基于扩散界面法的液态LBE中单个弹状气泡上升行为CFD研究

利用扩散界面法对液态铅铋合金中弹状气泡在垂直管中的上升过程进行数值模拟研究,用来验证扩散界面法在模拟液态铅铋合金(LBE)中弹状气泡上升行为的准确性和可行性,并对模拟结果进行进一步分析,来解释弹状气泡在LBE中的上升行为。数值模拟得到不同液体流速情况下弹状气泡在上升过程中的形态和速度变化,数值模拟结果与文献中的经验关系式以及实验中的结果都吻合良好,证明了扩散界面法在模拟液态铅铋合金中弹状气泡上升行为的准确性和可行性,为液态金属和弹状气泡的两相流提供了一种新方法。对数值模拟结果进行进一步分析,发现气泡尾部形态变化较大,会在尾部两端分裂出子气泡,尾部附近流场产生旋涡,受到强烈干扰而出现湍流,对于提高换热效率起到积极作用。气泡上升对尾部区域产生影响的最远距离约80 mm,为连续气泡的注入提供了气泡间距参考值。     

降膜流动行为的数值模拟研究

本文对降膜的流动行为进行了二维和三维的数值模拟。分析了不同雷诺数下降膜的液膜厚度、液膜速度、壁面剪应力。模拟结果与实验值以及理论模型的推导值进行了比较,吻合程度良好。模拟研究表明：三维模拟可再现降膜的表面波动,而二维模拟无法预测这种波动。三维数值模拟得到的瞬态结果也显示了表面波动对液膜速度和壁面剪应力的影响。     

液态金属内单个气泡上升行为的MPS法数值模拟

液态金属冷却核反应堆采用气泡泵的概念设计来提升堆芯自然循环能力。液态金属内气液两相流动特征将直接影响核反应系统一回路的自然循环能力及堆芯安全。本研究通过采用移动粒子半隐式（MPS）方法,对液态金属中单个上升气泡的气泡动力学行为进行数值模拟。分析了铅铋合金中3种初始直径不同的单个氮气泡在上升过程中的气泡形状和速度的变化趋势;对比了初始直径相同的单个氮气泡在液钾、液钠、铅铋合金、钾钠合金和锂铅合金5种液态金属中的上升行为;同时将模拟得到的气泡形状与Grace经验关系图进行了对比,验证了MPS方法数值模拟结果的正确性。     

竖直窄矩形通道内弹状流中液膜特性研究

气液两相弹状流广泛存在于工程领域,弹状流中液膜特性对弹状流模型的建立具有重要意义。为此利用高速摄像系统,对竖直窄矩形通道(3.25 mm×40 mm)内弹状流中液膜进行了可视化研究。实验中发现窄矩形通道中气弹左右两侧窄边液膜厚度不等且存在波动,但其对两侧液膜速度影响较小,两侧液膜速度相等。液膜脱离厚度主要受两相流速及气弹长度影响。液膜脱离速度随液相折算速度增加而增大;在低液相流速时,随气相折算速度增加而减小;当液相流速≥1.204 m/s时,液膜不下落,液膜脱离速度随气相速度变化较小,主要受液相流速影响。     

基于扩散界面法的液态铅铋合金中气泡上升行为模拟

基于扩散界面法,对单个氮气气泡在液态铅铋合金内从静止到充分发展整个过程中的动力学行为进行数值模拟,得到气泡形变特性和气泡上升速度随时间的变化关系,将模拟结果与Grace经验关系图对比,发现模拟得到的气泡形变结果在Grace经验关系图中均可找到且很好地吻合,从而验证了扩散界面法在模拟液态铅铋合金中气泡上升行为的可行性和准确性。同时基于界面扩散法的模拟,对比了5种不同初始直径的氮气泡在液态铅铋合金中的上升行为,发现初始直径较小的气泡在上升过程中扰动会更剧烈,初始直径较大的气泡在上升过程中易发生分裂现象。     

竖直窄矩形通道内弹状流液弹中分区特性研究

弹状流的液弹部分受气弹尾部影响,其水力特性参数沿流动方向存在分区的不一致性。本文对竖直窄矩形通道中弹状流液弹内参数的分布特性进行了研究。结果表明：液弹内气泡在近壁面附近所受径向力较为平衡,气泡频率较大;随着远离气弹尾部,管道中间气泡频率逐渐增大。根据气泡频率波动变化将液弹分为3个区域,尾流区占液弹长度的40%～45%,过渡区占10%～15%,主流区占40%～50%。尾流区和主流区内,空泡份额呈“三峰型”分布;随着气相流速的增加,尾流区内近壁面处峰值逐渐增大,管道中间峰值逐渐下降,但主流区内情况相反。气泡直径随气相流速的增大而变大,且液弹内气泡聚合和破碎现象较少。     

基于计算流体动力学的矩形通道内气泡运动机理研究

基于计算流体动力学(CFD)方法,利用体积流(VOF)模型对矩形流道内单气泡及双气泡运动进行了数值模拟,并分析了其运动规律。模拟结果与前人实验观测结果相符。单气泡上升过程中经历加速上升和摆动上升两个阶段,其稳定速度与Eo数相关,摇摆阶段的气泡轨迹类似于周期性的正弦曲线。上下分布的气泡会发生聚并现象,水平对称分布的气泡上升过程中会出现规律的分离、靠近现象。研究表明:该方法能够准确地模拟气泡的运动,获得气泡运动的机理,可以作为进一步研究自然循环中不同浮升力下气泡分布机理研究的基础。     

竖直窄矩形通道内空气-水两相流中气弹运动速度研究

以空气和水为工质,应用高速摄像仪,对竖直窄矩形通道(3.25 mm×40 mm)内气液两相弹状流进行了可视化实验研究。气、液相表观速度分别为0.1~2.51 m/s和0.16~2.62 m/s,工作压力为常压。实验中发现窄矩形通道内弹状流与圆管中存在较大差别,气弹多发生变形,高液相流速时变形更为严重。窄边液膜含气量较高,在高液相流速时窄边液膜不下落,宽边液膜中含有由气弹头部进入和气弹尾部进入的气泡。气弹速度受气弹头部形状和宽度影响较大,受气弹长度影响较小。气弹速度可由Ishii & Jones-Zuber模型计算,但在低液相折算速度时偏差较大,其主要原因为漂移速度计算值较实验值偏小。     

晃荡对气泡上升运动影响的数值研究

海洋条件中风浪等因素会导致船舶上存在自由液面的容器产生剧烈的晃荡运动。为了分析晃荡条件下液体中的气泡行为特性,在计算流体动力学软件Fluent平台下运用流体体积函数法模型,对晃荡条件下气泡在液体中的上升过程进行数值模拟。计算结果显示,晃荡条件下气泡呈周期性摆动上升,并伴随着聚合与分离现象。分析表明,晃荡对气泡运动行为特性产生的影响不可忽视,剧烈的晃荡运动会影响气泡的脱离尺寸,在加速部分气泡分离的同时阻碍部分气泡的运动而导致气泡聚合;晃荡运动还会致使气泡在水平方向偏离比较大的位移。     

溶液堆内气-液两相流流动及换热特性数值研究

在溶液堆台架模型数值模拟研究的基础上,对实际堆结构的堆芯内气-液两相流流动及冷却盘管与堆内溶液间的换热特性进行了数值模拟研究.采用欧拉两相流模型描述堆芯内气-液两相流流动,MUSIG(MUltiple-SIze-Group)模型描述堆芯内气泡尺度分布和相互作用,流-固耦合模型描述溶液与盘管间换热.数值计算得到了堆芯内的温度、速度、气泡组分等分布及冷却盘管的换热效率.数值计算结果表明:在有气泡扰动时堆内温度分布比没有气泡时均匀,冷却盘管可将堆内产生的85%热量带出,与试验测量结果一致.额定功率时,不同气体产生量对于冷却盘管换热影响的研究表明,随着堆内气泡产生量的增加,溶液与冷却盘管之间的换热得到强化.     

静水中单气泡运动特性实验研究

气泡运动特性是两相流研究的重要领域之一。本文基于图像识别技术对静水中单气泡的上升过程及其动力学特性进行研究，分析了不同等效体积直径下气泡上升过程中轨迹、变形程度、瞬时速度等参数。研究结果表明：椭球型单气泡上升过程中横向振荡幅度峰值与横向速度峰值呈正比，瞬时纵横比与瞬时纵向速度呈反比；单气泡横向运动位置、纵横比、上升速度具有周期性变化的现象。通过定量分析气泡末速度与等效体积直径的关系，对已有预测关系式进行了比较和评价，并基于实验数据拟合得到了一个更为精确的显式末速度关系式。     

Experimental Investigation on Characteristic of Single Bubble Motion in Stagnant Water

The bubble motion characteristic is one of the most important fields in two-phase flow research. Based on the image recognition method, the characteristic and dynamics of single bubble rising process in stagnant water were analyzed. The trajectory, deformation extent and instantaneous terminal velocity of single bubble rising process with different equivalent volume diameters were analyzed. The research results show that the peak value of lateral oscillation amplitude is proportional to the peak value of lateral velocity. The instantaneous aspect ratio is inversely proportional to instantaneous axial velocity. The phenomenon that the lateral motion position, aspect ratio and rising speed of a single bubble has periodical change. Based on the quantitative analysis of the relationship between bubble terminal velocity and bubble equivalent volume diameter, the existing prediction formulas were compared and evaluated. A more accurate explicit terminal velocity correlation based on experimental data was obtained.     

Two-dimensional numerical simulation of single bubble rising behavior in liquid metal using moving particle semi-implicit method

Gas-lift pump in liquid metal cooling fast reactor (LMFR) is an innovative conceptual design to enhance the natural circulation ability of reactor core. The two phase flow characteristics of gas–liquid metal make significant improvement of the natural circulation capacity and reactor safety. It is important to study bubble flow in liquid metal. In present study, the rising behaviors of a single nitrogen bubble in 5 kinds of common stagnant liquid metals (lead bismuth alloy (LBE), liquid kalium (K), sodium (Na), potassium sodium alloy (Na–K) and lithium lead alloy (Li–Pb)) and in flowing lead bismuth alloy have been numerically simulated using two-dimensional moving particle semi-implicit (MPS) method. The whole bubble rising process in liquid was captured. The bubble shape, rising velocity and aspect ratio during rising process of single nitrogen bubble were studied. The computational results show that, in the stagnant liquid metals, the bubble rising shape can be described by the Grace's diagram, the terminal velocity is not beyond 0.3 m/s, the terminal aspect ratio is between 0.5 and 0.6. In the flowing lead bismuth alloy, as the liquid velocity increases, both the bubble aspect ratio and terminal velocity increase as well. This work is the fundamental research of two phase flow and will be important to the study of the natural circulation capability of Accelerator Driven System (ADS) by using gas-lift pump.     

可变液膜厚度下垂直塞状流参数预测模型

基于流动机理的分析建立了塞状流参数预测模型;模型中考虑了液膜的厚度变化.分析了液膜厚度变化对预测结果产生的影响,并用公开发表的数据对模型进行了验证.分析表明,若忽略液膜厚度的变化,将Taylor泡简化为圆柱体,会使其长度的预测值偏小,导致压力梯度的预测出现正偏差,且偏差会随气相表观速度的增加而增大.新建模型反映了液膜的流动特性,可对不同来源的数据进行较为准确的预测.     

基于MPS方法的液态铅铋合金内气泡上升流数值模拟

移动粒子半隐式(MPS)方法捕捉多相流体动力学相界面的能力比传统网格方法具有明显优势.本研究采用MPS方法,使用FORTRAN语言自编程序,对单个氩气气泡在液态铅铋合金内从静止到充分发展整个过程中的瞬态动力学行为进行二维数值模拟,得到气泡变形特性与上升速度的关系.结果表明:在气泡上升过程中,气泡由球形先变成酒窝形状,最...     

Numerical investigation of bubble wake properties in the moving liquid with LES model

Wake flow caused by the relative motions between bubble and liquid phase influences bubble motions and enhances turbulent properties in the liquid phase. This phenomenon has been stressed for a better understanding of bubbly flow. In this paper, large eddy simulation (LES) is performed to simulate a single bubble rising in the moving liquid, with volume of fluid (VOF) method to capture the interface movements between bubble and liquid phase. The simulation results are firstly compared with the numerical and experimental data from the literature. A good agreement demonstrated the capability of the employed computational fluid dynamics (CFD) approach to predict turbulent properties in the liquid phase and capture interface movement as well as its induced wake flow. Consequently, the dynamic behaviors of a single bubble rising in the moving liquid were investigated. An ensemble averaged has been employed to evaluate the velocity distribution composed by wake velocity and liquid velocity quantitatively as well as the velocity fluctuations enhanced by bubble motion. Their dependency was also evaluated based on a systematic CFD simulation which covers a wide range of liquid velocity. With comparisons of the single phase flow, the influence from the existence of bubble on turbulent properties was determined.     

基于扩散界面法的较大气泡上升过程数值模拟

基于扩散界面法和有限元法,对较大气泡在上升阶段的形态和速度进行了模拟,结果与实验吻合较好,说明该方法能准确地模拟气泡的运动特性。利用该模型,对初始直径不同的较大气泡上升过程中的形态、速度和振荡随时间变化的规律进行了分析。并分析了14mm直径的气泡在不同尺寸通道中上升过程的形态、速度的变化规律。结果表明:气泡的稳定形态随着气泡初始直径的增大由椭球形变为球帽形,且达到稳定形状的时间更长。气泡初始直径越大,气泡的顶端速度越快,并稍有波动。而气泡的底端速度开始快速增大使气泡向内凹陷,随后回落并在气泡顶端速度上下振荡。气泡上升通道越窄,气泡达到稳定形态的时间越长,顶端速度越小,气泡的高宽比越大。     

Simulation of a Single Bubble Rising with Hybrid Particle-Mesh Method

A computational study of the dynamics on a gas bubble rising in a viscous liquid by a hybrid particle-mesh method is presented. The hybrid particle-mesh method has been developed for the simulation of a two-phase flow. One phase is represented by moving particles and the other phase is defined on a stationary mesh. The flow field is discretized by conservative finite volume approximation on the stationary mesh, and the interface is automatically captured by the distribution of particles moving through the stationary mesh. The moving particles are calculated by Moving Particle Semi-implicit (MPS) method. The effect of surface tension is evaluated by the continuum surface force model. In this study, we simulate the motion of a gas bubble rising in a viscous liquid. The buoyancy driven motion of the bubble in a wide range of flow regimes is simulated successfully by the present method. The deforming interface of the bubble is captured effectively by the moving particles although significant density and viscosity differences exist. By comparing the simulation results with experimental ones, we approve the possibility of applying the hybrid method to the two-phase flow with particles representing the gas phase and mesh representing the liquid phase.     

基于图像法的气液两相稀疏泡状流气泡参数分析

采用图像法对垂直上升管气液两相流中稀疏上升气泡进行实验测量研究。该方法使用高速摄像机拍摄气泡运动图像,经图像处理后,提取气泡的特征参数,分别绘制稀疏气泡上升过程中速度变化曲线和单个气泡上升过程中面积变化及形心位置变化曲线,分析气泡参数,总结运动规律。实验结果表明,采用图像法可以很好地完成对气泡参数的分析。