The effect of system boundaries on the mean free path for confined gases

Abstract

The mean free path of rarefied gases is accurately determined using Molecular Dynamics simulations. The simulations are carried out on isothermal argon gas (Lennard-Jones fluid) over a range of rarefaction levels under various confinements (unbounded gas, parallel reflective wall and explicit solid platinum wall bounded gas) in a nanoscale domain. The system is also analyzed independently in constitutive sub-systems to calculate the corresponding local mean free paths. Our studies which predominate in the transition regime substantiate the boundary limiting effect on mean free paths owing to the sharp diminution in molecular free paths near the planar boundaries. These studies provide insight to the transport phenomena of rarefied gases through nanochannels which have established their potential in microscale and nanoscale heat transfer applications.

Real gas transport through nanopores of varying cross-section type and shape in shale gas reservoirs

“…Nanopores type and shape affect gas transport mechanism and capacity, which can be explained from the microscopic point of view: (1) the interaction force between wall solid molecules and gas molecules is different from the gas intermolecular force, which causes the difference of gas molecules number density between the wall vicinity and away from the wall [34,35]; (2) gas molecules near wall prematurely collide with wall, which drastically reduces the mean free path [33]; (3) nanopores with different type and shape have different specific surface, which causes different ratios of the gas moleculeswall collision frequency to the total collision frequency [31]. In addition, this phenomenon can also be explained from a macro point of view.…”

“…For the slip region and transition region, the strong collision between the gas molecules and the nanopore wall affect the gas transport behavior [31], and nanopores with different cross-section types and shapes have different specific surface. Therefore, the cross-section type and shape also affect gas transport behavior in slip and transition regions [32][33][34][35]. Due to the diversity of nanopores in SGRs, finding analytical solutions for gas transport in nanopores with all cross-section types and shapes is complex and/or impossible [12,30,32].…”

Molecular free paths in nanoscale gas flows

“…While the channel height determines the characteristic dimension for fully developed internal flows, accurate prediction of Kn relies on correct calculation of MFP. Recent literature claims spatial variations of MFP in scales smaller than λ with gradients normal to the channel surfaces (Arlemark and Reese 2009;Dongari et al 2011aDongari et al , b, 2013aPrabha et al 2013;Qixin and Zhiyong 2014). In addition, confinement size-dependent MFP models have been developed to define an effective viscosity (Arlemark et al 2010;Dongari et al 2011bDongari et al , 2013aDongari and Agrawal 2012;Guo et al 2007;Peng et al 2004) and a corresponding phoretic velocity (Dongari et al 2010) in the derivation of extended Navier-Stokes equations.…”

“…The forth common error is in reporting spatial variations of MPF due to the confinement effects (Arlemark and Reese 2009;Dongari et al 2011aDongari et al , b, 2013aPrabha et al 2013;Qixin and Zhiyong 2014). Local MFP variations are physically admissible only if there are local density and temperature variations in the system, which is common in the streamwise direction for pressure driven gas flows in a channel.…”

“…MFP is the average distance traveled by molecules between intermolecular collisions. Given this fact, the first common cause of error is counting of the gas-wall collisions in determination of λ (Arlemark et al 2010;Arlemark and Reese 2009;Dongari et al 2010Dongari et al , 2011aDongari et al , b, 2013aDongari and Agrawal 2012;Guo et al 2007;Peng et al 2004;Prabha et al 2013;Qixin and Zhiyong 2014). In the case of gas adsorption on the surface, gas molecules will collide with immobilized gas molecules on the surface.…”