膜状冷凝初期过程的分子动力学模拟研究

采用分子动力学模拟方法,对氩蒸气在铂金属表面发生的膜状冷凝过程进行了研究.为保证冷凝过程在相对较长时间范围内持续稳定进行,提出了一种改进的气态分子补充方法.通过逐时对系统内局部温度及密度进行统计,获得了不同时刻的参数分布.结果显示:在模拟时间范围内,液膜厚度近似线性增加,壁面附近液相分子受固壁势能作用而呈现出密度振荡的"液体层状化"分布;液膜内产生温度梯度,固液界面处温度跳跃现象明显.考察了气体温度以及壁面润湿性变化产生的影响,结果表明:随着气体温度的升高,温度梯度以及温度跳跃均增大;液相密度略有下降,液体内层状区域的密度振荡范围略有减小,气液界面厚度增加;质量流率以及液膜厚度增长速率也都增大,反映出更大的气固温差加快了冷凝过程的进行,这一点与宏观规律一致.随着润湿性增强,液膜厚度增长加快,液体层状区内的密度振荡范围增加,液膜内温度梯度增大,温度跳跃大幅减小,冷凝过程得到显著强化.显然,近壁面区内的热传导对整个冷凝过程进行的速度具有重要影响.

Molecular Dynamics Study on Early Stage of Filmwise Condensation

Filmwise condensation of argon vapor on a platinum surface was studied with molecular dynamics simulation method.An improved molecule supply method was proposed in order to keep the condensation in progress for a comparatively long time.The temporal profiles of local temperature and density were obtained statistically.The results show that the liquid film grows about linearly and there exists an oscillating distribution called 'liquid layering' near the solid wall.The temperature jump at the solid-liquid interface and the temperature gradient in the liquid film were observed.The effects of the vapor temperature and the solid wettability were also studied,and the results indicate that with the increase of the vapor temperature,the temperature gradient,the temperature jump,the thickness of the vapor-liquid interface,the mass flux,and the film growth rate all increase,while the amplitude of the layering region decreases slightly.This indicates that larger temperature difference will strengthen the condensation process,which agrees with the macroscopic phenomenon.The growth rate,the amplitude,and the temperature gradient increase but the temperature jump decreases distinctly when the solid wall becomes more wettable.It can be concluded that the wall neighboring region,where the contact thermal resistance exists,has a dominant effect on the condensation process.