蒸汽冷凝近壁过渡区团簇演化特性

doi:10.11949/0438-1157.20220043

摘要/Abstract

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

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

中图分类号:

O 647

李雯, 兰忠, 强伟丽, 任文芝, 杜宾港, 马学虎. 蒸汽冷凝近壁过渡区团簇演化特性[J]. 化工学报, 2022, 73(7): 2865-2873.

Wen LI, Zhong LAN, Weili QIANG, Wenzhi REN, Bingang DU, Xuehu MA. Evolution characteristics of clusters in transitional region near subcooled wall during condensation process of steam[J]. CIESC Journal, 2022, 73(7): 2865-2873.

图/表 9

图1 近壁几百微米空间中团簇分布演化的研究方法的示意图和初始模拟体系的构建

Fig.1 Schematic of research method for cluster distribution evolution in hundreds of microns space near subcooled wall and setup of the initial simulation system

图2 δT=50 K体系的温度随模拟时间的变化

Fig.2 Variation of system temperature with time for δT=50 K

图3 δT=50 K时的模拟过程变化

Fig.3 Variations during the simulation process for system of δT=50 K

图4 不同δT体系中水分子的团聚演化

Fig.4 Evolution of water molecules aggregation for different δT systems

图5 不同δT体系的均方位移MSD随时间的变化

Fig.5 Variation of mean square displacement with time for different δT systems

图6 近壁空间中的团簇分布演化的特点

Fig.6 Characteristics of clusters distribution evolution in the near wall region

图7 蒸汽压力对近壁空间中的团簇分布的影响

Fig.7 Effect of vapor pressure on cluster distribution in the near wall region

图8 氮气存在下近壁空间中水分子的团聚演化与团簇分布特点

Fig.8 Evolution of water molecules aggregation and characteristics of clusters distribution in the near wall region with nitrogen

图9 不凝气对近壁空间中的团簇分布的影响

Fig.9 Effect of noncondensable gas on cluster distribution in the near wall region

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