Vapor Bubbles Research Articles

Radius evolution for the synchronous collapse of a circular vapor bubble cluster

The radius evolution for the synchronous collapse motion of a circular vapor bubble cluster in an ideal and infinite fluid is studied. With the assumption that all bubbles are evenly distributed around the circumference and have the same initial conditions, we establish the associated governing equation for this case from a modified Rayleigh–Plesset equation. The inverted analytical solution in the form of t=t(r) and analytical approximations in the form of r=r(t) for the radius evolution to the initial value problem of the dimensionless governing equation are derived. These novel analytical approximations extend the previous ones for a single bubble by adding the effects of surface tension and multiple bubbles. In addition, we point out that the analytical results and dynamic behaviors for multiple bubbles will degenerate into the corresponding ones for single bubbles when the radius of the bubble cluster approaches to infinity.

Wear Behavior of Epoxy Resin Reinforced with Ceramic Nano- and Microparticles.

Cavitation erosion poses a significant challenge in fluid systems like hydraulic turbines and ship propellers due to pulsed pressure from collapsing vapor bubbles. To combat this, various materials and surface engineering methods are employed. In this study, nano and micro scale particles of silicon carbide (SiC) or boron carbide (B4C) were incorporated as reinforcement at 6% and 12% ratios, owing to their exceptional resistance to abrasive wear and high hardness. Microparticles were incorporated to assess the damage incurred during the tests in comparison to nanoparticles. Wear tests were conducted on both bulk samples and coated aluminum sheets with a 1mm of composite. Additionally, cavitation tests were performed on coated aluminum tips until stability of mass loss was achieved. The results indicated a distinct wear behavior between the coatings and the bulk samples. Overall, wear tended to be higher for the coated samples with nanocomposites than bulk, except for the nano-composite material containing 12% SiC and pure resin. With the coatings, higher percentages of nanometric particles correlated with increased wear. The coefficient of friction remained within the range of 0.4 to 0.5 for the coatings. Regarding the accumulated erosion in the cavitation tests for 100 min, it was observed that for all nanocomposite materials, it was lower than in pure resin. Particularly, the composite with 6% B4C was slightly lower than the rest. In addition, the erosion rate was also lower for the composites.

Experimental study of the motion and shape of a vapor bubble rising in an annular channel at subatmospheric pressure

Experimental study of the motion and shape of a vapor bubble rising in an annular channel at subatmospheric pressure

A heat transfer model for liquid film boiling on micro-structured surfaces.

High heat transfer coefficient (HTC) and critical heat flux (CHF) are achieved in liquid film boiling by coupling vibrant vapor bubbles with a capillary liquid film, which has thus received increased interest for thermal management of high-power electronics. Although some experimental progress has been made, a high-fidelity heat transfer model for liquid film boiling is lacking. This work develops a thermal-hydrodynamic model by considering both evaporation atop the wick and nucleate boiling inside the wick to simultaneously predict the HTC and CHF. Nucleate boiling is modeled with microlayer evaporation theory, where a unified scaling factor is defined to characterize the change of microlayer area with heat flux. The scaling factor η is found to be independent of wicking structure and can be determined from a few measurements. This makes our model universal to predict the liquid film boiling heat transfer for various micro-structured surfaces including micropillar, micropowder, and micromesh. This work not only sheds light on understanding fundamental mechanisms of phase-change heat transfer, but also provides a tool for designing micro-structured surfaces in thermal management.

Factors affecting the formation of a cumulative jet after the collapse of a vapor bubble in a subcooled liquid

Factors affecting the formation of a cumulative jet after the collapse of a vapor bubble in a subcooled liquid

The violent collapse of vapor bubbles in cryogenic liquids

Vapor bubbles in cryogenic fluids may collapse violently under subcooled and pressurized conditions. Despite important implications for engineering applications such as cavitation erosion in liquid propellant rocket engines, these intense phenomena are still largely unexplored. In this paper, we systematically investigate the ambient conditions leading to the occurrence of violent collapses in liquid nitrogen and analyze their thermodynamic characteristics. Using Brenner’s time ratio χ, the regime of violent collapse is identified in the ambient pressure–temperature parameter space. Complete numerical simulations further refine the prediction and illustrate two classes of collapses. At 1 < χ < 10, the collapse is impacted by significant thermal effects and attains only moderate wall velocity. Only when χ > 10 does the collapse show more inertial features. A mechanism analysis pinpoints a critical time when the surrounding liquid enters supercritical state. The ultimate collapse intensity is shown to be closely associated with the dynamics at this moment. Our study provides a fresh perspective to the treatment of cavitation in cryogenic fluids. The findings can be instrumental in engineering design to mitigate adverse effects arising from intense cavitational activities.

Explosions of nanodroplets studied with molecular dynamics simulations

Explosions of droplets that are caused by superheating of the liquid phase occur in many combustion processes but are difficult to investigate experimentally. We have studied this process for nanodroplets using non-equilibrium molecular dynamics simulations. Starting from an equilibrium state in which a spherical droplet is surrounded by a vapor phase, a local thermostat is used to impose a high temperature in a small control volume in the droplet center and the following process is studied for varying set temperatures. The fluid is modeled using the Lennard–Jones truncated and shifted potential. Depending on the set temperature, three different system responses were observed: (i) Low set temperatures lead to a shrinking of the droplet due to evaporation that follows the well-known d2 law. (ii) At intermediate set temperatures, a vapor bubble emerges in the droplet center and the liquid phase is formed into spherical shell that expands as the bubble inside of it grows. However, that spherical shell is only temporarily stable and eventually breaks apart. (iii) For high set temperatures, the abrupt and violent formation of the vapor bubble leads to an immediate breakup of the droplet. For case (ii), unexpected phenomena were observed. Oscillations in the diameter of the vapor bubble surrounded by the liquid film occurred. In some simulations, small holes formed temporarily in the liquid shell during its expansion, which closed again over the course of the simulation. Moreover, for one specific set temperature, a transition of the spherical droplet shell into a torus-like object was observed.

Numerical predictions of underwater radiated noise from a non-cavitating model-scale propeller

Propeller-induced acoustic noise from marine vessels is the largest source of anthropogenic underwater radiated noise (URN) and a significant threat to marine ecosystems. Under typical operation, cavitation dominates the URN emissions. Cavitation, which is a pressure-driven phase-change process that results in the violent formation and collapse of vapor bubbles in the wake of the propeller, is often unavoidable during realistic, full-scale operating conditions. However, at a model scale, inducing cavitation requires a depressurized flow facility that makes acoustic measurements difficult due to confinement effects. Numerical simulation is, therefore, appealing as a tool for predicting URN, but the simulation of the cavitation phenomenon and the associated acoustics involves considerable uncertainty and a range of potential sources of error. In propeller operation, cavitation frequently occurs in the core of the vortices shed from the tips of propeller blades. In the present work, we developed a delayed detached-eddy simulation (DDES) of a model-scale ship with the focus on predicting fluctuating pressure due to shed vorticity. The solution is compared to hydrophone measurements from non-cavitating tow-tank experiments. Finally, we numerically introduce cavitation at model scale in the numerical solution and examine its effects.

Acoustic radiation of bubble clusters with different volume fractions

Cavitation noise, mainly from bubble cluster collapse, is a primary source of noise in underwater equipment. This study aims to elucidate the acoustic characteristics of bubble clusters with varying volume fractions (0.002%–43%), thereby illuminating the noise patterns generated during bubble cluster collapse. With the direct numerical simulation (DNS) method, we simulate the collapse of bubble clusters containing 352 vapor bubbles. Far-field sound pressure was calculated by using the Farassat 1 A integral solution of the Ffowcs Williams–Hawkings (FW–H) equation. The results show that when the volume fraction of bubble clusters is greater than 20%, the higher the volume fraction, the closer the far-field sound pressure radiated by its collapse is to that of the equivalent bubble (whose volume is equal to the volume of the bubble cluster), and when the volume fraction is greater than 30%, the sound pressure of the bubble cluster is basically the same as that of the equivalent bubble. Furthermore, when the volume fraction of bubble clusters is less than 1%, the lower the volume fraction is, the closer the sound pressure obtained through the superposition of a single bubble (each with an initial radius equal to that within clusters and in a free field) is to that of bubble clusters, and when the volume fraction is less than 0.1%, the bubble clusters' sound pressures agree with those obtained through superposition. This study reveals the far-field noise pattern generated by the collapse of bubble clusters, providing a theoretical basis for the prediction of cavitation noise.

Effect of gas content on cavitation nuclei

Cavitation inception originates from nuclei in a liquid. This paper proposes a Gibbs free energy approach that provides a smooth transition from homogeneous to heterogeneous nucleation when gas is present. The impact of gas content on nucleation is explored. It is found that the gas content stabilises nuclei, a phenomenon not present in pure liquid–vapour systems. This reduces the energy barrier over that required to nucleate a vapour bubble. Different gas saturation levels are studied. Gas content can significantly reduce the energy barrier required for nucleation, and under certain circumstances eliminate it. An analytic solution for the critical radius and activation energy is obtained that accounts for gas content. The classical Blake radius is recovered as a limiting case. The hysteresis between incipience and desinence is explained using the asymmetry observed in the critical radii. The solution is used to obtain the initial bubble radius, given a critical pressure condition in cavitation susceptibility meter experiments. The relationship between initial bubble diameter and critical pressure is described by an analytic solution that accounts for gas content. A model for the derivative of the cumulative nuclei histogram with respect to bubble diameter is proposed. An analytic expression is obtained that shows good agreement with decades worth of experimental data compiled by Khoo et al. (Exp. Fluids, vol. 61, issue 2, 2020, pp. 1–20) from ocean to water tunnels. The expression recovers the $-4$ power law that is observed experimentally.

Principle and antimicrobial efficacy of laser-activated irrigation: A narrative review.

In the last two decades, the activation of root canal irrigants with pulsed lasers as an adjunct in root canal treatment has become increasingly popular. This narrative review explains the physical basics and the working mechanism of laser-activated irrigation (LAI), explores the parameters influencing LAI efficacy, considers historical evolutions in the field and summarizes laboratory and clinical evidence with emphasis on the antimicrobial action of LAI. Cavitation is the driving force behind LAI, with growing and imploding vapour bubbles around the laser tip causing various secondary phenomena in the irrigant, leading to intense liquid dynamics throughout the underlying root canal. High-speed imaging research has shown that laser wavelength, pulse energy, pulse length and fibre tip geometry are parameters that influence this cavitation process. Nevertheless, this has not resulted in standardized settings for LAI. Consequently, there is significant variability in studies assessing LAI efficacy, complicating the synthesis of results. Laboratory studies in extracted teeth suggest that, with regard to canal disinfection, LAI is superior to conventional irrigation and there is a trend of higher antimicrobial efficacy of LAI compared to ultrasonic activation. Clinical evidence is limited to trials demonstrating similar postoperative pain levels after LAI versus no activation or ultrasonic activation. Clinical evidence concerning the effect of LAI on healing of apical periodontitis as yet is scarce.

Effect of Dissolved Carbon Dioxide on Cavitation in a Circular Orifice

In this work, we investigate the effect of dissolved gas concentration on cavitation inception and cavitation development in a transparent sharp-edged orifice, similar to that previously analyzed by Nurick in the context of liquid injectors. The working liquid is water, and carbon dioxide is employed as a non-condensable dissolved gas. Cavitation inception points are determined for different dissolved gas concentration levels by measuring wall-static pressures just downstream of the orifice contraction and visually observing the onset of a localized (vapor) bubble cloud formation and collapse. Cavitation onset correlates with a plateau in wall-static pressure measurements as a function of a cavitation number. An increase in the amount of dissolved carbon dioxide is found to increase the cavitation number at which the onset of cavitation occurs. The transition from cloud cavitation to extended-sheet or full cavitation along the entire orifice length occurs suddenly and is shifted to higher cavitation numbers with increasing dissolved gas content. Volume flow rate measurements are performed to determine the change in the discharge coefficient with the cavitation number and dissolved gas content for the investigated cases. CFD analyses are carried out based on the cavitation model by Zwart et al. and the model by Yang et al. to account for non-condensable gases. Discharge coefficients obtained from the numerical simulations are in good agreement with experimental values, although they are slightly higher in the cavitating case. The earlier onset of fluid cavitation (i.e., cavitation inception at higher cavitation numbers) with increasing dissolved carbon dioxide content is not predicted using the employed numerical model.

Lift force on single bubbles in subcooled pool boiling systems: Experimental measurements and CFD simulations

Lift coefficient has been evaluated for a subcooled pool boiling system with water as continuous phase and vapor (steam) bubble as dispersed phase generated on vertical walls with constant heat flux for a specific Rayleigh number (Ra) range (8x1012 < Ra < 2x1013) for the first time. Experimental investigation has been done with High Speed Camera while two phase CFD simulations have been performed using Volume of Fluid (VOF) method. The Morton number (M) for this system is log10M = -10.6 and Eötvös number (Eo) range is 0.2 < Eo < 1.3 while the shear rates have been maintained in the range 2 ≤ Sr ≤ 2.8. Bubble size of the bubbles arising from the sites on vertical surface have been determined by both experiments and simulations. The CFD predictions for bubble size and lift coefficients agree well with experimental measurements with a deviation of less than 10%. Further, empirical expressions from experimental data for the estimation of bubble size and lift coefficients have also been presented. The data for lift coefficients obtained is different from the ones of bubbles generated by single sparger in air-water systems.

Modeling of vapor bubble dynamics considering the metastable fluid state

This study investigates the dynamic modeling of a spherical vapor bubble in an infinite liquid. Initially, the liquid is in a state of saturation, but then, there is a change in ambient pressure. Unlike previous studies that assumed vapor saturation, our model fully considers the metastable fluid state of subcooled vapor and superheated liquid. The theoretical model is based on the state equation of vapor and the kinetic theory of gases, allowing the visualization of the effects of superheating and subcooling on the bubble pressure and the phase transition rate. The accuracy of the numerical solutions for the bubble collapse and growth is confirmed by experimental results using water as the working fluid. Two approaches of heat transfer simplification are validated in the numerical solution, and an adhesion coefficient within the range of 0.1–1 is recommended for the calculation. Additionally, this study provides insights into how the metastable fluid state affects the bubble pressure and phase transition, especially in the early stages of bubble collapse. Furthermore, the conservation of vapor mass inside the bubble is also demonstrated.

Spontaneous Boiling of Submerged Jets Generated during the Collapse of Vapor Bubbles

Spontaneous Boiling of Submerged Jets Generated during the Collapse of Vapor Bubbles

The Origin and Differentiation of CO2-Rich Primary Melts in Ocean Island Volcanoes: Integrating 3D X-Ray Tomography with Chemical Microanalysis of Olivine-Hosted Melt Inclusions from Pico (Azores).

ABSTRACT Constraining the initial differentiation of primary mantle melts is vital for understanding magmatic systems as a whole. Chemical compositions of olivine-hosted melt inclusions preserve unique information about the mantle sources, crystallisation behaviour and volatile budgets of such melts. Crucially, melt inclusion CO2 contents can be linked to mantle CO2 budgets and inform us on Earth's carbon fluxes and cycles. However, determining total inclusion CO2 contents is not straightforward, as they often need to be reconstructed from CO2 dissolved in melts and CO2 stored in a vapour bubble. Here, we improve upon existing reconstruction methods by combining 3D X-ray computed tomography (CT) with geochemical microanalyses of major, trace and volatile elements. We show that in comparison to CT data, traditional reconstruction methods using 2D photomicrographs can underestimate CO2 budgets by more than 40%. We applied our improved methods to basaltic olivine-hosted melt inclusions from Pico volcano (Azores) in order constrain the formation and differentiation of volatile-rich primary melts in the context of a mantle plume. Results for these inclusions yielded 1935 to 9275 μg/g reconstructed total CO2, some of the highest values reported for ocean island volcanoes to date. Using these CO2 concentrations, we calculate entrapment pressures of 105 to 754 MPa that indicate a magma reservoir comprising stacked sills straddling the crust–mantle boundary. In the magma reservoir, crystallisation of volatile saturated melts drives extensive degassing, leading to fractionated CO2/Ba ratios of 3.5 to 62.2 and a loss of over 79% of primary mantle-derived CO2. Variabilities in trace elements (La, Y) show that differentiation occurred by concurrent mixing and crystallisation of two endmember melts, respectively depleted and enriched in trace elements. Geochemical models show that enriched endmember melts constitute 33 wt % of all melts supplied to the crust at Pico and that primary melts underwent 60% crystallisation prior to eruption. Mantle melting models indicate that the enriched and depleted primary melt endmembers are low- and high-degree melts of carbon-poor lherzolite and carbon-rich pyroxenite, respectively. Moreover, since deep magmas at Pico island are dominantly pyroxenite derived, their CO2-enrichement is mainly controlled by mantle source carbon content. Overall, our study illustrates that by combining 3D imaging, geochemical microanalyses and numerical modelling, melt inclusions provide a unique record of differentiation and storage of deep magmas, as well as mantle melting.

Highly efficient pool boiling heat transfer on surfaces with zoned rose-petal-inspired hierarchical structures

Efficient boiling heat transfer depends on the rapid formation and detachment of vapor bubbles on solid surfaces. However, this process is hindered by the contradictory requirements of bubble nucleation and departure. Herein, a novel surface with zoned rose-petal-inspired hierarchical structures is prepared on copper by laser-based methods to promote bubble nucleation and detachment simultaneously. The surface features parahydrophobic rose zones surrounded by superwetting pump zones. The rose-petal-inspired hierarchical structures on the rose zones greatly enhance the bubble nucleation, resulting in very high heat transfer coefficients (HTC). The pump zones with hierarchical mini-nanostructures facilitate the formation of distinct pathways for liquid and vapor flows, enabling efficient liquid rewetting and bubble departure at high heat fluxes. The synergistic effects of the rose and pump zones enhance both bubble nucleation and departure, leading to superior boiling performance. The critical heat flux (CHF) and the HTC achieved at the CHF (HTC@CHF) increase by 100% and 360%, respectively, compared to a smooth surface. Our results provide new avenues for the development of metal-based highly efficient boiling surfaces, which may enhance the efficiency of energy utilization and heat dissipation of various thermal management devices.

Thermal and Visualisation Study of the HFE7100 Refrigerant Condensation Process

Abstract Technological advances are contributing to the search for highly efficient energy designs, and increasing interest in compact heat exchangers. Indeed, small channel diameters determine large heat transfer coefficients and condition a significant heat transfer area about the overall volume of the heat exchanger, as well as a smaller amount of refrigerant flowing in the system. Nevertheless, the operating stability and energy efficiency of compact heat exchangers are influenced by two-phase flow structures, which depend on thermal flow parameters. Knowledge of the structures formed during the condensation process is therefore essential for optimising the operation of refrigeration and air-conditioning equipment. This article presents the results from experimental studies of the HFE7100 refrigerant, from the hydrofluorocarbon group, condensation process in mini-channels with hydraulic diameters dh = 2.0 mm, 1.2 mm, 0.8 mm and 0.5 mm. Thermal flow characteristics were determined, and the forming structures of two-phase flow were recorded. The results of visualisation were subjected to morphological image analysis, based on a special algorithm written in MATLAB software. The algorithm makes it possible to determine the void fraction, which is necessary for calculating the vapour quality, as well as the area of vapour bubbles and their number, directionality and length along the x- and y-axes.

Enhanced heat transfer in microgravity from asymmetric sawtooth microstructure with engineered cavities

Nucleate pool boiling in microgravity is influenced by stagnant vapor bubble dynamics, leading to early dryout of the heated surface compared to terrestrial conditions. The Asymmetric Sawtooth and Cavity-Enhanced Nucleation-driven Transport (ASCENT) investigation studied an engineered microstructure in a nucleate pool boiling setup aboard the International Space Station (ISS) from November 2022-January 2023. The experiment was housed in the Microgravity Science Glovebox (MSG) and used degassed FC-72 as the test liquid. The engineered microstructure consisted of repeating mm-sized 60°-30° sawtooth structures located within a hermetically sealed square ampoule as the test chamber. Experimental investigations were conducted to explore a range of input heat fluxes, ranging from 0.5 to 2.3 W/cm². The resulting vapor dynamics and heat transfer were compared against a flat baseline surface. Both surfaces contained 250 µm square cavities spaced 1-mm apart. The constrained dimensions of the square chamber severely influenced vapor bubble dynamics across the heated surface. The presence of a liquid layer beneath vapor slugs on the microstructure was observed, in contrast to the baseline surface, where no liquid layer was visually detected. The transient heat transfer coefficient from the microstructure was higher than the baseline surface as the vapor bubbles grew larger at constant heat flux due to liquid film access provided by the asymmetric ridges on the microstructure. A significant increase in the heat transfer coefficient was observed at 1.8 W/cm2, potentially due to vigorous nucleation in the observed liquid pockets along the microstructure, increasing the heat transfer coefficient from 2900 to 6200 W/m2K. Nucleation was observed at heat fluxes as low as 0.5 W/cm2 from the engineered cavities, and a quasi-steady heat transfer coefficient of 2200 W/m2K was obtained at 2.3 W/cm2 from the microstructure surface. The quasi-steady analysis also indicated that both surfaces performed similarly in microgravity and terrestrial gravity in the same experimental setup. The results demonstrate that the liquid film dynamics underneath vapor slugs influence heat transfer to a large extent in microgravity conditions.

Water can trigger nuclear reaction to produce energy and isotope gases

This paper reports the discovery that water can trigger a peculiar nuclear reaction and produce energy. Cavitation may induce unusual reactions through implosion of water vapor bubbles. Many of this research has been published formally or informally. We have conducted experiments using two reactor types made from multiple-pipe heat exchanger and found that the heat exchange process of water produces peculiar excess heat and abnormally high pressure leading to rupture of the reactor. Recently, we have tested another eight reactors. Interestingly, these reactors produce non-condensable gas. We suspected that they include 22Ne and CO2. We used a mass spectrometer (MS) to analyze 14 gas samples collected from 8 reactors, including ten samples showing a coefficient of performance COPx > 1.05 (with excess heat) and four having COPx < 1.05 (without excess heat). Several methods were adopted to identify the gas content. For CO2 identification, two methods are employed. For 22Ne identification, three methods are employed. All the results confirm that isotope 22Ne and regular CO2 really exist in the output gas from reactors determined to have excess heat. We conjecture a possible mechanism to produce 22Ne and CO2 and find out that 12C and isotope 17O are the intermediate. They finally form isotope gases containing 17O, including H2O-17 (heavy-oxygen water), isotope O2 (16O–17O), and isotope CO2 (12C–16O–17O). In the excess heat producing reactors, all these gasses were detected by MS in the absence of 20Ne and 21Ne. The observed isotope gases produced from reactors having excess heat verifies that water can trigger a peculiar nuclear reaction and produce energy.