【摘要】 对自然对流条件下干燥和湿润2种初始状况平板表面上的初始结霜现象进行了显微观测.实验结果表明,表面稳定温度愈低,结霜发生愈早,临界温度也愈低.干燥表面上凝结的液滴及随后冻结的冰粒形状较湿润表面的规整且接近圆形.干燥表面发生结霜较湿润表面早,其差别随表面稳定温度的降低而减小.表面稳定温度高于-15℃时,2类表面的临界温度几乎相同;随着表面稳定温度降低,干燥表面的临界温度较湿润表面低,其差别随着表面稳定温度的降低而增大.分析表明结霜临界状态时冰粒的大小及其在冷表面上的分布具有分形特征,并提出一基于分形理论定量描述其特征的方法.干燥和湿润2种表面上临界结霜时冰粒的平均直径并无显著差别,而湿润表面上的冰粒表面覆盖率较干燥表面的大.更多还原

【Abstract】 A microscopic analysis of frost formation on the dry/wet flat surface under natural convection is conducted. The result indicates that the critical time is reached faster and the critical temperature is also lower under the condition of the lower steady-state temperature. The sharp of the droplets/ice particles under the initial condition of the dry surface is more regular (close to the circle) than that under the initial condition of wet surface. The critical time under the initial condition of dry surface is reached earlier than that under the initial condition of wet surface. The difference decreases with the decrease in the steady-state temperature. The critical temperatures are almost the same for the cases where the steady-state temperature is higher than-15 ℃. The critical temperature under the initial condition of dry surface is lower than that under the initial condition of wet surface in the cases where the steady-state temperature is lower than-15 ℃. For that case, the difference increases with the decrease of the steady-state temperature. The sizes and distributions of droplets/ice particles show fractal characteristics. A method based on the fractal analysis is proposed to quantitatively determine the characteristic sizes and distributions of droplets/ice particles during the onset of freezing when sub-cooled droplets are formed and changed to the ice particles. The ice particle fraction on the wet surface is greater than that on the dry surface at the onset of freezing while there is no significant difference on the mean diameter of ice particles on both.更多还原