In this study, the effect of pressure and diameter on the diffusion behavior of CH4 gas in CNTs, is investigated by using molecular dynamics (MD) simulation. In order to analyze the diffusion behavior of methane, the mean square displacement (MSD) of methane molecules is investigated. Moreover, radial distribution function (RDF) and density of molecules along CNT axis are computed. The results show that the diffusion coefficient of methane molecules inside CNTs is in the order of magnitude of 10-5 m2/s. This value increases by increasing the pressure up to 40 bar. But after that, increasing the pressure decreases the diffusion behavior. This behavior is due to the CNTs filling which was approved by density profile of gas molecules. Furthermore, RDF plot of methane molecules show the formation of one layer of adsorbed methane inside CNT. But, for CNT with middle diameter, the second layer around the CNT center is formed too.

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