Lattice Boltzmann simulation to study multiple bubble dynamics

Lattice Boltzmann method (LBM) has been used in this study to understand the behavior of bubble motion and bubble coalescence in liquids. Highly isotropic gradient vectors have been obtained on a lattice for two-phase simulations using LBM. For a fully periodic domain, bubble dynamics and shape for a single bubble and multiple bubbles are dependent on Eotvos number, Reynolds number and Morton number. For single bubble simulations, computations were done for high Eotvos and low to moderate Reynolds numbers, and the results are matched with the experimentally quantified flow visualization chart. The drag coefficient for single bubble motion under buoyancy for both two- and three-dimensional simulations compares well with existing correlations. For multiple bubbles, the bubble dynamics is dictated by the vortex pattern of the leading bubble, which allows the bubbles to coalesce. Coalescence can be described as a three stage process: collision; drainage of the liquid film between adjacent bubbles to a critical thickness; and rupture of this thin film of liquid. Such simulations have also been run for different configurations of the initial bubble distribution for both in-line and staggered bubble configuration to show the effect of vortex shedding on the oscillatory motion of the bubbles and subsequent coalescence.     

Dynamic characteristics of transient boiling on a square platinum microheater under millisecond pulsed heating

A series of experimental investigations of boiling incipience and bubble dynamics of water under pulsed heating conditions for various pulse durations ranging from 1 ms to 100 ms were conducted. Using a very smooth square platinum microheater, 100 μm on a side, and a high-speed digital camera, the boiling incipience was observed and investigated as a function of the bulk temperature of the microheater, pulse power level, and pulse duration. Given a specific pulse duration, for low pulse power levels, there would be no bubble nucleation or bubble mergence, for moderate pulse power levels, individual bubbles generated on the heater merged to form a single large bubble, while for high pulse power levels, the rapid growth of the individual bubbles and subsequent bubble interaction, resulted in a reduction in bubble coalescence into a single larger bubble, referred to as bubble splash. The transient heat flux range at which bubble coalescence occurs was identified experimentally, along with the temporal variations of bubble size, bubble interface velocity and interface acceleration.     

Bubble generation in quiescent and co-flowing liquids

A numerical simulation has been accomplished to analyze the problem of dynamic bubble formation from a submerged orifice in an immiscible Newtonian liquid under the condition of constant gas inflow. We have considered two cases for the surrounding liquid, namely the liquid in a quiescent condition and the liquid as a co-flowing stream with the gas. The full cycle, from formation to detachment of the bubbles and the corresponding bubble dynamics, was simulated numerically by using a coupled level-set and volume-of-fluid (CLSVOF) method. The role of the liquid to gas mean velocity ratio, the Bond number and the Weber number in the bubble formation process was studied and the order of magnitude of forces involved in bubble dynamics are presented. Our simulation results show that the minimum radius of the neck decreases with a power law behavior and the power law exponent in a co-flowing liquid is less than 1/2 as predicted by the Rayleigh–Plesset theory for quiescent inviscid liquids. Single periodic and double periodic bubbling (with pairing and coalescence) regimes are observed in the present investigation. It is identified that a moderate co-flowing liquid may inhibit the bubble coalescence. The volume of the bubble and the bubble formation time decrease with increasing liquid to gas mean velocity ratios. For small Bond numbers, significant differences pertaining to bubble dynamics are observed between the co-flowing liquid and the quiescent liquid. Furthermore, the generation and breakup of the Worthington jet after bubble pinch-off and formation of tiny drops inside the detached bubbles are observed.     

Bubble growth and horizontal coalescence in saturated pool boiling on a titanium foil,investigated by high-speed IR thermography

Growth of an isolated bubble and horizontal coalescence events between bubbles of dissimilar size were examined during pool nucleate boiling of water on a horizontal, electrically-heated titanium foil 25 μm thick. Wall temperature measurements on the back of the foil by high-speed IR camera, synchronized with high-speed video camera recordings of the bubble motion, improved the temporal and spatial resolution of previous observations by high-speed liquid crystal thermography to 1 ms and 40 μm, respectively, leading to better detailed maps of the transient distributions of wall heat flux. The observations revealed complex behaviour that disagreed with some other observations and current modelling assumptions for the mechanisms of heat transfer over the wall contact areas of bubbles and interactions between bubbles. Heat transfer occurred from the entire contact area and was not confined to a narrow peripheral triple-contact zone. There was evidence of an asymmetrical interaction between bubbles before coalescence. It was hypothesised that a fast-growing bubble pushed superheated liquid under a slow-growing bubble. Contact of this liquid with regions of the wall that had been pre-cooled during bubble growth caused local reductions in the wall heat flux. During coalescence, movement of liquid under both bubbles caused further changes in the wall heat flux that also depended on pre-cooling. Contraction of the contact area caused a peripheral reduction in the heat flux and there was no evidence of a large increase in heat flux during detachment. Boiling on very thin foils imposes special conditions. Sensitivity to the thermal history of the wall must be taken into account when applying the observations and hypotheses to other conditions.     

高剪切力下气泡聚并与破碎特性的数值研究

了解气泡在剪切力场中的聚并与破碎机理及生长与运动特性,对于气液搅拌槽中桨叶的优化设计具有重要意义。将欧拉-欧拉模型与群体平衡模型(population balance model, PBM)进行耦合,对不同剪切力下的气液两相流场进行求解,研究了剪切力、高剪切力下进气速度、气泡塔高度对气泡聚并与破碎的影响,并结合气泡的聚并与破碎模型对高剪切力下气泡的聚并与破碎机理进行了探究。研究表明,剪切力主要影响气泡的破碎,当剪切力较小时其对气泡破碎的影响较小,随着剪切力的增大其对气泡破碎的影响逐渐显著,使小气泡的含量大幅增多;高剪切力下进气速度、气泡塔高度对气泡的聚并与破碎的影响不明显。     

Internal flow structure description of slug flow-pattern in a horizontal pipe

Utility of the hot-film anemometry technique in describing the internal flow structure of a horizontal slug flow-pattern is discussed within the scope of intermittent nature of slug flow. It is shown that a single probe can be used for identifying the gas and liquid phases and for differentiating the large elongated bubble group from the small bubbles present in the liquid slug. Analyzing the nature of voltage signals, a signal processing scheme is developed for measurements of time-averaged void fractions of small and large bubbles as well as for the measurements of local mean axial velocity and turbulent intensity in the liquid phase. Some results of local measurements of time-averaged void fractions of small and large bubble groups, axial mean velocity and turbulent intensity are presented at relatively low and high gas and liquid flows for a horizontal slug flow-pattern in a 50.3 mm i.d. pipe.     

Hydrodynamic aspects of interaction between nucleation sites

The dynamics of twin and triple nucleation sites interaction has been investigated. Nonlinear data analysis techniques such as the dimension spectrum and largest Lyapunov exponent have been used for analysis of data recorded in experiment. It has been shown that properties of dynamics of nucleation sites changes nonlinearly together with changing the spacing between cavities. To explain appearance of such properties of nucleation sites interaction the simulation of gas bubble movement in the liquid in the last stage of bubble departures has been made with using the stabilized finite element method and level set method. It has been shown that the bubble departure velocities and structure of velocity field around the bubbles nonlinearly depend on the initial horizontal spacing between bubbles. The obtained results indicate that one of the reasons for nonlinear changes of properties of dynamics of nucleation sites interaction occurring with changes of initial spacing between them is hydrodynamic interaction between departing bubbles.     

Critical Heat Flux of Boiling Heat Transfer in a Moderate Narrow Space

INTanDUCTI0NBoilingheattransferandcriticalheatflux(CHF)inaconfinednarrowspacehavebeenstudiedexperi-melltallybyanumberofinvestigatorsinthepastfewdecades.However,thereisnoanypopularlyacceptedmodelintheheattransferinnarrowspaceboiling,althoughsomepopularknowledgeabouttheboilingheattransferinthenarrowspacehavebeenacceptedbymanyresearchers.Theknowledgecanbecon-cludedasthatthenucleateboilingheattransferisenhancedatlowheatfluxregionanddeterioratedathighheatfiuxregi0nespeciallyatCHF.Theenhanceme…     

Coalescence of bubbles in nucleate boiling on microheaters

In this paper, an experiment was performed which is based on a heating surface consisting of microheaters where the temperature of each heater can be individually controlled by an electronic feedback loop. The power consumed by the heaters throughout the cycle of individual bubble growth, coalescence, detachment and departure was measured at high frequencies, thus the heat flux and its variation were obtained. By a careful timing and control of two individual microheaters, we were able to produce two individual bubbles side-by-side. The coalescence would takes place when they grow to a certain size that allows them to touch each other. We have recorded two major heat flux spikes for a typical cycle of boiling with coalescence. The first one corresponds to the nucleation of bubbles; the second one is for the coalescence of the two bubbles. We found that the heat flux variation is closely related to the bubble dynamics and bubble-bubble interaction. By comparing with the single bubble results without coalescence, we also found that the heat transfer is highly enhanced due to the coalescence.     

Numerical Simulation of Bubbles Deformation,Flow, and Coalescence in a Microchannel Under Pseudo-Nucleation Conditions

This paper reports the results of numerical study on bubbles deformation, flow, and coalescence under pseudo-nucleate boiling conditions in horizontal mini-/microchannels. The numerical simulation, which is based on the multiphase model of volume of fluid method, aims to study the corresponding flow behaviors of nucleate bubbles generated from the tube walls in mini-/microchannels so as to understand the effect of confined surfaces/walls on nucleate bubbles and heat transfer. Under the pseudo- or quasi-nucleate boiling condition, superheated small vapor bubbles are injected at the wall to ensure that the bubbles generation is under a similar condition of real nucleation. The numerical study examined the fluid mechanics of bubble motion with heat transfer, but the mass transfer across the bubble–liquid interface is not simulated in the present work.     

Dynamic Behavior of Bubble Interface During Boiling

A brief review with discussions is conducted for some pertinent works, done and ongoing in the Laboratory of Phase-Change and Interfacial Phenomena at Tsinghua University, on interfacial behavior of vapor bubbles and interfacial transport phenomena during liquid nucleation boiling. From a sequence of experimental investigations, some new phenomena, particularly, the visually observed interfacial transport phenomena or processes including jet-like flows, bubble interaction and spatial scale effect, were described in this article. The interfacial effects and transport phenomena associated with surface tension gradients caused by temperature and concentration variations were theoretically analyzed to reveal the marked influence on bubble interfacial shape and dynamic behavior, the bubble dynamics including nucleation, bubble motion and coalescence. Several theoretical models and methods were proposed to describe the dynamic characteristics and explain the physics of interfacial phenomena/processes. The spe     

Investigation of heat transfer and bubble dynamics in a boiling thin liquid film flowing over a rotating disk

Nucleate boiling heat transfer and bubble dynamics in a thin liquid film on a horizontal rotating disk were studied. A series of experiments were conducted to determine the heat transfer coefficient on the disk. At low rotation and flow rates, vigorous boiling increased the heat transfer coefficients above those without boiling. Higher rotational speeds and higher flow rates increased the heat transfer coefficient and suppressed boiling by decreasing the superheat in the liquid film. The flow field on the disk, which included supercritical (thin film) flow upstream of a hydraulic jump, and subcritical (thick film) flow downstream of a hydraulic jump, affected the type of bubble growth. Three types of bubble growth were identified. Vigorous boiling with large, stationary bubbles were observed in the subcritical flow. Supercritical flow produced small bubbles that remained attached to the disk and acted as local obstacles to the flow. At low rotational rates, the hydraulic jump that separated the supercritical and subcritical regions produced hemispherical bubbles that protruded out of the water film surface and detached from the disk, allowing them to slide radially outward. A model of the velocity and temperature of the microlayer of water underneath these sliding bubbles indicated that the microlayer thickness was approximately 1/25th of that of the surrounding water film. This microlayer is believed to greatly enhance the heat transfer rate underneath the sliding bubbles.     

Numerical study on thermodynamic growth of single hydrogen bubble in an infinite space

In the liquid hydrogen storage and delivery, cavitation and boiling bubbles are prone to occur, which reduces the safety and economy of the liquid hydrogen delivery. For the bubble in liquid hydrogen, its growth process is different from that of room temperature media owing to the thermodynamic properties. In this paper, a single bubble growth model in liquid hydrogen is developed considering temperature distribution inside the bubble. The growth of single bubble in liquid hydrogen is described and predicted by solving Rayleigh-Plesset equation, thermal diffusion equation, thermal equilibrium equation, and heat conduction equation in semi-infinite space simultaneously. The growth trend of bubble radius, radius growth rate, vapor pressure, thermal boundary layer thickness and temperature difference between boundary and center are investigated by the model. The influence of superheat and ambient pressure on the growth of single bubble in liquid hydrogen is investigated by analysis of variance (ANOVA) and range analysis method. The mechanism of the single bubble transform from dynamic growth to thermal growth is clarified by comparing the critical time of the above physical indicators.     

Dynamics of bubble formation and detachment from an immersed micro-orifice on a plate

Visualization experiments were carried out in the present study to investigate formation and detachment of the bubbles developing from an immersed micro-orifice on a plate in a stagnant and isothermal liquid. A sub-fitting method was proposed to describe the bubble edges and to extrude the bubble characteristics. Taking the microscale effect into consideration, the dynamic behavior of bubbles emerging from various orifices with 0.5, 0.12 and 0.054 mm in diameter was analyzed and compared. The experimental results showed that the bubble waiting time, departure time and departure volume decreased with decreasing orifice diameter. Based on the analysis on actual gas flow rate at the orifice, the evolution of bubble formation process was described by three or four stages for different orifices. The bubble formation under the condition of 0.5 mm orifice mainly experienced the nucleation stage, while the steady growth stage was the dominator for the micro-orifices with 0.12 and 0.054 mm in diameter. The rupture scene and evolution of bubble contact ring at the detachment moment were found to significantly vary with the orifice diameter. The inrush of several trailing bubbles into the detached leading bubble was observed for the orifices with 0.12 and 0.054 mm in diameter, resulting in a significant fluctuation in the interface.     

Formation and breakup of compound pendant drops at the tip of a capillary and its effect on upstream velocity fluctuations

In this paper, the formation and breakup process of compound pendant drops (CPDs, pendant drops with smaller drops or bubbles in them) at the tip of a glass capillary and its effect on upstream velocity fluctuation are experimentally investigated. The formation process of an air/water compound drop from a CPD consists of four main stages. First, an air plug in the capillary flows into the small liquid pendant drop to initialize a small CPD. Next, a liquid slug flows into the CPD, and the liquid in the CPD accumulates. Subsequently, an air plug flows into the CPD, and it coalesces with the existing air bubble in the CPD. The accumulation and coalescence stages repeat, until the CPD reaches a critical weight, then the CPD finally breaks up to produce a compound drop. For the air/SDS-solution system, the bubbles in the CPDs do not coalesce, and the contact line of the CPDs initially climbs along the capillary and then moves downwards with the growth of the CPDs. The upstream velocity fluctuates during the periodical formation and breakup of the CPD due to Laplace pressure variation at the tip of the glass capillary. By adding surfactant into water, the fluctuation of the upstream velocity decreases. The size distribution of the compound drops produced by the breakup of CPDs is quantified, and the results show that the current system is able to produce monodisperse compound drops.     

Numerical investigation of bubble coalescence characteristics under nucleate boiling condition by a lattice-Boltzmann model

A numerical study was performed to investigate the characteristics of bubble growth, detachment and coalescence on vertical, horizontal, and inclined downward-facing surfaces. The FlowLab code, which is based on a lattice-Boltzmann model of two-phase flows, was employed. Macroscopic properties, such as surface tension (σ) and contact angle (β), were implemented through the fluid–fluid (G_σ) and fluid–solid (G_t) interaction potentials. The model predicted a linear relationship between the macroscopic properties (σ,β) and microscopic parameters (G_σ,G_t). The simulation results on bubble departure diameter appear to have the same parametric dependence as the empirical correlation. Hydrodynamic aspects of bubble coalescence are investigated by simulating the growth and detachment behavior of multiple bubbles generated on horizontal, vertical, and inclined downward-facing surfaces. For the case of horizontal surface, three distinct regimes of bubble coalescence were represented in the lattice-Boltzmann simulation: lateral coalescence of bubbles situated on the surface; vertical coalescence of bubbles detached in a sequence from a site; and lateral coalescence of bubbles, detached from the surface. Multiple coalescence was predicted on the vertical surface as the bubble detached from a lower elevation merges with the bubble forming on a higher site. The bubble behavior on the inclined downward-facing surface was represented quite similar to that in the nucleate boiling regime on a downward-facing surface.     

Numerical study on bubble behavior in magnetic nanofluid used for waste heat recovery power generation concept

Electrical energy can be generated by the bubble motion inside the magnetic nanofluid under the influence of an external magnetic field. The relative movement of the magnetic particles dispersed in the magnetic fluid is induced through the movement of the bubbles rising by buoyancy force. This disturbs the external magnetic field associated with the generator coil, and electrical energy can be generated. The bubble movement in this complex physical environment was studied through 2D numerical analysis. Commercial magnetic fluids EFH1 and EFH3, manufactured by Ferrotec, were selected as the working fluid for the investigation. A level set method was used to analyze the 2‐phase flow of bubbles motion in the magnetic fluid. The effect of magnetic particle concentration on the behavior of bubbles and the change of bubble flow patterns through interaction between bubbles were observed by analysis. In addition, the influence of the magnetic force caused by the external magnetic field on the behavior of the bubble was also investigated. The following results can be obtained through the analysis of this study. The high concentration of magnetic particles increases the viscosity and attenuates the rising velocity and the lateral oscillation of the bubbles. The interaction of the 2 bubbles depends on the initial relative distance. Merging occurs only between 2 bubbles within a certain initial distance, which maximizes disturbance of the surrounding magnetic fluid. The magnetic force exerted by the permanent magnets externally applied is relatively small in comparison with gravity. Therefore, the effect on the rise behavior of the bubble is not significant. In consideration of the overall external force and flow conditions, the pattern of the bubble flow that maximizes the efficiency in the present electric energy generation concept was found.     

Synchronous observation of rising soluble bubble through quiescent solution

An experimental method using computer image processing technology (CIPT) was proposed to observe and investigate the velocity, deformation, heat and mass transfer, etc. of a rising soluble gas (CO₂) bubble through a quiescent hot water. A model was set up to describe the behavior of the bubble in a visual experimental system in which a high-speed camera rose instantaneously with the movement of the bubble. A series of trajectory videos about the bubble were recorded by a computer linked to the camera. The trajectory, volume changes and rate of mass transfer of the bubble were obtained by the CIPT. It is found that the single bubble follows a rolling trajectory at the initial stage when there is mass transfer. With the volume decreasing, the disturbed behavior of the bubble becomes tempered. When the rising velocity of the bubble reaches the maximum, the velocity is nearly at a constant. The experimental and analysis results show that this method is useful for the research on the mass transfer and the movement of rising bubbles in liquid.     

Dynamics of boiling succeeding spontaneous nucleation on a rapidly heated small surface

The dynamics of boiling succeeding spontaneous nucleation on a small film heater immersed in ethyl alcohol are investigated at heating rates ranging from 10⁷ K/s to approximately 10⁹ K/s, under which spontaneous nucleation is dominant for the inception of boiling. Immediately after the concurrent generation of a large number of fine bubbles, a vapor film that covers the entire surface is formed by coalescence and rapidly expands to a single bubble. As the heating rate is increased, the coalesced bubble flattens and only a thin vapor film grows before cavitation collapse. Similar behaviors are also observed for water. Based on the observed results, a theoretical model of the dynamic bubble growth due to the self-evaporation of the superheated liquid layer, which develops before boiling incipience, is presented. The calculated results are compared with the observed results.