基于CLSVOF方法的三维单个上升气泡运动的数值模拟

运用CLSVOF方法数值模拟了三维单气泡在液体中的上升和变形过程,分别考察了液体的表面张力和浮力对上升气泡变形的影响。计算结果表明:CLSVOF方法能比较精确地跟踪三维单气泡在液体中的上升过程;在相同密度比和粘度比情况下,随着液体表面张力的减小和液体浮力的增大,气泡底部形成的射流使得气泡下缘凹陷变得明显,变形幅度加大,射流穿透气泡的时间提前。     

SURFACE TENSION EFFECTS ON THE BEHAVIOR OF TWO RISING BUBBLES

In the inviscid and incompressible fluid flow regime, surface tension effects on the behavior of two initially spherical bubbles with same size rising axisymmetrically in an infinite and initially stationary liquid are investigated numerically with the VOF method. The numerical experiments are performed for two bubbles with two different bubble distances. The ratio of gas density to liquid density is 0.001, which is close to the case of air bubbles rising in water. In the case of Dis = 2.5, where Dis is defined as the ratio of the distance between the bubble centroids to the radius of the bubble, it is found from numerical experiments that there exist four critical Weber numbers We1 , We2 , We3 and We4 , which are in between 10 and 100, 3 and 4, 1.5 and 1.8, and 0.2 and 0.3, respectively. In the case of Dis = 2.3, similar phenomena also appear but the corresponding four critical Weber numbers are lower than those in the case of Dis = 2.5. The mechanism of the above phenomena is analyzed theoretically and numerically.     

VIBRATION OF A SINGLE PROTEIN BUBBLE IN BINGHAM LIQUID

The vibration of a single protein bubble may take place under the action of high pressure difference. In this process, the bubble wall may experience a finite deformation. The equation describing the dynamics of the protein bubble with viscoelastic film in Bingham liquid is derived. A numerical solution to this equation is carried out to study the effect of liquid pressure, the characteristic parameters of Bingham liquid and the viscosity of the protein film on the finite deformation of the bubble. The results show that the vibration of the protein bubble wall is caused by the action of pressure difference, the elastic stress in finite deformation and the dissipation of viscosity of the protein film and Bingham liquid. The vibration is nonlinear. Decreasing the pressure difference between gas and Bingham liquid on both sides of the protein bubble will lead to a change of vibration performance. The frequency and amplitude are reduced, tegether with the speed of vibration damping. In addition, the deformation rate of the bubble is smaller when the amplitude of vibration is reduced, which means shorter time to reach a balance state. On the other side, the increase of the magnitude of viscosity of the protein film or the plastic viscosity of Bingham liquid can restrain the vibration of the protein bubble wall in the course of finite deformation, as a result, the load bearing capacity of bubble is enhanced.     

平口管口处气泡行为特征数值模拟

为研究平口管口处单个气泡行为特征,采用流体体积模型(Volume of Fluid, VOF)方法,数值模拟研究气泡的膨胀脱离过程,对比分析了浸没方式、管径、气体速度、液体密度和表面张力对气泡膨胀脱离时间和脱离直径的影响规律。研究结果表明:在3种不同管口浸没方式下,气泡膨胀脱离时间均随管口气体速度增大而降低,但降低速率逐渐减小;在底部和顶部浸没方式下,气泡膨胀脱离时间和脱离直径随表面张力增大而增加,随液体密度增大而降低;对于侧面浸没方式,气泡膨胀脱离时间和脱离直径随表面张力增大整体呈上升趋势,而受液体密度影响不大。气泡膨胀脱离时间与气泡脱离直径呈正相关。所建立的数学模型能够真实地模拟气泡形成及运动特征,从而提供一个有效的研究方法。     

汽液黏滞系数对空化泡溃灭过程影响的数值模拟

利用格子玻尔兹曼方法(LBM)伪势模型模拟了近壁区空化泡溃灭过程,分析了不同汽相黏滞系数和液相黏滞系数对空化泡溃灭过程中的最大微射流流速、最大溃灭压力、最大溃灭时间的影响。结果表明,相同液相黏滞系数条件下,改变汽相黏滞系数,空化泡溃灭时产生的最大微射流流速、最大溃灭压力和最大溃灭时间不变;保持汽相黏滞系数不变,空化泡溃灭时最大溃灭压力、最大微射流流速均随着液相黏滞系数的增大而减小,但最大溃灭时间随着液相黏滞系数的增大而增大。     

气泡从浸没孔中生成与上升的数值模拟

应用改进的Level Set方法和Navier-Stokes方程，结合ALE数值算法，三维模拟了常流量下气泡在静止黏性液体中从浸没孔中生成与上升的过程。考察了气泡在两种不同的黏性液体以及在单孔和多孔条件下的生长过程。模拟结果与实验观测定性相符。数值试验表明，改进的Level Set方法改善了气泡的容积守恒性。模拟结果显示，在不同的黏性液体中单孔气泡的生成行为很不相同，在Paratherm传热流体中生成的气泡直线上升，而在水中则是盘旋上升；多孔气泡的生成不同步，气泡的尺寸及空间分布极其不均匀。     

叶片式泵内气液两相泡状流的三维数值计算

为研究旋转叶片式泵中的气液两相流动规律及泵性能，本文根据气泡泡状流模型提出了一个两相流三维数值分析方法。该数值计算由两部分的计算迭代完成，第一部分是已知含气率分布的连续相(液相)流场的计算。液相流动方程的三维数值求解采用了流面坐标迭代法。第二部分是已知液相流场后的分散相(气泡)轨迹的计算。气泡运动方程中考虑了流场压力梯度差产生的力、气泡周围液体产生的阻力及由于液相质量产生的惯性力等影响气泡运动的因素。通过对气泡运动方程进行数值求解，可得到泵内的含气率分布。再将两部分的计算进行反复迭代，最终得到收敛解。本文将该三维两相流数值计算方法用于多级螺旋轴流式增压泵叶轮的计算、设计。计算结果表明：从叶轮回转面上看，气泡运动转轨并不明显地偏离液相流线。然而从叶轮子午面上看，几乎所有的气泡，不论其初始位置如何，它们最终都移向轮毂表面。该三维两相流数值计算以其简单实用的特点，有望成为工程中气液两相增压泵的一种有效设计工具。     

液固悬浮液中气泡生成的数值模拟

将Level Set方法与硬球离散颗粒方法相结合,建立了一个三相流混合模型。应用该模型对液固悬浮液中气泡从单孔及多孔中的形成过程进行了三维模拟,并与文献中已有的结果进行了定性比较。数值试验主要比较了在悬浮液与纯液体中的气泡生成与上升行为。模拟结果表明,颗粒的存在对气泡的形成与上升有重要影响;气泡诱导的流体流动对气泡与颗粒的运动均有显著地影响。     

下呼吸道重开的生物流体力学研究：理论分析

本文理论分析了受阻塞肺下呼吸道重开的生物流体力学问题，解控制流动的低雷诺数的Stokes方程，得出阻塞液前阵面和指状气泡前阵面的位置、速度等公式。所得结果和作者前文所发表的实验结果[17]吻合得相当好，进一步分析表明阻塞液粘度、管径和外加压强对流以动有较大影响，毛细数Ca同样受到这些参数的影响。     

水中气泡溃灭的理论与数值研究

从理论和数值两个方面研究了单个气泡在自由流场中的溃灭.采用Rayleigh方程推导出一个新的球形气泡演变的解析解.基于流体体积法( VOF),在考虑表面张力、粘度和可压缩性效应的情况下,对单个纯气体泡的运动进行了三维模拟.在第一次溃灭阶段,气泡体积演变的数值结果与解析解保持一致. 数值结果表明气泡体积到达最小值后开始回弹,气泡的形状、速度场和压力场在回弹点附近发生剧烈变化.     

静止水中单个气泡与倾斜壁面碰撞及反弹的实验研究

采用高速摄影和图像处理技术,实验研究了在静止水中单个气泡与倾斜壁面碰撞与反弹运动。设计了试验装置,给出了照明方案、试验程序及数字图像处理的技术。获得了气泡与壁面碰撞后的运动轨迹、上升及反弹速度等参数,初步得到了气泡与壁面碰撞及反弹的规律。     

EXPERIMENTAL INVESTIGATIONS ON BUBBLE COLLAPSE NEAR BOUNDARIES

Collapse of a spark-generated bubble near rigid or deformable surfaces is studied experimentally with a high speed cammera. The formation of a bubble wall jet has been observed and discussed. Results from experimental studies on the effect of liquid viscosity on bubble collapse near the rigid wall are also presented.     

EFFECT OF A PROPELLER AND GAS DIFFUSION ON BUBBLE NUCLEI DISTRIBUTION IN A LIQUID

A multi-bubble dynamics code accounting for gas diffusion in the liquid and through the bubble wall was developed and used to study the modification of a bubble nuclei population dynamics by a propeller.The propeller flow field was obtained using a Reynolds-Averaged Navier-Stokes(RANS) solver and bubble nuclei populations were propagated in this field.The numerical pro-cedure enabled establishment of the possibility of production behind the propeller of relatively large visible bubbles starting from typical ocean nuclei size distributions.The resulting larger bubbles are seen to cluster in the blade wakes and tip vortices.Parametric investigations of the initial nuclei size distribution,the dissolved gas concentration,and the cavitation number were conducted to ide-ntify their effects on bubble entrainment and the resultant void fractions and bubble distribution modifications downstream from the propeller.Imposed synthetic turbulence-like fluctuations unto the average RANS flow field were also used to study the effect avera-ging in the RANS procedure has on the results.     

THE NONLINEAR BEHAVIOR OF INTERFACE BETWEEN TWO-PHASE SHEAR FLOW WITH LARGE DENSITY RATIOS

1. INTRODUCTION Fluids with interface or free surface have potential applications in the fields of geophysics, engineering, and applied physics. In nature, when wind blows over a sea surface in oceanography, breaking waves play a significant role in the t…     

水下高速喷气气水流场研究

水下喷气是一个复杂的过程，本文在一定假设的基础上，将复杂的流场分为三部分：喷管内流场、气泡流场及流体水流场。喷管内流场采用准一维模型，用差分格式求解，气泡模型采用滞止等压泡模型；液体流场采用MAC方法，通过步进法实现三个流场的耦合求解。给出了几种环境压力及速度下的数值计算结果，初步掌握了水下喷气的特征。     

Significant intervals of energy transforms in bubbles freely oscillating in liquids

A wall motion of a bubble freely oscillating in a liquid is studied from the point of view of energy conversions at different instants. It is shown that the time of the bubble oscillation can be divided into two distinct intervals. In the first long interval(here called PK and KP intervals) the prevailing energy conversion is between the potential energy of the bubble and the kinetic energy of the liquid. In the second short interval(here called KI and IK intervals) the kinetic energy of the liquid is transformed into the internal energy of the gas/vapor in the bubble interior and into some other forms of energy. By observing the bubble wall motion in the PK and KP intervals, it is shown that only the value of the maximum bubble radius in the corresponding oscillation can be determined. However, only the knowledge of the maximum bubble radii is insufficient for formulation of a correct theoretical model. Unfortunately this fact is often not noticed in the literature.     

The effect of bubble plume on oxygen transfer for moving bed biofilm reactor

The movement of the bubble plume plays an important role in the operation of a moving bed biofilm reactor （MBBR）, and it directly affects the contact and the mixture of the gas-liquid-solid phases in the aeration tank and also the oxygen transfer from the gas phase to the liquid phase. In this study, the velocity field is determined by a 4-frame PTV as well as the time-averaged and timedependent velocity distributions. The velocity distribution of the bubble plume is analyzed to evaluate the operating efficiency of the MBBR. The results show that the aeration rate is one of the main factors that sway the velocity distribution of the bubble plumes and affect the operating efficiency of the reactor.     

曝气池中气液两相流数值模拟

应用数值模拟的方法,对曝气池气液两相流运动规律进行了探讨。研究表明:当孔径相同,空气流量越大,紊动强度越大,气液混合越充分,曝气效果越好。对于相同的水深和相同的孔径,流量是影响气泡大小和速度的主要因素。当气体流量或速度增加时,气泡体积增大,气相速度也明显增大。     

Bubble hydrodynamic influence on oxygen transfer rate at presence of cationic and anionic surfactants in electroflotation proces

In this work, the effects of the presence of surfactants in the liquid phase and the hydrodynamic regime of the bubble flow on the oxygen transfer rate were investigated in an electroflotation process in batch mode. The volumetric mass transfer coefficient KLα and the oxygenation capacity were evaluated to improve the performances of the electroflotation process in terms of oxygenation. In order to evaluate the liquid-side mass transfer coefficient KLα the volumetric mass transfer coefficient KLα was dissociated into KL and the specific interracial area （a） since the last one was obtained from the gas hold-up and the bubble diameter. The effect of Reynolds number which define the hydrodynamic of the bubble flow has been also studied. Models of KLa and KL have been established to show the effects of the hydrodynamic parameters and liquid phase characteristics on the oxygen transfer rate.     

两相流相界面迁移的数值模拟

该文采用VOF法追踪了重力流液液和气液相界面的迁移，发现VOF法可很好地追踪强非线性、大形变的相界面随时间的变化。重点讨论了不同相密度比、黏性比条件下，液液相界面和气液相界面的运动形式，验证了液液相界面主要受Kelvin-Helmholtz不稳定性控制。结果显示：液相密度比越大，液液相界面随时间变化越快，气液交界面不再保持水平位置，而以摆动的形式存在；在黏性比小于5时，黏性比不太影响液液相界面随时间变化。