冻结液滴融化过程中的表面及界面演化特性分析

Analysis of surface and interface evolution characteristics of freezing droplet during melting

液滴在血浆储存、航空航天等技术领域广泛存在,而其机理研究主要集中在冻结阶段,对融化阶段的研究则相对较少。故此,本文通过液滴可视化实验,发现并归纳了冻结液滴在不同材料表面、不同基底温度下融化过程的动态表面及界面演化模式,总结了液滴表面扩散系数、高度系数、相界面偏离度等形态演化参数与相变时间之间的变化规律并对其展开分析。结果表明:冻结液滴存在3种不同的表界面演化模式;在熔融中后阶段,金属材料(纯铝板、镀锌板)表面冻结液滴的冰相区以颗粒群状分布态融化,冰晶结合度低,而高分子聚合物材料[有机玻璃(PMMA)及聚氯乙烯(PVC)试板]表面冻结液滴的冰相区呈块状分布态融化,冰晶结合度高;金属类材料表面冻结液滴的相变速率高于聚合物类材料表面冻结液滴的相变速率,金属表面相变时间在100s以内,而聚合物表面冻结液滴的相变时间在300s以内;金属表面最大扩散系数分布区间为0.950~1.021,聚合物表面最大扩散系数分布区间为1.000~1.076,温度高,则各类材料表面液滴的微观前驱膜移动受阻,液滴的表面润湿过程受阻;金属表面冻结液滴的高度系数及冰相高度变化率受冰相区变化影响,聚合物表面则主要受温度影响;温度升高会使热量传递过程不稳定,加剧聚合物表面冻结液滴偏离度位移的波动性。

Droplets exist widely in plasma storage,aerospace and other technical fields.The research on the mechanism of droplets mainly focuses on the freezing stage,and the research on melting stage is relatively rare.Through the droplet visualization experiment,the dynamic surface and interface evolution modes of the melting process of frozen droplets on different material surfaces were studied and summarized under different substrate temperatures.Furthermore,the changing rules between the morphological evolution parameters,such as surface diffusion coefficient,height coefficient,phase interface deviation,and the phase transition time were summarized.The experimental results showed that there were three different surface and interface evolution modes for frozen droplets.In the middle and later stages of melting,the ice phase area of frozen droplets on the surface of metal material(pure aluminium,galvanized sheet)melted in a particle group distribution state.The hardness of its inner ice crystals was very weak.While the ice phase area of the frozen droplets on the surface of polymer materials(PMMA and PVC)melted in a massive distribution state,and it had a high ice crystal binding degree.The phase transition rate of the frozen droplets on the metal surface was faster than that of the polymer surface.The phase transition time of metal surface was less than 100s,while the phase transition time of the frozen droplets on the polymer surface was less than 300s.The maximum diffusion coefficient distribution range of the metal surface was 0.950-1.021,and the maximum diffusion coefficient distribution range of the polymer surface was 1.000-1.076.As the temperature increased,the movement of the microscopic precursor films and the surface wetting process of droplets on the surface of various materials was hindered.The height coefficient of the frozen droplet on the metal surface and the change rate of the ice phase height were affected by the change of ice phase region,while the polymer surface was mainly affected by the temperature.The increase of temperature would make the heat transfer process unstable and exacerbate the fluctuation of the deflection displacement of frozen droplets on the polymer surface.