气泡破裂产生膜液滴理论模型的建立与验证

本文研究了常温、常压条件下自由液面单气泡破裂产生膜液滴现象。在明确该过程物理机理及气泡破裂环状物模型基础上,应用瑞利射流不稳定性分析理论结果,通过合理假设,建立了自由液面单气泡破裂产生膜液滴的物理模型。通过引入瑞利断裂时间判据,对时间变量进行离散,数值求解该理论模型,可获得气泡破裂产生膜液滴的初始参数,包括膜液滴数量、尺寸、速度、初始位置。将模型计算结果同已有实验数据对比,二者符合较好,证明了模型的正确性。     

一种新型的液滴分布测量方法

介绍了一种新型液滴空间分布测量技术及实验装置,该技术可测得气-水系统中气泡在自由液面处破裂产生液滴的空间分布及运动轨迹。本工作利用染色液滴撞击到水敏试纸(熟宣纸)即被吸收这一原理记录液滴运动位置。通过设计一系列不同直径的纸筒,统计同一大小气泡破裂产生液滴分别在这些圆纸筒上的撞击位置,运用数理统计原理对得到的所有数据进行综合分析,得到气泡破裂产生液滴的空间分布和运动轨迹。     

压水堆蒸汽发生器自由液面膜液滴产生情况估算

确定蒸汽发生器内液滴的来源信息至关重要,是汽水分离机理研究工作的基础。自由液面汽泡破裂产生膜液滴现象是蒸汽发生器内液滴重要来源之一。本文在验证单气泡破裂产生膜液滴模型正确性的前提下,围绕岭澳二期核电站压水堆蒸汽发生器进行建模,使用膜液滴产生模型估算单位时间内两相自由液面处汽泡破裂产生膜液滴情况,获得总体膜液滴产生情况,以及沿蒸汽发生器液面半径方向膜液滴初始参数,包括膜液滴数量分布、尺寸分布、速度分布、初始位置分布等。     

气泡破裂产生膜液滴现象可视化实验研究

气泡破裂产生液滴的现象普遍存在于核电厂蒸汽发生器中，由此产生的液滴是蒸汽流夹带液滴的主要来源之一。本工作利用可视化装置及高速摄像技术拍摄气泡在自由液面处破裂产生液滴的实验现象。结果表明，在实验气泡尺寸范围内，气泡液帽破裂产生膜液滴形式都是单点破裂，首先液膜进行排液，其后液帽顶部或底部产生孔隙，然后孔隙迅速扩大、液膜卷曲形成液体环，最终发生不稳定射流形成细小的膜滴。对实验数据进行拟合，得到气泡等效半径在3~25 mm范围内膜液滴数量同气泡尺寸关系式，实验结果与前人结果变化趋势相似。     

气泡破裂点位置及液膜卷曲速度实验研究

利用高速摄像技术构造可视化装置拍摄纯水中气泡在自由液面处破裂产生膜液滴的整个过程，包括气泡浮动、液膜排液、破裂点形成、液膜卷曲、瑞利不稳定射流形成膜液滴等阶段。气泡破裂点产生相对位置及液膜卷曲速度对于气泡破裂产生膜液滴过程非常重要，通过图像后处理得到两者实验数据。结果表明：对于实验中气泡曲率半径在2～18 mm范围内的137个数据，其破裂点产生位置主要位于气泡底部，气泡顶部同中间有破裂点产生，但发生频率远低于气泡底部。对曲率半径在7～28 mm范围内的气泡数据进行拟合，得到液膜卷曲速度同气泡曲率半径的关系式。     

气泡破裂及其诱发射流液滴释放过程的实验研究

本文基于实验方法,通过高速摄像的方法捕捉不同直径下的气泡破裂过程及射流液滴的释放过程,获得了气泡破裂后气泡空腔的演变过程,捕捉了射流液滴的速度,探究了气泡直径和气泡表面寿命对射流液滴释放过程的影响规律。实验结果表明,气泡表面寿命对气泡破裂产生射流液滴的过程有着重要影响。随气泡表面寿命的增加,破裂气泡产生的射流液滴的速度也随之增加。当气泡直径较小时,气泡表面寿命呈现Rayleigh分布的特征,射流液滴的释放概率也较高。随气泡直径的增加,气泡表面寿命逐渐转变为指数衰减分布的特征,射流液滴的释放概率也随之下降。基于现有实验数据给出了一个精度更高的射流液滴速度与气泡直径关系式。     

气泡破裂及其夹带特性行为测量综述

自由液面处气泡破裂造成的液滴夹带现象广泛存在于水冷反应堆中,在事故工况下液池中滞留的气溶胶可通过夹带的方式进入气相中,对释放量的准确计算是源项评估的重要基础。气溶胶最终的释放特性由气泡寿命、液膜厚度及产生的液滴尺寸等参数决定。由于气泡破裂是一复杂且随机的过程,因此液相表面气泡破裂关键参数的测量没有普遍适用的方法,且各方法的测量结果并不互相适用,这就给气溶胶释放特性的研究带来困难。为对该现象进行研究,本文总结了上述物理参数的多种测量方法及部分经验公式,比较了不同测量方法的适用范围,最后展望适用于此领域的测量方法及后续的研究方向。     

气泡破裂及其诱发射流液滴释放过程的实验研究

本文基于实验方法,通过高速摄像的方法捕捉不同直径下的气泡破裂过程及射流液滴的释放过程,获得了气泡破裂后气泡空腔的演变过程,捕捉了射流液滴的速度,探究了气泡直径和气泡表面寿命对射流液滴释放过程的影响规律。实验结果表明,气泡表面寿命对气泡破裂产生射流液滴的过程有着重要影响。随气泡表面寿命的增加,破裂气泡产生的射流液滴的速度也随之增加。当气泡直径较小时,气泡表面寿命呈现Rayleigh分布的特征,射流液滴的释放概率也较高。随气泡直径的增加,气泡表面寿命逐渐转变为指数衰减分布的特征,射流液滴的释放概率也随之下降。基于现有实验数据给出了一个精度更高的射流液滴速度与气泡直径关系式。     

基于丝网传感器的5×5棒束通道空泡分布测量研究

为研究压水反应堆燃料组件棒束通道内的两相分布规律,设计并制造了适用于棒束通道的丝网传感器模块,开展了5×5棒束通道内空气-水泡状流的空泡分布测量实验,分析了棒束通道内空泡份额的分布规律及气泡尺寸对空泡分布的影响。实验结果表明,发生横升力方向反转的小气泡在壁面附近聚集、大尺寸气泡则聚集在子通道中心;常温常压下发生横升力方向反转的临界气泡直径在4~6 mm之间,证明了横升力模型在棒束通道中的适用性。      

竖直矩形窄缝通道内静水条件下气泡运动特性实验研究

基于图像自动识别方法对矩形窄缝通道(2 mm×60 mm×790 mm)内常温常压静态流体中的气泡运动特性进行可视化实验研究,获得不同气泡当量直径对应的气泡形态和运动速度;定量分析气泡运动速度与气泡当量直径间的关系,并基于本研究获得的实验数据对现有关于气泡运动速度的预测关系式进行定量评价。     

Void fraction distributions of inert gas jets across a single cylinder with non-wetting surface in liquid sodium

Little work on the void fraction behaviors along structural materials with poor-wettability for liquid metals has been performed. In the present study, void fraction behaviors around a single cylinder with non-wetting surface condition were quantitatively discussed by using a gas jet–cylinder system where the impinging jet flow, the boundary layer flow, the separation flow, and the wake flow appear. One cylinder with a non-wetting surface and two cylinders with a wetting surface were used to vary the wettability for liquid sodium, and void fraction distributions were measured around the cylinders. In the case of wetting condition, void fraction distributions around the cylinder decrease clearly in the backward region of the cylinder, and liquid-rich region is formed due to bubble separation from the cylinder surface. On the other hand, under non-wetting condition, because of two-phase flow without bubble separation on the cylinder surface, void fraction distributions show almost steady values around the cylinder compared to those with wetting surface. The void behaviors on a non-wetting surface were also confirmed by a visualization experiment conducted in water. The observed differences can be basically attributed to the work of adhesion required for liquid–solid interfacial separation.     

离散液滴运动模型研究

汽水分离器和安全壳喷淋系统是核电厂安全可靠运行的关键设备和设施,其运行过程涉及液滴在流场中的运动、碰撞和相变等物理现象。本文采用欧拉 拉格朗日耦合方法首先建立了离散液滴运动模型,包括单/多液滴运动模型、液滴碰撞、流场耦合、液滴相变、液滴产生和液滴消亡等单立模型,及相应的边界条件和空间条件,来描述多液滴在流场中的运动行为;其次给出了离散液滴运动模型的应用实例,来描述自然界蒸发、冷凝和喷雾等物理现象中的离散液滴运动行为;最后指出了现阶段离散液滴运动模型和算法存在的问题,希望感兴趣的学者在离散液滴运动模型研究方面进一步完善和推动其发展。     

振荡区下蒸汽冷凝过程的热工水力模型建立及验证

蒸汽射流冷凝过程具有强烈的凝结换热能力,广泛应用于先进非能动反应堆安全系统中,但该过程会产生强烈的压力振荡现象。为研究蒸汽浸没射流冷凝振荡现象的本质,从基本守恒方程式出发,建立了气泡边界层质量交换模型、气泡控制方程模型、气泡内蒸汽压力计算模型、水池中任意位置处压力计算模型等关键模型,构建了模拟水池中蒸汽气泡冷凝振荡过程的热工水力模型。运用建立的气泡半径和水池内压力的计算模型获取气泡半径和压力随时间变化的规律,并与Chun实验和Fukuda实验的实验数据进行比对,验证了模型的有效性,为后续开展冷凝振荡机理研究打下理论基础。     

Measurements of Natural Frequency and Damping Constant of Single Steam Bubble Oscillating in Water

The natural frequency f_n and damping constant δ of a bubble in liquid have been determined by observing the resonance of the bubble to forced oscillation. The bubble was retained under a rigid plate horizontal disk, and the oscillation was applied by underwater speaker. The applied frequency f was kept constant while letting the bubble increase its volume and vary its radius R. Bubble resonance was detected by observing wrinkles appearing on the bubble due to surface waves. Resonance curves relating the amplitude of bubble radius variation to the intensity of applied oscillation is derived theoretically.

Good agreement was seen between the data obtained from experiment and the theoretically derived resonance curves at test to the validity of the method proposed of determining fⁿ and δ from bubble resonance. The values of δ and of the resonant bubble radius R_o of large steam bubbles (8.5 mm>R>11.5 mm) in water were determined at f =270, 290 and 358 Hz. The results support the assumption that for large bubbles the value of f_n is little influenced by the exchange of mass between liquid and gaseous phases through evaporation and condensation accompanying bubble pressure oscillation. On the other hand, δ is found to be one order of magnitude higher than calculated for steam bubbles without taking into evaporation and condensation the interphase exchange of mass. The effect brought on δ by the interphase mass exchange can be taken into account by adding a new constant δ_pn to the terms constituting the total damping constant.     

Fragmentation mechanisms based on single drop steam explosion experiments using flash X-ray radiography

Flash X-ray and high-speed regular photography were used to investigate the fragmentation processes during the vapor explosion of single drops of molten metal immersed in water. For relatively low ambient flow velocities (< 5 m/s), a comparison of the breakup of hot and cold drops shows that whereas cold drops breakup due to the stripping of fragments by the relative flow, the fragmentation of a hot drop is dominated by the growth and collapse of a vapor bubble. X-ray radiographs show during the growth of the bubble, that fine filaments of metal protrude from the drop and the drop surface becomes highly convoluted. Using a simple model for the bubble dynamics, an estimate of the energy budget shows that the majority (about 80%) of the thermal energy transfer occurs during bubble collapse. For hot drops at higher flow velocities (> 45 m/s), vapor bubble growth is diminished and high-speed motion of vapor within the bubble leads to an enhanced fragmentation rate.     

Steam bubble condensation in sub-cooled water in case of co-current vertical pipe flow

The structure of a steam-water flow in a vertical pipe of 195.3 mm inner diameter was studied using novel wire-mesh sensors for high-pressure/high-temperature operation (max 7 MPa/286 °C). Tests were carried out at pressures of 1 and 2 MPa under nearly adiabatic conditions as well as with slightly sub-cooled water (6 K at max). Steam was injected into sub-cooled water and condensed during the upwards flow. The evolution of radial gas fraction profiles and bubble size distributions along the pipe in a high-pressure steam-water flow was measured for the first time. The experimental data allow correlating the intensity of steam condensation in contact with sub-cooled water with the structure of the interfacial area and the bubble size distribution, which is very important for the model development. The data were used to test the complex interaction of local bubble distributions, bubble size distributions and local heat and mass transfer. The model considers a large number of bubble classes (50). This allows the investigation of the influence of the bubble size distribution. The results of the simulations show a good agreement with the experimental data. The condensation process is clearly slower, if the injection nozzle diameter is increased (from 1 to 4 mm orifices). Also bubble break-up has a strong influence on the condensation process because of the change of the interfacial area. Some modelling errors arise from the uncertainty of the interfacial area for large bubbles and the heat transfer coefficient.