基于MATLAB的超声空化气泡动态仿真

推导出考虑了液体粘滞性、表面张力和辐射阻尼的单一空化气泡运动方程,在Matlab下仿真得到了单一空化气泡的动态曲线,为用数值方法进一步研究空化气泡的运动过程提供了参考。     

非牛顿流体中的单气泡行为研究

通过实验研究了非牛顿流体ＣＭＣ水溶液－Ａｉｒ中的单气泡行为，并重点考察了操作条件，液体性质，通所型式等因素的影响，结果表明：操作条件、液体性质对气泡的形态、大小及其运动均有直接影响，通敢型式对气泡形态的影响并不明显；体系非牛顿性的增强，气泡间的聚并加剧。     

晃荡条件下气泡上升速度特性研究

海浪、地震等条件都会导致核动力设备中储存自由液面的设备产生剧烈的晃荡。为了分析晃荡条件下液体中的气泡行为特性,本文运用CLSVOF(coupled level-set and volume-of-fluid)模型追踪两相流体交界面,数值模拟气泡在液体中受余弦简谐激励和自由液面影响的上升过程。计算结果显示余弦简谐激励降低了气泡纵向运动速度,而横向速度略大于激励速度;自由液面的波动在纵向上会使气泡上升速度骤然降低,甚至出现负速度,越接近自由液面横向速度峰值越大。计算结果表明晃荡运动的影响是余弦简谐激励与自由液面波动影响的叠加,晃荡条件下自由液面波动是气泡速度变化的主要因素,气泡的本身属性起次要作用。     

螺旋内构件强化排液的泡沫分离塔内上升泡沫的流体力学

为了探索带有螺旋内构件的泡沫分离塔强化泡沫排液的机理,对塔内螺旋上升泡沫的流体力学行为进行了研究.结果表明,螺旋通道内泡沫的错流排液和旋转运动是促使气泡聚并加快的主要原因.离心力的存在不利于重力导致的排液,但可使间隙液流到塔壁上进而回流到液池当中.泡沫由竖直通道进入到螺旋通道后,表观液体流速增大而液体分数下降.螺旋通道内表观液体流速和液体分数受气泡大小的影响较大.气泡越大,螺旋构件强化排液的效果越明显.     

射流搅拌发酵罐中循环液量气含率和气泡直径的研究

本文测定了射流搅拌发酵罐中自由射流气泡区的气含率、气泡直径和液体循环量。研究了混合器、喷嘴尺寸、气速、清液层高度、液体粘度、表面张力对自由射流气泡区的液体循环量、气含率、气泡直径的影响。由理论分析及实验数据获得了液体循环量、气含率、气泡直径的的关联式。模型关联式与实验值吻合良好。     

粒子图像测速技术在两相流测量中的应用研究

为实现对两相流中各相速度的同步测量，提出了一种两相流数字粒子图像测速(PIV)方法.该方法选用合适的示踪粒子显示液体的运动，并用高速摄像机采集气液两相流动数字图像.从图像处理着手，根据图像灰度的不连续性和相似性，对采集到的两相流数字粒子图像进行分离，实现液体示踪粒子和气泡的标定.最后采用基于改进了的互相关算法得到液体流场速度分布，同时利用跟踪法得到气泡的速度分布，并对液体速度场中产生误矢量的原因进行了分析，消除了误矢量.经实例研究表明，该测量方法对硬件的依赖性小，测量成本低，可进一步推广至气固两相流的测量.     

垂直上升弹状流中Taylor气泡上升流速的研究

用势流理论研究了Ｔａｙｌｏｒ气泡在垂直管内的上升速度．给出了液体速度分布对气泡上升速度的影响,导出了气泡头部形状方程;提出了液体流动为层流和紊流时气泡上升速度的表达式,研究结果与实验数据进行比较,符合较好．     

基于运动场分离水下图像增强处理方法研究

对于处于气泡等运动信息干扰情况下的水下视频图像,传统的增强手段不能达到令人满意的要求。本文提出了一种在水下视频图像中去除气泡场的方法。采用独立成分分析算法,将序列图像中的背景和运动目标进行分离处理,在灰度域上区分开气泡和其他运动目标并去除气泡,然后,在已经去除了气泡信号的图像中重新赋上其相邻序列的视频图像中相同位置但清晰的像素值,并使用光流法对实验结果进行了分析。结果证明,此方法能够成功的从水下图像中分离出气泡信号,达到了削弱、去除水下图像中气泡场的目的。     

基于气泡跟踪与相位相关的浮选表面气泡平移运动估计

针对浮选表面气泡运动估计难以兼顾准确度和速度的问题,提出了结合气泡跟踪与相位相关的浮选表面气泡运动估计方法。首先对序列的相邻2帧图像按灰度最小值进行尺度缩小,然后通过阈值分割法提取其气泡高亮区域以完成气泡跟踪,最后采用相位相关法得到气泡平移运动信息。同时提出一种分块相位相关法用于进一步获取各个气泡子区域的运动特征。多组运动估计仿真测试证实了算法的有效性和准确度。铅锌浮选厂的现场实验结果表明,算法能快速地获取气泡运动矢量,分块相位相关法能够提供各子块的运动特征。浮选各槽的气泡运动曲线数据表明：每个浮选槽表面气泡的平移运动均表现出了一定的周期性;同一等级的浮选槽里的气泡平移运动曲线和运动紊乱程度曲线具有类似的分布规律;保持一定程度的气泡运动紊乱性有助于提高精矿的品质。     

气液固三相逆流化床内气泡行为的实验研究

应用自行开发的微电导探针技术,对气液固三相逆流化床内的气泡尺寸、气泡上升速度和气泡频率等气泡特性进行了系统研究,考察了床内气泡特性的轴径向分布规律及气液表观速度、液体黏度、固体加入量、颗粒密度及表面活性剂的加入等对气泡特性的影响规律。结果发现,随距离气体分布板的轴向距离增加或由床器壁向中心区域的距离增加,气泡大小和频率都增加,前者分布从窄变宽,后者相反。气泡上升速度与气泡大小的轴径向分布很相似。随固体加入量、颗粒密度增加或水中乙醇的加入,气泡大小和上升速度减小,气泡频率增加;随表观气速增加,三者都增加;随表观液速增加,气泡大小和频率增加,气泡上升速度减小;随液体黏度增加,气泡大小和上升速度增加,气泡频率减小。利用操作变量和流体物性对各气泡特性进行了关联。     

双吸离心泵叶轮空化流动预测的数值分析

基于CFD数值模拟和外特性试验并重的方法,结合RNG k-ε湍流模型、欧拉两相流模型和Zwart-Gerber-Belamri空化模型,采用非结构化网格和SIMPLEC算法,对一台单级双吸泵内的空化流场特性进行多工况下的定常数值研究,得到以下主要结论:不同流量泵内部空化发展速度不同,小流量发展速度较慢,各阶段空化数相对较小,大流量时空泡发展速度更快;液体流经双吸式叶轮结毂处与双涡壳隔板起始端时会产生液体之间或液体与固体之间的相互作用,导致流体液流角发生变化,出现脱流与漩涡,使这些位置及附近区域由于能量耗散造成压力减小,进而导致空泡析出;此现象在0.6Q_d工况下最明显,说明小流量工况液流角更易发生变化。     

High-speed photographic observation of collapse of two cavitation bubbles

Cavitation as a hydrodynamic phenomenon exists widely in water conservancy, shipbuilding, chemical and many other industries. Previous cavitation bubble dynamic studies mainly focused on single cavitation bubbles and their interaction with the wall. This paper studies the interaction between two cavitation bubbles under conditions with or without a wall. The results show that if the inception of two cavitation bubbles is not synchronized, the cavitation bubble of early inception collapse backwards the cavitation bubble of later inception; if the inception of two cavitation bubbles is synchronized, the two bubbles collapse towards each other; if a wall exists nearby, no matter whether the line connecting the centers of the two cavitation bubbles is vertical or parallel to the wall, the two cavitation bubbles collapse towards each other and then gradually merge, and the merged collapse body quickly moves to the wall. It is suggested that, as the number of cavitation bubbles increases, the cavitation erosion effect is not simply increased proportionally. Instead, mutual inhibitory effect may be demonstrated.     

节流槽结构对气穴噪声的影响

为了消除液压阀中的噪声，对不同节流槽内部压力分布、气穴形态和噪声频谱进行了研究.在对节流槽结构特征及流动模型进行分析的基础上，预测了节流阀内气穴噪声的声压模型.通过高速观测及噪声频谱分析对模型进行了验证，并估计了节流阀内的气泡尺寸.结果表明，节流槽结构特征对阀内压力分布、气穴与噪声特性有直接的影响，U形槽内部压力超调量明显高于V形槽，可以显著抑制气穴的析出与生长.不同类型节流槽通过压力分布决定了气泡的成长过程，与U形槽相比，V形槽内部气泡尺寸较大，气泡尺度大小是影响节流槽气穴噪声的主要因素.     

扩展角对收缩－扩散型喷嘴空化效果的影响

喷嘴扩展角是影响收缩-扩散型喷嘴空化效果的重要参数之一。运用连续方程、动量方程和Rayleigh-Plesset方程,对流经不同扩展角的收缩-扩散型空化喷嘴的泡液流进行稳态求解。计算结果表明:扩展角的存在对液泡流的流动特性有强烈影响,随着扩展角的变化,喷嘴空隙率和气泡半径呈现"剧烈振荡"。空泡在临界值附近大幅值、高频率振荡导致的空泡多次溃灭过程,将有利于释放更大的空化能量。     

Validation of dynamic cavitation model for unsteady cavitating flow on NACA66

Unsteady cavitating flow is extremely complicated and brings more serious damages and unignorable problems compared with steady cavitating flow.CFD has become a practical way to model cavitation;however,the popularly used full cavitation model cannot reflect the pressure-change that the bubble experiences during its life path in the highly unsteady flow like cloud cavitating.Thus a dynamic cavitation model(DCM)is proposed and it has been considered to have not only the first-order pressure effects but also zero-order effect and can provide greater insight into the physical process of bubble producing,developing and collapsing compared to the traditional cavitation model.DCM has already been validated for steady cavitating flow,and the results were reported.Furthermore,DCM is designed and supposed to be more accurate and efficient in modeling unsteady cavitating flow,which is also the purpose of this paper.The basic characteristic of the unsteady cavitating flow,such as the vapor volume fraction distribution and the evolution of pressure amplitude and frequency at different locations of the hydrofoil,are carefully studied to validate DCM.It is found that not only these characteristics mentioned above accord well with the experimental results,but also some detailed transient flow information is depicted,including the re-entrant jet flow that caused the shedding of the cavity,and the phenomenon of two-peak pressure fluctuation in the vicinity of the cavity closure in a cycle.The numerical results validate the capability of DCM for the application of modeling the complicated unsteady cavitating flow.     

Effect of air bubble size on cavitation erosion reduction

Over the past 60 years, the air concentration in water has been considered as a control index of cavitation erosion reduction and widely used in the designs of hydraulic structures. However, the mechanism of air entrainment against cavitation erosion has been paid good attention to. In the present work, the effect of air bubble size on cavitation erosion reduction was experimentally investigated. A device with micron-scale orifice diameters(10, 20 and 50 μm in size) was specially designed to introduce air bubbles into water. The experiments about the effect of air bubble size were conducted by means of a vibratory apparatus, including the behavior of formation and movement for single air bubble, the characteristics of cavitation erosion reduction at different air entrainment conditions. The findings demonstrate that high air concentration has significant effects on cavitation erosion reduction.But, a notable problem was that the size of air bubbles is of outstanding effect on cavitation erosion reduction. Small air bubbles support to alleviate cavitation erosion, even at same air concentration.     

超空泡射弹尾翼流体动力特性实验分析

为研究尾翼对超空泡流型发展及射弹流体动力的影响,在高速水洞中进行了通气条件下的超空泡射弹尾翼流体动力特性实验.首先基于高速水洞搭建了通气空化绕流实验测试系统,同时根据射弹基本外形,按照相似理论设计了尾翼测力实验模型,为改变空泡壁面与尾翼的相对位置关系,用以研究穿刺高度对射弹尾翼流体动力特性的影响,实验中采用调整空泡尺寸或模型安装姿态来改变尾翼穿刺高度,最后对实验方案的合理性进行了充分的论证.通过改变测试工况对超空泡射弹尾翼的流体动力特性开展实验研究,得到了不同穿刺条件下尾翼对空泡流型的影响规律及其流体动力特性.研究结果表明:尾翼穿刺空泡以后,在其顶部和侧面生成的二次空泡将显著改变主体空泡的形态,且随着穿刺高度的增加,尾翼对主体空泡的影响更加明显;尾翼的流体动力主要产生于前缘沾湿部位,且随着穿刺高度的增加升力系数和阻力系数均显著增加;在尾翼穿刺空泡的条件下,其阻力系数和升力系数随攻角的增大而线性增加.     

离心泵的空化流数值模拟与空化余量预测

空化余量是泵非常重要的性能指标之一,目前主要依靠试验来确定。如何在离心泵设计过程中较为准确地预测出必须的空化余量对优化设计和提高运行稳定性等方面十分重要。针对离心泵运行过程中发生空化时的流动特点,基于Rayleigh—Plesset方程来描述空泡生长和溃灭过程的空泡动力学模型,采用混合空化两相流模型和三维全流道两相流流动数值模拟技术,探索通过数值试验来预测空化余量的方法。对一低比转速离心泵进行全流道空化流数值模拟,通过改变NPSHa来模拟试验工况,数值模拟预测出各模拟试验工况下的扬程、叶片表面压力分布、叶片表面空化发生区域以及流道内空泡体积率分布,从而预测该泵的NPSHr,其预测结果与试验值的误差小于10％。     

Dynamic Characteristics of Supercavitating Flow Around a Hydrofoil

Supercavitation dynamic characteristics around a hydrofoil are studied with both high-speed visualization and drag and lift measurements. In the supercavitation condition, the cavitation area covers the entire surface of the foil. There is a distinct interface between the main flow and the supercavitating flow regions. The cavitatin gregion is filled with cavitation bubble and water mixture. Small fluctuations of the interface are observed, which indicates that even a relative smooth surface is formed in this state, the interface is unsteady. The minimum drag is to be obtained in the supercavitation condition.