Identifying the dominant transport mechanism in single nanoscale pores and 3D nanoporous media.

Gas transport mechanisms can be categorized into viscous flow and mass diffusion, both of which may coexist in a porous media with multiscale pore sizes. To determine the dominant transport mechanism and its contribution to gas transport capacity, the gas viscous flow and mass diffusion processes are analyzed in single nanoscale pores via a theoretical method, and are simulated in 3D nanoporous media via pore-scale lattice Boltzmann methods. The apparent permeability from the viscous flow and apparent diffusivity from the mass diffusion are estimated. A dimensionless parameter, i.e., the diffusion-flow ratio, is proposed to evaluate the dominant transport mechanism, which is a function of the apparent permeability, apparent diffusivity, bulk dynamic viscosity, and working pressure. The results show that the apparent permeability increases by approximately two orders of magnitude when the average Knudsen number ( ) of the nanoporous media or Knudsen number () of single nanoscale pores increases from 0.1 to 10. Under the same conditions, the increment in the apparent diffusivity is only approximately one order of magnitude. When  < 0.01, the apparent permeability has a lower bound (i.e., absolute permeability). When  > 10, the apparent diffusivity has an upper bound (i.e., Knudsen diffusivity). The dominant transport mechanism in single nanoscale pores is the viscous flow for 0.01 <  100, where the maximum diffusion-flow ratio is less than one. In nanoporous media, the dominant transport relies heavily on and the structural parameters. For nanoporous media with the pore throat diameter of 3 nm,  = 0.2 is the critical point, above which the mass diffusion is dominant; otherwise, the viscous flow is dominant. As increases to 3.4, the mass diffusion is overwhelming, with the maximum diffusion-flow ratio reaching ∼4.