Impact freezing modes of supercooled droplets determined by both nucleation and icing evolution

Abstract

Supercooled large droplet (SLD), which can cause abnormal icing phenomenon like run-back ridged ice, poses a serious threat to aircraft safety. In impact freezing of a supercooled droplet, the coupling of impact dynamics and ice growth determines its freezing rate and heat transfer efficiency, which is responsible for the abnormal icing phenomenon. The knowledge of supercooled droplet impact freezing is vital for the development of aircraft anti-icing and safety technologies. However, the effect of icing evolution on impact freezing is not well understood yet, especially on surfaces with different properties. This work experimentally investigates the freezing process of supercooled droplets impinging on surfaces with different heat conduction properties. By observing the phenomenon of droplet impact-nucleation and ice growth with high-speed camera, the frozen morphology, spreading ratio and freezing time are recorded with different temperatures and surface heat conduction properties. The mechanism of impact freezing of supercooled droplet is analyzed in different nucleation and ice growth conditions. In experiment, two frozen morphologies of droplets are found: irregular shape on plexiglass and poached egg shape on metal surface, besides the previously discovered frozen morphologies (basin, pancake, semisphere). The former shape exists when the horizontal growth rate of ice is lower than retraction rate of droplet, while the latter one exists when the vertical growth rate of ice is low enough. These two morphologies (irregular shape and poached egg) would evolve into shapes of basin or pancake with the increase of horizontal and vertical growth rate of ice, when the supercooling and surface thermal conductivity rise. During this transformation, the freezing of droplet is mainly influenced by nucleation rather than ice growth. Theoretical analysis shows that the freezing of supercooled droplet is determined by both nucleation and icing evolution. The freezing mode is nucleation-dominated if the growth rate of ice is higher than retraction rate, and the freezing mode is ice growth-dominated in the opposite case.