Experimental investigation of concentration diffusion in a rarefied supersonic free jet

Precise control of gas concentration in vacuum conditions is vital for advanced semiconductor processing. While rarefied supersonic free jets are widely used in vacuum chambers, concentration diffusion under low-pressure conditions remains poorly understood due to experimental challenges. Previous studies focused on flow shock structure characterization but lacked quantitative data on scalar diffusion. In this study, we conducted an experimental investigation of an acetone-seeded supersonic free jet under rarefied conditions using laser-induced fluorescence (LIF). A sonic nozzle discharged tracer gas into a vacuum chamber, and 266 nm UV excitation enabled visualization of both near-field shockwaves and far-field scalar profiles. A one-barrel shockwave, including a Mach disk and oblique shocks, was clearly observed and matched theoretical predictions. Downstream, a laminar annular shear layer developed with a spreading rate significantly lower than that of turbulent jets, confirming that diffusion was governed by the outlet Peclet number and nozzle pressure ratio, rather than by turbulent mixing. This study provides quantitative insight into concentration diffusion in vacuum conditions and will improve gas flow design in semiconductor processing.