No abstract

Gas flow with surface reaction in porous media appears in various regions of engineering. In porous media with holes as small as a molecular mean free path, Kn of gas flow in the narrow channel is on the order of unity. Therefore, the direct simulation Monte Carlo (DSMC) method is suitable to solve transport phenomena in such kind of porous media. We perform 2D DSMC simulations of such a flow. The shape of narrow channel in porous media is complicated. To reduce complexity, we propose the simplification for porous structures by cubes and polyhedra. Results for the simplification by polyhedra agree well with the result obtained in the case without simplification. Results for the simplification by cubes also show good agreement with the result without simplification when surface reaction probability is modified.

The dissociation probabilities of H2 and D2 molecules on a Pt(111) surface with thermal motion were analyzed using the molecular dynamics (MD) method. The potential constructed using the embedded atom method was used as the interaction potential between a gas molecule and the surface. The effects of changing the translational energy and incident polar angle of D2 molecules impinging on a Pt(111) surface were analyzed using MD simulations. The effect of initial orientation, incident azimuthal angle, rotational energy of gas molecules, and the impinging points on the surface were averaged by setting the initial values in a random manner. When the molecules approach normal to the surface, the dissociation probability increases with the initial translational energy. At larger incident angles, the probability becomes smaller. The impinging processes were categorized in terms of reaching the chemisorption layer by analyzing the repulsion forces from the surface. The effective translational energies for impingement, both normal and parallel to the surface, play important but different roles in terms of molecules reaching the chemisorption layer and the dissociation probability. The results were compared to those obtained by molecular beam experiments to check the validity of the simulations. The results indicate that the dependence of the dissociation probability on the translational energy and incident angle is in approximate agreement with that from experiments.

In the paper a new way of Kinetic Force Method application for modeling rarefied gas flows is suggested. This way is founded on a kinetic equation for auxiliary two-particle distribution function of quasiparticle pairs. One-particle distribution function satisfying the classical Boltzmann equation can be obtained from the auxiliary distribution function by a simple integration. The using of quasiparticle pairs guarantees energy and momentum conservation in the course of the rarefied gas flows modeling automatically. Comparison of the results obtained by Kinetic Force Method and DSMC method is carried out on the examples of numerical simulations of the homogeneous relaxation and the vacuum pump micro flows.

Molecular Dynamics (MD) was used to simulate dissociative adsorption of a hydrogen molecule on the Pt(111) surface considering the movement of the surface atoms and gas molecules. The Embedded Atom Method (EAM) was applied to represent the interaction potential. The parameters of the EAM potential were determined such that the values of the dissociation barrier at different sites estimated by the EAM potential agreed with that of DFT calculation results. A number of MD simulations of gas molecules impinging on a Pt(111) surface were carried out randomly changing initial orientations, incident azimuth angles, and impinging positions on the surface with fixed initial translational energy, initial rotational energy, and incident polar angle. The number of collisions in which the gas molecule was dissociated were counted to compute the dissociation probability. The dissociation probability was analyzed and expressed by a mathematical function involving the initial conditions of the impinging molecule, namely the translational energy, rotational energy, and incident polar angle. Furthermore, the utility of the model was verified by comparing its results with raw MD simulation results of molecular beam experiments.