Abstract
When the channel height is reduced to a small value such as on the scales of 10 nm or 100 nm, the physical adsorbed layers on the channel walls will participate in the flow, although intermediate between them is a continuum fluid flow. The multiscale simulation results are presented for this multiscale mass transfer in a narrow slit pore based on the derived flow equations. The results are respectively compared with those calculated from conventional continuum flow theory and from the theory based on the solid layer assumption, when the fluid-wall interaction is respectively weak, medium and strong. The total mass flow rate of the two adsorbed layers is also compared with the mass flow rate of the intermediate continuum fluid. The obtained results show the importance of the incorporation of the adsorbed layer flow by the multiscale scheme when calculating the transferred mass in a microchannel.
Highlights
When the channel height is reduced to a small value such as on the scales of 10 nm or 100 nm, the physical adsorbed layers on the channel walls will participate in the flow, intermediate between them is a continuum fluid flow
For a pressure driven nanochannel flow, it was found that the volume flow rate through the channel calculated from molecular dynamics simulation is considerably smaller than that calculated from conventional hydrodynamic theory when the fluid-wall interaction is quite s trong[9]; the reduced flow rate in the nanochannel was ascribed to the formation of the strongly solidified layer on the wall surface, which makes the channel narrower[9]
A conventional multiscale approach may calculate both the adsorbed layer flows by full atomistic molecular dynamics simulation and calculate the continuum fluid flow by the continuum fluid model[14,16,17,18]. As commented above, such an approach usually requires an unaffordable computational time and storage for an engineering flow. The reason for this difficulty is that when carrying out a molecular dynamics simulation for the adhering layer flows, there are a huge amount of molecules both in the adhering layer zones and in the solid wall zones involved because of the macroscopic length in the flow direction; accounting for the pairwise interactions between all the fluid molecules and between all the fluid and solid wall molecules by molecular dynamics simulation is really a massive computational work as having been well known
Summary
For a low value of the continuum fluid film thickness h which gives the value of bf as high as over 0.1, the physical adsorbed layer still can not be treated as a solid layer even for the medium fluid-wall interaction, as the value of rq is significantly greater than that of rq,s ; it suggests that for this case the adsorbed layer should still be considered as a flowing layer, otherwise the total mass flow rate through the channel will be pronouncedly underestimated This corresponds to the considerable proportion of the flow rate of the adsorbed layer as shown by the values of rb/h. The present study indicates that when calculating the flow rate through the channel for small channel heights i.e. high bf values, the flowing property of the adsorbed layer should still be considered even for the strong fluid-wall interaction
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