William D. Nicholls

Water transport through carbon nanotubes with defects

Non-equilibrium molecular dynamics simulations are performed to investigate how changing the number of structural defects in the wall of a (7,7) single-walled carbon nanotube (CNT) affects water transport and internal fluid dynamics. Structural defects are modelled as vacancy sites (missing carbon atoms). We find that, while fluid flow rates exceed continuum expectations, increasing numbers of defects lead to significant reductions in fluid velocity and mass flow rate. The inclusion of such defects causes a reduction in the water density inside the nanotubes and disrupts the nearly frictionless water transport commonly attributed to CNTs.

Boundary conditions for molecular dynamics simulations of water transport through nanotubes

This article compares both new and commonly used boundary conditions for generating pressure-driven water flows through carbon nanotubes in molecular dynamics simulations. Three systems are considered: (1) a finite carbon nanotube membrane with streamwise periodicity and ‘gravity’-type Gaussian forcing, (2) a non-periodic finite carbon nanotube membrane with reservoir pressure control, and (3) an infinite carbon nanotube with periodicity and ‘gravity’-type uniform forcing. Comparison between these focuses on the flow behaviour, in particular the mass flow rate and pressure gradient along the carbon nanotube, as well as the radial distribution of water density inside the carbon nanotube. Similar flow behaviour is observed in both membrane systems, with the level of user input required for such simulations found to be largely dependent on the state controllers selected for use in the reservoirs. While System 1 is simple to implement in common molecular dynamics codes, System 2 is more complicated, and the selection of control parameters is less straightforward. A large pressure difference is required between the water reservoirs in these systems to compensate for large pressure losses sustained at the entrance and exit of the nanotube. Despite a simple set-up and a dramatic increase in computational efficiency, the infinite length carbon nanotube in System 3 does not account for these significant inlet and outlet effects, meaning that a much smaller pressure gradient is required to achieve a specified mass flow rate. The infinite tube set-up also restricts natural flow development along the carbon nanotube due to the explicit control of the fluid. Observation of radial density profiles suggests that this results in over-constraint of the water molecules in the tube.

Modelling Of Metal-to-metal Seals In A Pressure Relief Valve Using Advanced FE-analysis

This study investigates the behaviour of the contact faces in the metal-to-metal seal of a typical pressure relief valve. The valve geometry is simplified to an axisymmetric problem. A cylindrical nozzle, which has a valve seat on top, contacts with a disk, which is preloaded by a compressed linear spring. All the components are made of the steel AISI type 316N(L) defined using the multilinear kinematic hardening material model based on monotonic and cyclic tests at 20◦C. Analysis considerations include the effects of the Fluid Pressure Penetration (FPP) across the valve seat which exists at two different scales. There is certain limited fluid leakage through the valve seat at operational pressures, which is caused by the fluid penetrating into surface asperities at the microscale. At the macroscale, non-linear FE analysis using the FPP technique available in ANSYS revealed that there is also a limited amount of fluid penetrating into gap. Accurate prediction of the fluid pressure profile over the valve seat is addressed in this study by considering the FPP interaction on both scales. The shape of this pressure profile introduces an additional component of the spring force, which needs to be considered to provide a reliable sealing. The analysis showed that the evolution of the profile, which is caused by the isotropic softening of the material, is significant during the cyclic operation of the valve.

Molecular dynamics simulations of liquid flow in and around carbon nanotubes

Using recently-developed fluid state controllers [1], we apply continuum fluid boundary conditions to molecular dynamics (MD) simulations of liquid argon flow past a carbon nanotube (CNT) and through a CNT membrane. Advantages of this method are that it: is not dependent on periodic boundary conditions; can accurately generate fluid transport without any geometrical constraints; and is capable of performing as an essential part of a hybrid continuum/atomistic technique. In our simulations, a pressure gradient is applied across a CNT membrane by controlling the densities of two reservoirs located either side of the membrane. Fluid velocity and density distributions are reported and compared to other published data where possible.

PERSPECTIVES ON THE SIMULATION OF MICRO GAS AND NANO LIQUID FLOWS

Micro- and nano-scale fluid systems can behave very differently from their macro-scale counterparts. Remarkably, there is no sufficiently accurate, computationally efficient, and — most importantly — generally agreed fluid dynamic model that encapsulates all of this important behaviour. The only thing that researchers can agree on is that the conventional Navier-Stokes fluid equations are unable to capture the unique complexity of these often locally non-thermodynamic-equilibrium flows. Here, we outline recent work on developing and exploring new models for these flows, highlighting, in particular, slip flow as a quintessential non-equilibrium (or sub-continuum) phenomenon. We describe the successes and failures of various hydrodynamic and molecular models in capturing the non-equilibrium flow physics in current test applications in micro and nano engineering, including the aerodynamic drag of a sphere in a rarefied gas, and the flow of water along carbon nanotubes.Copyright