基于分形理论的水汽在燃煤细颗粒表面异质核化数值研究

Numerical Analysis of Water Vapor Nucleation on Fine Particles From Coal Combustion Based on Fractal Model

利用Fletcher模型对过饱和水汽在燃煤细颗粒表面异质核化特性进行了数值预测，对不同粒径段燃煤细颗粒的形态进行扫描电镜（scanning electron microscopy，SEM）分析，并用分形理论对细颗粒物的结构特征进行描述，考察其对细颗粒异质核化性能的影响。结果表明，实验用的燃煤细颗粒表面具有典型的分形结构，分形维数在2．21-2．63；燃煤细颗粒的不规则结构能使液滴胚胎形成临界吉布斯自由能降低、成核速率增大、核化所需的临界过饱和度降低，可提高过饱和水汽在其表面的成核能力；此外，核化所需临界过饱和度随颗粒粒径增大而降低，特别是小于0．1μm的颗粒，粒径对过饱和度的影响更为显著，随温度提高核化所需的临界过饱和度相应降低。

A numerical prediction of water vapor heterogeneous nucleation behavior on fine particles from coal combustion was carried out according to Fletcher＇s theory under the condition with and without considering the fractal characteristics of the fine particle. Microstructures of fine particles in different sizes were analyzed by scanning electron microscopy （SEM）. Microstructures of fine particles were investigated using fractal model. The results show that it has a classical fractal structure of fine particles from coal combustion, with dimension between 2.21 and 2.63. The heterophilic properties of fine particles can reduce the Gibbs free energy of embryo formation and cause the magnitude of nucleation rate to increase dramatically and lead to an obviously reduction of critical supersaturation. Thus, the nucleation capabilities of supersaturated vapor are enhanced greatly for heterophilic particles. Moreover, critical supersaturation decreases with particles size, especially for those smaller than 0.1μm, and it seems a significant drop of critical supersaturation as the temperature increase.