Axial propagation and radial spreading characteristics of pulsed underexpanded helium jets in low vacuum

Le résumé fourni par la source

Pulsed underexpanded jets in low vacuum environments are relevant to vacuum gas injection, propulsion plumes, and transient gas transport, but their axial and radial diffusion characteristics remain difficult to quantify because the reduced ambient density weakens Schlieren contrast. In this study, high speed Schlieren imaging is used to investigate pulsed underexpanded helium jets in a low vacuum chamber. Two image based descriptors are extracted from boundary layer features: the propagation velocity of vortex ring structures, Vvr, and the radial spreading width at a fixed axial location, Wrs. The experiments show that decreasing the ambient pressure increases Vvr and substantially enlarges Wrs, with the radial enhancement being especially pronounced in the low pressure range. Semi-empirical models based on a global expansion velocity scale, the Mach disk location, and the relative measurement position capture the main pressure dependent trends of both quantities. A comparison of two nozzle pressure ratio (NPR) pathways further shows that increasing the upstream pressure at fixed ambient pressure and decreasing the ambient pressure at fixed upstream pressure produce similar axial velocity responses, but markedly different radial spreading responses. These results indicate that NPR is an effective descriptor for the axial propagation of boundary layer vortex structures, whereas radial spreading is additionally controlled by the normalized ambient density and the associated weakening of transverse confinement in low vacuum environments.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.