Single Bubble Dynamics Research Articles

Influence of cavity geometry on the bubble dynamics of nucleate pool boiling

Nucleate pool boiling is known for its exceptional heat transfer coefficients, with the use of cavities further improving bubble nucleation and heat transfer rate. To promote this heat transfer enhancement technique, a thorough understanding of the influence of cavity geometry on single bubble dynamics is required. The influence of depth and radius of cylindrical and conical cavities on the bubble dynamics of nucleate pool boiling of R1234yf were numerically investigated. The cavity radius was varied between 50 and 400 μm and the cavity depth between 100 and 1000 μm at a fixed heat flux of 28 kW/m2. It was found that the maximum equivalent diameter prior to departure was constant for cavities with radii smaller than 120 μm, while it increased linearly when increasing the cavity radius further. Cylindrical cavities exhibited high stability regardless of cavity radius or depth whereas conical cavities showed a decrease in vapor retention with increasing cavity angle. During the necking phase, the bubble interface became pinned at the cavity edge, depending on conical cavity angle, implying that smaller radii allowed for enhanced surface rewetting. Conical cavities could be considered as cylindrical cavities when the cavity angle was less than a quarter of the interface contact angle. When translating the single cavity findings to cavity array design, cylindrical cavities were recommended as they allowed for stable bubble behavior. For increased nucleation zones and rewetting, a sub-critical radius was recommended. Wider cavities were recommended for high superheat conditions as larger bubbles could enhance bubble growth.

Critical assessment of the lattice Boltzmann method for cavitation modelling based on single bubble dynamics

The lattice Boltzmann Method (LBM) is recognised as a popular technique for simulating cavitation bubble dynamics due to its simplicity. In the validation of LBM results, the Rayleigh-Plesset (R-P) equation is commonly employed. However, most studies to date have neglected the impact of simulation settings on the predictions. This article sets out to quantify the impact of LBM domain size and bubble size, and the initial conditions of the R-P equations on the predicted bubble dynamics. First, LBM results were validated against the classical benchmarks of Laplace’s law and Maxwell’s area construction. LBM results corresponding to these fundamental test cases were found to be in satisfactory agreement with theory and previous simulations. Secondly, a one-to-one comparison was considered between the predictions of the LBM and the R-P equation. The parameters of the two models were matched based on careful considerations. Findings revealed that a good overlap between the predictions is observable only under certain conditions. The warming-up period of the LBM simulations, small domain size, and small bubble radius were identified as key factors responsible for the measured differences. The authors hope that the results will promote good simulation practices for cavitation simulation including both single bubbles and bubble clusters.

Experimental investigation of the impact of surface tension and artificial nucleation site geometry on the bubble growth

The present work provides an experimental assessment of the impact of liquid surface tension on the single bubble dynamics generated by artificial nucleation cavities during boiling in a quiescent liquid. De-ionized water is used with and without added non-ionic surfactant (Dynol 800) to modify the fluid’s surface tension while keeping the other thermo-physical fluid properties unchanged. Furthermore, two different cavity geometries are investigated: a vertical and a 30°-inclined cone produced via femtosecond laser texturing. This paper shows preliminary results related to the influence of surface tension and cavity geometry on the single bubble dynamics. A methodological baseline is provided to enable future studies aiming to answer still-open questions about vapour bubble nucleation phenomena.

Direct Numerical Simulation of single bubble dynamics in nucleate pool boiling with micro-region modeling and thermal coupling to a solid wall

In this study, we conduct two-dimensional, axisymmetric simulations to investigate the growth and departure of a single bubble, with a particular focus on the conjugate heat transfer between the fluid and the heating wall. A multiscale modeling approach is employed to account for nanoscale effects near the liquid-vapor-solid triple contact line (CL). At the macro (bubble size) scale, the interface dynamics is captured with the front-tracking method by using the open-source code TRUST/TrioCFD. The macro scale algorithm is coupled with a sub-grid micro-region model. It is driven by the wall superheating at the CL as input, and predicts the macroscopic apparent contact angle and heat fluxes. To validate our modeling approach, we first conduct quantitative comparisons using the data on the bubble growth experiment RUBI and its simulations. Next, an investigation of the case of water under atmospheric pressure and gravity is performed. Notably, we observe a significant spatial variation in temperature near the nucleation site during the expansion and contraction of the bubble base. This variation greatly affects the thermal boundary layers in the fluid and solid domains and the bubble growth, demonstrating the need to resolve the conjugate heat transfer in the considered cases.

The effective use of focused shadowgraphy for single bubble nucleate pool boiling investigations

Nucleate pool boiling is known for its high heat transfer coefficients. Despite being widely implemented, the prediction of nucleate pool boiling mechanisms remains complex and single bubble heat transfer analysis is helpful to simplify the problem. Owing to the interesting bubble behaviour during nucleate pool boiling, publications tend to focus on the bubble dynamics while the experimental setup is only briefly discussed. Small but critical information on the experimental setup is often omitted, which makes it challenging or even impossible to reproduce or compare experimental data. Therefore, the purpose of this study is to provide a systematic approach to using focused shadowgraphy for the investigation of single bubble dynamics using R245fa. A pool boiling experimental setup has been built and equipped with temperature, pressure and heat flux sensors, as well as a high-speed camera and light source. The nucleate pool boiling takes place at a single cavity on a copper block. The influences of the heat transfer surface area, light source, and diffuser films were investigated to provide a systematic approach to the use of focused shadowgraphy for the investigation of single bubble dynamics.

On efficient modelling of radical production in cavitation assisted reactors

Process intensification by cavitation is gaining widespread attention due to the benefits that the intense bubble collapse conditions can provide, yet, several knowledge gaps exist in the modelling of such systems. This work studies the numerical prediction of single bubble dynamics and the various approaches that can be employed to estimate the changes in the chemical composition of cavitating bubbles. Specific emphasis is placed on the prediction of the radical production rates during bubble collapse and the computational performance, with the aim of coupling the single bubble dynamics to flow models for reactor hydrodynamics. The results reveal that the choice of chemical reaction approach has virtually no effect on the bubble dynamics, whereas the predicted radical production rates can differ substantially. It is found that evaluating the radical production only on temperature peaks, an approach commonly followed in literature, may result in the most erroneous estimations (on average 12.8 times larger than those of the full kinetic model), while a simplified kinetic model yields more accurate predictions (2.3 times larger) at the expense of increased computational times. Continuous evaluation of the bubble content by assuming equilibrium when the bubble temperature is above a certain threshold (≈1500K) is shown to be capable of predicting total radical production values close to those estimated by solving the kinetics of a detailed reaction model (19.8% difference), as well as requiring only 22.2% more computational costs compared to simulations without chemical reaction modelling. Such an equilibrium approach is therefore recommended for future studies aiming to couple flow simulations with single bubble dynamics to accurately predict radical production rates in cavitation devices, involving numerous bubbles following different flow trajectories. Furthermore, an algebraic expression that successfully approximates the full kinetic simulation results is proposed as a function of the initial nucleus size and the time integral of the liquid pressure when it is under vapor pressure. Such a model can be applied in modelling efforts that do not require local instantaneous radical concentrations, and paves the way for efficient closure modelling of radical production in CFD simulations of hydrodynamic reactors.

Theoretical estimation of sonochemical characteristics in a single cavitation bubble under various static pressure conditions

Cavitation bubbles can be generated by introducing ultrasonic waves into the liquid. The growth and collapse of the bubbles transfer energy from ultrasonic waves to the gas mixture, resulting in the formation of an extreme environment with local high pressure and temperature, and then, multiple products (H2, ·OH, H·, O, H2O2, and so on) are produced. In the present study, the sonochemical characteristics inside an oxygen bubble have been investigated by using single bubble dynamics equations taking mass transfer, heat exchange, and chemical reactions into account. The effects of the equilibrium radius and static pressure on the temperature and the yields of H2, ·OH, and total oxidants inside the bubble are analyzed. There are optimal equilibrium radii that maximize gas temperature and the amounts of H2, ·OH, and total oxidants under different static pressures. The results of this paper are in good agreement with the previous results and can be used to explain sonochemical phenomena observed in experiments.

BIMBAMBUM: A potential flow solver for single cavitation bubble dynamics

In the absence of analytical solutions for the dynamics of non-spherical cavitation bubbles, we have implemented a numerical simulation solver based on the boundary integral method (BIM) that models the behavior of a single bubble near an interface between two fluids. The density ratio between the two media can be adjusted to represent different types of boundaries, such as a rigid boundary or a free surface. The solver allows not only the computation of the dynamics of the bubble and the fluid-fluid interface, but also, in a secondary processing phase, the computation of the surrounding flow field quantities. We present here the detailed implementation of this solver and validate its capabilities using theoretical solutions, experimental observations, and results from other simulation softwares. This solver is called BIMBAMBUM which stands for Boundary Integral Method for Bubble Analysis and Modeling in Bounded and Unbounded Media. Program summaryProgram Title: BIMBAMBUMCPC Library link to program files:https://doi.org/10.17632/89vv35pmhr.1Licensing provisions: GPLv3Programming language: C++ and PythonNature of problem: The code solves the axisymmetric dynamics of single cavitation bubble in the vicinity of different types of boundaries. The boundaries are treated as an interface between two fluids, where the fluids can have different density ratios. The two fluids are considered inviscid and incompressible and the associated flows irrotational so that Laplace's equation is valid everywhere.Solution method: A boundary integral method is used to calculate the velocities on the discretized bubble surface and the fluid-fluid interface. The position of these boundaries can then be updated in time using a Lagrangian approach. In the fluid domain surrounding the bubble, the velocities and pressure are estimated using a combination of the boundary integral method and finite differences.Additional comments including restrictions and unusual features: The code solves the bubble dynamics as long as its surface remains simply connected.

Exploring viscosity influence mechanisms on coating removal: Insights from single cavitation bubble behaviours in low-frequency ultrasonic settings

The role of acoustic cavitation in various surface cleaning disciplines is important. However, the physical mechanisms underlying acoustic cavitation-induced surface cleansing are poorly understood. This is due to the combination of microscopic and ultrashort timescales associated with the dynamics of acoustic cavitation bubbles. Here, we have precisely controlled single-bubble cavitation in both space and time. Ultrasonic excitation leads to the cavitation of generated single bubbles. A synchronous ultrafast photomicrographic system simultaneously records the dynamics of single acoustic cavitation bubbles (SACBs) and the cleaning process of the nearby surface in liquids with varying viscosities. Finally, we analysed the correlation between bubble dynamics and surface cleaning situations. The differences in the typical dynamic characteristics of the bubbles during collapse in liquids with varying viscosities reveal two main mechanisms underlying surface cleaning by acoustic cavitation, which are respective the Laplace pressure during the bubble’s movement and liquid jets during bubble collapse. Our study provides a better physical understanding of the ultrasonic cleaning process based on acoustic cavitation, and will help to optimize and facilitate the applications of surface cleaning, especially for the cleaning of substrates with tightly attached dirt.

Volume of fluid simulation of single argon bubble dynamics in liquid steel under RH vacuum conditions

Volume of fluid simulation of single argon bubble dynamics in liquid steel under RH vacuum conditions

Single cavitation bubble dynamics in a stagnation flow

Jetting of collapsing bubbles is a key aspect in cavitation-driven fluid–solid interactions as it shapes the bubble dynamics and additionally due to its direct interaction with the wall. We study experimentally and numerically the near-wall collapse and jetting of a single bubble seeded into the stagnation flow of a wall jet, i.e. a jet that impinges perpendicular onto a solid wall. High-speed imaging shows rich and rather distinct bubble dynamics for different wall jet flow velocities and bubble-to-wall stand-off distances. The simulations use a volume-of-fluid method that allows us to numerically determine the microscopic and transient pressures and shear stresses on the wall. It is shown that a wall jet at moderate flow velocities of a few metres per second already shapes the bubble ellipsoidally inducing a planar and convergent jet flow. The distinct bubble dynamics allow us to tailor the wall interaction. In particular, the shear stresses can be increased by orders of magnitude without increasing impact pressures the same way. Interestingly, at small seeding stand-offs, the bubble during the final collapse stage can lift off the wall and migrate against the flow direction of the wall jet such that the violent collapse occurs away from the wall.

NUMERICAL MODELING OF BUBBLE DYNAMICS USING INTERFACE CAPTURING METHOD

In the present numerical study, a dynamics of single gas bubble (circular in 2D and spherical in 3D) rising in a stagnant viscous liquid due to the buoyancy is presented using various volume of fluid (VOF) based computational fluid dynamics (CFD) solvers such as commercial Converge and Star CCM+, and open source OpenFOAM<sup>®</sup> platform. To capture the interface dynamics, either an interpolated curved interface based on the high-resolution interface framework or a mass conservative VOF approach with a planar sharp interface based geometric reconstruction of the piecewise-linear interface calculation (PLIC) scheme was used. Both qualitative and quantitative analysis of air bubble rising upward inside the quiescent water column at ratios of low density, ρ<sub>r</sub> = 10 and high density, ρ<sub>r</sub> = 1000 are simulated and evaluated similar to report by Hysing et al. The proposed numerical models can simulate a wide range of density and viscosity ratios. In this study, a robustness and accuracy of the solvers are evaluated and comparative study between open source OpenFOAM<sup>®</sup> solver with commercial solvers such as Converge and Star CCM+. Based on the present numerical results, the gas bubble base undergoes severe deformations for the high density ratio, ρ<sub>r</sub> = 1000 and high viscosity ratio, μ<sub>r</sub> = 100 compared to low density ratio, ρ<sub>r</sub> = 10 and low viscosity ratio, μ<sub>r</sub> = 10. Any of the solvers can be used to simulate complex multiphase flow situations encountered in many industrial applications.

Mechanisms underlying the influence of skin properties on a single cavitation bubble in low-frequency sonophoresis

As a safe and effective method for systemic transdermal drug delivery (TDD), sonophoresis has drawn much attention from researchers. Despite numerous studies confirming cavitation as the main reason for sonophoresis, the effect skin has on cavitation bubble dynamics remains elusive due to the difficulty of experimental challenges. For a start, we reveal how single cavitation bubble (SCB) dynamics are affected by skin properties, including elasticity, hydrophilicity and texture. We use polydimethylsiloxane (PDMS) to simulate human skin and record the temporary evolution of SCBs with synchronous ultrafast photography. The influences of skin properties on SCBs are concluded: 1) SCBs collapse later near walls with better elasticities and generate microjets with higher speed; 2) SCBs collapse later near hydrophilic walls with slower microjets; and 3) the existence of a texture structure on walls also delays the time of bubble collapse near them and slows the velocities of microjets (v) during collapses.

The relevance of bubble dynamics in ultrasonic/sonochemical processes

Acoustic cavitation bubble dynamics has been extensively studied by physicists and mathematicians. Process intensification is primarily studied by chemical engineers. Chemists tend to focus on bubble dynamics in a multibubble field to optimize the physical and chemical forces generated during acoustic cavitation with an intention to maximise/intensify chemical processes. To achieve process intensification successfully a multidisciplinary approach is required. Our early work on multibubble cavitation unveiled various factors that contribute to process optimization. For example, while single bubble dynamics can estimate the maximum bubble temperatures, they may not be relevant to achieving higher chemical yield, which in turn depends on varus other factors such as average bubble temperatures, number of active cavitation bubbles, etc. Other factors critical for process intensification include the generation of a homogeneous cavitation field, control over mass transfer effects caused by bubble oscillations, reactor design, etc. The presentation will provide an overview on the relevance of single and multibubble bubble dynamics in ultrasonic/sonochemical processes.

The impacts of material acoustic impedance and thickness on single laser-induced bubble dynamics and determining factors in resulting pressure

The objective of this paper is to experimentally identify the primary sources of pressure when a laser-induced cavitation bubble is collapsing to a wall with specific emphases on the material acoustic impedance and thickness. Both high-speed videos and local wall pressure measurements were performed for various standoff ratios γ, bubble diameters, and wall materials. In the case of a rigid wall, in addition to the known high pressure for γ<0.6 where the bubble attaches and collapses on the wall (ring collapse), at γ≈1.12 where the jet is dominant, and low pressure obtained at γ≈0.913, where neither effect is significant, we further captured similar pressure profiles during the collapse after the first rebound at γ≈1.16 for the ring collapse, γ≈1.79 for the jet, and γ≈1.41 for the minimal, respectively. This indicates a strong jet is typically followed by a strong ring collapse. Generally, the pressure from the second collapse increases faster with the bubble size than that of the first collapse. For walls featuring smaller acoustic impedance or thickness, which cannot be approximated as rigid bodies or accessed by pressure sensor, our unique bubble edge analyzing tool shows that the ring collapse and jet effects are moved to smaller values of γ. The maximum pressure exerted on the wall in these cases is smaller than that on the rigid wall. Finally, we summarized the asymptotic evolution curves of each edge which bound the bubble dynamics at different standoff ratios.

Computational modelling and investigation of nucleate boiling with periodic exponential heat flux-based power-transients

The present work proposes a numerical methodology to perform single bubble nucleate boiling simulation subjected to power transients with exponential heating. Cyclic variation in transient heat flux is utilized which enables—proper characterization of the power transient parameters, parametric investigation of power transients on single bubble dynamics, and comparison between steady and transient heating condition. For constant time averaged heat flux in power cycle, it was observed that heating parameters are not having any influence on bubble departure diameter and bubble growth period whereas the bubble departure frequency is enhanced until vertical coalescence is observed. Parametric investigations based on time period of periodic heat flux and time averaged heat flux reveal the existence of a range in excursion time period which cause increase in bubble departure frequency, with applicability for controlling bubble dynamics with power transients and improving efficiency of thermal management devices using nucleate boiling.

Collapsing and Splashing Dynamics of Single Laser-Induced Cavitation Bubbles within Droplets

In the present paper, the cavitation bubble dynamics model for a single bubble oscillating within a droplet is improved based on the classical Rayleigh–Plesset bubble dynamics equation and the effects of liquid surface tension and viscosity are both considered. In the aspect of the experiment, the collapsing dynamic process of a bubble within a droplet is carried out by building a high-speed photography experimental platform. In addition, the numerical solution of the dynamic equation for the collapse time of the bubble within the droplet is also carried out. The findings are given as follows: (1) The bubble dynamic equation considering liquid surface tension and viscosity of bubble within droplet is proposed. (2) The surface of liquid droplets induced by the bubble motion could be divided into three modes: no splashing, scattered splashing, composite splash consisting of scattered and flaky splash. (3) The bubble interface during the first collapsing stage could be divided into three types: spherical, conical, and fungiform. (4) The numerical solution shows an accurate prediction of the bubble collapse time within the droplet especially under the condition of medium radius ratio.

Analyses of bubble dynamics in subcooled boiling flow using Euler-Lagrange method

Subcooled boiling flow is widely used for heat and mass transfer in industries such as nuclear reactor, electronic equipment, and spacecraft, where the bubble dynamics is essential for understanding the gas-liquid flow behavior. Current studies focused on the gas void fraction or bubble size distribution at a global scale, but the dynamics of single bubbles are still obscure, which inhibits the fundamental understanding of gas-liquid flow and the precise prediction of heat transfer. Here, we develop a numerical method that captures the motion and condensation of bubbles in subcooled boiling flow using Euler-Lagrange method, with a proposed model for spatiotemporal lift-off bubble distribution on the heated wall as boundary condition. Our numerical method is validated by the agreement between the calculated gas void fraction distributions with the existing experimental data in vertical subcooled boiling flow. We further investigate the distribution of bubble position and size in the tube and find that the bubble size distribution shows different modes at different radial positions. Analysis of the single bubble dynamics shows that small and large bubbles suffer from centrifugal and centripetal lift forces, and thus tend to accumulate at a radial position near the wall and near the tube center, respectively, which reveals the mechanism of bubble size distribution. Besides, we find that the centripetal wall lubrication force is essential for the path instability of small bubbles near the wall. These findings promote the fundamental understanding of the gas-liquid phase distribution in the subcooled boiling flow and provide a numerical strategy for the precise prediction of mass and heat transfer in subcooled boiling flow.

Experimental investigation to quantify the influence of single artificial indentations on vapour bubble dynamics and the associated heat transfer rates

Dependence of single vapour bubble dynamics and the associated heat transfer phenomena on the type of artificial indentations created on the heated substrate has been studied. Experiments under nucleate boiling regime have been carried out for conical indentations of varying cone angles (2γ = 30, 60 and 90o) and cylindrical cavity keeping the depth of features constant (hc = 500 μm). Rainbow schlieren deflectometry has been employed for simultaneous mapping of bubble dynamics and thermal field in a completely non-intrusive and real time manner. Direct comparison of the rainbow schlieren images shows a relatively large partition of thermal energy through natural convection for conical indentation of small cone angle and cylindrical cavity. For large-angled conical indentation, increased availability of thermal energy for bubble growth process results in the formation of relatively larger-sized vapour bubble at a relatively slower rate. In addition, due to the relative differences in the amount of vapour entrapped inside the cavities between any two bubbling cycles, the consistency of bubbling features was found to be much higher for larger cone angled cavity. The evidence for increased partition of thermal energy transfer through natural convection has been seen in the form of an increase in the portion of superheat layer in the vicinity of vapour bubble for conical cavity with small cone angles from the whole-field temperature reconstructions. For ΔTsup = 10 K, the heat transfer performance of surfaces with conical cavity with large cone angle (2γ = 90o) is significantly much better than the other conical and cylindrical cavities.

Mass transfer dynamics of single CO2 bubbles rising in monoethanolamine solutions: Experimental study and mathematical model

Carbon dioxide (CO2) absorption into amine-based absorbents is a widely used method for carbon emission reduction to deal with global warming problems. The intensification of the absorption process relies on an in-depth understanding of the mass transfer mechanisms between CO2 gas and the reaction media. However, the influence of products of a chemical reaction on mass transfer dynamics is usually underestimated. In this study, we perform research studies on the absorption process of single CO2 bubbles rising in monoethanolamine (MEA) solutions. Firstly, we perform an experimental study to visualize and measure the spatial position and the diameter of the bubbles using the high-speed camera. We observe a special phenomenon that the CO2 bubble diameter rapidly decreases to a nearly constant value in MEA solutions, and the terminal bubble diameter increases with the MEA concentration. Based on the phenomenon, we divide the absorption process into two stages. One is the mass transfer enhancement stage, where the bubble diameter decreases rapidly. The other is the mass transfer deterioration stage, where the bubble diameter remains almost constant. Furthermore, we propose an elaborate mass transfer dynamics model considering the combined effect of chemical reaction kinetics and reaction-induced bubble surface contamination. Finally, a well-predictive performance by the mass transfer dynamics model is validated by experiment. The findings of this study are of theoretical and practical significance for the mass transfer in gas–liquid reactive flow and multiphase reactor design.