Abstract:
Molecular dynamics (MD) simulations have been performed to investigate the boiling phenomena of thin liquid film adjacent to a solid surface with particular emphasis on (i) the effect of material and wetting condition of the solid surface, and (ii) the effect of nanostructures under various surface wetting conditions. The molecular system consists of liquid and vapor argon, and solid wall. To examine the effect of surface material on boiling phenomena, three different solid materials have been considered which are platinum, silver and aluminum. The solid-liquid interfacial wettability, in other words whether the solid surface is hydrophilic or hydrophobic has been altered for different cases to examine its effect on boiling phenomena. To explore the effect of the nanostructures along with the surface wettability, nanostructures having the shape of rectangular block have been considered in this study and the height of the nanostructures has been altered for both hydrophilic and hydrophobic cases. The initial configuration of the simulation domain was equilibrated at 90 K and after equilibrium period, the wall temperature was suddenly increased to 130 K to resemble the evaporation of thin liquid film, while in another case it was set to 250 K (which is far above the critical point of argon) to initiate rapid or explosive boiling. The spatial and temporal variation of temperature and density as well as the variation of system pressure and energy with respect to time were closely monitored for each case. The heat flux normal to the solid surface was also calculated to illustrate the effectiveness of heat transfer under different cases. The results show that the wetting condition of surface has significant effect on both the normal and explosive boiling of the thin liquid film rather than surface material. The surface with higher wettability i.e. hydrophilic condition provides more favorable situation for boiling than the low-wetting surface (hydrophobic) and therefore, liquid argon responds quickly and shifts from liquid to vapor phase faster in case of hydrophilic surface. The change of system energy in case of evaporation after the increase of wall temperature from 90 K to 130 K were almost same for both hydrophilic and hydrophobic surface. Similarly, for explosive boiling case the jump of energy was same regardless of surface wetting condition but larger in magnitude. In conjunction with the wettability of the surface, the height of the nanostructures has significant effect on boiling phenomena. The magnitude of heat flux during the evaporation of the thin film liquid has been found to be in the same order of magnitude of the theoretical maximum value of heat flux as defined by Gambill and Lienhard [47]. But in case of explosive boiling, it was one order of magnitude greater than the theoretical maximum value of heat flux.
