Non-equilibrium dynamics of dense gas under tight confinement

Wu, Lei and Liu, Haihu and Reese, Jason M. and Zhang, Yonghao (2016) Non-equilibrium dynamics of dense gas under tight confinement. Journal of Fluid Mechanics, 794. pp. 252-266. ISSN 0022-1120 (https://doi.org/10.1017/jfm.2016.173)

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Abstract

The force-driven Poiseuille flow of dense gases between two parallel plates is investigated through the numerical solution of the generalized Enskog equation for two-dimensional hard discs. We focus on the competing effects of the mean free path (Formula presented.), the channel width (Formula presented.) and the disc diameter (Formula presented.). For elastic collisions between hard discs, the normalized mass flow rate in the hydrodynamic limit increases with (Formula presented.) for a fixed Knudsen number (defined as (Formula presented.)), but is always smaller than that predicted by the Boltzmann equation. Also, for a fixed (Formula presented.), the mass flow rate in the hydrodynamic flow regime is not a monotonically decreasing function of (Formula presented.) but has a maximum when the solid fraction is approximately 0.3. Under ultra-tight confinement, the famous Knudsen minimum disappears, and the mass flow rate increases with (Formula presented.), and is larger than that predicted by the Boltzmann equation in the free-molecular flow regime; for a fixed (Formula presented.), the smaller (Formula presented.) is, the larger the mass flow rate. In the transitional flow regime, however, the variation of the mass flow rate with (Formula presented.) is not monotonic for a fixed (Formula presented.): the minimum mass flow rate occurs at (Formula presented.). For inelastic collisions, the energy dissipation between the hard discs always enhances the mass flow rate. Anomalous slip velocity is also found, which decreases with increasing Knudsen number. The mechanism for these exotic behaviours is analysed.

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