Gas concentration in rarefied flows: Experiments and modeling

The relationship between gas-surface scattering dynamics and flow through conical systems in the rarefied regime has been explored with experimental and computational tools. Molecular beam-surface scattering experiments were used to characterize the inelastic scattering dynamics of Ar and N₂ on a highly oriented pyrolytic graphite (HOPG) surface. These data were then used to develop a gas-surface scattering approach for kinetic fluid simulations, which combines the diffuse scattering and Cercignani-Lampis-Lord (CLL) models. The translational energy and angular flux distributions from the beam-surface scattering experiments were used to obtain the normal energy accommodation (α_n) parameter as well as the effective surface mass (M) in the CLL model. Constant values of α_n=0.9, M = 40 amu for Ar and α_n=0.75, M = 28 amu for N₂ were found to capture the experimental angular flux scattering distributions over a wide range of incident energies and angles. The gas-surface model was subsequently used in direct simulation Monte Carlo (DSMC) simulations of the flux of directed gas flow through a conical concentrator and compared with measurements of gas concentration with a prototype concentrator in separate molecular beam experiments. Comparisons of predicted concentration pressure ratios (pressure in an accommodation chamber with vs. without concentrator) with measured values for three free-stream velocities of Ar and N₂ showed maximum deviations of 4.8% and 9.8% for Ar and N₂, respectively. The simulations show that gas-surface collisions resulting in the loss of the normal component of the incident energy while preserving the tangential momentum of the impinging atom or molecule, resulting in super-specular scattering, lead to higher concentration of gases than purely specular, energy conserving gas-surface interactions. For conical concentrators, with inlet-to-outlet area ratios of 133 and 600, specular reflections result in concentration factors (flux through an orifice with vs. without concentrator) of only ∼8, while super-specular scattering results in concentration factors of 64 and 127, respectively.