A diffusion-enhancing icing theory for the freezing transition of supercooled large water droplet in impact

Abstract

The freezing behavior of supercooled large droplet(SLD) in impact causes severe threats to civil aircraft. However, the mechanism by which the impact dynamics of SLD accelerates the icing still remains unclear. In this paper the impact and freezing of water droplets with different supercooling on inclined surfaces of different wettability were investigated experimentally and theoretically. Compared to one/two freezing morphologies and linear supercooling-freezing time relation reported previously, four freezing morphologies were observed in different supercoolings, and the transition of freezing morphologies resulted in the piecewise nonlinear supercooling-freezing time relation. By comparing the characteristic times of droplet dynamics and horizontal icing, it was found that the dynamics-icing interaction could lead to higher heat conduction area in higher supercooling. Theoretical analysis shows that the unsteady heat conduction enhances the freezing rate in a short time initially. The coupled effect of large contact area and initial heat flux greatly enhances the diffusion of latent heat in freezing of droplet with high supercooling. Based on this machanism, a diffusion-enhancing icing theory is proposed. Compared with previous icing theories, this theory can give much better predictions about the freezing rate of impinging supercooled water droplets, especially in conditions of high supercooling and hydrophobic surface.