Infinitely-fast diffusion in Single-File Systems

We have used Dynamic Monte Carlo (DMC) methods and analytical techniques to analyze Single-File Systems for which diffusion is infinitely-fast. We have simplified the Master Equation removing the fast reactions and we have introduced a DMC algorithm for infinitely-fast diffusion. The DMC method for fast diffusion give similar results as the standard DMC with high diffusion rates. We have investigated the influence of characteristic parameters, such as pipe length, adsorption, desorption and conversion rate constants on the steady-state properties of Single-File Systems with a reaction, looking at cases when all the sites are reactive and when only some of them are reactive. We find that the effect of fast diffusion on single-file properties of the system is absent even when diffusion is infinitely-fast. Diffusion is not important in these systems. Smaller systems are less reactive and the occupancy profiles for infinitely-long systems show an exponential behavior.Comment: 8 pages, 5 figure

Modeling thermochemical reactions in thermal energy storage systems

\u3cp\u3eThe focus of this chapter is mainly on molecular modeling techniques for the hydration and dehydration (sorption and desorption) processes occurring in salt hydrates at the nano-scale. Modeling techniques such as density function theory, molecular dynamics and monte carlo are briefly introduced. Some attention is also given to micro- and macro-scale modeling techniques used at larger length scales, such as Mampel's model and the continuum approach. Before introducing all the length (and time) scales involved when modeling a heat storage system, a qualitative description is given of the hydration and dehydration processes on the nano/micro-scale.\u3c/p\u3

Measurement of the interfacial temperature jump during steady-state evaporation of a droplet

Evaporation is an important phenomena that occurs in a wide range of natural and industrial processes. Although this phenomena has been a subject of research for many years, it is still not fully understood.\u3cbr/\u3eExperimental results of the last few decades seem to contradict with each other, and with the theory which describes this process, e.g. the kinetic theory of gasses (KTG) and non-equilibrium thermodynamics (NET). Temperature jumps of about 3.2-8.1oC at the interface of a steady state evaporating water droplet\u3cbr/\u3eat a pressure of about 245 Pa were measured . In order to determine whether this temperature jump exists and what influences this temperature jump, an experimental setup has been developed and the results are compared to theory

The influence of gas-wall interactions on the accommodation coefficients for rarefied gases:a molecular dynamics study

The energy accommodation coefficient (EAC) and the momentum accommodation coefficient (MAC) are two significant parameters determining the gas-solid energy and momentum exchange efficiencies. In this work,\u3cbr/\u3emolecular dynamics (MD) simulations were employed to study the impact of gas-wall interaction potential on energy and momentum accommodation coefficients between Gold and monoatomic gases (Argon and\u3cbr/\u3eHelium). The MD simulation setup consists of two infinite parallel plates of unequal temperature positioned at certain distance (12 nm and 102 nm for Argon and Helium gases, respectively) apart from each other, and\u3cbr/\u3eof gas molecules confined between them. A pairwise Lennard-Jones 12-6 potential was considered at the solid-gas interface. The interaction potential parameters were obtained using the Lorentz-Berthelot (LB) and Fender-\u3cbr/\u3eHalsey (FH) mixing rules, as well as based on existing ab-initio computations. Comparing the obtained results for the accommodation coefficients with empirical values revealed that the interaction potential based on abinitio\u3cbr/\u3ecalculations is the most reliable one for computing ACs. Besides, in the case of Au-Ar, the LB mixing rule substantially overpredicts the potential well depth which leads to sticking gas atoms on the solid surface. As a result, computing accommodation coefficients in this case from numerical point of view was not possible

Gas-wall interactions under rarefied conditions

The past decade has seen a considerable growth in portable devices with mobile connectivity. This growth has been enabled by the development of high capacity telecommunication networks globally. Individuals require high data transfer capabilities to remotely stream large information sets (i.e. HD video) and this is leading to greater demands for next generation networks (i.e. 5G). To ensure this growth continues, hardware devices must be smaller, more energy-efficient and provide greater functionality. This requirement poses a thermal management challenge, increasing heat transfer density significantly. Novel materials and cooling methods, which are engineered at the micro- and nanoscale, are necessary to address this. In this paper the focus is on the numerical modelling of the gas-wall interactions that determine the heat transfer

Velocity correlations and accommodation coefficients for gas-wall interactions in nanochannels

\u3cp\u3eIn order to understand the behavior of a gas close to a channel wall, it is important to model the gas-wall interactions correctly. When using Molecular Dynamics (MD) simulations these interactions are modeled explicitly, but the computations are time consuming. Replacing the explicit wall with an appropriate wall model reduces the computational time, but should still remain the same characteristics. In this paper the focus lies with an argon gas confined between two platinum walls at different temperature. Several wall models are investigated for their feasibility as a replacement of the MD simulations and are mainly compared using the velocity correlations between impinging and reflecting particles. Moreover, a new method to compute the accommodation coefficient using the velocity correlations is demonstrated.\u3c/p\u3

Thermal contact resistance in carbon nanotube enhanced heat storage materials

Solid-liquid phase change is one of the most favorable means of compact and economical heat storage in the built environment. In such storage systems, the vast available solar heat is stored as latent heat in the storage materials. Recent studies suggest using sugar alcohols as seasonal heat storage materials for their large storage capacity, moderate melting point, and evident supercooling effects. However, the heat transfer in such materials is sluggish and hence carbon structures are proposed to enhance their overall heat conductivity. In this work, we focus on sugar alcohol - carbon nanotube system, analyze the heat transfer in the radial direction of the nanotube using molecular dynamics simulations. The thermal contact resistance is calculated using Nos´e-Hoover dynamics and is found dependent on the diameter of the tubes. We validate our results using water - nanotube simulations. Then the simulation method is extended to sugar alcohol - nanotube systems

Computation of accommodation coefficients and the use of velocity correlation profiles in molecular dynamics simulations

For understanding the behavior of a gas close to a channel wall it is important to model the gas-wall interactions as detailed as possible. When using molecular dynamics simulations these interactions can be modeled explicitly, but the computations are time consuming. Replacing the explicit wall with a wall model reduces the computational time but the same characteristics should still remain. Elaborate wall models, such as the Maxwell-Yamamoto model or the Cercignani-Lampis model need a phenomenological parameter (the accommodation coefficient) for the description of the gas-wall interaction as an input. Therefore, computing these accommodation coefficients in a reliable way is very important. In this paper, two systems (platinum walls with either argon or xenon gas confined between them) are investigated and are used for comparison of the accommodation coefficients for the wall models and the explicit molecular dynamics simulations. Velocity correlations between incoming and outgoing particles colliding with the wall have been used to compare explicit simulations and wall models even further. Furthermore, based on these velocity correlations, a method to compute the accommodation coefficients is presented, and these newly computed accommodation coefficients are used to show improved correlation behavior for the wall models

Nanoscale heat transfer in carbon nanotube - sugar alcohol composites as heat storage materials

Nanoscale carbon structures such as graphene and carbon nanotubes\u3cbr/\u3e(CNTs) can greatly improve the effective thermal conductivity of thermally sluggish heat storage materials, such as sugar alcohols (SAs). The specific improvement depends on the heat transfer rate across the carbon structure. Besides, the heat transfer rate is further dependent on the material and the CNT diameter. In this paper, molecular dynamics simulations are applied to graphene/CNT-SA interfacial systems. Using erythritol and xylitol as model materials, we find the cross-plane thermal contact conductance to decrease as the CNT diameter decreases, with an exception for CNT(7,7). A phonon mode analysis is carried out to explain the general decreasing trend. The larger phonon mode mismatch observed between the molecules on both sides of smaller diameter CNTs is found to be a finite size effect of the confinement, instead of an interfacial effect. From the molecular collision point of view, a higher molecular density promotes heat transfer. In the case of CNT(7,7), the effective density of molecules enclosed in the CNT is found to be much higher than that of CNT(8,8). This may be the cause of the higher heat transfer rate across CNT(7,7). Molecular orientations and hydrogen bond structures of the molecules inside the CNTs are investigated to demonstrate the finite size effect\u3cbr/\u3eof the confinement. For graphene-SA composites, five model materials are considered and their cross-plane thermal contact conductance values fall into a narrow range