, full flooding of the hydrophobic surface might ensue, which is deemed responsible for temporary deactivation of the hydrophobic spot as a viable nucleation site. Furthermore, based on the diffuse-interface simulations of TPCL propagation across heterogeneous wettabilities, a comparison of cases with different bubble expansion rates offered qualitative evidence supporting the critical role of such accelerated bubble growth rate in driving the TPCL to overcome the energy barrier raised at the wettability divide.

Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film

Abstract He-charged FeCrNi films were prepared by radio frequency (RF) magnetron sputtering technique, and helium desorption behavior, growth process and evolution mechanism of helium bubbles in FeCrNi film were studied by thermal desorption spectroscopy (TDS) spectra combined with microstructure analysis under different annealing treatments. Three different He desorption peaks were observed in TDS spectra, which can be related to the release of interstitial helium atoms from the near-surface regions, helium-vacancy clusters with different sizes in FeCrNi film. TEM analysis showed that helium bubbles can be uniformly distributed in He-charged FeCrNi nanocrystalline films. Under annealing treatments at different temperatures, the size of helium bubbles obviously increases with increasing annealing temperature, corresponding to the significantly decrease in the number density of helium bubbles in FeCrNi films, and the growth mechanism of helium bubbles was suggested to originate from the migration and coalescence (MC) mechanism.