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。     

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.     

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。     

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。     

Numerical analysis of bubble motion with the VOF method

Numerical analyses of a two-dimensional single bubble in a stagnant liquid and in a linear shear flow were conducted in the present study using the volume of fluid method, which is based on the local-instantaneous field equations. It was clarified that this method gives qualitatively appropriate predictions for the effects of the Morton number and the Eotvos number on fluctuating bubble motion in a stagnant liquid. Calculated velocity and pressure distributions indicated that the Karman vortex causes a sinuous movement of the bubble. As for the bubble motion in a linear shear flow, calculated bubbles migrated in a lateral direction. The direction of the lateral migration agreed to available experimental data. It was also confirmed that (i) the direction or the magnitude of the lateral migration is affected by the Eotvos and the Morton numbers, and (ii) the interaction among the internal flow of the bubble, the wake of the bubble and the external shear flow plays an essential role for the lateral migration.     

单个浸没孔气泡动力学特性的数值模拟

核电厂发生严重事故时可利用池洗效应去除泄漏的放射性气溶胶。对池洗过程进行两相数值模拟研究是有必要的，在使用两相计算流体力学（CFD）程序计算之前需要确定气泡注入点处的边界条件。基于整合池洗研究（IPRESCA）项目框架和流体体积法（VOF），对单个浸没孔气泡动力学特性进行数值模拟研究，捕捉浸没孔处气泡的大小、形状和脱离频率，并对气泡注入速度对气泡脱离频率的影响进行敏感性分析。利用DBSCAN聚类算法获得了气泡质心高度，并计算得到了不同高度的气泡上升速度。给出了平均空泡份额沿z轴方向的分布，以及平均空泡份额和平均混合速度在不同高度平面沿水平及径向的分布。      

单个浸没孔气泡动力学特性的数值模拟

In severe accidents of a nuclear power plant, the released radioactive aerosols can be removed by pool scrubbing effect. Two-phase numerical simulation of the pool scrubbing process is necessary. The boundary conditions at the bubble injection point need to be determined before using the two-phase CFD program. Based on the framework of Integration of Pool scrubbing Research to Enhance Source-term Calculations (IPRESCA) project and the volume of fluid (VOF) method, a numerical simulation of gas injected bubble dynamics from single submerged orifice was carried out. Bubble size, shape, and detachment frequency at the orifice were captured. Sensitivity analysis of the influence of bubble injection speed on bubble detachment frequency was carried out. The bubble centroid height was obtained by DBSCAN clustering algorithm, and the bubble rising velocity at different heights was calculated. The distribution of the mean void fraction along the z-axis direction and the distribution of mean void fraction and mean mixture velocity along the horizontal lines and radius in central plane at different height are given.     

过冷度对窄缝通道内单蒸汽泡运动特性影响的实验研究

对窄缝通道内过冷条件下单蒸汽泡运动特性进行了实验研究,分析了过冷度对单蒸汽泡运动特性的影响。实验结果表明,过冷条件下,单蒸汽泡在上升过程中,其尺寸不断减小,形状也不断改变;单蒸汽泡界面存在冷凝现象,过冷度越大,直径减小越快,同一直径蒸汽泡的纵横比在一个范围内波动;过冷条件下,单蒸汽泡的z向速度和x向速度都随着直径的增大先增加后减小,均在直径约10 mm时具有最大值;单蒸汽泡z向速度则随着过冷度的增大而增大,而x向速度在零上下波动,随过冷度增大略有增大。过冷度会影响窄缝通道内单蒸汽泡的行为特性,并进一步影响流型形成与演变。      

上升气泡内气溶胶沉降机理研究

气溶胶池洗过滤是反应堆严重事故中去除放射性源项的重要手段。本文以严重事故条件下上升气泡中气溶胶的滞留过程为背景，设计搭建了可视化单气泡鼓泡实验装置。通过该装置研究了气溶胶在上升气泡中的沉降效率，并与MELCOR中的气溶胶沉降模型计算结果进行了对比。结果表明，气溶胶沉降效率对气泡尺寸的变化较为敏感，当气体流量大于0.1 L/min时，气泡等效直径迅速增加，相应的气溶胶沉降效率快速降低；与MELCOR模型计算结果的对比表明，两者在总体趋势上呈现出较好的一致性，但计算结果低估了液相对气溶胶的实际去除能力，导致这种偏差的主要原因是气泡在上升过程中存在无规则的晃动以及气液界面的波动。     

倾斜管内上升泡状流界面参数分布特性实验研究

采用双头光纤探针对倾斜圆管内空气-水两相泡状流界面参数分布特性进行了实验研究,包括局部空泡份额、气泡通过频率、界面面积浓度及气泡当量直径径向分布特性。实验段内径为50 mm,液相表观速度为0.144 m/s,气相表观速度为0~0.054 m/s。结果表明倾斜管内向上泡状流气泡明显向上壁面聚集。局部界面浓度、空泡份额及气泡通过频率径向分布相似。倾斜条件下局部界面参数分布下壁面附近峰值相对于竖直状态被削弱甚至消失,上壁面附近峰值被加强,中间区域从下壁面往上逐渐增大,且随倾斜角度的增加变化更加剧烈。气泡等价直径随径向位置、气相速度及倾斜角度的不同无明显变化,气泡聚合和破碎现象较少发生。通过气泡受力分析解释了倾斜对泡状流局部界面参数分布的影响机理。     

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。     

基于MPS方法模拟液态铅铋合金中弹状气泡上升

基于移动粒子半隐式(MPS)方法对液态铅铋合金中单个弹状气泡的垂直上升行为进行数值模拟,得到弹状气泡上升过程的形态变化、气泡最终上升速度拟合直线及下降液膜厚度与下降液膜中的轴向速度分布。将部分数值模拟结果与文献中的实验结果进行比较,揭示了弹状气泡最终上升速度、下降液膜厚度及其中轴向速度分布所满足的规律。比较结果证明了所选取计算模拟模型的正确性与合理性,以及MPS-MAFL方法用于模拟弹状气泡上升行为的准确性与可靠性。     

Void distribution and bubble motion in bubbly flows in a 4×4 rod bundle. Part II: numerical simulation

Numerical simulations of bubbly flows in a four by four rod bundle are carried out using a multi-fluid model to examine effects of the numerical treatment of phase distribution and drag model. The transport equations of bubble number density and void fraction are used as the continuity equation of the gas phase. Two drag models are tested: one of them accounts for the bubble deformation (aspect ratio), whereas the other does not. The rod diameter, the rod pitch and the hydraulic diameter of the rod bundle are 10, 12.5 and 9.1 mm, respectively. The gas and liquid volume fluxes are J_G = 0.06 m/s and J_L = 0.9 and 1.5 m/s, respectively. The bubble diameter ranges from 1 to 5 mm. Comparisons between the numerical and measured data show that (1) the restriction on bubble lateral motion due to the presence of rods can be taken into account by using the transport equation of bubble number density, whereas that of the void fraction cannot deal with the restriction and causes large errors in the distribution of void fraction and (2) the reduction in the bubble-relative velocity near the wall is predictable by using the drag model accounting for the bubble deformation effect.     

Numerical simulation of bubble condensation using CF-VOF

In present study, subcooled boiling is simulated using color function volume of fluid (CF-VOF) method. For this purpose, energy equation and Tanasawa mass transfer model accompanied with some suitable source terms are implemented in OpenFOAM solver (interFoam). The surface tension between vapor–liquid phases is considered using continuous surface force (CSF) method. In order to reduce spurious current near interface, a smoothing filter is applied to improve curvature calculation. The variation of saturation temperature in vapor bubble with local pressure is considered with Clausius–Clapeyron relation. The numerical model is validated with one-dimensional Stefan problem.The shape and life time history of single vapor bubble condensation are verified with existing experimental data. Computational study shows bubble life time is nearly proportional with bubble size and it is prolonged at bubble swarm motion. The present study reveals some fundamental characteristics of single and multiple vapor bubble condensation and is expected to be instructive for further applications.     

Analysis of Control Rod Effect on Neutronics in FBR by Three-Dimensional Calculation

Experiments were carried out to study the mass transfer of iodine from a xenon-iodine mixed gas bubble to a liquid sodium pool. The bubble was generated in the pool by cracking a quartz glass ball which contains the xenon-iodine mixed gas. The mixed gas released into the argon cover gas was collected to determine the transferred iodine mass into the sodium pool. A rising velocity of the bubble was measured by Chen-type sensors arranged vertically in the pool. From the measured rising velocity and another observation of bubble behavior in simulated water experiments, it is found that the generated bubble breaks up into several smaller bubbles of spherical cap type during the rising period. Transferred iodine mass per the unit initial bubble volume from the bubbles to the sodium pool shows increases with increasing time and the initial iodine concentration. A mass transfer rate obtained by differentiating the transferred iodine mass with respect to the time indicates a rapid decrease just after the bubble generation and a slow decrease for the successive period. Measured decontamination factors are described as a function of the time and the iodine concentration.     

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...     

竖直圆管内泡状流界面参数分布特性

采用双头光纤探针对内径为50 mm竖直圆管内空气-水两相泡状流界面参数径向分布特性进行了实验研究。气液两相表观速度变化范围分别为0.004～0.05 m/s和0.071～0.283 m/s。结果表明,竖直管内向上泡状流局部界面面积浓度（IAC）、空泡份额及气泡频率径向分布相类似,即气相流速较低时管道中间很大范围内以上3个局部界面参数几乎恒定,近壁区迅速下降到较低值;随气相流速的增加,局部界面参数在管道中心出现峰值。本实验中气泡聚合与破碎现象较少发生,索特平均直径沿径向近似均匀分布,且随气液两相流速变化很小。通过气泡横向受力解释了局部界面参数分布的影响机理。     

Study on flow characteristics in gas-molten metal mixture pool

As part of basic research on the flow characteristics of a two-phase mixture pool under severe accident of fast breeder reactor (FBR), visualization and measurement of nitrogen gas-molten lead/bismuth two-phase flow in a rectangular pool were performed by using the neutron radiography technique. Measurements of drag coefficient of a single bubble and bubble shape regime showed that the relationship between the shape, size and the rising velocity of a single isolated nitrogen bubble in the molten lead/bismuth was not much different from that for an ordinary one. Appropriate correlation for drift velocity and drag coefficient between phases were recommended based on the drift flux correlation of measured pool void fraction. One- and two-dimensional analyses were performed by using a next generation computational code for safety analysis of severe accident of FBRs, SIMMER-III with various drag coefficient models. It was revealed that Kataoka–Ishii’s equation was suitable basically for estimation of drift velocity, namely, drag force between phases.