Experimental investigation of the flow regimes in a rarefied supersonic free jet according to complex Reynolds number

Recent technological advancements necessitate an in-depth understanding of fluid dynamics in rarefied gas environments, vital for semiconductor and aerospace industries. Free jets used in semiconductor deposition and satellite control exhibit complex shockwave structures in these conditions, complicating understanding of the flow. Previous studies focused mainly on near-field shockwaves. However, understanding the far-field zone post-shockwave dissipation is crucial for vacuum applications. This study quantitatively visualizes 2D flow fields of supersonic circular free jets in rarefied conditions. We analyzed shockwave and flow structures in the near-field and far-field zones to establish a reliable experimental basis for vacuum environments. Using nanometer-sized particles for particle image velocimetry (PIV) and acetone for molecular tagging velocimetry (MTV), the study measured supersonic jet flows at 1 torr. A one-barrel shockwave in the near-field and a long annular viscous layer beyond the Mach disk was observed. The far-field zone transitioned to a laminar regime, matching theoretical predictions based on the complex Reynolds number. These findings provide essential experimental evidence for designing equipment for vacuum environments.