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。     

竖壁降液膜流动纵向及水平发展规律研究

采用高频率电容式测厚仪测量并分析了降液膜膜厚和波动的统计规律,得到了降液膜沿竖直大平板流动平均膜厚经验关系式,发现了液膜流动的横向扩展和纵向发展规律。结果表明,降膜在竖直发展2.6 m后可能已基本充分发展。     

非能动安全壳中降膜流动与波动特性数值模拟研究

基于非能动安全壳建立了1:45的简化的二维数值模型,在保持无量纲数不变的前提下,利用流体体积函数（VOF）模型捕捉气-液交界面,分析了不同雷诺数（Re）条件下的液膜铺展状况。通过与实验结果的对比,模拟结果能够较好地反映出实验结果中的液膜波动现象,且液膜厚度的变化与Re一致;竖壁降膜过程受多种力的综合作用,波动的液膜可以分成明显的层流底层与独立波两部分,层流底层流速较低而独立波沿流动方向移动较快,随着液膜的铺展,独立波之间发生碰撞和融合并加剧液膜的波动性;随着Re的增加,层流底层的液膜厚度及独立波的振幅都有所增加,同时波动性加剧。      

竖壁冷态降液膜流动统计特性实验研究

对冷态降液膜在大平板上的流动特性进行了实验研究,运用电容式精密测微仪测定垂直降液膜的厚度,考察了在无蒸发情况下雷诺数对降液膜厚度和流量对液膜覆盖率的影响规律,得到平均膜厚经验关系式,并认为液膜在达到临界膜厚后,随流量增加只会使覆盖率增大而厚度几乎不变,发现同流量下接触角较小的平板上液膜覆盖率较大。     

气液并流向下通过筛板孔口的液膜厚度模型

通过孔口的收缩-膨胀增强气液接触是强化气液两相传热传质的重要方式，其中孔口边缘的液膜厚度则是建立气液两相传递过程动力学模型的关键参数。基于孔口流动时的气液相力平衡原理，建立了气液两相并流向下通过孔口的液膜厚度模型，并将模型预测的液膜厚度与管道环状流液膜厚度进行了对比分析，表明了新模型的有效性和广泛适用性。基于新建液膜模型，从流体受力的角度分析了气液流量、液相运动黏度以及孔口直径等参数对液膜厚度的影响。结果表明：随气相流量提高，气液界面速度增大，气液界面剪切力增强，无因次液膜厚度随之减薄；液相流量增大，气相流通面积减小，气相流速增大，尽管气液界面剪切力及气液界面速度均有所增大，但液量的直接增量最终导致无因次液膜厚度增厚；液相运动黏度增加，尽管气液界面剪切作用增强，但气液界面速度减小，液膜厚度增厚；相同流量下，孔口直径增大，气液界面剪切力及气液界面速度均减小，液膜厚度随之增加。     

稳定同位素~(13)C分离用高效丝网波纹填料表面降膜流动研究

由于碳同位素分离系数小,分离难度大,需要采用高效规整填料实现~(13)C的分离。本文通过计算流体力学(CFD)数值模拟研究,采用流体体积函数(VOF)方法,研制了用于~(13)C分离的高比表面积丝网波纹规整填料(PACK-~(13)C),建立了PACK-~(13)C填料表面伴随有气相逆流的局部液体降膜流动模型,选用CO(l)-CO(g)为模拟计算物系,考察了板面结构、丝网目数等因素对液膜流动的影响,并对填料表面气液相界面进行追踪,探究了气液相界面波动对传质效率的影响,研究表明,改善填料壁面结构能够增强气液相界面波动,可以实现强化传质过程。填料表面局部降膜流动的研究方法,可应用于填料气液传质过程中涉及的多尺度流动及传质现象的可视化研究,为优化填料结构提供基础性理论指导。     

不同壁面特性下液膜铺展性能数值模拟

液膜在安全壳表面的流动铺展性能对非能动安全壳冷却系统有重要的影响。采用三维数值模拟方法研究了降膜板表面形貌、接触角与液相雷诺数对液膜铺展性能的影响,并将结果与前人的实验和理论结果对比,吻合良好。研究发现,当降膜板为横向波纹板时液膜完全铺展时间、液膜厚度及界面湍动程度明显大于平板与纵向波纹板,此时波纹板波谷处会有循环流动产生。随接触角的增加或液相雷诺数的减小,液膜逐渐从完整流转变为片状流、溪流、滴状流。在纵向波纹板对液膜的导流与撕裂综合作用下,随接触角的变化,液膜的铺展性能与平板相比也发生较大的变化。     

平面激光诱导荧光技术在液膜厚度波动实验研究中的应用

针对传统接触式测量方法探头干扰液膜流动特性导致测量精度难以提高以及传统光学测量方法中光线折射作用限制其应用范围的问题,提出利用平面激光诱导荧光技术测量膜厚的新方法,并对该技术的测试原理、实现方式进行了详细的介绍。通过对沿流向不同位置液膜厚度的实时测量,研究了液膜波动的时序特征及纵向演化规律。结果表明,液膜厚度的概率密度分布的特征差异可作为诊断液膜波动特性的判据。     

离心力对旋叶式分离器壁面液膜界面不稳定性的影响

本文对旋转流场条件下离心力对旋叶式分离器壁面液膜界面不稳定性的影响规律进行了理论研究。首先利用势函数对汽液两相的动量方程和连续方程进行线性化处理。然后通过对无黏条件下液膜界面的受力分析,获得汽液界面的动力学边界条件和运动学边界条件,结合线性化方程组建立液膜界面波的色散方程。根据力平衡原理获得了液膜运动规律,并结合色散方程建立了界面不稳定发生的判定准则关系式。基于所获得的理论模型编写计算程序,对界面稳定性进行计算分析。研究发现,旋转流场条件下的离心力能够抑制旋叶式分离器壁面液膜界面不稳定的发生。     

延展表面凝结液膜的动量积分方法

采用动量积分方法分析压水堆发生失水事故时在安全壳的内表面上的液膜凝结、再浸润和蒸发过程。由凝结液膜的质量和动量守恒方程导出了凝结液膜在延展表面的子午线方向平均速度的积分-微分方程。假设液膜以层流的方式流动，把导出的积分-微分方程变成容易进行数值积分的液膜速度的一阶常微分方程，由此求得液膜厚度分布。液膜能量守恒方程的解给出了安全壳内壁面的温度分布。     

Hydrodynamically-controlled rewetting

Transient cooling of a hot tube by a falling liquid film is analysed. Vapor production from the liquid film and the sputtered droplets can produce a countercurrent vapor velocity which exceeds the flooding limit, and rewetting becomes hydrodynamically-controlled rather than heat conduction-controlled. A criterion shows that conduction-controlled rewetting prevails at higher coolant flow rates and flooding conditions at lower flow rates. A solution is obtained for the liquid coolant vaporized during its fall from the sputtering film front. The required thermal radiation properties are also presented. Detailed calculation based on this analysis shows good agreement with experimental results.     

Estimation of Shear Stress in Counter-Current Annular Flow

The accuracy to which the friction factor is correlated is important in counter-current flow (CCF) analysis using a two-fluid model. Current two-fluid model codes use correlations of friction factors developed for co-current flow, or correlations developed on the assumption of zero wall shear stress. An assessment of two-fluid model codes with these existing correlations shows that the downward flow rate of water is overestimated. An analytical model is developed to calculate the shear stress profile in a liquid film of the CCF in order to obtain information on the shear stress between the interface and the wall. Both this analysis and Bharathan's CCF data show that the wall shear stress acting on the falling water film is approximately of same order as the interfacial shear stress, and the correlation for co-current flow cannot be applied to CCF. Tentative correlations of interfacial and wall friction factors are developed based on the results of the present study.     

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。     

Recovery of Electrical Resistivity in Neutron Irradiated Uranium Monocarbides

An experimental study was performed on the heat transfer to a turbulent water film which is falling down the outside surface of a heated vertical tube. The test section was made of stainless steel tube 13 mm in outside diameter and 1,500 mm long. It was found that a nearly constant heat transfer coefficient was obtained for the lower half of the test section covering a length of about 1 m, which was direct-heated by electric current. The resulting values of the Nusselt number in this region were correlated to the Reynolds number, and the plots fell in a region intermediate between those given by Wilke and by McAdams. An analytical model taking account of the undulation on the film surface is proposed. The predictions from the model give fairly good agreement with the experimental data.     

Vertically Downward Two-Phase Flow, (I)

Fully developed vertically downward two-phase flow of air-water mixture was investigated on void distribution and average void fraction among the three basic flow regimes; bubbly, slug and annular flows. The annular flow further was divided into two regimes of falling film flow and annular drop flow. Test channel is in form of inverted U-tube and tests were carried out at 100 tube diameters downstream from the curved part. Distributions of local void fraction were measured by means of a conductance needle probe method and the average void fraction was obtained from numerical integration of the measured local void fraction According to the results, profiles of local void fraction in bubby and slug flows showed characteristic natures with a peak in the middle region between the center and the wall of tube The average void fraction in downward flow depended greatly on the flow regimes. Accordingly correlation for each flow regime was developed to predict the average void fraction, based on flow mechanisms and experiments. The correlations were compared with experimental results for atmospheric air-water flow and showed satisfactory agreement.     

Experiments on sodium fires and their aerosols

A number of new sodium fire and aerosol experiments were undertaken to provide data for LMFBR safety analyses: (1) Experiments on the burning of single drops of liquid sodium falling in air have been performed to aid in model development for sodium spray fire codes. (2) The leakage of sodium oxide aerosols through a straight smooth capillary tube, representative of the maximum size of a hypothetical gas leak in the wall of the secondary containment of an LMFBR, has been studied. Even in those cases in which the capillary did not plug, <11% of the entering mass was of a respirable size as it emerged from the capillary. In addition, there were a number of conditions under which the capillary plugged. (3) Experiments on the behavior of high temperature, high concentration aerosols have shown a rapid depletion of the aerosol concentration in the first 6 sec following injection of 800 g/m³ aerosols at 1000°C into a closed vessel. This depletion has been correlated with the early formation of 100 to 200 μm agglomerates which fall out promptly.     

Experimental study of high temperature particle dropping in coolant liquid

A series of experiments of the premixing stage of fuel-coolant interactions (FCI), namely the particles falling into water, were carried out. The force on the particles during the course of falling has been studied. The dropping character of hot particle was influenced by three main parameters, i.e., particle temperature, particle diameter and coolant subcooling that varied over a wide range. A high-speed camera recorded the falling speed of the particle and the moving curves were obtained. The experimental results showed that for the film boiling on the surface of particle and water, the temperature increase of either particle or coolant would slow down the particle falling velocity. The falling velocity of particle in small diameter is lower than that of the bigger particle. The present work can provide an experimental foundation for further investigation of high-speed transient evaporation heat transfer.     

摇摆对过冷沸腾相分布特性的影响机理分析

采用双探头光学探针测量了摇摆条件下圆管内过冷沸腾局部空泡份额、界面面积浓度及汽泡尺寸等局部相界面参数径向分布特性,根据实验及计算结果,从汽液相界面作用力角度对摇摆运动条件下过冷沸腾相分布机理进行了分析。结果表明：摇摆条件下,浮力径向分量、升力、湍流分散力和壁面润滑力量级约为10³ N/m³,附加惯性力与其余诸力相比小2～3个量级。因此摇摆条件下过冷沸腾相分布特性主要取决于周期性波动的升力、湍流分散力、壁面润滑力及浮力径向分量之间的平衡关系。