Abstract:

To investigate the heterogeneous nucleation characteristics of supersaturated water vapor on the surface of fine particles, and accurately predict the nucleation parameters, a molecular kinetic model of heterogeneous nucleation of supersaturated water vapor on the surface of coal-fired fine particles was established based on the theory of molecular kinetic heterogeneous nucleation. The promotion effects and relative importance of two diffusion condensation mechanisms of water and vapor molecules to accelerate embryo growth and the influences of supersaturation and macroscopic contact angle on nucleation rate are systemically analyzed. Furthermore, the critical supersaturation for fine particles under different temperature and macroscopic contact angles are also predicted. The results show that when the radius of embryo is less than critical embryo radius, the ratio of diffusion condensation rate of water molecules absorbed on particle surface to direct vapor deposition condensation rate is always above 100, it indicates that the surface diffusion mechanism of absorbed water molecules plays a dominated role in embryo growth. The nucleation rate is remarkable increased with the increase of supersaturation or the decrease of macroscopic contact angle. Additionally, the nucleation rate increases exponentially with the increase of supersaturation. Increase the vapor temperature or decrease macroscopic contact angle can lead to a lower critical supersaturation. For fine particles with a particle size of less than 0.1 μm, as the fine particle size increases, the critical supersaturation of heterogeneous nucleation decreases significantly.

Key words: fine particles, water vapor, heterogeneous condensation, nucleation, numerical simulation

摘要：

为研究过饱和水汽在细颗粒表面异质核化特性，准确预测成核参数，基于分子运动学异质核化理论建立了过饱和水汽在燃煤细颗粒表面异质核化的运动学模型。数值分析了液滴晶核长大过程中水汽分子和水分子两种扩散凝结机制对晶核长大的促进作用及其相对重要性，数值预测了水汽过饱和度和宏观接触角对成核速率的影响；数值计算了不同温度和宏观接触角下细颗粒的临界过饱和度。结果表明：当液滴晶核尺寸小于临界晶核半径时，颗粒表面吸附水分子扩散凝结速率与水蒸气分子直接扩散凝结速率的比值大于100，颗粒表面吸附水分子的扩散凝结机制对晶核长大起主导作用。提高水汽过饱和度或减小宏观接触角均可显著提高液滴晶核的成核速率；成核速率随水汽过饱和度增大呈指数型增长。提高气相主体中水汽温度或减小细颗粒物的宏观接触角均可显著降低异质成核的临界过饱和度；对于粒径小于0.1 μm的细颗粒物，随着细颗粒粒径的增大，异质核化的临界过饱和度显著减小。

关键词: 细颗粒物, 水汽, 异质凝结, 核化, 数值模拟