Ab Initio Molecular Dynamics of Dimerization and Clustering in Alkali Metal Vapors

Thin film liquid-vapor phase change phenomena over nano-porous substrates: A molecular dynamics perspective

Surfaces with nano-pores have significant effect in enhancing heat transfer during phase change process. In this study, Molecular dynamics simulations have been performed to investigate thin film evaporation over different nano-porous substrate. The molecular system consists of argon as the working fluid and Platinum as the solid substrate. To study the effect of the nano-pores in phase change process, the nano-porous substrates had been structured with four different hexagonal porosity with three different heights. The structures of the hexagonal nano-pore were characterized through variation of void fraction as well as height to arm thickness ratio. Qualitative heat transfer performance has been characterized by closely monitoring the temporal variation of temperature and pressure, net evaporation number, wall heat flux of the system for all cases under consideration. The quantitative characterization of heat and mass transfer performance has been done by calculating the average heat flux and evaporative mass flux. Diffusion coefficient of argon is also evaluated to illustrate the effect of these nano-porous substrate in enhancing the movement of argon atoms thus heat transfer. It has been found that the presence of hexagonal nano-porous substrates significantly increases heat transfer performance. Structures with lower void fraction offers better enhancement of heat flux and other transport characteristics. Increment in nano-pores height also significantly enhances heat transfer. Present study clearly points out the noteworthy role associated with nano-porous substrate in modulating heat transfer characteristics during liquid-vapor phase change phenomena both from qualitative and quantitative perspectives.

Ab initio Gibbs ensemble Monte Carlo simulations of the liquid-vapor equilibrium and the critical point of sodium

The ab initio (ai) Gibbs ensemble (GE) Monte Carlo (MC) method coupled with Kohn-Sham density functional theory is successful in predicting the liquid-vapour equilibrium of insulating systems. Here we show that the aiGEMC method can be used to study also metallic systems, where the excited electronic states play an important role and cannot be neglected. For this we include the electronic free energy in the formulation of the effective energy of the system to be used in the acceptance criteria for the MC moves. The application of this aiGEMC method to sodium yields a good agreement with available experimental data on the liquid-vapour equilibrium densities. We predict a critical point for sodium at 2338 ± 108 K and 0.24 ± 0.03 g cm-3. The liquid structure stemming from aiGEMC simulations is very similar to the one from ab initio molecular dynamics. Since this method can determine phase transition without computing the Gibbs free energy, it may offer a new possibility to study other materials with a reasonable computational cost.