Visualization study of phase distribution on a boiling surface

Abstract

In this study, the liquid and vapor phase distributions on a boiling surface are visualized using a simple visible ray optic setup with high-speed and high-resolution. We find that the thermal and fluidic nature of the boiling process originates from the behaviors of the dry spot, microlayer, and liquid wetting, and their intertwined dynamics. At a high surface heat flux condition, large coalesced dry spots appear on the boiling surface, which deteriorates the boiling heat transfer. They can be wetted and cooled by the surrounding liquids based on two mechanisms: liquid driven by adjacent nucleation and gravitational liquid supply. However, liquid wetting to the dry spots is significantly restricted by the appearance of a particular dry spot, called the irreversible dry spot. In particular, the liquid film formed around the dry spot bursts immediately, and the liquid stream is abruptly scattered without wetting. Our numerical simulation results show that the formation of an irreversible dry spot is caused by the overheating of a spot on the dry spot, and the surface temperature yielding the blockage of liquid wetting is found to be in the range of 130–140 °C under the present experimental conditions. In other words, a dry spot with its surface temperature higher than the temperature limit might not be wetted anymore by the surrounding liquids, and this triggers the critical heat flux phenomenon. Finally, we propose a theoretical model to predict the critical temperature limit based on the balance between the pressures imposed at the triple contact line by the liquid weight (gravity) and vapor recoil force (evaporation).