闪蒸中蒸汽携带效应机理的研究

The mechanism of steam-carrying effect during flash evaporation of pure water and aqueous sodium chloride solution is carried out in this project through experiment and theoretical analysis. Phenomenon, such as boiling type during flash, formation of foam mixture during foam-like boiling, as well as distribution of droplet during pool boiling are observed by visualization system. Transient parameters, such as expansion height of foam-like mixture, transient steam speed, transient steam-carrying ratio, and stable parameters, such as duration of foam-like boiling, evaporated mass, total be-carried mass and mean steam-carrying ratio are measured in experiment. According to experimental data, mathematical and physical model for steam-carrying effect in different boiling type are set up. Combined with theoretical analysis and numerical simulation, the mechanism of steam-carrying effect in different boiling type is studied. Quantitative relationship of steam-carrying ratio with initial condition, such as superheat, waterfilm concentration, waterfilm speed, as well as structure size of flash chamber and separating height are deduced. Finally, a set of calculation method for steam-carrying parameters are built up. This research will enrich the theory of multiphase flow and heat transfer, and provide theoretical support for optimization design, technology development for flash-related industrial equipment or system.

本项目采用实验和理论分析的方法对纯水和不同浓度NaCl水溶液闪蒸中蒸汽携带效应的机理进行系统地研究。通过可视化观察不同初始条件下液膜的沸腾形态，观察泡沫状沸腾下泡沫混合物的形成与膨胀过程，以及池态沸腾下液滴的形成与分布规律；准确测定泡沫状混合物膨胀高度、蒸汽瞬态流速、蒸汽瞬态携带率等瞬态参数，以及泡沫状沸腾持续时间、闪蒸蒸发量、被携带液相总量、平均蒸汽携带率等稳态参数；以实验数据为基础，针对不同沸腾形态下液相被携带的过程建立具体的数学物理模型；结合理论分析和数值模拟，分析不同闪蒸状态下蒸汽对液相携带的机理，得出蒸汽携带率等参数与液膜初始过热度、盐浓度以及闪蒸腔尺寸、分离高度的定量关系，建立一套针对闪蒸过程中蒸汽携带特性参数的计算方法。本研究将丰富多相流动与传热的理论体系，可为涉及闪蒸的相关工业设备和热力系统的优化设计、技术开发提供理论支持。

本项目采用实验和理论分析的方法对纯水和不同浓度NaCl水溶液闪蒸中蒸汽携带效应的机理进行系统地研究。闪蒸是非稳态的多相流动和传热过程，蒸汽携带效应与沸腾传热，以及准饱和溶液闪蒸中的晶体析出相互耦合。围绕闪蒸沸腾传热的规律，观察了不同初始条件下液膜的沸腾形态的演变规律。测量了闪蒸腔、真空腔的压力、温度变化，以及闪蒸前后液膜浓度的变化。通过引入不平衡分数和体积传热系数来分别描述闪蒸的温度变化和沸腾传热强度。结果指出，过热度的增大可强化闪蒸沸腾传热，而液膜初始厚度或浓度的提高均抑制了闪蒸沸腾传热。据此拟合了不平衡分数的半经验实验关联式。围绕蒸汽携带效应，通过比较实际液位降和理论液位降发现蒸汽携带是导致液膜厚度降低的主要原因。定义蒸汽携带率为被携带液相与闪蒸蒸汽的质量之比。结果指出，蒸汽携带率随过热度的增大有峰值存在；液膜初始厚度、初始浓度的提高都强化了蒸汽携带效应。根据实验结果，以瞬时蒸汽携带率为计算目标建立了液滴托举模型。此外，根据工程需要又以平均蒸汽携带率为计算目标建立了简化液滴托举模型，实现了对平均蒸汽携带率、液膜液位降、液膜平衡浓度的快速计算和分析。围绕准饱和溶液的晶体析出，观察了析出晶体的分布。分析了晶体析出对沸腾传热和蒸汽携带的影响。结果指出，随着液膜初始浓度的增大，闪蒸的沸腾传热被削弱、蒸汽携带效应被加强。而当浓度增大到准饱和时，晶体的析出反而强化了沸腾传热，即二次沸腾现象。根据实验结果，得出了准饱和溶液最小结晶过热度的半经验关联式。通过引入析出晶体影响因子，对原有不平衡分数的半经验关联式、蒸汽携带率的计算式在晶体析状态下进行了修正。总之，本研究详细分析了闪蒸中的蒸汽携带效应及其与沸腾传热和晶体析出的耦合关系，建立了较为完备的计算和分析体系，可为涉及闪蒸的相关工业设备和热力系统的优化设计、技术开发提供理论支持。