Solidification process of a water droplets freely dropped on a cooled substrate and development of crack initiation and delamination models

Abstract

The icing phenomenon, which is due to continuous deposition of water droplets or super-cooled droplets, causes serious problems, such as airplane crashes and power line tower collapse, in various fields. It is important to understand the fundamental icing process to overcome these problems. In this study, a free-fall drop test was conducted to investigate the effects of substrate temperature and falling height on the deposition process for a water droplet. A water droplet with a temperature of 20°C was freely dropped onto an A2017 aluminum alloy substrate, which has been cooled to a temperature between -20°C and -70°C, from a height of 600, 800, or 1000 mm. The deposition process was continuously observed via a high-speed video camera. Consequently, cracking occurred in solidified droplets at substrate temperatures below -22.5°C. In addition to cracking, partial delamination of a solidified droplet from the substrate was also observed to occur at substrate temperatures below -32.5°C. These critical temperatures were independent of the falling height. Theoretical prediction models were established to predict these critical temperatures, variation of the water droplet diameter during deposition, and the time required for crack initiation. The validity of the established models was demonstrated via comparison with the experimental results.

Keywords

• icing
• free-fall water droplet
• deposition process
• cracking
• delamination