Molecular dynamics study of the wettability effect on the evaporation of thin liquid sodium film

Abstract

The nanoscale liquid sodium film exists in the porous wick of the sodium heat pipe. Understanding its evaporation is essential for further exploring the heat transfer inside the heat pipe. Due to the solid–liquid interaction, surface wettability has significant effects on evaporation. In this work, molecular dynamics is adopted to study these effects. A cuboid evaporation system is constructed. It consists of the bottom solid surface and thin liquid sodium film. By changing the potential well depth between solid and liquid atoms, four surfaces with different wettability are built. The 400 K equilibrium evaporation is achieved. The self-diffusion coefficients of liquid atoms on four surfaces are acquired. The non-equilibrium is introduced by increasing the heat source wall’s temperature to 600 K. The main phase of evaporation is in 0–8 ns. The variation of gas atoms number is observed. The evaporation rate, average heat flux, and heat transfer coefficient are acquired. The results indicate that evaporation could be improved with surface wettability. For the 0°, 58° hydrophilic surface, the heat transfer efficiency could reach 96.9 %, 96.3 % respectively. The surface wettability effects are achieved by two paths: it enhances the heat transfer in the solid–liquid contact region and intensifies the atoms’ collision heat transfer inside the liquid film.