A new equation for the mean free path of air

DG Tsalikis and VG Mavrantzas and SE Pratsinis, AEROSOL SCIENCE AND TECHNOLOGY, 58, 930-941 (2024).

DOI: 10.1080/02786826.2024.2333859

A recent rigorous methodology for determining the mean free path, lambda, of air at ambient conditions Tsalikis, D. G., Mavrantzas, V. G., & Pratsinis, S. E. (2023). Physics of Fluids, 35, 097131 is extended to pressures, P, from 0.5 to 5 atm and temperatures, T, from 100 to 3000 K that are of environmental and industrial relevance, allowing to derive a new simple equation for lambda as a function of T and P. This entails molecular dynamics (MD) simulations of air accounting for the actual shape and force field of nitrogen and oxygen and analysis of the computed microcanonical ensemble of free paths to derive lambda at any P and T. Simulations are rigorously validated by comparing MD-predicted air densities, diffusivities and viscosities at various temperatures and pressures against experimental measurements, theoretical expressions and ab initio simulations. At all T and P, the MD-computed lambda are systematically smaller (at least 40%) than those from classic kinetic theory and its variants. The new equation for lambda is: lambda(T,P)=0.033916xT1.23P-1 and can be also expressed in terms of the standard Jennings' expression, lambda=pi/8 mu u1 rho P Jennings, S. G. (1998). Journal of Aerosol Science, 19(2), 159-166, relating lambda to air viscosity, mu, and density, rho, using just a new value for its numerical factor u, u = 0.81475 +/- 0.00288, which is 63% larger than the current u = 0.4987445.

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