Density-dependent drag coefficient for gas-adsorbed particles in free-molecule flows

Gas adsorption by the spherical particles in gas-particle flows has been recently studied by Yu et al. ["Direct simulation Monte Carlo of the gas adsorption of particles in gas-particle flows," Phys. Fluids 34, 083302 (2022)]. However, the gas-adsorption distribution on the particle surface has heretofore remained unknown. This paper addresses this knowledge gap by introducing a numerical method to calculate the gas-adsorption distribution for ellipsoidal particles in gas-particle flows. We split the particle surface into internal flat plates and calculate the gas adsorption for each internal flat plate in the gas flow. Based on this numerical method, the gas adsorption distribution for the ellipsoidal particles is reconstructed by using the direct simulation Monte Carlo method. The results show that the average adsorption by prolate particles with particle eccentricity reverses as the particle temperature increases. Moreover, we show that the density-dependent drag coefficient for particle motion in the free-molecule flow may evince gas adsorption at the particle surface. Those points could inspire the studies of dust physics in rarefied gas spaces.