Molecular dynamics simulation of thermodiffusion and mass diffusion in structureless and atomistic micropores

International audienceIn this work, we have studied the effect of surface roughness on thermodiffusion in simple "isotopic" mixtures confined in a slit nanopore. To do so, we have performed non-equilibrium molecular dynamics simulations of Lennard-Jones binary equimolar mixtures confined in structureless (in which the interaction with the fluid is described by a Lennard-Jones 9-3 potential) and atomistic walls for various widths, from 5 to 35 times the size of a molecule, in the NP//T ensemble. For that purpose, a new algorithm is proposed in atomistic pore. Different super-critical conditions have been explored, ranging from low to moderate densities. In addition to the thermal diffusion factor, we have also estimated the mass diffusion and thermodiffusion coefficients separately. The results show that the two types of walls lead to noticeably different results. The thermal diffusion factor tends to increase in atomistic wall and slightly decrease in structureless wall when the pore width is decreasing, this being related to the average density behaviour. More precisely, both mass and thermodiffusion coefficients are weakly affected by the pore width for structureless walls, whereas both quantities largely decrease (up to 70% and 55% respectively compared to bulk fluid) when pore size decreases in the case of a rough solid surface because of the friction on the walls

Thermodiffusion dans les fluides de Lennard-Jones par dynamique moleculaire

Ce travail porte sur l'étude de la thermodiffusion, ou effet Soret, par simulation numérique à l'échelle microscopique. Ce processus de transport croisé couple flux de masse et gradient thermique et est encore largement incompris. Pour cette étude, nous avons appliqué un algorithme de dynamique moléculaire hors équilibre à des mélanges de sphères de Lennard-Jones libres ou confinées. Après avoir testé la validité de nos simulations, nous avons montré que les résultats obtenus permettaient d'estimer la thermodiffusion dans ces fluides modèles à partir de corrélations simples sur les paramètres moléculaires. Cette démarche a été également validée sur des mélanges ternaires. Par ailleurs, les résultats de l'influence du milieu poreux sur la thermodiffusion ont montré la prépondérance des effets d'adsorption sur ceux liés au confinement. Cette influence restant faible dans la majorité des cas, exceptée pour les pores les plus fins et les plus attractifs

Shear Viscosity of Inhomogeneous Hard-Sphere Fluids

International audienceUsing molecular dynamics on Hard-Sphere-like fluids subject to an external sinusoidal field inducing density inhomogeneities and undergoing a bi-periodical shear flow, we have studied the local viscosity of the inhomogeneous fluid. It has been shown that for a slowly varying density profile the local average density model combined with the well-known models proposed in the density function theory yields a good description of the viscosity profile obtained by molecular simulation. However, for a rapidly varying density profile these models are unable to describe correctly the viscosity profile obtained by molecular simulations. So, to overcome the weakness of these models we have proposed a simple model that takes into account the effect of the angle formed by the colliding molecules and the direction of the flow

Couplage mécanique des milieux continus / Dynamique moléculaire

Dans ce travail, nous présentons un schéma de couplage spatial et temporel entre les équations de Navier-Stokes (échelle macroscopique) et la dynamique moléculaire (échelle microscopique). Le couplage de ces deux approches est basé sur la décomposition de domaine de Schwarz. Dans cette méthode, le domaine d'étude est décomposé en deux sous-domaines qui se recouvrent partiellement: un sous-domaine traité par dynamique moléculaire et l'autre sous-domaine où les équations de Navier-Stokes en formulation incompressible sont résolues par la méthode des volumes finis

High Pressure Acid Gas Viscosity Correlation

International audienceAcid gases containing H2S are often encountered in the petroleum industry. However, reliable experiments on their thermophysical properties in reservoir conditions, in particular viscosity, are very scarce. From a modeling point of view H2S (and CO2) are polar compounds and are so often considered as rather difficult to model accurately. In this work, we propose a correlation based on a corresponding states approach in order to predict the viscosity of acid gas mixtures, among others, with a strong physical background. This correlation is based on the Lennard-Jones fluid model, which has been studied extensively thanks to molecular dynamics simulations over a wide range of thermodynamic conditions. This fluid model can be extended to deal with polar molecules such as CO2 or H2S without a loss of accuracy. In a first part, we demonstrate that the proposed physically based correlation is able to provide an excellent estimation of the viscosity (with average absolute deviations below 5 %) of pure compounds including normal-alkanes, CO2 or even H2S whatever the thermodynamic conditions, gas, liquid or supercritical. Then, using a one-fluid approximation and a set of combining rules, the correlation is applied to various mixtures in a fully predictive way, i.e. without any additional fitted parameters. Using this scheme, the deviations between predictions and measurements are as low as on pure fluids. The viscosity of natural and acid gas mixtures in reservoir conditions is shown to be very well predicted by the proposed scheme. In addition, it is shown that this correlation can also be applied to predict reasonably the viscosity of asymmetric high pressures mixtures even in the liquid phase. This physically based approach is easy to plug in any simulation software as long as the only inputs, the molecular parameters, are directly related to the critical temperature and volume

Contribution to the modeling of the shear viscosity of sulfur hexafluoride (SF6): Comparative study of some representative models

International audienceThree recent physically based models (Lennard-Jones, free volume, thermodynamic scaling) for representing the viscosity of sulfur hexafluoride (SF6) are discussed together with two models (friction theory and Enskog 2r) that have been recently applied to this fluid. The experimental database employed for adjustment (1562 data points) considers a large temperature (225.18 to 473.15 K) and pressure intervals (0.0264 to 51.21 MPa). The absolute average deviation is 3.8% for the Lennard-Jones model (one parameter), 1.7% for the free volume model (three parameters) and 1.5% for the thermodynamic scaling model (six parameters). Thus, it is shown that when physically based approaches are employed, a limited number of parameters is sufficient to represent accurately the shear viscosity of SF6. Furthermore it has been confirmed, using the thermodynamic scaling approach, that the repulsive steepness of the SF6 interaction potential is higher than usually found for fluids composed by non polar spherical molecule