Frosting characteristic measurement and defrosting performance improvement

作为结霜结冰的前提条件，本课题首先对水滴的凝固进行了研究。设计了一种Hele-Shaw细胞，用于观察二维水滴的凝固特性。包括非均相成核、液滴形态、冻结锋演变、被困气泡、液滴内部温度分布等。本研究通过定量比较泵送空气蒸馏水和新型蒸馏水在冻融循环过程中水滴的空气释放过程，研究了冷冻二维疏水型水滴中的气泡。首先，在熔化阶段，溶解空气从气泡中逸出，导致过冷度降低。对于新制备的蒸馏水滴，直到表面温度从-20℃变为-30℃的实验条件下，才发生三次循环的成核。其次，在冻融循环过程中，气泡尺寸变小，气泡总数增多。(3)由于启动时空气溶解含量不同，两个水滴的总逸出气量有明显差异，但三个循环的总逸出气量相似，两个水滴的总逸出气量分别为15.87%和14.02%。最后，在三个冻融循环中，逸出空气含量、水滴温度下降持续时间和两个被油包围的水滴过冷度呈现相同的波动趋势。

As the prerequisite of frosting and icing, solidification of water droplets was firstly investigated in this project.A Hele-Shaw cell was designed and used to observe the solidification characteristics of two-dimensional water droplets. including heterogeneous nucleation, droplet morphology,freezing front evolution, trapped air bubbles, and temperature distribution inside a droplet, etc.Air bubbles in frozen two-dimensional hydrophobic type water droplets were investigated in this year by quantitatively comparing the air releasing process from water droplets during freezing-melting cycles using pumped air distilled water and new made distilled water. Firstly, the subcooling degree would decrease as the dissolved air escaped from the bubbles in the melting stage. For new made distilled water droplet, the nucleation for three cycles did not occur until the experimental condition of the surface temperature was changed from -20 oC to -30 oC. Secondly, during the freezing-melting cycles, the air bubbles become smaller in dimension, and more in total number. Thirdly, the total escaped air volumes for two water droplets are obvious different due to different air dissolved content at start, while the total proportions for three cycles are similar, at 15.87% and 14.02% for two water droplets. Finally, the escaped air content, water droplet temperature decrease duration, and the subcooling degree for two water droplets surround with oil show the same fluctuation trend in the three freezing-melting cycles.