Gas separation through carbon nanotubes

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Layering phenomena of carbon dioxide and methane transported through carbon nanotubes are being examined through molecular dynamics. The layering formation is investigated for carbon nanotubes ranging from (6,6) to (20,20) subjected to pressures spanning between 5-20 bar at 300 K. Well defined layers are developed both in the internal and external surface of the nanotubes for all the examined cases. It is also shown that the number of layers along with their absolute strength varies as a function of the nanotube's diameter, carbon dioxide and methane's density and gas-structure interactions. Finally, the diffusion inside the interior of the nanotubes has been examined showing a Fickian diffusion mode

Multiscale simulation of nanofluidic networks of arbitrary complexity

We present a hybrid molecular-continuum method for the simulation of general nanofluidic networks of long and narrow channels. This builds on the multiscale method of Borg et al. (Microfluid Nanofluid 15(4):541–557, 2013; J Comput Phys 233:400–413, 2013) for systems with a high aspect ratio in three main ways: (a) the method has been generalised to accurately model any nanofluidic network of connected channels, regardless of size or complexity; (b) a versatile density correction procedure enables the modelling of compressible fluids; (c) the method can be utilised as a design tool by applying mass-flow-rate boundary conditions (and then inlet/outlet pressures are the output of the simulation). The method decomposes the network into smaller components that are simulated individually using, in the cases in this paper, molecular dynamics micro-elements that are linked together by simple mass conservation and pressure continuity relations. Computational savings are primarily achieved by exploiting length-scale separation, i.e. modelling long channels as hydrodynamically equivalent shorter channel sections. In addition, these small micro-elements reach steady state much quicker than a full simulation of the network does. We test our multiscale method on several steady, isothermal network flow cases and show that it converges quickly (within three iterations) to good agreement with full molecular simulations of the same cases

A hybrid molecular-continuum simulation method for incompressible flows in micro/nanofluidic networks

We present a hybrid molecular-continuum simulation method for modelling nano- and micro-flows in network-type systems. In these types of problem, a full molecular dynamics (MD) description of the macroscopic flow behaviour would be computationally intractable, or at least too expensive to be practical for engineering design purposes. Systems that exhibit multiscale traits, such as this, can instead be solved using a hybrid approach that distinguishes the problem into macroscopic and microscopic dynamics, modelled by their respective solvers. The technique presented in this study is an extension and addition to a hybrid method developed by Borg et al. (J Comput Phys 233:400–413, 2013) for high-aspect-ratio channel geometries, known as the internal-flow multiscale method (IMM). Computational savings are obtained by replacing long channels in the network, which are highly scale-separated, by much smaller, but representative, MD simulations, without a substantial loss of accuracy. On the other hand, junction components do not exhibit this length-scale separation, and so must be simulated in their entirety using MD. The current technique combines all network elements (junctions and channels) together in a coupled simulation using continuum conservation laws. For the case of steady, isothermal, incompressible, low-speed flows, we use the conservation of mass and momentum flux equations to derive a set of molecular-continuum constraints. An algorithm is presented here that computes at each iteration the new constraints on the pressure differences to be applied over individual MD micro-elements (channels and junctions), successively moving closer to macroscopic mass and momentum conservation. We show that hybrid simulations of some example network cases converge quickly, in only a few iterations, and compare very well to the corresponding full MD results, which are taken as the most accurate solutions. Major computational savings can be afforded by the IMM-type approximation in the channel components, but for steady-state solutions, even greater savings are possible. This is because the micro-elements are coupled to a steady-state continuum conservation expression, which greatly speeds up the relaxation of individual micro-components to steady conditions as compared to that of a full MD simulation. Unsteady problems with high temporal scale separation can also be simulated, but general transient problems are beyond the capabilities of the current technique

Molecular dynamics simulation on flows in nano-ribbed and nano-grooved channels

We present molecular dynamics simulation results on fluid and transport properties for nanochannel flows. The upper channel wall is constructed from periodic roughness elements and flows are simulated both in longitudinal (ribs) and transverse (grooves) direction and are compared to respective flat-wall channel flows. Various wall/fluid interaction strength ratios are considered for the simulations, covering typical hydrophilic and hydrophobic channels. We show that groove orientation (ribs and grooves) has a primitive effect on flow mainly due to slip length increase in a ribbed-wall channel. The transport properties of the fluid are significantly affected by wall wettability, as, in flows past an hydrophobic wall, the diffusion coefficient presents anisotropy, shear viscosity attains a minimum value and thermal conductivity increases. © 2015 Springer-Verlag Berlin Heidelber