Simulations Of Bubble Dynamics Research Articles

Subgrid moving contact line model for direct numerical simulations of bubble dynamics in pool boiling of pure fluids

This contact line vicinity model is conceived as a subgrid model for the DNS of bubble growth in boiling. The model is based on the hydrodynamic multiscale theory and is suitable for the partial wetting case. On the smallest length scale (distance from the contact line) ∼ 100 nm, the interface slope is controlled by the Voinov angle. It is the static apparent contact angle (ACA) that forms due to evaporation, similarly to previous models neglecting the contact line motion. The calculation of the Voinov angle is performed with the generalized lubrication approximation and includes several nanoscale effects like those of Kelvin and Marangoni, vapor recoil, hydrodynamic slip length and interfacial kinetic resistance. It provides the finite values of the heat flux, pressure and temperature at the contact line. The dynamic ACA is obtained with the Cox-Voinov formula. The microscopic length of the Cox-Voinov formula (Voinov length) is controlled mainly by the hydrodynamic slip. The integral heat flux passing through the contact line vicinity is almost independent of the nanoscale phenomena, with the exception of the interfacial kinetic resistance and is mostly defined by the dynamic ACA. Both the dynamic ACA and the integral heat flux are the main output parameters of the subgrid model, while the local superheating and the microscopic contact angle are the main input parameters. The model is suitable for the grid sizes > 1 µm.

Bubble dynamics properties of B-particles in a quasi-2D gas-solid fluidized bed: Computational particle fluid dynamics numerical simulation and post-processed by digital image analysis technique

Bubble dynamics properties play a crucial and significant role in the design and optimization of gas-solid fluidized beds. In this study, the bubble dynamics properties of four B-particles were investigated in a quasi-two-dimensional (quasi-2D) fluidized bed, including bubble equivalent diameter, bubble size distribution, average bubble density, bubble aspect ratio, bubble hold-up, bed expansion ratio, bubble radial position, and bubble velocity. The studies were performed by computational particle fluid dynamics (CPFD) numerical simulation and post-processed with digital image analysis (DIA) technique, at superficial gas velocities ranging from 2umf to 7umf. The simulated results shown that the CPFD simulation combining with DIA technique post-processing could be used as a reliable method for simulating bubble dynamics properties in quasi-2D gas-solid fluidized beds. However, it seemed not desirable for the simulation of bubble motion near the air distributor at higher superficial gas velocity from the simulated average bubble density distribution. The superficial gas velocity significantly affected the bubble equivalent diameter and evolution, while it had little influence on bubble size distribution and bubble aspect ratio distribution for the same particles. Both time-averaged bubble hold-up and bed expansion ratio increased with the increase of superficial gas velocity. Two core-annular flow structures could be found in the fluidized bed for all cases. The average bubble rising velocity increased with the increasing bubble equivalent diameter. For bubble lateral movement, the smaller bubbles might be more susceptible, and superficial gas velocity had a little influence on the absolute lateral velocity of bubbles. The simulated results presented a valuable and novel approach for studying bubble dynamics properties. The comprehensive understanding of bubble dynamics behaviors in quasi-2D gas-solid fluidized beds would provide support in the design, operation, and optimization of gas-solid fluidized bed reactors.

Three-dimensional numerical simulation of bubble dynamics and design optimization of microchannel in proton exchange membrane water electrolyzers

Proton exchange membrane (PEM) water electrolysis is considered as a promising and sustainable solution for energy storage and hydrogen production. The dynamic behaviors of gas bubble in microchannels have an important impact on the gas/liquid two-phase transport process during electrochemical reaction, consequently affect the performance and efficiency of PEM water electrolyzers. This paper aims to investigate and understand the phenomena of bubble detachment in microchannels during the operation of PEM water electrolyzers. The detachment process is captured in-situ/operando, and the four stages of bubble detachment are qualitatively analyzed. Three-dimensional numerical simulation is conducted to study the dynamics of bubble detachment in a Poiseuille flow of microchannel. A strategy of determine the critical flow velocity, pressure drop, and power consumption of bubble detachment in a microchannel with consideration of main forces acting on a bubble during detaching process is developed. The simulation results reveal that various factors, including the contact angle, microchannel size, and the shape and parameters of the upper wavy wall, significantly influence the behavior of bubble detachment and the associated power consumption. Based on these findings, a design strategy is proposed to enhance bubble detachment efficiency in microchannels. The suggested approach involves the implementation of a rectangular upper wavy wall with appropriate wavelength and amplitude, resulting in the lowest power consumption during the bubble detachment process. The present analysis results provide a better understanding of bubble dynamics behaviors and offer practical insights for optimizing the design of microchannels in PEM water electrolyzers.

Numerical simulation of bubble dynamics and segregation in binary heptane/dodecane mixtures

A mathematical model for the simulation of spherical bubble dynamics in binary alkane mixtures is presented. Detailed heat and mass transfer and phase transition are resolved, and air absorption and desorption are considered. As example mixtures, high-volatile heptane and low-volatile dodecane are investigated. The low-volatile component shows a convective counter-intuitive mass transport opposite to the diffusion flux. A staggered mass flux of heptane and dodecane is associated with a local segregation of mixture components in the surrounding liquid and a distinctive species distribution within the bubble. A comparative study with a pseudo-fluid where alkane species interdiffusion is absent and whose components cannot segregate reveals that for an oscillating bubble, the local segregation hardly affects bubble dynamics, while for a continuously growing bubble in a superheated liquid, the growth rate is considerably affected, particularly for a low heptane mixture percentage. This study demonstrates limitations of treating mixtures by a single-component surrogate fluid and may serve as a starting point for the development of multi-component cavitation models for computational fluid dynamics applications in real fluid mixtures as, e.g. fuels or hydraulic oils.

Production and dispersion of free radicals from transient cavitation Bubbles: An integrated numerical scheme and applications

As an advanced oxidation process with a wide range of applications, sonochemistry relies on acoustic cavitation to induce free radicals for degrading chemical contaminants. The complete process includes two critical steps: the radical production inside the cavitation bubble, and the ensuing dispersion of these radicals into the bulk solution. To grasp the physicochemical details in this process, we developed an integrated numerical scheme with the ability to quantitatively describe the radical production-dispersion behavior. It employs coupled simulations of bubble dynamics, intracavity chemical reactions, and diffusion–reaction-dominated mass transport in aqueous solutions. Applying this method to the typical case of argon and oxygen bubbles, the production mechanism for the main radicals is revealed. Moreover, the temporal-spatial distribution of the radicals in the liquid phase is presented. The results demonstrate that the enhanced radical production observed in oxygen bubbles can be traced to the initiation reaction O2 + H2O → OH+HO2, which requires relatively low activation energy. In the outside liquid region, the dispersion of radicals is limited by robust recombination reactions. The simulated penetration depth of OH is around 0.2 μm and agrees with reported experimental measurements. The proposed numerical approach can be employed to better capture the radical activity and is instrumental in optimizing the engineering application of sonochemistry.

Investigation of bubble dynamics in a micro-channel with obstacles using a conservative phase-field lattice Boltzmann method

Simulating bubble dynamics impacting on obstacles is challenging because of large liquid-to-gas density ratio and complex interface deformation. In this study, a conservative phase-field model, based on a modified Allen–Cahn equation, is employed to accurately capture the bubble interface, and the lattice Boltzmann model is applied to solve the flow field. The bubble rises under the influence of buoyancy force and surface tension force, and complex topology changes, such as rotation, breakup, and squeeze deformation, are predicted in the presence of obstacles. Three dimensionless numbers, including Reynolds, Eötvös, and Morton numbers, are used to characterize bubble dynamics, and two shape indicators, including the revised Blaschke coefficient and the oblateness degree, are introduced to obtain a more systematic assessment of the bubble shape. Effects of flow parameters and obstacle geometries on bubble dynamics impacting on obstacles are investigated to render a quantitative investigation with physical insights. Model extension to the 3D case, the low-viscosity flow and non-pure fluid is further remarked, which can shed light onto future development of physically informed models for predicting the bubble behavior in more real scenarios.

Estimating bubble interfacial heat transfer coefficient in pool boiling

Interfacial heat transfer coefficient (IHTC) is one the most deterministic factors of nucleate boiling characteristics in pool boiling. IHTC and bubble dynamics have a reciprocal effect on each other, which makes accurate estimations of IHTC even more challenging, but important for nucleate boiling studies. In this study, by considering interfacial heat resistance, micro-layer heat transfer and conjugate heat transfer, a comprehensive model that is capable of simulating phase change phenomena and IHTC has been developed and a modified form of Volume of Fluid method (VOF) is used to simulate pool boiling phenomena. In this study, three major challenges in simulating bubble dynamics and related phase change heat transfer using a VOF method have been successfully addressed to achieve more accurate simulations. First, spurious currents resulting from approximating curvature in VOF and interface diffusion have been minimized by introducing curvature smoothing and employing a simplified coupled level set volume of fluid method (S-CLSVOF). Second, simulation instability due to the concentration of source terms on the interface has been addressed by smearing source terms around the interface. Third, micro-layer thickness, which is noticeably smaller than computational cells, has been calculated based on a linear depletable micro-layer method. Effects of three-phase contact line were adopted by introducing a dynamic contact angle model and temperature variation inside the heater. The model shows great agreement with available experimental and numerical results in nucleate boiling of water and R113 in terms of bubble growth and departure rate, frequency and temperature distribution, and micro-layer thickness. The new model developed in this study simulates IHTC in two water cases and its relationship to interfacial relative velocity.

Numerical simulation of bubble dynamics and heat transfer in the 2D saturated pool boiling from a circular surface

In this paper, the saturated pool boiling heat transfer from a heated circular surface is investigated numerically based on the popular lattice Boltzmann phase-change method. The crucial interfacial phenomena, including the bubble growth, departure, coalescence, and rising under different wall superheats, wettabilities, and heater sizes are studied, and the corresponding heat transfer characteristics within these bubble dynamics are also revealed. The numerical experiments indicate that at low wall superheats the bubble nucleation occurs only at the top of the heated circle. With the increase in the wall superheat, the sides and the bottom of the heated circle also become nucleate sites. As the wall superheats continue to increase, more and more nucleate sites are activated so that the heated circle is wrapped by the vapor film early in the boiling process. And the average heat flux varies periodically with time corresponding to bubble dynamics in nucleation boiling regime. It reaches a steady state of film boiling regime after the stable vapor film is generated from the heated circular. It is also found that hydrophobic surface is conducive to the onset of boiling. However, it leads to a low critical heat flux and incurs film boiling at a lower wall superheat, which is also observed in both experimental and numerical studies. On the other hand, the boiling regimes can undergo the transition from nucleate boiling regime to film boiling regime at the same wall superheat with the increase in the heater size. In addition, under various wall superheats and wettabilities, the bubble departure diameters from the circular surface are smaller than those from the flat surface, while the bubble departure periods from the circular surface are less than those from the flat surface one. Finally, the saturated boiling curves from the circular surface for different wall wettabilities, heater sizes, liquid–vapor density ratios, and heating modes are also achieved.

Numerical simulation of bubble dynamics in subcooled flow boiling in a channel

In this article, we report bubble dynamics in a subcooled flow boiling in a vertical boiling channel based on numerical simulation using four-equation drift flux model (DFM4). The focus of this work is to validate the bubble phenomena computed by DFM4 in high inlet subcooling, low heat fluxes and low flow rates. The computed bubble parameters such as bubble growth time tg, bubble waiting time tw, bubble departure diameter Dd based on the available correlations were compared with the experimental results obtained from our boiling loop. We also propose an analytical solution for tg and Dd from the asymptotic growth kinetics and the force balance at the time of bubble departure. We have explained some important insights of the axial variation of bubble phenomena under varying heat flux. Unal correlation for bubble departure works extremely well to a highly subcooled boiling in low heat fluxes and low flows.

Direct numerical simulation of bubble dynamics and heat transfer during nucleate boiling on the micro-pin-finned surfaces

The pool boiling on the micro-pin-finned surfaces was numerically investigated using the curvilinear coupled Volume of Fluid and level set method (VOSET) with phase change. The information interaction problem on the boundary of a complex domain was solved by the partition identification function and unstructured storage. The experiment and simulation of single bubble growth on the micro-pin-finned surface were conducted, whose results agree with each other and validate the accuracy of the numerical method. The bubble characteristics such as bubble shape, temperature distribution, pressure field and micro flow were analyzed and compared with that on the smooth surface, which demonstrates that the vortex near the three-phase contact line plays a significant role in the bubble growth and boiling heat transfer. The effects of wall superheat, gravity level, contact angle and the height of micro-pin-fins were investigated. With the increase of superheat, the average heat flux and bubble diameter increase while the departure time decreases. Under microgravity, the bubble diameter and departure time are larger compared with terrestrial gravity, which deteriorates the heat transfer performance. The fin height basically has no effect on the initial bubble growth. However, there exists a critical fin height leading to the smallest departure diameter and time. Besides, the double bubble coalescence on the micro-pin-finned surface was also investigated. The bubble growth rate reduces after the coalescence. During the coalescence, the surface energy converts into kinetic energy causing the violent deformation of the bubble shape, which promotes the liquid supply and increases the wall heat flux.

Numerical Simulation of Bubble Dynamics in Subcooled Boiling Along Inclined Structured Surface

In this Paper, the lattice Boltzmann method (LBM) is adopted to simulate bubble dynamics and heat transfer characteristics along inclined structured surfaces in subcooled boiling. First, the growth of the bubble in the liquid without consideration of gravity is simulated to validate the selected LBM multiphase model. Then, the current research is concerned with the bubble motion and its thermodynamic behavior on the inclined structured surface. Particularly, the effects of inclination angle and surface geometry are explored. Next, the simulation is performed at the surface with inclination angles of 50, 60, and 70 deg to the horizontal. In general, it is easier for the bubble to escape from the concave as the inclination angle is increased. At the same time, the heat transfer coefficient increases as the inclination angle increases. Moreover, in terms of the semicircle structure, trapezoid structure, rectangular structure, and inverted trapezoid structure, it has been found that the rectangular structured surface is advantageous to the bubble motion and surface heat transfer when the inclination angle is 60 deg. Numerical results demonstrate that the heat transfer efficiency is relevant to both the inclination angle and surface geometry.

Investigations in physical mechanism of ultrasound-assisted antisolvent batch crystallization of lactose monohydrate from aqueous solutions.

Sonication is known to enhance crystallization of lactose from aqueous solutions. This study has attempted to reveal the mechanistic features of antisolvent crystallization of lactose monohydrate from aqueous solutions. Experiments were conducted in three protocols, viz. mechanical stirring, mechanical stirring with sonication and sonication at elevated static pressure. Mechanical stirring provided macroconvection while sonication induced microconvection in the system. Other experimental parameters were initial lactose concentration and rate of antisolvent (ethanol) addition. Kinetic parameters of crystallization were coupled with simulations of bubble dynamics. The growth rate of crystals, rate of nucleation, average size of crystal crop and total lactose yield in different protocols were related to nature of convection in the medium. Macroconvection assisted nucleation but could not give high growth rate. Microconvection comprised of microstreaming due to ultrasound and acoustic (or shock) waves due to transient cavitation. Sonication at atmospheric static pressure enhanced growth rate but reduced nucleation. However, with elimination of cavitation at elevated static pressure, sonication enhanced both nucleation and growth rate resulting in almost complete lactose recovery.

Numerical simulation of bubble dynamics in pool boiling at heated surface

In this work, we investigate numerically the dynamics of bubble growth and departure in pool boiling at surface in low pressures using the asymptotic solution derived from our semi analytical method and considering both macrolayer and microlayer evaporation. Our asymptotic solution for bubble-growth kinetics is capable of computing bubble departure diameter Dd and growth time tg more accurately in higher superheats at atmospheric pressure, in which the existing asymptotic growth-kinetics are not adequate. It does not apply in low pressures, particularly below atmospheric pressure. We therefore propose a new asymptotic solution of growth kinetics for computing bubble departure in low pressures. A steady-state analysis of force balance equation in low pressure yields a power-law variation of Dd with Jakob number and pressure. The bubble departure with and without microlayer evaporation reveal that the method with dynamic microlayer evaporation predicts Dd and tg more accurately. The effect of superheat on the shape of growing bubbles reveals that spherical bubbles form at low superheat and hemi-spherical bubbles at high superheat.

An assessment of multicomponent flow models and interface capturing schemes for spherical bubble dynamics

Numerical simulation of bubble dynamics and cavitation is challenging; even the seemingly simple problem of a collapsing spherical bubble is difficult to compute accurately with a general, three-dimensional, compressible, multicomponent flow solver. Difficulties arise due to both the physical model and the numerical method chosen for its solution. We consider the 5-equation model of Allaire et al. [1] and Massoni et al. [2], the 5-equation model of Kapila et al. [3], and the 6-equation model of Saurel et al. [4] as candidate approaches for spherical bubble dynamics, and both MUSCL and WENO interface-capturing methods are implemented and compared. We demonstrate the inadequacy of the traditional 5-equation model for spherical bubble collapse problems and explain the corresponding advantages of the augmented model of Kapila et al. [3] for representing this phenomenon. Quantitative comparisons between the augmented 5-equation and 6-equation models for three-dimensional bubble collapse problems demonstrate the versatility of the pressure-disequilibrium model. Lastly, the performance of the pressure-disequilibrium model for representing a three-dimensional spherical bubble collapse for different bubble interior/exterior pressure ratios is evaluated for different numerical methods. Pathologies associated with each factor and their origins are identified and discussed.

Simulating bubble dynamics in a buoyant system

SummaryMultiphase flows are critical components of many physical systems; however, numerical models of multiphase flows with large parameter gradients can be challenging. Here, two different numerical methods, volume of fluid (VOF) and smoothed particle hydrodynamics (SPH), are used to model the buoyant rise of isolated gas bubbles through quiescent fluids for a range of Bond and Reynolds numbers. The VOF is an Eulerian grid–based method, whereas the SPH is Lagrangian and mesh free. Each method has unique strengths and weaknesses, and a comparison of the two approaches as applied to multiphase phenomena has not previously been performed. The VOF and SPH simulations are compared, verified, and validated. Results using two‐dimensional VOF and SPH simulations are similar to each other and are able to reproduce numerical benchmarks and experimental results for sufficiently large Morton and Reynolds numbers. It is also shown that at low Reynolds numbers, the two methods, SPH and VOF, diverge in the transient regime of the bubble rise. Regimes that require simulations capable of representing three‐dimensional drag are identified as well as regimes in which results from VOF and SPH diverge.

Evaluation of Cavitation Erosion Intensity in a Microscale Nozzle Using Eulerian–Lagrangian Bubble Dynamic Simulation

In the present study, cavitation erosion is investigated by implementing an Eulerian–Lagrangian approach. Three-dimensional two-phase flow is simulated in a microscale nozzle using Reynolds-averaged Navier–Stokes (RANS) solver along with realizable k−ε turbulence model and Schnerr–Sauer cavitation model. The numerical results are in agreement with experimental observations. A modified form of Rayleigh–Plesset–Keller–Herring equation along with bubble motion equation is utilized to simulate bubble dynamics. Average values of mixture properties over bubble surface are used instead of bubble-center values in order to account for nonuniformities around the bubble. A one-way coupling method is used between Lagrangian analysis and RANS solution. The impact pressure resulted from bubble collapse is calculated for evaluation of erosion in diesel and soy methyl ester (SME) biodiesel in different situations. The results show that the initial size of the bubbles is an important factor for determining the intensity of erosion. So, the bubbles erosive power increases when their initial radius increases. It is also found that the intensity of erosion in diesel is much higher than that of biodiesel and this is because of the differences in fuels properties, especially in viscosity and vapor pressure. The effect of bubbles initial position on erosion intensity is also investigated in this study, and it is found that bubbles with the highest distance from sheet cavity termination have the highest contribution in erosion rate.

Development and application of a high density ratio pseudopotential based two-phase LBM solver to study cavitating bubble dynamics in pressure driven channel flow at low Reynolds number

Abstract A pseudo-potential based single component multiphase Lattice Boltzmann flow solver was developed. Peng–Robinson equation of state was coupled with Exact Difference Method to incorporate interaction forces. This solver was developed for a D2Q9 lattice structure. The solver was thoroughly validated against several canonical problems like estimation of coexistence curve, surface tension estimation from Laplace Law and growth of a cavitating bubble in quiescent flow. The solver was found to be numerically stable for density ratios up to 103. The solver was then used to simulate bubble dynamics for cavitating flows in different flow configurations. Fluid flow characteristics of a single cavitating bubble for Poiseuille flow with stationary and non-stationary walls at Reynolds number of 1 and 10 were investigated. Temporal evolution of the bubble when it is placed along the channel centreline and at an offset is presented here.

Mechanical damage induced by the appearance of rectified bubble growth in a viscoelastic medium during boiling histotripsy exposure

In boiling histotripsy, the presence of a boiling vapour bubble and understanding of its dynamic behaviour are crucially important for the initiation of the tissue fractionation process and for the control of the size of a lesion produced. Whilst many in vivo studies have shown the feasibility of using boiling histotripsy in mechanical fractionation of solid tumours, not much is known about the evolution of a boiling vapour bubble in soft tissue induced by boiling histotripsy. The main objective of this present study is therefore to investigate the formation and dynamic behaviour of a boiling vapour bubble which occurs under boiling histotripsy insonation. Numerical and experimental studies on the bubble dynamics induced in optically transparent tissue-mimicking gel phantoms exposed to the field of a 2.0 MHz High Intensity Focused Ultrasound (HIFU) transducer were performed with a high speed camera. The Gilmore-Zener bubble model coupled with the Khokhlov-Zabolotskaya-Kuznetsov and the Bio-heat Transfer equations was used to simulate bubble dynamics driven by boiling histotripsy waveforms (nonlinear-shocked wave excitation) in a viscoelastic medium as functions of surrounding temperature and of tissue elasticity variations. In vivo animal experiments were also conducted to examine cellular structures around a freshly created lesion in the liver resulting from boiling histotripsy. To the best of our knowledge, this is the first study reporting the numerical and experimental evidence of the appearance of rectified bubble growth in a viscoelastic medium. Accounting for tissue phantom elasticity adds a mechanical constraint on vapour bubble growth, which improves the agreement between the simulation and the experimental results. In addition the numerical calculations showed that the asymmetry in a shockwave and water vapour transport can result in rectified bubble growth which could be responsible for HIFU-induced tissue decellularisation. Strain on liver tissue induced by this radial motion can damage liver tissue while preserving blood vessels.

Numerical Simulation of Bubble Dynamics and Heat Transfer during Pool Boiling in Microgravity

基于标准动量传输方程、连续性方程以及能量方程，建立了单个气泡的二维数值模型，考察了微重力过冷核态池内沸腾中的气泡动力学和传热行为.在动量传输方程中耦合了表面张力和Marangoni力，连续性方程和能量方程中耦合了相变效应，考虑了加热面上过热液体层的影响，并引入相场函数捕捉气液界面的动态变化.结果表明：气泡在生长过程中，形状由最初的半球形变为椭球形，最后变为脱附时的梨形；气泡在加热面上自由迁移，呈非轴对称结构，并且气泡上部的温度场呈现为蘑菇云状；气泡的脱附直径正比于<i>g</i>^-0.488，脱附周期正比于<i>g</i>^-1.113，加热面上的平均热通量正比于<i>g</i>^0.229.

A Comparative Study of Multiphase Lattice Boltzmann Methods for Bubble-Dendrite Interaction during Solidification of Alloys

This paper presents a comparative study between the pseudopotential Shan-Chen model and the phase field multiphase lattice Boltzmann method for simulating bubble dynamics during dendritic solidification of binary alloys. The Shan-Chen method is an efficient lattice Boltzmann multiphase method despite having some limitations, including the generation of large spurious currents. The phase field model solves the Cahn-Hilliard equation in addition to the Navier-Stokes equation to track the interface between phases. The phase field method is more accurate than the Shan-Chen model for simulation of fluids with a high-density ratio since it generates an acceptable small spurious current, though at the expense of higher computational costs. For the simulations in this article, the multiphase lattice Boltzmann model was coupled with the cellular automata and finite difference methods to solve temperature and concentration fields. The simulated results were presented and compared regarding the ability of each model to simulate phenomena at a microscale resolution, such as Marangoni convection, the magnitude of spurious current, and the computational costs. It is shown that although Shan-Chen methods can replicate some qualitative features of bubble-dendrite interaction, the generated spurious current is unacceptably large, particularly for practical values of the density ratio between fluid and gas phases. This occurs even after implementation of several enhancements to the original Shan-Chen method. This serious limitation makes the Shan-Chen models unsuitable to simulate fluid flow phenomena, such as Marangoni convection, because the large spurious currents mask completely the physical flow.