Effects of surface tension on bubble growth in an extensive uniformly superheated liquid

The study of bubble growth in an extensive pool of liquid provides considerable insight into the mechanisms that play a role in bubble growth near a heated surface and in the cavitation phenomenon. This work focuses on analyzing the effects of surface tension on the growth rate for the thermally controlled stage of a single bubble in such a liquid. The conservation of energy equations, including the internal energy term for the bubble and that within boundary layer around it, are numerically solved. The complete temporal variations of the bubble in water and liquid nitrogen are investigated based on the assumption that the bubble growth is controlled only in sequence by inertia and heat. Thus, the two stages are subject to the continuity of the bubble growth, while the inertia-controlled stage is only formulated by the well-known Rayleigh solution. The thickness of the boundary layer around the bubble is also determined. The results are comparable with the Plesset-Zwick models and Forster-Zuber models, as well as available experimental data. It is found that the influence of internal energy on the rate of bubble growth is small enough to be ignored; however, the accumulative effects of the surface tension are significant and increase with a decrease in the degree of superheat.     

Effect of charge on growth of droplets with ionic condensation nuclei

A growth kinetics model of droplets with ionic condensation nuclei based on the chemical potential and the kinetic equation of mass transfer is established. The model is simplified and the effect of charge on droplet growth is examined. The theoretical results show that the critical radius for droplet growth with an ionic condensation nucleus is less than that of those without an ionic nucleus. Furthermore, our results also indicate that if the initial droplet with an ionic nucleus has a radius shorter than the critical radius, the droplet will not vanish, but will reach a steady-state radius. As the ionic charge increases, the critical radius for droplet growth will decrease and the corresponding steady-state droplet radius will increase. In addition we show that once a critical charge is reached, all droplets will grow regardless of initial radius.     

Molecular dynamics simulation of nanoscale contact and sliding processes for probes with different tip radius of curvature

Three-dimensional simulations were carried out molecular dynamics （MD） to study the contact and sliding processes between diamond points with different tip radius of curvature and surfaces of single crystal copper. The material deformation, abrasion mechanism, lattice defects, the force of contact process, and the sliding friction process were investigated. The simulation results show that the contact force, dislocations, and stacking fault defects, increase during the contact process with increasing contact depth or tip radius of curvature. The dislocations emit along the [10i-] and [i-01] direction and then a glide band is formed. It was also found that a greater tip radius of cur- vature results in a larger groove and more material defor- mation. The normal force and friction increase with increasing tip radius of curvature, but the coefficient of friction decreases. The stacking faults spread along the sliding direction and increase with increasing tip radius of curvature. In addition, the number of upheaval atoms increases as the radius of tip curvature or sliding distance increases.     

The mathematical model and simulation of the electrostatic suspension system in the vertical direction

In order to research the vibration law of electrostatic suspension systems in the vertical direction, the mathematical model as a nonlinear differential equation is established. A series of simulation is carried out. The results show that the solution of the differential equation is a periodic function. The amplitude becomes bigger with the original velocity increased. The period becomes smaller with the original velocity increasing. The numerical methods are presented to derive the amplitude and the frequency, and the results coincide with that of the simulation. The condition during which the simple harmonic vibration arises is pointed out. The expressions for the amplitude and the period of simple harmonic vibration are derived respectively, and the results are the same with that of the simulation. This study is helpful for researching the vibration characteristics of the electrostatic suspension system. The external disturb should be controlled to lower the amplitude and the frequency of the vibration.     

Effect of reactor pressure on the growth rate and structural properties of GaN films

The effect of reactor pressure on the growth rate, surface morphology and crystalline quality of GaN films grown on sapphire by metalorganic chemical vapor deposition is studied. The results show that as the reactor pressure increases from 2500 to 20000 Pa, the GaN surface becomes rough and the growth rate of GaN films decreases. The rough surface morphology is associated with the initial high temperature GaN islands, which are large with low density due to low adatom surface diffusion under high reactor pressure. These islands prolong the occurrence of 2D growth mode and decrease the growth rate of GaN film. Meanwhile, the large GaN islands with low density lead to the reduction of threading dislocation density during subsequent island growth and coalescence, and consequently decrease the full width at half maximum of X-ray rocking curve of the GaN film.     

Heat Transfer Process of Bubble during the Occurrence of Cavitation in Hydraulic System

Thermal characteristic of cavitation has great influence on the process of occurrence,development and collapse of bubble in hydraulic system. By choosing the stage of bubble growth as the research object,combining with the characteristic of the process of bubble occurrence and development in hydraulic system, and ignoring the impact of thermal radiation,the heat transfer situation of bubble growth was analyzed under appropriate assumptions of thermodynamic conditions in the bubble generation and development process. The mathematical expression of the temperature change of bubble was deduced using thermodynamic principle. Through combining the expression with classic Rayleigh-Plesset Equation,numerical calculation was carried out and the temperature variation over time（ or bubble radius） was obtained. The influences of convective heat transfer coefficient of bubble and polytropic exponent on the thermodynamic process of bubble were analyzed. Finally,the thermal characteristic of bubble growth after cavitation occurrence was summarized.     

Two-dimensional investigation of forced bubble oscillation under microgravity

Recent referential studies of fluid interfaces subjected to small vibration under microgravity conditions are reviewed. An experimental investigation was carried out aboard the American Space Shuttle Discovery. Two-dimensional (2-D) modeling and simulation were conducted to further understand the experimental results. The oscillation of a bubble in fluid under surface tension is governed by the incompressible Navier-Stokes equations. The SIMPLEC algorithm was used to solve the partial differential equations on an Eulerian mesh in a 2-D coordinate. Free surfaces were represented with the volume of fluid (VOF) obtained by solving a kinematic equation. Surface tension was modeled via a continuous surface force (CSF) algorithm that ensures robustness and accuracy. A new surface reconstruction scheme, alternative phase integration (API) scheme, was adopted to solve the kinematic equation, and was compared with referential schemes. Numerical computations were conducted to simulate the transient behavior of an oscillating gas bubble in mineral oil under different conditions. The bubble positions and shapes under different external vibrations were obtained numerically. The computed bubble oscillation amplitudes were compared with experimental data.     

Testing and Modeling for Energy Dissipation Behavior of Velocity and Displacement Dependent Hydraulic Damper

A passive energy-dissipating device, velocity, and displacement dependent hydraulic damper (VDHD), is developed to reduce the seismic response of structure. This device is comprised of a hydraulic jack, check valve, relief valve, and throttle valve. The numerical analysis model for SAP2000 nonlinear analysis program is proposed to simulate the energy-dissipating characteristics of VDHD. The analysis results of this model compared with the seismic resistant tests reveal that this proposed model can accurately describe the actual energy-dissipating behavior of VDHD. The efficiency of VDHD is confirmed using this proposed model for carrying out numerical analyses of bare building, building added with bulking resistant bracing (BBR), and VDHD. The energy-dissipating capabilities of VDHD are performing excellent displacement and acceleration control with various ground magnitudes; being an energy absorber to absorb mechanical energy in the structure and resist structural movement; and gathering the advantage of BRB.     

Effect of Mg and Si on infiltration behavior of Al alloys pressureless infiltration into porous SiCp preforms

The effect of Mg and Si additon to Al matrix on infiltration kinetics and rates of Al alloys pressureless infiltration into porous SiCp preform was investigated by observing the change of infiltration distance with time as the Al alloys infiltrate into SiCp preforms at different temperatures. The results show that infiltration of SiCp preforms by Al melt is a thermal activation process and there is an incubation period before the infiltration becomes stable. With the increase of Mg content in the Al alloys from 0wt% to 8wt%, the infiltration will become much easier, the incubation period becomes shorter and the infiltration rate is faster, but these effects are not obvious when the Mg content is higher than 8wt%. As for Si addition to the Al alloys, it has no obvious effect on the incubation period, but the infiltration rate increases markedly with the increase of Si content from 0wt% to 12wt% and the rate has no obvious change when the content is bigger than 12wt%. The effect of Mg and Si on the incubation period is related to the infiltration mechanism of Al pressureless infiltration into SiCp preforms and their impact on the infiltration rate is a combined result from viscosity and surface tension of Al melt and SiC-Al wetting ability.     

烟火药水下燃烧高温残渣气泡的动力学模型

为研究烟火药水下燃烧高温残渣与水作用形成的气泡动力学特性,建立了传热、传质数学模型,推导了单个高温残渣气泡的生成模型,计算了生成气泡的半径以及半径的变化率与时间的关系。将计算结果与文献结果进行了比较;并分析了高温颗粒特性对气泡动力学的影响。结果表明,随着时间增加,高温残渣颗粒不断冷却,气泡半径不断增大;但增长速度不断变慢,计算结果与文献结果吻合较好。     

单泡空化振动系统的动力学数值模拟

根据微气泡的动力学方程,用数值计算的方法,采用改进的初始半径对单泡超声空化现象进行了研究。分析表明：气泡的振动对初始半径这个参量很敏感;气泡崩溃速率随气泡初始半径的增加而增大,在一定范围内能保证空化泡稳定的振动,在初始半径为1.6μm处空化程度最强,如果继续增大初始半径则空化程度减弱甚至消失、同时,将分析结果与前人的实验数据比较,发现在考虑液体的可压缩性以后对单泡最佳空化区域有很大的改进。     

超声场中双气泡非线性动力学参数

从气泡动力学出发,建立了双气泡在外加声场作用下的运动方程,对气泡间的相互作用力进行了研究.研究结果表明：考虑到气泡之间相互作用后,气泡的运动方程、谐振频率、谐振时半径的变化情况明显与单个气泡不同,这些参数不只与气泡各自的初始半径,外加声压强度,液体的黏滞性等因素有关,还与它们之间距离和相对另一气泡的初始半径有关.研究发现：气泡的谐振频率与它们之间距离有关,随着气泡之间距离的增大,其谐振频率在不断减小.但当它们之间的距离增大到某一值时,其谐振频率趋于一定值.这也可以理解为当气泡相对较远时它们之间的相互作用对其影响可以忽略,此时的谐振频率即为单泡的谐振频率.对于谐振时它们的半径变化而言,两气泡之间的作用是相互抑制的,但气泡的初始半径的不同,这种抑制作用的强、弱不同.气泡初始半径相同的气泡,相互抑制作用较弱,数值计算表现出半径相同时其谐振半径的变化幅度要大于半径不同时的结果.     

单个三维气泡运动的直接数值模拟

采用VOF（Volume—of-Fluid）中的PLIC（Piecewise Linear Interface Calculation）界面重构方法模拟了三维气泡在另一种静止流体中的上升和变形运动;在数值模拟的例子中分别考察了不同黏度和表面张力对气泡在上升过程中的变形及上升速度等的影响．计算结果表明,PLIC界面重构方法可以正确地模拟气泡的变形、破裂等过程．黏性和表面张力在气泡运动过程中的作用可以用Eoetvoes数和Reynolds数来描述．在相同的密度和黏度比的情况下,表面张力越大,则气泡的形状变化越小,上升的速度越快,表面张力起着使气泡保持原状的作用;黏性越小,则气泡在上升过程中射流穿透上表面的时间越早,变形速度越快．     

过热液中汽泡生长初期阶段特性

依据非平衡热动力学相变理论建立了在均匀过热液中的汽泡生长模型．该模型中引入多变热,以保证汽泡在正化学势差作用下长大．采用数值计算方法,对汽泡生长的初始阶段特性进行了研究．计算表明,汽泡在此生长阶段存在滞后和自适应调节现象,滞后时间与引入的不同温度、压力、速度等扰动量的强度有关．在一定范围内,扰动大小并不明显影响汽泡生长曲线     

文丘里管反应器空化泡的动力学特性

应用四阶Runge-Kutta法,对空泡径向非线性方程进行数值模拟,分析了文丘里管反应器内空泡的成长与溃灭特性以及湍流作用、空化泡初始半径、入口压力对空化泡运动特性与形成压力脉冲的影响规律.结果表明:在湍流作用下,气泡崩溃时压力脉冲远大于非湍流的效果,初始半径越小,压力脉冲越大,入口压力变化对压力脉冲影响有一最佳值.     

气泡堆积法生成曲线多边形区域非结构化网格及其应用

对气泡堆积法进行改进,发展了一种基于气泡堆积法生成曲线多边形区域非结构化网格的算法,将曲线边界映射为直线完成气泡添加和位置动态调整,再通过弧长参数化的方法将气泡位置映射回曲线边界,避免了移动调整过程中气泡偏离曲线的问题.提出了一种简便的判断点在复杂曲线多边形区域内的方法,简单且易于编程.通过添加不同大小的顶点气泡以及在内部设置人工点源,利用加权平均法实现了网格的局部加密.应用基于非结构化同位网格的SIMPLE算法对环扇形空腔顸盖驱动流进行了数值模拟,不同雷诺数下的计算结果与文献结果吻合较好.     

声场中气泡界面稳定性研究

声致空化广泛应用于表面清洁、生物成像和材料合成等领域,气泡的界面稳定性对空化作用效果有重要影响.考虑液体黏性和表面张力的影响,采用扰动法获得气泡径向运动控制方程的解析解,建立单/双频激振声场中气泡界面稳定性预测模型并求解,获得气泡振荡的临界半径.当气泡初始平衡半径小于临界半径时,气泡界面保持稳定;大于临界半径时,气泡界面失稳.在单频激振声场中,低频激振下气泡界面失稳的低阶临界半径在其共振半径附近,高阶临界半径不受频率影响,与同阶的高频激振下临界半径相当,给出了不同激振幅值下高低频的临界频率值.在双频激振声场中,分3种情况（即双低频激振、高频+低频激振和双高频激振）讨论声场中关键参数（即激振频率、振幅分配比和总的声压幅值）对气泡界面失稳临界半径的影响.     

气体性质对单泡声致发光平衡参数空间的影响

构造了一个描述气泡宏观运动的均匀非绝热模型,结合气泡的平衡机制,分析了空气和惰性气体的各种特性对单泡声致发光的平衡参数空间的影响．发现气体性质通过对平衡参数空间的影响对单泡声致发光起着重要作用．其中热传导系数起着主要作用,它的减小将导致气泡的平衡半径与压缩比增大,并导致发光亮度的增强．     

Modeling and Simulation of a Gas-Liquid Coupling Excitation-Induced Cavitation Bubble

A gas-liquid coupling excitation mode is proposed and the gas-liquid excitation experimental system is developed. Air from pulse generator is mixed with liquid,through which the generated cavitation bubbles can strip contaminants adhered to the pipe inner wall rapidly. The kinematics equation of the bubble inside the hydraulic oil is established and the numerical simulations are carried out. The influential factors such as gas pressure, excitation frequency,initial bubble radius and fluid viscosity are analyzed.The results show that the cavitation will evolve from steady state to transient state with the increasing gas pressure and initial bubble radius. The pulse generator frequency has a slightly effect on the growth of the bubble radius,and the breakup time of the bubble is shortened with the rising frequency. Similarly, the increasing viscosity of liquid has minimal impact on cavitation effect,which can weaken the growth and the collapse of the bubble. Moreover,the temperature inside the cavitation bubble is investigated,indicating that the instantaneous temperature inside the bubble increases with the rising gas pressure. Once the gas pressure is raised to a certain value greater than the fluid static pressure, the instantaneous temperature inside the bubble will rise sharply. So, it can be concluded that the gas-liquid coupling excitation-induced cavitation process is controllable, and some theoretical basis of the new excitation mode is presented,which is expected to be applied in the online cleaning of the complex hydraulic system.