Experimental study of initial state of frost formation on flat surface

A microscopic analysis of frost formation on the dry/wet flat surface under natural convection is conducted. The result indicates that the critical time is reached faster and the critical temperature is also lower under the condition of the lower steady-state temperature. The sharp of the droplets/ice particles under the initial condition of the dry surface is more regular (close to the circle) than that under the initial condition of wet surface. The critical time under the initial condition of dry surface is reached earlier than that under the initial condition of wet surface. The difference decreases with the decrease in the steady-state temperature. The critical temperatures are almost the same for the cases where the steady-state temperature is higher than-15°C. The critical temperature under the initial condition of dry surface is lower than that under the initial condition of wet surface in the cases where the steady-state temperature is lower than-15°C. For that case, the difference increases with the decrease of the steady-state temperature. The sizes and distributions of droplets/ice particles show fractal characteristics. A method based on the fractal analysis is proposed to quantitatively determine the characteristic sizes and distributions of droplets/ice particles during the onset of freezing when sub-cooled droplets are formed and changed to the ice particles. The ice particle fraction on the wet surface is greater than that on the dry surface at the onset of freezing while there is no significant difference on the mean diameter of ice particles on both.