Analysis of Flow Behavior of Micro- and Conventional Nozzles Operating in Vacuum

Flow behaviors of micro- and conventional nozzles used for spacecraft applications significantly differ. Although the differences are noted, a detailed comparison exploring the influence of nozzle size remains limited. Based on the current results, for nozzles operating in vacuum, this paper proposes to define micronozzles as having microscale throats with significant rarefaction, internal continuum breakdown, and low Reynolds numbers Re while defining conventional nozzles as having bigger dimensions, continuum internal flow, external breakdown, and higher Reynolds numbers Re. The direct-simulation Monte Carlo (DSMC) method is employed to resolve the rarefied and nonequilibrium flow in the micronozzle, whereas a hybrid Navier–Stokes/DSMC approach is applied to the conventional nozzle. The comparison highlights strong scale-dependent flow. In the micronozzle, viscous effects dominate, leading to reduced acceleration, lower exit velocity, and suppression of shock structures. The plume produced is more collimated with a higher normalized centerline velocity. Plume–surface interaction is characterized by smooth shock-free deceleration and minimal reverse flow, resulting in a more concentrated impingement region with higher axial particle density and localized heat flux. A conventional nozzle exhibits supersonic expansion, higher exit velocities, and distinct shock-cell formations near the exit. Plume–surface interactions reveal the presence of strong normal and oblique shocks, significant backflow, and potential contamination risks.