In Knudsen pumps with geometric configuration of rectangle, gas flows are induced by temperature gradients along channel walls. In this paper, the direct simulation Monte Carlo (DSMC) method is used to investigate numerically the flow characteristics of H₂–N₂ mixtures in the Knudsen pump. The variable soft sphere (VSS) model is applied to depict molecular diffusion in the gas mixtures, and the results obtained are compared with those calculated from a variable hard sphere (VHS) model. It is demonstrated that pressure is crucial to affecting the variation of gas flow pattern, but the gas concentration in H₂–N₂ mixtures and the collision model do not change the flow pattern significantly. On the other hand, the velocity of H₂ is larger than that of N₂. The velocities of H₂ and N₂ increase if the concentration of H₂ rises in the gas mixtures. The results of velocity and mass flow rate obtained from VSS and VHS models are different. Finally, a linear relation between the decrease of mass flow rate and the increase of H₂ concentration is proposed to predict the mass flow rate in H₂–N₂ mixtures.

Rarefied gas mixtures can be separated by thermally induced flows in microchannels, a technique of relevance to microelectromechanical systems (MEMSs). The paper proposes a microchannel that exploits rightward/leftward flow as its primary mechanism; every component of the channel is held at a constant temperature, so that separation is achieved without wall temperature gradients or surface-property gradients, and no special surface material or treatment is required. By connecting such units periodically, a multi-stage separator is formed. The separation of He–Xe mixtures in a single microchannel unit and in the multi-stage separator is analyzed using the direct simulation Monte Carlo (DSMC) method, focusing on the influences of the Knudsen number, temperature difference, and geometric parameters. Results show that, within a microchannel unit, the lighter gas accumulates in hot regions and the heavier gas in cold regions; the diffusive velocities of He and Xe are oppositely directed, with He diffusing considerably faster, demonstrating the separation capability of the proposed configuration. In the multi-stage separator, separation degrades when the aspect ratio, row distance, or Knudsen number deviates too far from its optimum. The separation factor reaches about 1.28 at a Knudsen number of about 0.9 in the transitional flow regime, about 1.329 at an aspect ratio a / b of about 0.375, and about 1.29 at a row distance h / b of about 0.1, while it increases monotonically with the temperature difference, up to about 1.77 at the largest temperature difference tested. These results may inform the design of thermally driven gas separators in MEMS-related applications.

This study employs the Cercignani-Lampis-Lord (CLL) gas-surface interaction model with the direct simulation Monte Carlo (DSMC) method to systematically investigate how surface accommodation coefficients affect gas separation of He–Ne binary mixtures in a ratchet-type Knudsen pump. The normal energy accommodation coefficient (NEAC) and the tangential momentum accommodation coefficient (TMAC) are varied under different Knudsen numbers and wall temperature differences. Through analysis of species molar fraction distributions, velocity fields, temperature fields, and Knudsen forces, the physical mechanisms by which surface reflection properties govern thermally induced flow separation are elucidated. The results reveal that the accommodation coefficients directly determine the type and relative intensity of thermally induced flow by modifying the scattering properties of the triangular ratchet walls. Under perfectly specular reflection, radiometric flow dominates; increasing either coefficient progressively activates thermal edge flow. The separation factor is consistently negatively correlated with both coefficients, reaching its maximum when both are zero, with the NEAC exerting a stronger influence than the TMAC at equal magnitude. Large temperature differences and transitional flow constitute the optimal operating regime, while under small temperature differences or free-molecular conditions the regulatory effects become negligible. The Knudsen force difference between the two species is identified as the direct mechanical origin of differential species transport, with its variation highly consistent with that of the separation factor. These findings provide quantitative guidance for optimizing microscale gas separators through surface engineering strategies.