Equilibrium Molecular Dynamics Computer Simulations of the Transport Coefficients of Ar-CH₄ Mixtures

Abstract

Equilibrium molecular dynamics computer simulations have been used to determine the transport coefficients of model Ar-CH₄ mixtures at state points for which experimental data are available. Both species are represented by single-site Lennard-Jones pair potentials with Lorentz-Berthelot rules for the cross-species interactions. We calculate the self-diffusion coefficients for each species and also mutual-diffusion coefficients using time correlation functions in the Green-Kubo formulae and mean square displacements. Time correlation functions are used to evaluate the shear and bulk moduli and viscosities, thermal conductivity and the thermal diffusion coefficient in [NVE] and [NVT] ensembles. Only for the bulk viscosity is there a significant ensemble dependence. In order to evaluate the thermo-transport coefficients, we use a rigorous definition for the heat flux, which includes the partial enthalpy of the two species. This was obtained from separate computations carried out at constant pressure in the [NPT] ensemble.

The average densities (obtained under [NPT] conditions) and thermal conductivities agree well with experiment over a wide temperature and composition range. The simulated shear viscosity of near pure methane is typically ∼50% higher than experiment. This is seen as a clear shortcoming of a single site model for methane, which fails to account for its orientational properties which appear to be significant for the shear viscosity. The bulk viscosity also shows a similar qualitative discrepancy between experiment and simulation, which we conjecture may be for a different reason. The infinite frequency shear moduli are, in contrast, in very good agreement with experiment.

Key Words:

• Binary mixtures
• argon-methane
• MD
• partial thermodynamic quantities
• transport coefficients
• Green-Kubo