Ultrathin liquid film phase change heat transfer on fractal wettability surfaces

Surface wettability is significant for the liquid-vapor phase change heat transfer. In this paper, patterned wettability surfaces are constructed from the perspective of fractal theory to comprehensively investigate the microscopic mechanism of surface wettability patterns affecting evaporation and bubble nucleation. The evaporation heat transfer rates of the hydrophilic fractal wettability surfaces are close to that of the pure hydrophilic surface. The solid-liquid interfacial heat transfer dominates the evaporation heat transfer. In contrast, the liquid-vapor interfacial heat transfer's effect is insignificant. The hydrophobic points of the hydrophilic fractal wettability surfaces provide stable and abundant nucleation sites for the bubble nucleation process. The nucleation times of the hydrophilic fractal wettability surfaces are earlier than that of the pure hydrophilic surface. Hydrophobic fractal wettability surfaces can improve bubble nucleation. With increased hydrophilic points, nucleation sites are mainly concentrated in hydrophobic regions near the hydrophilic/hydrophobic contact boundary. The expanding low potential energy region near the contact boundary is the main reason for improving bubble nucleation. Critical heat flux (CHF) increases as the area proportion of the hydrophilic part increases, and surface superheats decrease when CHF occurs. The fractal wettability surfaces improve CHF, and the CHF of the hydrophilic fractal wettability surface is close to that of the pure hydrophilic surface.