激光空泡在文丘里管中运动的动力学特性

以水为工作介质,在不同文丘里管入口压力下,利用YAG激光器产生的激光轰击水中的金属靶材产生空泡,借助高速摄像系统记录激光空泡在文丘里管中的运动过程,并采用流体动力学模拟对文丘里管中的流场特性和空泡的溃灭特性进行分析.结果表明:激光空泡在文丘里管中的运动,其形状变化可分为产生阶段、挤压阶段、溃灭初始阶段和溃灭阶段等四个阶段.空泡的溃灭取决于流场状态,当流动为层流时,空泡不发生溃灭;当流动为湍流时,空泡发生溃灭,且湍流程度越剧烈,溃灭现象越显著.搭建的激光空泡生成与运动系统能够实现空泡的定点溃灭.     

Bubble nucleation and growth anomaly for a hydrophilic microheater attributed to metastable nanobubbles

Nanobubbles on a hydrophilic surface immersed in water and ethanol are inferred from the response of the surface to two consecutive heat pulses with a variable separation time. Bubble nucleation occurs at specific positions on the surface during the first heat pulse but at lower nucleation temperatures and random locations on the second. Nanobubbles are hypothesized to form on collapse of the bubble from the first pulse.     

Experimental investigation on the effects of the standoff distance and the initial radius on the dynamics of a single bubble near a rigid wall in an ultrasonic field

Bubble behaviors near a boundary in an ultrasonic field are the fundamental forms of acoustic cavitation and of substantial importance in various applications, such as industry cleaning, chemical engineering and food processing. The effects of two important factors that strongly affect the dynamics of a single acoustic cavitation bubble, namely, the initial bubble radius and the standoff distance, were investigated in this work. The temporal evolution of the bubble was recorded using high speed microphotography. Meanwhile, the time of bubble collapse and the characteristics of the liquid jets were analyzed. The results demonstrate that the intensity of the acoustic cavitation, which is characterized by the time of bubble collapse and the liquid jet speed, reaches the optimum level under suitable values of the initial bubble radius and the normalized standoff distance. As the initial bubble radius and the normalized standoff distance increase or decrease from the optimal values, the time of the bubble collapse increases, and the first liquid jet’s speed decreases substantially, whereas the speeds of the second and third liquid jets exhibit no substantial changes. These results on bubble dynamics in an ultrasonic field are important for identifying or correcting the mechanisms of acoustic cavitation and for facilitating its optimization and application.     

Modeling of surface cleaning by cavitation bubble dynamics and collapse

Surface cleaning using cavitation bubble dynamics is investigated numerically through modeling of bubble dynamics, dirt particle motion, and fluid material interaction. Three fluid dynamics models; a potential flow model, a viscous model, and a compressible model, are used to describe the flow field generated by the bubble all showing the strong effects bubble explosive growth and collapse have on a dirt particle and on a layer of material to remove. Bubble deformation and reentrant jet formation are seen to be responsible for generating concentrated pressures, shear, and lift forces on the dirt particle and high impulsive loads on a layer of material to remove. Bubble explosive growth is also an important mechanism for removal of dirt particles, since strong suction forces in addition to shear are generated around the explosively growing bubble and can exert strong forces lifting the particles from the surface to clean and sucking them toward the bubble. To model material failure and removal, a finite element structure code is used and enables simulation of full fluid–structure interaction and investigation of the effects of various parameters. High impulsive pressures are generated during bubble collapse due to the impact of the bubble reentrant jet on the material surface and the subsequent collapse of the resulting toroidal bubble. Pits and material removal develop on the material surface when the impulsive pressure is large enough to result in high equivalent stresses exceeding the material yield stress or its ultimate strain. Cleaning depends on parameters such as the relative size between the bubble at its maximum volume and the particle size, the bubble standoff distance from the particle and from the material wall, and the excitation pressure field driving the bubble dynamics. These effects are discussed in this contribution.     

High-speed imaging of ultrasonic emulsification using a water-gallium system

Aiming at elucidating ultrasonic emulsification mechanisms, the interaction between a single or multiple acoustic cavitation bubbles and gallium droplet interface was investigated using an high-speed imaging technique. To our best knowledge, the moment of emulsification and formation of fine droplets during ultrasound irradiation were observed for the first time. It was found that the detachment of fine gallium droplets occurs from the water-gallium interface during collapse of big cavitation bubbles. The results suggest that the maximum size of cavitation bubble before collapsing is of prime importance for emulsification phenomena. Previous numerical simulation revealed that the collapse of big cavitation bubble is followed by generation of high-velocity liquid jet directed toward the water-gallium interface. Such a jet is assumed to be the prime cause of liquid emulsification. The distance between cavitation bubbles and water-gallium interface was found to slightly affect the emulsification onset. The droplet fragmentation conditions are also discussed in terms of the balance between (1) interfacial and kinetic energies and (2) dynamic and Laplace pressure during droplet formation.     

Observations of the collapses and rebounds of millimeter-sized lithotripsy bubbles

Bubbles excited by lithotripter shock waves undergo a prolonged growth followed by an inertial collapse and rebounds. In addition to the relevance for clinical lithotripsy treatments, such bubbles can be used to study the mechanics of inertial collapses. In particular, both phase change and diffusion among vapor and noncondensable gas molecules inside the bubble are known to alter the collapse dynamics of individual bubbles. Accordingly, the role of heat and mass transport during inertial collapses is explored by experimentally observing the collapses and rebounds of lithotripsy bubbles for water temperatures ranging from 20 to 60 °C and dissolved gas concentrations from 10 to 85% of saturation. Bubble responses were characterized through high-speed photography and acoustic measurements that identified the timing of individual bubble collapses. Maximum bubble diameters before and after collapse were estimated and the corresponding ratio of volumes was used to estimate the fraction of energy retained by the bubble through collapse. The rebounds demonstrated statistically significant dependencies on both dissolved gas concentration and temperature. In many observations, liquid jets indicating asymmetric bubble collapses were visible. Bubble rebounds were sensitive to these asymmetries primarily for water conditions corresponding to the most dissipative collapses.     

Role of the channel geometry on the bubble pinch-off in flow-focusing devices

The formation of bubbles by flow focusing of a gas and a liquid in a rectangular channel is shown to depend strongly on the channel aspect ratio. Bubble breakup consists in a slow linear 2D collapse of the gas thread, ending in a fast 3D pinch-off. The 2D collapse is predicted to be stable against perturbations of the gas-liquid interface, whereas the 3D pinch-off is unstable, causing bubble polydispersity. During 3D pinch-off, a scaling w_(m) approximately tau(1/3) between the neck width w_(m) and the time tau before breakup indicates that breakup is driven by the inertia of both gas and liquid, not by capillarity.     

Pulsed holmium laser ablation of tissue phantoms: correlation between bubble formation and acoustic transients

5 Pa served as tissue phantoms to evaluate such effects. Holmium laser pulses (wavelength: 2.12 μm, duration: 180 μs FWHM), were delivered through 400 and 600 μm diameter optical fibers inserted into cubes of clear gel. Bubble effects were investigated using simultaneous flash micro-videography and pressure recording for radiant exposures of 20–382 J/cm². Bubble formation and bubble collapse induced pressure transients were observed regardless of phantom stiffness. Bubbles of up to 4.2 mm in length were observed in gels with a Young’s modulus of 2.9×10⁵ Pa at a pulse energy of 650 mJ. An increase of Young’s modulus (reduction in water content) led to a monotonic reduction of bubble size. In the softest gels, bubble dimensions exceeded those observed in water. Pressure amplitudes at 3 mm decreased from 100±14 bars to 17±6 bars with increasing Young’s modulus over the studied range. Theoretical analysis suggested a major influence on bubble dynamics of the mass and energy transfer through the bubble boundary. Received: 26 August 1996/Revised version: 10 February 1997     

Dynamic behavior of a single bubble between the free surface and rigid wall

The objective of this paper is to apply high-speed photography and schlieren method to investigate the bubble dynamics between the free surface and a rigid wall. The temporal evolution of the bubble shape and the free surface motion are recorded by two synchronous high-speed cameras. Experiments are carried out for a single bubble generated at various normalized stand-off distances from bubble center to the free surface and to the rigid wall. The results show that (1) three distinctive patterns are identified with the morphology of the bubble and free surface, namely single toroidal bubble without spike (STB), single toroidal bubble with a spike (STBS) and double toroidal bubbles with a spike (DTBS). (2) The dynamic characteristics of the bubble at collapse and rebound stage vary evidently at different patterns, including the bubble shape variations and free surface motion. In detail, the schlieren images show the formation and propagation of shock waves, which explains the radiative process of bubble collapse energy. (3) Qualitative comparisons are carried out for the bubble and free surface at the same pattern. And quantitative analyses are conducted for the jet velocity, bubble collapse position, bubble collapse time and spike height, etc. for different values of bubble-rigid wall distance.     

Luminescence from acoustic-driven laser-induced cavitation bubbles

The influence of a continuous sound field on the first oscillation cycle and on the cavitation luminescence of a transient laser-induced bubble is investigated experimentally. The variation of the collapse phase is predicted with a simple numerical model and compared with experiment. Bubble dynamics is mainly influenced by three parameters: the phase of bubble generation, the size of the bubble, and the amplitude of the sound field. The experimentally found enhancement and reduction of the luminescence is discussed and several suggestions are made for further boosting of the collapse strength.     

无泵吸收制冷系统气泡泵的性能分析

根据两相流流型转换理论,推导出了气泡泵从弹状流向泡状流转变和从弹状流向块状流转变时液体流量、气体流量与管径的关系式;根据空气提升理论、能量平衡、质量平衡推导出了气泡泵的性能关系式。根据上述关系式,具体分析了爱因斯坦制冷循环工况下气泡泵的性能,分析结果表明,在弹状流下限、大的沉浸比时液体循环量较大。     

Dynamics of a cavitation bubble near a solid wall

The cavitation bubble dynamics, the variation of pressure and velocity fields of the surrounding liquid in the process of the bubble axisymmetric compression near a planar solid wall are considered. It is assumed that the liquid is at rest at the initial moment of time, and the bubble has a spheroidal shape. The liquid is assumed inviscid and incompressible, its motion being potential. The bubble surface deformation and the liquid velocity on the surface are computed by the Euler scheme using the boundary element method until the moment of the collision of some parts of the bubble surface with one another. The influence of the distance of the bubble from the wall and its initial nonsphericity on the liquid pressure and velocity fields, the bubble shape, and the pressure inside the bubble at the end of the time interval under consideration are studied. The maximum pressure in liquid is shown to realize at the bottom of the cumulative jet arising at the bubble collapse with direction to the wall. In the upper part of this jet, the velocity and pressure are practically constant, and the pressure in the jet is approximately equal to the pressure in the bubble.     

Dynamics of tandem bubble interaction in a microfluidic channel

The dynamics of tandem bubble interaction in a microfluidic channel (800 × 21 μm, W?×?H) have been investigated using high-speed photography, with resultant fluid motion characterized by particle imaging velocimetry. A single or tandem bubble is produced reliably via laser absorption by micron-sized gold dots (6 μm in diameter with 40 μm in separation distance) coated on a glass surface of the microfluidic channel. Using two pulsed Nd:YAG lasers at λ?=?1064 nm and ～10 μJ/pulse, the dynamics of tandem bubble interaction (individual maximum bubble diameter of 50 μm with a corresponding collapse time of 5.7 μs) are examined at different phase delays. In close proximity (i.e., interbubble distance?=?40 μm or γ?=?0.8), the tandem bubbles interact strongly with each other, leading to asymmetric deformation of the bubble walls and jet formation, as well as the production of two pairs of vortices in the surrounding fluid rotating in opposite directions. The direction and speed of the jet (up to 95 m/s), as well as the orientation and strength of the vortices can be varied by adjusting the phase delay.     

圆锥边界附近激光空泡溃灭行为的研究

为了研究刚性圆锥边界锥角对激光空泡溃灭行为的影响,文章建立了虚拟平面边界模型, 同时采用阴影摄影术、光偏转法以及数值计算的手段对边界附近空泡溃灭过程进行了研究. 结果表明边界的锥角对空泡的形状、溃灭时间以及液体射流形成均有明显影响. 空泡形状偏离球形的程度和溃灭时间均随锥角的增大而增大,且增大锥角度可以促使射流的形成. 空泡溃灭时间的实验值同理论值具有较高的一致性,验证了虚拟平面边界假设及无量纲距离修正的有效性.     

三维双气泡融合的格子Boltzmann模拟

针对基于自由能模型的格子Boltzmann方法,推导D3Q15格子模型对应的平衡态分布函数;采用该模型模拟双气泡的融合过程.结果表明,气泡的融合不仅与它们之间的初始间距有关,还与表面张力有关.表面张力越大,气泡融合的临界距离也越大.此外,研究气泡融合速度与初始间距、表面张力及粘性系数的关系.     

液体介质中的空泡发光现象及其他

空泡广泛存在于自然界中，理论和实验表明，空泡在坍塌时可以将能量密度提高-10^12倍，发出皮秒级超短脉冲闪光，文章对液体介质中的空泡规律和常用研究方法进行了描述，主要内容包括空泡动力学、声致发光、冲击波产生、激光空泡及空泡应用前景介绍。     

Gas bubble pulsation in a semiconfined space subjected to ultrasound

In the case of ultrasound application in biological tissues, gas bubbles might form and collapse within cells, in the intercellular spaces and on tissue surfaces. In this work the effect of confined space on the behavior of the gas bubble in the presence of ultrasonic field is studied. A numerical model for bubble pulsation in a planar liquid layer, bounded by two rigid walls, is developed. Surface tension at the interface between the host liquid and the gas in the bubble is considered as well. A mathematical statement and solution technique based on the boundary integral method are presented. In some cases, the bubble divides into two symmetrical parts and high-velocity jets are generated, aimed at the walls. The final velocity of the jets strongly depends on the surface tension of the host liquid. Two new parameters that predict the occurrence of jet formation are developed.     

Time-resolved measurements of shock-induced cavitation bubbles in liquids

A novel experimental method for the measurement of cavitation bubble dynamics is presented. The method makes use of a collimated cw HeNe laser beam that is focused onto a photodiode. A cavitation bubble centered in the laser beam leads to refraction and thus changes the diode signal. With sufficient temporal resolution of the measurement, the evolution of the bubble dynamics, and in particular, the collapse, could be well resolved (limitation is only due to diode response and oscilloscope bandwidth). In the present work this is demonstrated with cavitation bubbles generated with high-power nanosecond and femtosecond laser pulses, respectively. Bubble evolution is studied in two different liquids (water and glycerine) and at different temperatures and pressures.     

Numerical analysis of the effects of radiation heat transfer and ionization energy loss on the cavitation Bubble?s dynamics

A numerical scheme for simulating the acoustic and hydrodynamic cavitation was developed. Bubble instantaneous radius was obtained using Gilmore equation which considered the compressibility of the liquid. A uniform temperature was assumed for the inside gas during the collapse. Radiation heat transfer inside the bubble and the heat conduction to the bubble was considered. The numerical code was validated with the experimental data and a good correspondence was observed. The dynamics of hydrofoil cavitation bubble were also investigated. It was concluded that the thermal radiation heat transfer rate strongly depended on the cavitation number, initial bubble radius and hydrofoil angle of attack.