Determination of the type of nanoconfined gas transport and quantification of their velocity contributions from molecular perspectives

Understanding the gas flow in nanoconfined spaces is crucial for industrial applications. In this work, we analyzed the type of gas transport in nanopores and quantified their velocity contributions to the total gas flux by performing molecular dynamics simulations. The methods of calculating the mean free path (MFP) of gas molecules, determining the gas transport type based on collisions, quantifying the velocity of gas transport were proposed. Results show that the MFP of gas in nanopores is smaller than that of bulk phase gas resulting from the interaction between gas and pore wall. The Knudsen number in the nanopores is in the range of 0.04 to 0.16 with the pressure within 45 to 1 MPa. The proposed collision-based classification extends the upper Kn limit for viscous flow from 0.1 to 0.16 and raises the lower Kn limit for slip flow from 0.001 to 0.06 compared to empirical classifications. The main type of the gas transport in nanopores is viscous flow and surface diffusion with the pressure within 1 to 45 MPa. Slip flow and Knudsen diffusion contribute minimally to the overall gas transport. The results obtained in this study provide new insights on the types of gas transport in nanopores of different flow regimes.