[Objective] The atmosphere-breathing electric propulsion (ABEP) system has become a highly promising candidate for drag compensation in spacecraft operating in very low Earth orbit. To improve the inlet design of ABEP systems, this study performs a comprehensive numerical investigation of gas flows inside the inlet. The primary objective is to gain insight into the effects of the gas-surface interaction (GSI) model on the flow features, compression, and collection performances. [Methods] This paper explores ABEP inlet flows using the direct simulation Monte Carlo (DSMC) method. A typical altitude of 180 km in the upper atmosphere is considered, and four GSI accommodation coefficients (σ=1, 0.8, 0.5, and 0.2) are selected. The DSMC method simulates gas flows according to the motion of a cluster of simulation particles, where each particle represents a large number of real gas molecules. In the DSMC method, particle motions are computed deterministically, whereas intermolecular collisions are calculated statistically. Each simulation particle travels at a constant velocity until it collides with another simulation particle or a solid surface. In the event of an intermolecular collision, an appropriate molecular collision model is employed to compute post-collision velocities, and in the event of gas-surface collisions, a suitable GSI model is adopted to calculate the molecular velocity after reflection. In this work, the internal energy exchange is modeled using the Larsen-Borgnakke scheme. Further, the intermolecular collision is handled using the variable hard sphere model and the no time counter-collision sampling technique. The simulation is always evaluated as an unsteady flow, and a steady result is obtained as the large-time state of unsteady simulation. After achieving a steady flow, the simulation particles in each cell are sampled for a sufficient duration to decrease statistical scattering. All macroscopic field quantities (such as mass density, velocity, and temperature) and surface quantities (such as surface pressure, shear stress, and heat flux) are calculated based on these time-averaged data. [Results] Numerical results show that the distributions of gas pressure and mass flux are considerably affected by the GSI models. The lower the GSI accommodation coefficient, the higher the gas pressure and the larger the mass flux. Consequently, the GSI accommodation coefficients play a vital role in the compression factor and collection efficiency of the inlet. Furthermore, the decrease in the GSI accommodation coefficient from 1.0 to 0.2 leads to an increase in the compression factor and collection efficiency by a factor of 7 and 4, respectively. In addition, as the GSI accommodation coefficient decreases, the high-pressure region moves toward the ionization section, facilitating the ionization of neutral gas molecules. The following mechanism underlies this effect: after reflecting in a specular manner from the concave surface, the gas molecules congregate at the focus and enter the ionization section. [Conclusions] To improve the inlet design of an ABEP system, a combination of the geometric design and surface-material design should be adopted. A concave compression section should be employed, and at the same time, the inlet surface should be smoothened to decrease the GSI accommodation coefficient.
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