Buttersack Tillmann, Bauerecker Sigurd
Institut für Physikalische und Theoretische Chemie, Technische Universität Braunschweig , Hans-Sommer-Strasse 10, 38106 Braunschweig, Germany.
J Phys Chem B. 2016 Jan 28;120(3):504-12. doi: 10.1021/acs.jpcb.5b09913. Epub 2016 Jan 19.
The freezing of freely suspended supercooled water droplets with a diameter of bigger than a few micrometers splits into two rather different freezing stages. Within the first very fast dendritic freezing stage a spongy network ice with an ice portion of less than one-third forms and more than two-thirds of liquid water remain. In the present work the distribution of the ice portion in the droplet directly after the dendritic freezing phase as well as the evolution of the ice and temperature distribution has been investigated in dependence of the most relevant parameters as droplet diameter, dendritic freezing velocity (which correlates with the supercooling) and heat transfer coefficient to the surroundings (which correlates with the relative droplet velocity compared to the ambient air and with the droplet size). For this purpose on the experimental side acoustically levitated droplets in climate chambers have been investigated in combination with high-speed cameras. The obtained results have been used for finite element method (FEM) simulations of the dendritic freezing phase under consideration of the beginning second, much slower heat-transfer dominated freezing phase. A theoretical model covering 30 layers and 5 shells of the droplet has been developed which allows one to describe the evolution of both freezing phases at the same time. The simulated results are in good agreement with experimental as well as with calculated results exploiting the heat balance equation. The most striking result of this work is the critical radius of the droplet which describes the transition of one-stage freezing of the supercooled water droplet toward the thermodynamically forced dendritical two-stage freezing in which the droplet cannot sufficiently get rid of the formation heat anymore. Depending on the parameters named above this critical radius was found to be in the range of 0.1 to 1 μm by FEM simulation.
直径大于几微米的自由悬浮过冷水滴的冻结过程分为两个截然不同的冻结阶段。在第一个非常快速的枝晶冻结阶段，会形成一种冰部分占比小于三分之一的海绵状网络冰，且超过三分之二的水仍为液态。在本研究中，根据最相关的参数，如液滴直径、枝晶冻结速度（与过冷度相关）以及与周围环境的传热系数（与液滴相对于周围空气的相对速度和液滴大小相关），研究了枝晶冻结阶段结束后液滴中冰部分的分布以及冰和温度分布的演变。为此，在实验方面，结合高速摄像机对气候箱中声悬浮的液滴进行了研究。所获得的结果已用于枝晶冻结阶段的有限元方法（FEM）模拟，并考虑了开始的第二个慢得多的以传热为主的冻结阶段。已开发出一个涵盖液滴30层和5壳层的理论模型，该模型能够同时描述两个冻结阶段的演变。模拟结果与实验结果以及利用热平衡方程计算得到的结果吻合良好。这项工作最显著的结果是液滴的临界半径，它描述了过冷水滴的单阶段冻结向热力学强制的枝晶两阶段冻结的转变，在两阶段冻结中，液滴无法充分散发形成热。通过有限元模拟发现，根据上述参数，该临界半径在0.1至1μm范围内。
