挥发性化合物的分子结构对水-气界面传质的作用

利用能精确捕获水自由液面的VOF (Volume of fluid,体积百分比)法,建立挥发性化合物在无化学反应条件下的耦合扩散时空模型. 用W*(分子拓扑指数)表征化合物分子结构,将化合物传质系数——Schmid数与W*进行关联,结合亨利常数、Schmidt数与W*的关联式,预测乙醇、苯、己醛与2,2,4-三甲基戊点源泄漏后的质量浓度时空分布,预测结果与试验吻合较好. 烷基苯、醇、醛、烃类化合物的Schmidt数与W*0.25成线性关系. Schmidt数随分子间距减小而增大,烷基苯、醇、醛、烃类化合物在水中的Schmidt数约为空气中的600倍以上. 相同W*的化合物,Schmidt数随分子量增加而增大. 泄漏初期,亨利常数的作用高于Schmidt数,挥发强度随亨利常数的增加而增大,水中乙醇峰值浓度下降的平均速率仅是苯的45.1%；泄漏后期,Schmidt数起主要作用,挥发强度随Schmidt数减少而增大,苯的峰值浓度下降平均速率降低,乙醇峰值浓度下降平均速率比苯高120.0%. 己醛的W*是2,2,4-三甲基戊的16.27倍,但二者的亨利常数和Schmidt数很接近,挥发迁移过程极其相似. 

The Effect of Volatile Compounds' Molecular Structure on Mass Transfer at Air-Water Interface

A temporal and spatial coupling diffusion model based on volume of fluid (VOF) method was developed without chemical reaction. VOF is a numerical technique acquiring and tracking the free surface of water flow. Molecular structure of compounds was characterized by enlarging topological index of molecule W*. The relationships of mass transfer coefficient Schmidt Numbers and W* were developed. Based on the relationships of enlarged topological indices of molecule W* and Henry's constant and Schmidt Numbers, the temporal and spatial coupling diffusion model was applied to predict concentration distributions of ethanol, benzene, hexanal and 2,2,4-trimethlypentane after point source leakage. The predicted results were well consistent with the experimental results. Schmidt Numbers of alkylbenzene, alcohol, aldehyde and hydrocarbon are in linear relations with the enlarged topological index of molecule W*0.25. Schmidt Numbers increase with the decline of the molecule distance. Schmidt Numbers of alkylbenzene, alcohol, aldehyde and hydrocarbon in water are 600times as high as those of in the air. For the compounds with the same topological index of molecule W*, Schmidt Numbers increase with the molecular weights. At the initial stage of leakage, the effect of Henry's constant was stronger than Schmidt Numbers. The volatilization of compounds increases with Henry's constant. The average declining rate of the peak concentration of ethanol in water is only 45.1% as high as that of benzene. At the later stage of leakage, the effect of Schmidt Numbers was stronger than Henry's constant. The volatilization of compounds increased with the reduction of Schmidt Numbers. The decline rate of the peak concentration of benzene drops. The declining rate of the peak concentration of ethanol is 120% higher than that of benzene. The topological index of molecule W* of hexanal is 16.27times of that of 2,2,4-trimethlypentane. However, Henry's constant and Schmidt Numbers of hexanal and 2,2,4-trimethlypentane were close to each other, the processes of volatilization were similar.