On the binary diffusion coefficients of n-alkanes in He/N2

Abstract

The binary diffusion coefficients of fuel molecules in bath gasses are key parameters for accurate predictions of flame properties such as extinction strain rates. In combustion simulations, they are often calculated using the Hirschfelder-Bird-Spotz (HBS) equation, which requires the availability of effective Lennard-Jones parameters, i.e. collision diameters and well depths, which can be obtained from the intermolecular potentials of two interacting species by applying an orientation-averaging rule. Besides the HBS equation, the Green-Kubo formula can be used to obtain binary diffusion coefficients, using velocity autocorrelation and cross-correlation functions from molecular dynamics simulations. In this work, we propose a new orientation-averaging rule, called VolW-σ-ε, based on the characteristic of intermolecular potentials, and evaluate different approaches to computing the diffusion coefficients of n-alkanes in He/N2 mixtures. Several orientation-averaging rules are evaluated and compared with experimental data of n-alkanes in He and N2 and the results of molecular dynamics simulations. Finally, using the binary diffusion coefficients of n-dodecane in N2 evaluated by the VolW-σ-ε method, we reproduce previously reported experimental extinction strain rates reported for counterflow n-dodecane/N2 versus O2 jets.