A molecular dynamics simulation study on condensation of R600a refrigerant

Abstract

Molecular Dynamics (MD) simulations have been employed to study the homogeneous condensation phenomena of isobutane used as a R600a refrigerant in the vapor compression refrigeration system. The molecular system consists of the superheated vapor isobutane in a cubic simulation cell with periodic boundary conditions in all three dimensions. A more physically reliable thermostat and a barostat are applied to control the temperature and pressure of the cell containing the vapor phase molecules and then to cool and compress the molecules placed in the confined simulation domain. The simulation started from an initial configuration of vapor phase isobutane system, and once the equilibration is established at 330 K, the system is suddenly cooled to the lower ambient temperature of 295 K below the saturated condensing temperature 317.86 K. The density, internal potential energy changing with time in the homogeneous condensation process of the superheated vapor molecules are investigated. Subcritical clusters of isobutane molecules are formed in the preliminary part of the simulations, which then aggregate to give the critical condensate nucleus over time. The results show that vapor isobutane molecules are instantly cooled and compressed, and underwent a rapid phase transition to the subcooled liquid at a certain critical time period. The calculated density of both phases and enthalpy of condensation agree well with the NIST standard reference thermodynamic and transport property data of isobutane.