Multiscale simulation of slip flow and adsorption in carbon nanopores
File(s)
Author(s)
Ross, Daniel Alexander
Type
Thesis
Abstract
Molecular dynamics and the lattice Boltzmann method (LBM) are combined as part of a multiscale procedure for translating information from nanoscopic molecular simulations up to the level of continuum models. The larger scales attainable with LBM allow for the scale-up of nanoscale phenomena, observed in individual pores, to larger complex porous structures. One such phenomena is that of slip flow, quantified by a slip length, whereby fluid in contact with a solid surface exhibits a non-zero velocity. Initially, adsorption and pore filling mechanisms in carbon nanotubes and graphitic slit pores with varying sizes are investigated. The fluids of interest are water, utilising the TIP4P model, and n-heptane, modelled with the OPLS-AA forcefield. Models are validated against experimental literature for macroscopic properties as functions of temperature at a pressure of 1 bar. The final densities in the pores are shown to be highly dependent on the level of packing of the fluids, which in turn depends on the surface-fluid wettability. As such, the fluid density varies drastically with relatively small changes in the pore dimensions.
The friction coefficient, which is proportional to the inverse of the slip length, is calculated for each equilibrated system. This is shown to be independent of separation for flat surfaces. However, with increasing confinement and wettability, large deviations are observed due to enhanced fluid densities. There is also a clear relationship between the slip length and curvature that differs depending on whether the surface is concave or convex.
Finally, two methods of implementing a slip boundary condition into LBM are presented and compared with analytical solutions to the Navier-Stokes equations for flows in channels and pipes. These are a specular-bounce-back combination boundary and an on-site velocity boundary. The latter is found to be preferable for implementation of slip in complex geometries, however further work is still required.
The friction coefficient, which is proportional to the inverse of the slip length, is calculated for each equilibrated system. This is shown to be independent of separation for flat surfaces. However, with increasing confinement and wettability, large deviations are observed due to enhanced fluid densities. There is also a clear relationship between the slip length and curvature that differs depending on whether the surface is concave or convex.
Finally, two methods of implementing a slip boundary condition into LBM are presented and compared with analytical solutions to the Navier-Stokes equations for flows in channels and pipes. These are a specular-bounce-back combination boundary and an on-site velocity boundary. The latter is found to be preferable for implementation of slip in complex geometries, however further work is still required.
Version
Open Access
Date Issued
2017-07
Date Awarded
2018-03
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
License URL
Advisor
Muller, Erich
Boek, Edo
Sponsor
Engineering and Physical Sciences Research Council
Shell
Publisher Department
Chemical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)