Numerical Simulation 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.

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.

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.

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.

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.

Numerical simulation of bubble dynamics in central region of streamer

A mathematical model and a numerical technique for studying strong expansion and collapse of cavitation bubbles located in the central region of a streamer where the bubbles are almost motionless are developed. They are essentially efficient combinations of the models and techniques previously created by the authors for calculating the dynamics of interacting weakly-non-spherical bubbles in a streamer and the dynamics of a single axisymmetric bubble. The first model and technique are applied at the low-speed stage of expansion and compression of bubbles where their hydrodynamic interaction is significant. The second ones are used at the final high-speed stage of their collapse where the interaction is inessential. The simplest case of the streamer comprising three bubbles is considered as an example to illustrate the features of the developed model and numerical technique. It is shown that under the strong expansion and collapse of an initially spherical cavitation bubble, the presence of neighboring bubbles can substantially deflect the bubble cavity vapor dynamics from what is realized inside a similar but single bubble.

From underwater thunder to cavitation from pulsed ultrasound: One graduate student’s perspective of the years leading up to the formation of NCPA

The Physical Acoustics Research Group at the University of Mississippi (PARGUM) fostered a vibrant, multidisciplinary research environment in the early 1980s that incubated the concept of the National Center for Physical Acoustics (NCPA). It also provided graduate students, new to physical acoustics, an enticing glimpse into the breadth of the field and an exposure to a range of important contemporary problems. The research programs included terrestrial lightning detection and location, the underwater sound generated by lightning striking the ocean surface, long range sound propagation, propagation through porous materials, sound amplification by selective molecular excitation, acoustic levitation of bubbles and droplets, studies of acoustic cavitation thresholds and sonoluminescence, numerical simulation of bubble dynamics, and others. Beyond the scientific activities, PARGUM introduced a number of new acousticians to the ASA, many of whom have contributed to the leadership of the Society. The purpose of this presentation is to review some of the research results produced by the members of group from the time period as well as their contributions to the ASA.

Effect of periodic reverse flow on the heat transfer performance of microchannel evaporators

Periodic flow reversal are commonly present in microchannel heat exchangers, but have been studied in air conditioning systems only recently. This paper presents the effect of periodic reverse flow on the heat transfer performance of a heat exchanger. Two heat exchangers with identical geometries in the heat transfer areas are employed and an artificial upstream flow resistance is added for one of them. The heat exchanger without artificial flow resistance is subject to more severe boiling instabilities and consequently generates four times more reverse vapor flow than the other one. The comparison of capacities under identical operating conditions reveals that higher intensity of reverse flow helps to improve cooling capacity by up to 13.3%. Meanwhile, numerical simulations of bubble dynamics coupled with heat transfer are carried out for both heat exchangers. Results show that in the heat exchanger with more reverse flow, the refrigerant side heat transfer coefficients are enhanced, especially in the upstream part of a channel where the flow velocity is relatively low.

Numerical simulation of bubble dynamics in the gravitational and uniform electric fields

ABSTRACTA coupled volume-of-fluid, level set, and smoothed physical parameter (VOF+LS+SPP) method based on FLUENT is used to simulate bubble dynamics in the gravitational and uniform electric fields. Both of the bubble and surrounding medium are assumed to be perfect dielectrics with constant but different permittivities. The effects of electric Bond number, permittivity ratio, Morton number, and Eotvos number on the deformation and rising motion of a single bubble are systematically investigated. Simulation results show that a vertical electric field elongates the bubble along the electric field direction and accelerates the bubble rising. The electric Bond number has a much greater effect on bubble deformation and rising velocity than the permittivity ratio. The bubble behaviors in the electric field are similar for different Morton numbers but totally different for various Eotvos numbers. The influence of electric field on bubble Reynolds number changes a little for different Morton numbers but decreases distinctly with the increase of Eotvos number.

Three-Dimensional Numerical Simulation of Bubble Dynamics in Microgravity under the Influence of Nonuniform Electric Fields

A three-dimensional VOSET method is used along with the adaptive mesh refinement (AMR) method to simulate the behaviors of a bubble departing from the outside wall of a horizontal square-cross-section tube in microgravity under the influence of nonuniform electric fields. The effects of gravity, electric field intensity, fluid permittivity, and bubble initial position on the bubble detachment and rising are investigated and analyzed. Computational results show that the gravity and electric fields have significant influences on the bubble detachment and rising velocity and rising trajectory. Decrease in gravity results in the decrease in the buoyancy exerted on the bubble, considerably mitigating the rising capability of the bubble and delaying the bubble detachment. Imposing a nonuniform electric field, which exhibits physically the strongest intensity in regions near the tube wall, can supply an additional driving force as a replacement of the buoyancy to accelerate the bubble detachment and rising. It is also shown that a larger electric field intensity or larger ratio of liquid permittivity to gas permittivity leads to a larger deformation, easier detachment, and larger rising velocity, of the bubble. The nonuniformity of the electric fields can also affect the bubble motion trajectory and result in the asymmetric deformation of the bubble.

Numerical Simulation of Bubble Dynamics in a Uniform Electric Field by the Adaptive 3D-VOSET Method

This article presents a three-dimensional numerical simulation of the effect of a uniform electric field on the dynamics of bubbles in a viscous fluid. The two-phase interface is captured utilizing a coupled volume-of-fluid and level set (VOSET) method by solving the full Navier–Stokes equations coupled with electric field equations. To track the interface more accurately, the dynamically adaptive octree grids are used to refine the grids around the interface. The effects of different parameters such as the electric Bond number, the ratio of electrical permittivity, the gravitational Bond number, and the Reynolds number on the motion and deformation of the bubble are investigated. According to the computational results, it is found that the electric field has a significant influence on the bubble dynamic behavior. Increase of the electric Bond number or the ratio of electrical permittivity results in the larger deformation and rising velocity of the bubble. For a higher electric Bond number and the Reynolds number, separations of the tail of the bubble are observed. In this case, the jet above the bubble is strong enough to turn the spherical bubble to a toroidal shape.

Numerical simulation of bubble dynamics and heat transfer with transient thermal response of solid wall during pool boiling of FC-72

Using the Ghost Fluid Method for sharp interface representation, bubble dynamics and heat transfer during single bubble pool boiling of saturated FC-72 are simulated numerically with transient thermal response of the heated SiO2 solid wall. A constant and uniform temperature is fixed on to the bottom surface of the solid wall in the simulations, and thus, both the spatio-temporal averaged heat flux and superheat on the top surface, which contacts the working fluid directly, are dependent variables instead of controllable ones. Multi-cycle simulations are carried out to eliminate the influence of unreal initial conditions. Steady periodical processes of single bubble pool boiling can be reached on the wall with a thickness of 1mm, while only quasi-steady ones on the wall with a thickness of 5mm due to the limited simulation time, which results in a smaller thermal penetration depth inside the solid wall compared with its thickness. Comparing with the prediction of the correlation by Zuber for the discrete bubble region and experimental data of single bubble pool boiling, the numerical results of boiling curves in both steady and quasi-steady cases exhibit the same trend. Transient heat conduction inside the solid wall is analyzed in detail. A sharp drop of the wall temperature is evident in the vicinity of the contact line due to violent evaporation in this tiny region. The area of the temperature drop moves with the contact line, resulting in a pseudo-periodical process of heat storage and release inside the solid wall, which exhibits a coupling effect with bubble dynamics and heat transfer. The thermal penetration depth caused by the processes of bubble growth and departure in a single bubble cycle is about 0.5mm in both steady and quasi-steady cases, which is much small than the heater thickness.

Direct numerical simulation of bubble dynamics in subcooled and near-saturated convective nucleate boiling

With the long-term objective of Critical Heat Flux (CHF) prediction, bubble dynamics in convective nucleate boiling flows has been studied using a Direct Numerical Simulation (DNS). A sharp-interface phase change model which was originally developed for pool boiling flows is extended to convective boiling flows. For physical scales smaller than the smallest flow scales (smaller than the grid size), a micro-scale model was used. After a grid dependency study and a parametric study for the contact angle, four cases of simulation were carried out with different wall superheat and degree of subcooling. The flow structures around the growing bubble were investigated together with the accompanying physics. The relation between the heat flux evolution and the bubble growth was studied, along with investigations of bubble diameter and bubble base diameter evolutions across the four cases. As a validation, the evolutions of bubble diameter and bubble base diameter were compared to experimental observations. The bubble departure period and the bubble shapes show good agreement between the experiment and the simulation, although the Reynolds number of the simulation cases is relatively low.

CFD Simulation of Bubble Dynamics in Bubble Columns with Internals

Numerical simulation of bubble dynamics in bubble columns with internals is investigated, and the results show that internal solid walls significantly affect bubble behavior. The trajectories of the selected diameter bubbles tend to be rectilinear, and there are rocking and oscillation for small bubbles which are associated with vortex shedding in the bubble wake. Bubble axially elongates and bubble aspect ratio increases with the increase of internals covered cross-sectional area (CSA). Simultaneously, bubble rise velocity is reduced, and it is related with internals covered CSA and pitch type. Moreover, simulation of bubble dynamics in different diameter internals indicates that hydraulic diameter is the key parameter for scale-up of bubble columns with internals. In addition, bubble breakup and turbulence structure are profoundly impacted by the internal solid walls.

Numerical simulation of bubble dynamics in an elastic vessel

The nonlinear evolution of a gas bubble in the middle of an elastic vessel is investigated numerically. The fluids inside and outside the vessel are assumed to be incompressible and potential. A boundary element method (BEM) is adopted to solve the Laplace equation for the velocity potential. The gas inside the bubble is described by the Boyle Law. The fluid outside the vessel is assumed to contain the elasticity. The dynamic boundary condition on the vessel interface i is obtained from the derived Bernoulli Equation. This numerical model is validated through the comparisons with both the analytic solution of Rayleigh-Plesset equation and spark bubble experiment. The bubble dynamic behavior with different elastic parameters and vessel inner radius is further discussed.