Vapor Bubbles Research Articles

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.

Viscoelastic vapor bubble collapse near solid walls and corresponding shock wave formation

This study investigates the influence of viscoelasticity on the collapse of aspherical vapor bubbles near a solid boundary through numerical simulations. A fully compressible three-dimensional finite volume method is employed, incorporating a single-fluid homogeneous mixture cavitation model and the simplified linear Phan-Thien Tanner viscoelastic constitutive model. The collapse dynamics, liquid jetting, shock wave formation, and associated pressure impact are analyzed, and the viscous and viscoelastic stress fields are presented. A comparison of viscoelastic to Newtonian dynamics reveals significant differences in collapse behavior and shock wave formation due to viscoelasticity. Viscoelasticity can induce jet piercing, which is not observed in the Newtonian collapse, and increases vapor re-evaporation after the first collapse. The effect of changing the initial standoff distance is examined for both viscoelastic and Newtonian fluids, where a second jet formation is present only for the viscoelastic collapse, and the second collapse's intensity is increased due to increased vapor production during rebound. Additionally, the variation of elasticity in the viscoelastic case demonstrates a correlation between the amount of vapor produced during rebound and the relaxation time for the investigated cases.

Laser beam properties and microfluidic confinement control thermocavitation

Thermocavitation, the creation of a vapor bubble by heating a liquid with a continuous-wave laser, has been studied for a wide range of applications. Examples include the development of an actuator for needle-free jet injectors, as the pumping mechanism in microfluidic channels and nanoparticle synthesis. Optimal use in these applications requires control over the bubble dynamics through the laser power and beam radius. However, the influence of the laser beam radius on the bubble characteristics is not fully understood. Here, we present a way to control the beam radius from an optical fiber by changing the distance from the glass–liquid interface. We show that the increase in the beam size results in a longer nucleation time. Numerical simulations of the experiment show that the maximum temperature at nucleation is 237 ± 5 °C and independent of laser parameters. Delayed nucleation for larger beam sizes results in more absorbed energy by the liquid at the nucleation instant. Consequently, a larger beam size results in a faster growing bubble, producing the same effect as reducing the laser power. We conclude that the bubble energy only depends on the amount of absorbed optical energy and it is independent of the beam radius and laser power for any amount of absorbed energy. This effect contrasts with pulsed lasers, where an increase in the beam radius results in a reduction of bubble energy. Our results are of relevance for the use of continuous-wave laser-actuated cavitation in needle-free jet injectors as well as other applications of thermocavitation in microfluidic confinement.

NUMERICAL STUDY OF THE EFFECT OF TUBE WALL ON SUBCOOLED BOILING AT THE END OF A LASER WAVEGUIDE

Results of numerical simulations of the laser-induced boiling at the end of a waveguide placed inside the tube are presented. The effect of the tube wall on the vapor bubble evolution and characteristics of the cumulative jet forming as a result of its collapse was studied. Particularly, it was found that while the tubes of large radii insignificantly affect the velocity of the cumulative liquid jet, in the relatively narrow tubes the jet may not form at all. The effect of the tubes of moderate radii comes down to the decrease of the jet velocity compared to the case without the tube. A possible physical explanation of such influence of tube walls is proposed. Numerical results on laser-induced boiling inside the tubes are summarized in the regime diagram in the tube radius-waveguide radius plane.

Constrained Vapor Bubble Experiment (CVB) in the Light Microscopy Module (LMM)

Abstract This short article describes the major findings from the CVB experiment performed in the LMM on the International Space Station from 2010–2012. CVB was the first experiment to run in the new facility and focused on understanding the heat transfer and fluid mechanics occurring inside a wickless miniature heat pipe. The LMM was used to map the location of the vapor-liquid interface inside the device and to measure the film thickness profile on the walls of the device. Several interesting and unexpected phenomena were observed in microgravity including flooding of the heater end with liquid as the heat input increased, explosive nucleation of vapor bubbles at the heater end in the shortest version of the heat pipe tested, condensation on highly superheated surfaces, and the spontaneous formation of rip currents as the device tried to enhance the contact line area available for evaporation of the liquid.

Durability of the bubble-jet sorter enables high performance bio sample isolation.

Sorting cells while maintaining their viability for further processing or analysis is an essential step in a variety of biological processes ranging from early diagnostics to cell therapy. Sorting techniques such as fluorescence-activated cell sorting (FACS) have evolved considerably and provide standard ways of sorting. Nevertheless, the search for compact, integrated, efficient, and high throughput microfluidic sorting platforms continues due to challenges such as cost, cell viability, and biosafety. In our previous work, we introduced a technology with the potential to become such a platform: the bubble-jet sorter. It is a silicon-based sorter chip relying on cell deflection through micro vapor bubble formation. In this work, we present a new version of the sorter chip that emphasizes durability and continuous sorting operation. To characterize the sorter, we first focus on the technical performance and show a sorter lifetime that repeatedly exceeds 80 million actuation cycles. In addition, we show continuous operation at high firing rates, but also discuss limitations due to heat buildup. In a second step, we present continuous sorting runs of millions of beads and CD3 positive T cells at rates surpassing 1000 sorting events per second, while maintaining high purity (>90%) and recovery (>85%). Dedicated viability tests show that the gentle sorting process maintains cell viability in this closed, aerosol-free device. The remarkable combination of high lifetime, sorting rate, and sorting efficiency, along with the potential for on-chip parallelization show the promise of this technology to meet the growing demand for large-scale sample isolation in drug and immunotherapy development.

Effects of cover plate wettability on pool boiling heat transfer in a narrow gap

An experimental study is conducted to evaluate the effects of cover plate wettability (hydrophobic to hydrophilic) on boiling heat transfer in a narrow horizontal gap (0.64mm to 5mm thick). When the gap thickness is greater than 1mm, the cover plate wettability plays a significant role. A hydrophobic cover plate causes vapor bubbles to move from the heating surface and get attached to the cover plate, maintaining liquid water on the heating surface. When the vapor layer does not reach the heating surface, the nucleate boiling heat transfer coefficient is hardly affected by the gap. However, the critical heat flux (CHF) is reduced relative to the unconfined case as vapor fills the narrow gap. Contrarily, when a hydrophilic cover plate is used with a gap thickness greater than 1mm, a vapor slug forms and hovers over the heating surface with transient lateral movement. Thin liquid film evaporation occurs under the slug, causing an enhanced heat transfer coefficient at low heat flux (q'' < 250kW/m2). As the heat flux increases, the vapor slug movement causes fluctuations in the heater temperature due to partial dryout at the heating surface. Consequently, a rapid decrease in the heat transfer and a reduced CHF are observed relative to the unconfined case. When the gap thickness is reduced to below 1mm, a film boiling-like behavior is observed regardless of the cover plate wettability. However, there is an increase in the heat transfer coefficient at low heat flux (q'' < 250kW/m2).

The effect of three-type Vortex Generators (VGs) on NACA-0015 hydrofoil cavitation formation at different angles of attack

Cavitation, the formation of vapor bubbles on hydrofoil surfaces, poses a significant challenge to the performance and durability of marine vessels utilizing hydrofoils. Vortex generators, small structures embedded on the hydrofoil surface, have emerged as a potential solution for mitigating cavitation, but the optimal shape of vortex generators for achieving maximum cavitation resistance remains an open question. This study aims to address this challenge by systematically investigating the impact of rectangular, triangular, and circular vortex generators on the cavitation characteristics of a NACA-0015 hydrofoil. Utilizing computational fluid dynamics (CFD) simulations, the influence of these vortex generator shapes on cavitation inception and critical cavitation numbers will be evaluated under varying angles of attack and flow velocities. The findings of this study will provide valuable insights into the design and optimization of vortex generators for hydrofoil applications. Despite the effectiveness of vortex generators in mitigating cavitation, the optimal vortex generator shape for specific hydrofoil designs and operating conditions remains unclear. The precise mechanisms by which vortex generators suppress cavitation are not fully understood, requiring further investigation to optimize their design and placement. The durability and long-term performance of vortex generators under various environmental conditions, including turbulence and corrosion, warrant further evaluation. By addressing these unsolved challenges, this study will contribute to the advancement of vortex generator technology and its practical application in hydrofoil systems, leading to more efficient and reliable marine vessels.

The extended Discontinuous Galerkin method for two-phase flows with evaporation

In this work, we present an extended Discontinuous Galerkin method for simulating transient, incompressible two-phase flows, which include heat transfer and thermally driven evaporation at the interface of single-component systems. This expands our previous work to include the consideration of non-material interfaces and a coupling between velocities and temperature gradients at the interface. The phase boundary is represented by the zero-set of a level set function, while effects due to surface tension are treated by the Laplace-Beltrami formulation. This sharp interface model allows for a sub-grid accurate representation of the solution fields. By using compactly supported polynomial solutions, discontinuities at the interface can be sharply represented without employing additional reconstruction schemes. The approach is validated through well-known evaporation test cases. This includes two 1D test cases, known as Stefan and Sucking problem, a 2D film boiling and finally the 3D growth of a vapor bubble, known as Scriven test case.