Abstract:

Considering the phenomenological characteristic of steady state condensation process, this work mainly focused on the evolution conformation and laws of nanoscale clusters within the near-wall space with a thickness of hundreds of microns. Resorting to molecular dynamics simulation, the schema of cluster evolution and temperature distribution within the near-wall space was constructed through multiple sampling system simulations under different supersaturations. The results showed that there was a “transition point” in the curve of steam temperature in response to the distance from the wall, that was, the near wall space could be divided into two regions: the dense distribution region of clusters near the wall and the diffusion development transition region far from the wall. Specifically, the transition point could be changed. As the initial vapor pressure decreased, the transition point position moved closer to the subcooled wall, resulting in a relatively thinner molecule dense region. In addition, with the increase of noncondensable gas content, the corresponding cluster diffusion development region became wider. This indicated that a higher subcooling was needed so as to achieve a similar thickness of molecule dense region to that in the pure steam system without noncondensable gas, which explained the considerable influence of noncondensable gas on the condensation heat transfer efficiency from phenomenological perspective. Finally, according to these characteristics of cluster distribution evolution in the near-wall region, a new concept of enhancing or regulating heat and mass transfer is pointed out, that is, not only the micro-nano functional structure on the wall surface can be designed, but also the material structure design in the near wall space can also be considered, starting from the vapor phase space to control the cluster evolution.

Key words: condensation, clusters in near-wall space, nucleation, noncondensable gas, molecule dense region, interface

摘要：

基于近壁几百微米厚度空间中纳米尺度团簇的演化特点，综合定态凝结过程的唯象特点，采用分子动力学模拟的方法，通过多个不同过饱和度下的抽样体系模拟，构建出近壁空间中的团簇演化和温度渐变的图景。分析发现，近壁空间中蒸汽温度随离壁距离的渐变曲线存在一个特征转折点，并以该点为界，近壁空间可分为靠近壁面的团簇的稠密分布区和靠近蒸汽体相的扩散发展的过渡区。随着初始蒸汽压力的降低，转折点位置向过冷壁面靠近，导致相对更薄的分子稠密区。而随着不凝气含量的增加，相应的团簇扩散发展的区域变宽，这说明不凝气存在时，要达到与纯蒸汽条件下相似厚度的分子稠密区，需要更高的过冷度，也从唯象角度解释了不凝气存在对凝结换热效率的极大影响。最后，根据近壁区团簇分布演化的这些特点，指出了强化或调控传热传质的新概念，不仅可以对壁面上的微纳功能结构进行设计，还可考虑近壁空间中的材料结构设计，从蒸汽体相空间入手来调控团簇演化。

关键词: 凝结, 近壁团簇, 成核, 不凝性气体, 分子稠密区, 界面