Bubble Evolution Research Articles

Rapid bubble growth within 200 μs in flat microchannels under “hot spot” heating conditions at ultra-high heat fluxes

The two-phase flow boiling in microchannels is an attractive cooling strategy for high-power density electronic/optoelectronic devices, while the exist of sub-millimeter “hot spots” characterized by ultra-high heat fluxes would degrade the reliability and safety for commercial and consumer applications. In this paper, the bubble nucleation and evolution characteristics of subcooled flow boiling in high aspect ratio flat microchannels were experimentally studied with the aid of high-speed visualization and infrared temperature measurement at localized heat fluxes exceeding 1.5 kW/cm2. Dielectric liquid HFE-7100 was used at mass flow rates ranging from 11.1 to 178.2 kg/(m2·s). Results show that the explosive boiling process was accompanied by an approximately linear bubble growth rate of about 20 m/s at about the first half of growth stage, and the time duration for overall bubble growth was less than 200 μs. The bubble nucleation was suppressed dramatically on the smooth bottom surface of the channel even at a localized “hot spot” heat flux above 2.5 kW/cm2. The introduction of staggered short micro-pin fin arrays at the bottom wall significantly lowered the threshold heat flux and onset temperature of bubble nucleation and reinforced the bubble growth to larger sizes. It is attributed to the reduction in the temperature gradient of thermal boundary layer adjacent to the heated wall, and it would be further reduced by increasing the micro-pin fin denseness. Even at quite high superheats on smooth and micro-structured surfaces, the heterogeneous boiling mechanism was proved to still work.

Pore-scale Ostwald ripening of gas bubbles in the presence of oil and water in porous media

HypothesisOstwald ripening of gas bubbles is a spontaneous mass transfer process that can impact the storage volume of trapped gas in the subsurface. In homogeneous porous media with identical pores, bubbles evolve toward an equilibrium state of equal pressure and volume. How the presence of two liquids impacts ripening of a bubble population is less known. We hypothesize that the equilibrium bubble sizes depend on the surrounding liquid configuration and oil/water capillary pressure. Method and numerical experimentsWe investigate ripening of nitrogen bubbles in homogeneous porous media containing decane and water using a level set method that alternately simulates capillary-controlled displacement and mass transfer between bubbles to eradicate chemical-potential differences. We explore impacts of initial fluid distribution and oil/water capillary pressure on the bubble evolution. FindingsRipening in three-phase scenarios in porous media stabilizes gas bubbles to sizes that depend on their surrounding liquids. Bubbles in oil decrease in size while bubbles in water increase in size with increasing oil/water capillary pressure. Bubbles in oil reach local equilibrium before the three-phase system stabilizes globally. A potential implication for field-scale gas storage is that the trapped gas fractions in oil and water vary with depth in the oil/water transition zone.

The Mechanisms of Inhibition Effects on Bubble Growth in He-Irradiated 316L Stainless Steel Fabricated by Selective Laser Melting.

The AISI 316L austenitic stainless steel fabricated by selective laser melting (SLM) is considered to have great prospects for applications in nuclear systems. This study investigated the He-irradiation response of SLM 316L, and several possible reasons for the improved He-irradiation resistance of SLM 316L were systematically revealed and evaluated by using TEM and related techniques. The results show that the effects of unique sub-grain boundaries have primary contributions to the decreased bubble diameter in SLM 316L compared to that in the conventional 316L counterpart, while the effects of oxide particles on bubble growth are not the dominant factor in this study. Moreover, the He densities inside the bubbles were carefully measured using electron energy loss spectroscopy (EELS). The mechanism of stress-dominated He densities in bubbles was validated, and the corresponding reasons for the decrease in bubble diameter were freshly proposed in SLM 316L. These insights help to shed light on the evolution of He bubbles and contribute to the ongoing development of the steels fabricated by SLM for advanced nuclear applications.

High-quality Ge/Si hetero-bonding by sputtered microcrystalline Ge interlayer

We report a high-quality germanium (Ge) and silicon (Si) wafer bonding based on an intermediate layer of 5 nm-thick microcrystalline Ge. The bubble evolution, crystalline state evolution, and oxide layer evolution at the wafer-bonded interface are investigated. The mechanisms of bubble elimination and oxide layer disappearance are systematically analyzed. The sputtered microcrystalline Ge film consists of an amorphous Ge (a-Ge) layer and a low-quality single-crystal Ge (c-Ge) layer. The bubbles can be eliminated by the loose structure of the low-quality c-Ge at the bonded interface. Thus, the bubble-free bonded interface can be obtained. Besides, the bonding strength decreases with the increase of the sputtering temperature. The pressure on the bonded wafers before post-annealing can eliminate large bubbles and improve bonding strength. It is worth mentioning that the oxide layer and the threading dislocations are not observed at the bonded interface. This study can give a guidance for the subsequent manufacture of high-performance Si-based Ge optoelectronic devices.

Laser-Induced Broadband White Emission of NdAlO3 Nanocrystals in Alcohols

Laser-induced white emission (LIWE) is a phenomenon in which a broadband emission is generated through focused laser excitation and described in several different materials; however, the vast majority was explored only in vacuum or air. Herein, LIWE for NdAlO3 nanocrystals immersed in different alcohols is described. It was observed as an evolution of bubbles in the focused point of excitation, which was responsible for the intermittence of the white emission due to changes in the excitation beam going through the bubbles. LIWE, although intermittent, presents a similar behavior regarding the nonlinear dependence of the excitation power in every alcohol used; however, a clear increase in the threshold of the LIWE phenomenon was observed, which was closely related to the relative permittivity of the alcohol in which the sample was immersed. This may open up the possibility to explore LIWE in solvents by properly choosing the material, relative permittivity of the solvent, and appropriate excitation power.

Bubble management in PEM water electrolysis via imprinting patterned grooves on catalyst layer

The demand for cost reduction in proton exchange membrane water electrolyzer (PEMWE) is driving it to increase the operating current density and higher current densities require better bubble management. In this work, we demonstrate that introducing patterned grooves to the catalyst layer surface (CL) is able to enhance the bubble evolution and improve electrolyzer performance. The patterned grooves microstructure on CL provides additional bubble nucleation sites and faster bubble evolution. Smaller average bubble size and higher bubble detachment frequency during water electrolysis are observed through a bubble evolution visualization system and further confirmed by a frequency analysis method. In addition, patterned grooves also improve the hydrophilicity and aerophobicity of CL surfaces. In the PEM water electrolyzer cell, the patterned grooves on CL boost the cell performance and reach high current densities up to 5.0 A cm-2. With better bubble management, patterned grooves significantly reduce the mass transport overpotential at each current density. At 5.0 A cm-2, the mass transport overpotential is reduced by 59%. This work provides a new idea of bubble management in PEMWE and is highly informative for designing CLs for PEMWE and other gas-evolving electrochemical energy conversion applications.

Global sensitivity analysis of a coupled multiphysics model to predict surface evolution in fusion plasma–surface interactions

We construct a global sensitivity analysis framework for a coupled multiphysics model used to predict the changes in material properties and surface morphology of helium plasma-facing components in future fusion reactors. The model combines the particle dynamics simulator F-TRIDYN, that predicts the helium implantation profile, with the cluster dynamics simulator Xolotl, that predicts the growth and evolution of subsurface helium gas bubbles. In order to keep the sensitivity analysis tractable, we first construct a sparse, high-dimensional polynomial chaos expansion surrogate model for each output quantity of interest, which allows the efficient extraction of sensitivity information. The sensitivity analysis is performed for two problem settings: one for ITER-like conditions, and one that resembles conditions inside the PISCES-A linear plasma device. We present a systematic comparison of important parameters, for both F-TRIDYN and Xolotl in isolation as well as for the coupled model, and discuss the physical interpretation of these results.

Study of surface morphology in GaAs by hydrogen and helium implantation at elevated temperature

In this work, the surface morphology and internal defect evolution process of GaAs substrates implanted with light ions of different fluence combinations are studied. The influence of H and He ions implantation on the atomic mechanism of the blister phenomenon observed after annealing is investigated. Raman spectroscopy is used to measure the surface stress change of different samples before and after implantation and annealing. Optical microscopy and atomic force microscopy are used to characterize the morphology changes of the GaAs surface under different annealing conditions. The evolution of bubbles and defects in GaAs crystals is revealed by transmission electron microscopy. Through this study, it is hoped that ion implantation fluence, surface exfoliation efficiency and exfoliation cost can be optimized. At the same time, it also lays a foundation for the heterointegration of GaAs film on Si.

Splitting of microbubble mediated by power-law carrier fluid inside a symmetric bifurcating channel

We investigate the dynamics of bubble propagation in a symmetric bifurcating Y-channel by varying the power-law index (n) of the carrier fluid from 0.3 to 1.5, in the presence of gravity. To characterize the bubble evolution, the unsteady two-phase flow is solved numerically, employing a suitable phase-field model. Based on the flow rate ratio between the upper and lower branch channels and the neck-width evolution, the bubble bifurcation process is divided into three distinct stages, namely, squeezing, transition, and pinch-off. Temporal variation of neck-width demonstrates that the bubble pinch-off is somewhat delayed for shear-thickening (n > 1) fluids, while a shear-thinning carrier fluid (n < 1) triggers faster pinch-off. Our study reveals that for a large n (say, n = 1.5), viscous force strongly counters the buoyancy effect, resulting in symmetric (equal) bifurcation of the bubble. Conversely, for shear-thinning fluids, the bubble evolution is dictated primarily by the buoyancy force, leading to an asymmetric bubble breakup. We investigate the role of n on wall shear variation and determine the wall-location that is susceptible to the maximum damage. Performing simulations over wide ranges of capillary numbers (Ca) and Bond numbers (Bo), we unveil important regimes of bubble splitting phenomena, e.g., symmetric breakup, asymmetric breakup, buoyancy dominated no-breakup, and surface tension dominated no-breakup regimes. Numerically predicted regime plots, which comprehensively illustrate the roles of Ca, Bo and, n on various breakup regimes, may act as fundamental design basis of branching networks in classic applications, such as microfluidics, biofluid mechanics, and flow through porous media.

Eulerian–Lagrangian modeling of phase transition for application to cavitation-driven chemical processes

Hydrodynamic cavitation is a promising technology for several applications, like disinfection, sludge treatment, biodiesel production, degradation of organic emerging pollutants as pharmaceutical, and dye degradation. Due to local saturation conditions, cavitating liquid exhibits generation, growth, and subsequent collapse of vapor-filled cavities. The cavities' collapse brings very high pressure and temperature; this last aspect is essential in some chemical processes because it induces the decomposition of water molecules into species with a high oxidation potential, which can react with organic substances. Properly exploiting this process requires a highly accurate prediction of pressure peak values. To this purpose, we implemented a multi-phase Eulerian–Lagrangian code to solve the fluid-dynamic problem, coupled with the Rayleigh–Plesset equation, to capture the evolution of bubbles with the required accuracy. The algorithm was validated against experimental data acquired with optical techniques for different cavitation-shedding mechanisms. Then, we used the developed tool to investigate the decoloration of organic substances from a cavitation Venturi tube operating at different pressure. We compared the obtained results with the experimental observation to assess the reliability of the developed code as a predictive tool for cavitation and the possibility of using the code itself to assess scale-up criteria for possible industrial applications.

The growth and coalescence of helium bubbles in bicrystal copper under tension

Molecular dynamics (MD) simulations are used to study the growth and coalescence of helium (He) bubbles in bicrystal copper (Cu) on an atomistic scale. The influences of He bubble positions, GB's characteristics, and initial temperatures on the coalescence mechanism are clarified. Our results reveal that the He bubbles distributed perpendicular to the loading direction are easier to coalesce than the parallel ones, and GB will accelerate this process due to the high diffusivity of the GB atoms. A spindle-shaped structure is observed during coalescence for the He bubbles in GB, due to the faster atomic diffusion along the GB plane. Three stages of He bubbles evolution are interpreted during tension loading. For He bubbles distributed in the GB plane, the GBs composed of fewer structural units show a reduced ability to promote the growth and coalescence of He bubbles. What's more, the temperature is found to accelerate the coalescence process due to the faster diffusion of surface atoms at higher temperatures.

Bubbling Phenomena in Liquid-Filled Transformers: Background and Assessment

The degradation of the insulation system in liquid-filled power transformers is a serious concern for electric power utilities. The insulation system’s ageing is accelerated by moisture, acids, oxidation products, and other decay particles (soluble and colloidal). The presence of these ageing by-products is detrimental to the insulation system and may further lead to premature ageing and serious consequences. The ageing mechanisms of oil-paper insulation are complex, highly interrelated, and strongly temperature-dependent. The operating temperature of the transformer insulating system has a direct relationship with the loading profile. The major aspect that is witnessed with the fluctuating temperatures is moisture migration and subsequent bubble evolution. In other words, gas bubbles evolve from the release of water vapor from the cellulosic insulation wrapped around the transformer windings. The models presented in the existing standards, such as the IEC Std. 60076-7:2018 and the IEEE Std. C57.91:2011, are mainly based on the insulation temperature, which acts as a key parameter. Several studies have investigated the moisture dynamics and bubbling phenomenon as a function of the water content in the paper and the state of the insulation system. Some studies have reported different prototypes for the estimation of the bubble inception temperatures under selected conditions. However, there are various attributes of the insulation system that are to be considered, especially when expanding the models for the alternative liquids. This paper reviews various evaluation models reported in the literature that help understand the bubbling phenomenon in transformer insulation. The discussions also keep us in the loop on the estimation of bubbling behavior in alternative dielectric liquids and key attributable factors for use in transformers. In addition, useful tutorial elements focusing on the bubbling issue in transformers as well as some critical analyses are addressed for future research on this topic.

Molecular dynamics simulation of bubble nucleation and growth during CO2 Huff-n-Puff process in a CO2-heavy oil system

Foamy oil is the main production mechanism in the CO2 Huff-n-Puff process in heavy oil reservoir. However, the micro-formation mechanism of foamy oil in heavy oil recovery is not yet clarified. It is crucial to gain in-depth knowledge about the evolution process of bubbles and its influencing factors to improve the stability of foamy oil. In this work, molecular dynamics simulation (MD) was used to study the microscopic mechanisms of bubble nucleation and growth, and the effect of the depressurization rate on gas evolution in the CO2-heavy oil system. The microscopic process of bubble evolution during depressurization is as follows: CO2 molecules assemble inside the cavities formed in the oil phase and develop into bubbles, then the bubbles further grow, merge, or disappear. The bubbles evolution is influenced by the binding degree of the surrounding asphaltenes. The macromolecular asphaltene structure contributes to the emergence of cavities in the oil phase for CO2 aggregation. The transfer rate of dissolved gas to gas phase increases with time before forming a continuous phase. The potential energy barrier of bubbles is further analyzed to illuminate the morphological mechanism of bubbles in the oil phase. In addition, the bulk phase properties indicate that higher depressurization rates increase the effective pressure range for bubble oil performance. This work reveals the bubble evolution process and the influence of asphaltene, providing molecular-level insights into gas transport and foamy oil stability.

Single-size and cluster dynamics modeling of intra-granular fission gas bubbles in UO[formula omitted

We perform simulations of intra-granular fission gas bubble evolution in UO2 using both a relatively simple, computationally inexpensive single-size model and a detailed cluster dynamics model. Simulations encompass 36 experimental cases from 4 different databases, covering various temperature and burnup levels. We systematically compare results from the two models to each other and to post-irradiation experimental data of bubble average size and number density. Overall, the model-to-model comparisons reveal an excellent agreement across the set of simulations. This outcome indicates that, in spite of the underlying assumptions, the single-size model provides a good approximation of the complex physical behavior that is more rigorously described by the cluster dynamics model. Qualitatively, both models reproduce the trends of the experimental data with temperature and burnup correctly. Quantitatively, calculated results are either in good agreement with the data or within errors that appear consistent with the inherent uncertainties. Moreover, for the single-size model, we demonstrate and assess a multiscale approach whereby values for the fission gas atom diffusion coefficient from separate atomistic calculations are used. Systematic comparisons to experimental data point out a credible accuracy of the multiscale model.

Directing Exfoliation, Slipping, and Corrugation in WS2 through Bubbling with 15 keV He2+ Ion Irradiation

Herein, effects of normal and 55° oblique angle incidence of 15 keV He2+ ion irradiation on polycrystalline WS2 are reported. Bright‐field high‐resolution transmission electron microscopy reveals exfoliated rectangular sheets, slipped to varying extents while edge angle maintains 90° for oblique angle irradiation (5 × 1015 ions cm−2). At similar fluence, normal ion irradiation gives polyhedral inorganic‐fullerene‐like morphologies occasionally. Moreover, uniformly distributed globular surface construct is quite evident at a critical fluence of 1 × 1016 ions cm−2, captured in semi‐contact mode of atomic force microscopy. Attributed to the formation of blisters and leading to microcracks through nucleation and evolution of bubbles by way of coprecipitation of He2+ ions in the target, these cracks result in corrugated surfaces. To determine the oxidation states of the elements and to examine the purity of phase upon irradiation, X‐ray photoelectron spectroscopy is employed that offers the core‐level spectra of W and S elements of WS2. In addition, vibrational modes are probed through Raman spectroscopy studies. The scope of exfoliated and slipped‐off sheets by ion beams can offer new insights and serve as a complementary window to process and visualize 2D materials for further use.

Dynamic evolution of polyhedral bubbles in Fe9Cr1.5W0.4Si F/M steel under 30keV H2+& He+ synergetic irradiation

Large polygon bubbles can cause significant swelling and embrittlement in irradiated materials. Understanding their formation and growth process is thus important to alleviate the mechanical property degradation of nuclear materials. Here, based on in-situ 30 keV H2+ and He+ synergetic irradiation in Fe9Cr1.5W0.4Si ferritic/martensitic steels, polygonal bubble evolution was investigated at grain boundaries, matrix-precipitate interfaces and matrices. Two threshold sizes were first found for bubble growth in the matrix. One determined the shape between sphere and rectangle, while the other determined whether growth was continuous or reached a saturation size. Large polygonal bubbles formed at interfaces due to abnormal growth rates and coalescence between bubbles. Finally, the bubble growth at sinks and matrices was explained using a bubble growth rate model.

Experiments to understand microlayer and dry patch dynamics under subcooled nucleate flow boiling in a vertically oriented rectangular channel

Nucleate flow boiling offers high heat transfer rates and is considered an effective mode of heat transfer in many systems involving high heat loads. The phenomenon is characterized by the inception of vapor bubble(s) and its growth, followed by its departure in a periodic manner. The evolution of the nucleating bubble's footprint—microlayer and dry patch dynamics—is important in understanding the heat transfer capability and limiting heat flux values. However, efforts toward developing a fundamental understanding of this phenomenon during the nucleate flow boiling regime under subcooled bulk conditions are scarce in the open literature. Toward bridging this gap, we report flow boiling experiments on a hydrophilic surface for investigating the plausible influence of subcooling and minimize the influence of the hydrodynamic movement of contact lines on the dry patch dynamics. Experiments have been conducted in a vertically oriented rectangular channel with water as the working fluid for a Reynolds number of Re = 2400. Real-time microlayer dynamics have been mapped using thin-film interferometry, while the bubble evolution has been captured using one of the gradients-based imaging approaches employed from the side view. Experiments revealed a noticeable influence of subcooling on dry patch and microlayer dynamics. The size of the dry patch and the radial spread of the microlayer showed a decreasing trend with increasing subcooling level. Experimental conclusions are also supported with theoretical considerations.

A Robust Bubble Growth Solution Scheme for Implementation in CFD Analysis of Multiphase Flows

Although the full form of the Rayleigh–Plesset (RP) equation more accurately depicts the bubble behavior in a cavitating flow than its reduced form, it finds much less application than the latter in the computational fluid dynamic (CFD) simulation due to its high stiffness. The traditional variable time-step scheme for the full form RP equation is difficult to be integrated with the CFD program since it requires a tiny time step at the singularity point for convergence and this step size may be incompatible with time marching of conservation equations. This paper presents two stable and efficient numerical solution schemes based on the finite difference method and Euler method so that the full-form RP equation can be better accepted by the CFD program. By employing a truncation bubble radius to approximate the minimum bubble size in the collapse stage, the proposed schemes solve for the bubble radius and wall velocity in an explicit way. The proposed solution schemes are more robust for a wide range of ambient pressure profiles than the traditional schemes and avoid excessive refinement on the time step at the singularity point. Since the proposed solution scheme can calculate the effects of the second-order term, liquid viscosity, and surface tension on the bubble evolution, it provides a more accurate estimation of the wall velocity for the vaporization or condensation rate, which is widely used in the cavitation model in the CFD simulation. The legitimacy of the solution schemes is manifested by the agreement between the results from these schemes and established ones from the literature. The proposed solution schemes are more robust in face of a wide range of ambient pressure profiles.

Study on the behavior of single oxygen bubble regulated by salt concentration in photoelectrochemical water splitting

The increase of electrolyte resistance near the electrode due to bubble evolution is the main obstacle to significantly improve the efficiency of photoelectrochemical water splitting. The oxygen bubble evolution on the TiO2 nanorod photocatalytic electrode was observed in situ by micro high-speed camera, and its corresponding current curve was measured synchronously by electrochemical workstation. The effect of single oxygen bubble evolution on current under different Na2SO4 electrolyte salt concentrations (0.01 M–1.0 M) was studied. When the salt concentration increases from 0.01 M to 0.8 M, the current in the bubble growth stage increases by about 2.5 times, while the bubble period and detachment diameter decrease by about 78.7% and 28.6%, respectively. When the salt concentration further increases, the current and bubble detachment diameter and period remain almost unchanged. The bubble growth coefficient reaches the optimum at 0.5 M, while the gas production rate reaches the optimum at 0.6 M. As the increase of electrolyte salt concentration, the increase rate of photocurrent density is lower than that of mass transfer coefficient, resulting in the decrease of solutal Marangoni force and the smaller bubble detachment diameter. The results show that regulating the concentration of electrolyte is an effective method to remove bubbles on the photocatalytic electrode.

The complex interplay of AGN jet-inflated bubbles and the intracluster medium

ABSTRACTWe use SWIFT, a smoothed particle hydrodynamics code, to simulate the evolution of bubbles inflated by active galactic nuclei (AGNs) jets, as well as their interactions with the ambient intracluster medium (ICM). These jets inflate lobes that turn into bubbles after the jets are turned off (at t = 50 Myr). Almost all of the energy injected into the jets is transferred to the ICM very quickly after they are turned off, with roughly 70 per cent of it in thermal form and the rest in kinetic. At late times (t > 500 Myr) we find the following: (1) the bubbles draw out trailing filaments of low-entropy gas, similar to those recently observed, (2) the action of buoyancy and the uplift of the filaments dominates the energetics of both the bubbles and the ICM, and (3) almost all of the originally injected energy is in the form of gravitational potential energy, with the bubbles containing 15 per cent of it, and the rest contained in the ICM. These findings indicate that feedback proceeds mainly through the displacement of gas to larger radii. We find that the uplift of these filaments permanently changes the thermodynamic properties of the ICM by reducing the central density and increasing the central temperature (within 30 kpc). We propose that jet feedback proceeds not only through the heating of the ICM (which can delay cooling), but also through the uplift-related reduction of the central gas density. The latter also delays cooling, on top of reducing the amount of gas available to cool.