基于MPS方法的汽泡凝结数值模拟研究

采用移动粒子半隐式(MPS)方法对静止过冷水中单个汽泡的凝结现象进行了数值模拟,研究了不同初始压力和初始直径时饱和蒸汽汽泡凝结过程,获得了凝结过程中汽泡形状、当量直径和压力的变化规律;汽泡初始压力为0.1～0.5MPa,初始直径为2mm、3mm和5mm;过冷水压力为0.1MPa,温度为70℃。结果表明两相界面不存在压差时,凝结过程中汽泡始终保持球形,汽泡当量直径逐渐减小,压力近似不变;相界面存在压差时,凝结过程中汽泡从球形逐渐变为心形、半月形,汽泡当量直径和压力会出现周期性振荡,且初始压力越大振荡幅度越大。     

竖直环形通道内液氮自然循环沸腾可视化实验研究

采用基于高速摄像手段的可视化方法研究了竖直环形通道内的液氮自然循环沸腾过程,建立了汽泡脱离直径及汽泡脱离频率的实验关联式,以新建方程结合双流体模型进行了数值计算。与实验数据的对比表明,建立的实验关联式对于液氮自然循环沸腾过程中的汽泡脱离直径及汽泡脱离频率具有很好的预测精度。     

液滴碰撞对液膜内汽泡运动与换热影响

采用数值模拟的方法,研究了沸腾雾化喷射过程中热壁面薄液膜层受到液滴碰撞扰动时液膜层内汽泡运动,相界面变化和由此引起的壁面换热特性.模拟结果显示汽泡生长初期相界面变化与液膜层内二次核化特征与文献结果吻合良好,汽泡生长后期相界面变化存在滞后.讨论了液滴下降速度.液滴直径与初始位置,多液滴碰撞对液膜层内流动与壁面换热的影响.     

电场作用下单个沸腾汽泡动态特性研究

电场可强化池态沸腾换热已经普遍得到研究者的认可,在电场强化换热的实验研究中,以往研究者主要采取统计学的方法研究电场作用下沸腾汽泡的动态特性,也有研究者研究了冷态注入气泡在电场作用下的动态特性,而直接针对单个沸腾汽泡的研究鲜有报道。本文采用针状电极以R113为实验工质,研究了小加热面上不同热流密度和不同电场强度的情况下单个沸腾汽泡的动态特性,实验中发现在电场的作用下汽泡脱离半径和脱离体积变小,而汽泡成长时间和汽泡等待时间随着电场强度的增加而增大。     

核态沸腾中汽泡动力学及传热机理分析

汽泡的传热及生长特性研究对于揭示核态沸腾的机理具有重要作用。本文介绍了利用高速摄像技术对核态沸腾中汽泡生长及运动现象进行实验观测,并用高速数据采集系统记录汽泡一个生长周期不同阶段的热流密度的方法。在不同的过冷度及壁面温度条件下,观测了汽泡的生长、周期性滑移、颈化和脱离现象。计算并绘制出不同条件下一个汽泡生长周期内的热流密度曲线,与汽泡图片相对应,分析并讨论了造成这些现象的机理。     

微重力下汽泡底部微层的对流和蒸发

本文数值研究了微重力下沸腾时汽泡底部微层区的多维传热及流动，不仅考虑由温度梯度引起的汽泡表面张力梯度而诱发的Marangoni流动，而且考虑汽泡表面的凝结和蒸发．微层厚度自0．01μm到10μm的计算结果表明，微层中流动很小，温度分布与纯导热几乎相同．在界面传热小于理想传热的情况下，Marangoni对流对传热增加几个百分比，微层中的流动与薄平界面的滑移近似相吻合．     

单个汽泡周围的电场数值研究

为明晰电场对汽—液两相系统的效应,本文针对均匀电场作用下的汽液两相流中附着于壁面的单个汽泡,建立了数学模型,考虑了汽泡的存在对电场分布的影响。通过求解电场控制方程,得到了均匀电场作用下汽泡周围及其内部的电势及电场分布的数值解。这为进一步研究 EHD 作用下的两相系统中汽泡的行为,揭示 EHD强化沸腾换热机理奠定了基础。     

一个新的核态沸腾汽泡动力学模型

本文在经典汽泡动力学理论基础上，提出了描述汽泡生长过程的综合界面模型．本模型的核心在于汽泡内部的热力学过程的详细分析及汽液界面的传热、传质过程的详细描述．并对汽泡生长过程进行了模拟计算，给出了动力学控制阶段的时间范围．本模型对汽泡生长、汽膜发展的理论分析及数值模拟提供了良好的基础．     

基于汽泡附壁接触直径的窄流道汽泡脱离预测模型

汽泡脱离是沸腾系统中非常重要的现象,对沸腾流动和传热都有重要的影响。汽泡脱离过程是非常复杂的,受到系统工况参数以及物性参数的综合影响;但汽泡在生长过程中的受力状况是决定汽泡发生脱离的根本因素。对于窄流道内的汽泡,由于受到流道壁面的限制,汽泡与加热壁面间将存在较大的接触面。本文基于汽泡生长过程中的附壁接触直径,详细分析作用在汽泡上的各个受力,并建立各个受力的计算关系式,从而得到汽泡脱离的预测模型。模型采用本研究小组在竖直窄流道内得到的实验数据进行了验证,预测结果与实验研究吻合很好。     

汽泡在电场作用下的变形

为探明外加电场对汽泡形状的影响规律,本文采用人工注射汽泡的方式,对均匀电场作用下单个汽泡的形状进行了可视化试验研究,计算了汽-液两相系统中的电场分布及汽泡所受的电应力。结果表明:电场作用下汽泡表面所受电应力分布的不均匀性导致汽泡沿与电场相平行的方向拉长,变成扁长椭球形。随着场强的增加,汽泡变形量加剧,汽泡与壁面的接触角逐渐变大。此外,讨论了汽泡变形对电场强化沸腾换热的影响。     

Effect of Bubble Contact Diameter on Bubble Departure Size in Flow Boiling

The growth and departure of bubbles on a vertical heated wall of an annular channel in flow boiling were observed by a high-speed camera, and the bubble contact diameter and bubble departure diameter were measured. It was found that bubble departure diameters were different among different nucleation sites in the same boiling area (same test condition, same surface roughness). Force balance on a single bubble attached to the heated surface was also analyzed to explain this phenomenon. The theoretical research results show that bubble contact diameter has a significant influence on bubble departure diameter, and the corresponding departure diameter is larger as the bubble contact diameter is larger. This agrees with the visual experimental results.     

含气量对空泡脉动特性影响的理论研究

分析了含气量对粘性液体中球形单空泡脉动特性的影响。研究结果表明：随着泡内初始含气量的增加,空泡达到最大泡半径的时间延长;空泡膨胀的最大泡半径、收缩的最小泡半径和脉动周期均随初始含气量的增加而增加;同时,空泡膨胀和溃灭时泡壁的运动速度均随空泡初始含气量的变化而变化。无论含气量如何,在空泡收缩到最小泡半径附近,泡壁运动速度（收缩或膨胀）要明显快于其在最大泡半径附近;此外,受液体粘性影响,空泡膨胀和收缩过程明显变缓。     

气泡在横流中的生长与脱离特性实验研究

为了系统研究气泡在横流中的运动学特性,利用水循环回路形成横流,采用高速数码摄像技术和图像处理方法对横流中气泡的演化过程进行描述。通过调节横流的平均流速和气体流量,获得了6种横流中的气泡形态,即单气泡、颈状泡、条带泡、泡囊、离散泡和雾状泡;解释了各类气泡形态的特征,对气泡尺寸及分布进行了统计,对气泡在横流中的发展过程进行了描述,发现气泡形态及气泡尺寸与横流速度密切相关,并受气体流量的影响,其中颈状泡和条带泡对横流流速的变化最为敏感。     

Modified Bubble Core Fields and Bubble Shape in Laser Driven Plasma

Bubble core fields as well bubble shape modification due to the nondepleted electrons inside the bubble is investigated theoretically. It is found that the slope of transverse fields are reduced significantly, however, the slope of longitudinal electric field, which plays a key role on electrons acceleration in bubble, changes little. Moreover a modified longitudinal compressed bubble shape leads to a shorter dephasing distance which makes the electrons acceleration energy reduced to some extent. As a comparison we perform particle-in-cell simulations whose results are consistent with that of our theoretical consideration.     

非球形水下爆炸气泡坍塌机制

 为研究非球形水下爆炸气泡的坍塌机制和射流特性,采用水下爆炸实验和数值模拟相结合的手段,研究了典型的柱形装药自由场水下爆炸气泡的运动过程。实验结果与数值模拟结果符合较好,通过对比发现,非球形水下爆炸气泡坍塌所形成的射流与球形气泡坍塌射流的行为存在差异。研究结果表明：非球形气泡的坍塌过程与气泡初始形态的局部曲率半径有关,初始气泡表面曲率半径大的区域的初始膨胀速度较曲率半径小的区域的小,坍塌时刻也较早,导致气泡产生非球形坍塌。     

Bubble population phenomena in acoustic cavitation

Theoretical treatments of the dynamics of a single bubble in a pressure field have been undertaken for many decades. Although there is still scope for progress, there now exists a solid theoretical basis for the dynamics of a single bubble. This has enabled useful classifications to be established, including the distinction between stable cavitation (where a bubble pulsates for many cycles) and transient cavitation (where the bubble grows extensively over time-scales of the order of the acoustic cycle, and then undergoes an energetic collapse and subsequent rebound and then, potentially, either fragmentation, decaying oscillation or a repeat performance). Departures from sphericity, such as shape and surface oscillations and jetting, have also been characterized. However, in most practical systems involving high-energy cavitation (such as those involving sonochemical, biological and erosive effects), the bubbles do not behave as the isolated entities modelled by this single-bubble theory: the cavitational effect may be dominated by the characteristics of the entire bubble population, which may influence, and be influenced by, the sound field.

The well established concepts that have resulted from the single-bubble theory must be reinterpreted in teh light of the bubble population, an appreciation of population mechanisms being necessary to apply our understanding of single-bubble theory to many practical applications of ‘power’ ultrasound. Even at a most basic level these single-bubble theories describe the response of the bubble to the local sound field at the position of the bubble, and that pressure field will be influenced by the way sound is scattered by neighbouring bubbles. The influence of the bubble population will often go further, a non-uniform sound field creating an inhomogeneous bubble distribution. Such a distribution can scatter, channel and focus ultrasonic beams, can acoustically shield regions of the sample, and elsewhere localize the cavitational activity to discrete ‘hot spots’. As a result, portions of the sample may undergo intense sonochemical activity, degassing, erosion, etc., whilst other areas remain relatively unaffected. Techniques exist to control such situations where they are desirable, and to eliminate this localization where a more uniform treatment of the sample is desired.     

动态气泡直径的远场干涉测量方法

利用气泡远场干涉对动态气泡直径进行测量,结果表明,当透明液体中的动态气泡经过激光束时必然在远场产生圆环状干涉条纹.对传统分光仪进行改制,用数码照相机代替望远镜,并连接计算机,组合成动态气泡直径测量系统,拍摄得到干涉条纹,对干涉条纹的角半径进行测量,并计算出气泡直径,测量相对差为1.1%.该系统还可对单个气泡束流或多个气泡束流进行气泡直径的统计.     

Bubble nonlinear dynamics and stimulated scattering process

A complete understanding of the bubble dynamics is deemed necessary in order to achieve their full potential applications in industry and medicine. For this purpose it is first needed to expand our knowledge of a single bubble behavior under different possible conditions including the frequency and pressure variations of the sound field. In addition, stimulated scattering of sound on a bubble is a special effect in sound field, and its characteristics are associated with bubble oscillation mode. A bubble in liquid can be considered as a representative example of nonlinear dynamical system theory with its resonance, and its dynamics characteristics can be described by the Keller–Miksis equation. The nonlinear dynamics of an acoustically excited gas bubble in water is investigated by using theoretical and numerical analysis methods. Our results show its strongly nonlinear behavior with respect to the pressure amplitude and excitation frequency as the control parameters, and give an intuitive insight into stimulated sound scattering on a bubble. It is seen that the stimulated sound scattering is different from common dynamical behaviors, such as bifurcation and chaos, which is the result of the nonlinear resonance of a bubble under the excitation of a high amplitude acoustic sound wave essentially. The numerical analysis results show that the threshold of stimulated sound scattering is smaller than those of bifurcation and chaos in the common condition.