Molecular dynamics simulations on homogeneous condensation of R600a refrigerant

Abstract

Molecular Dynamics (MD) simulations have been employed to study the homogeneous condensation phenomenon of R600a (isobutane) refrigerant in the vapor compression refrigeration system. The molecular system consists of superheated vapor isobutane in a cuboid simulation cell with periodic boundary conditions in all directions. A more physically reliable thermostat and a barostat are applied to control the temperature and pressure of the cell containing the molecules. Based on AHRI standard 460 rating conditions, the simulations started from the initial configuration of vapor phase isobutane system, and once the equilibration is established at 361K with 1atm, the system is suddenly cooled to the ambient temperature of 308K and is set under the condensing pressure of 7.1658bar corresponding to the saturation temperature at 325K of the compound at supersaturation ratio 1.548. The density, internal potential energy, and volume changing with time of superheated vapor molecules in the homogeneous condensation process of the system are investigated. The results show that vapor molecules have underwent a rapid phase transition to the subcooled liquid at a certain critical time period. Subcritical clusters of isobutane molecules are formed in the preliminary part of the simulations, which then aggregate to generate the critical condensate nucleus over time. An examination of the radial distribution functions (RDFs) shows that the bonding and non-bonding interactions are formed in the vapor and liquid phases of isobutane. It is truly consistent with the internal potential energy and phase change properties. The computed densities of both phases agree well with the NIST standard reference thermodynamic and transport property data of the compound. A comparative study is executed through several simulations for various values of supersaturation and undersaturation ratios with different numbers of molecules to investigate the critical time of phase transition in the homogeneous condensation process.