Emission spectroscopy of the interior of optically dense post-detonation fireballs

In recent years, emission spectroscopy has been applied to the study of post-detonation combustion in explosives, often yielding valuable information on temperatures and chemical dynamics. The post-detonation fireballs that form as under-oxidized detonation products burn in the surrounding air are optically dense and the corresponding emission spectra sample only the material at or near the surface of the fireball. In the present study, we exploit the large optical density in order to probe the dynamics occurring in the interior of the fireball. Emission spectra are collected following detonation of 20 g aluminized Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) charges using fiber optics located behind the flame front and then compared with the corresponding spectra of the surface layer collected from outside the fireball. We find that in the early evolution of the fireball (t ≤ 60 μs and r ≤ 10 cm in the current study), combustion and light emission are predominantly confined to the surface, while the interior is dark. Later, after the fireball expands and mixes with the surrounding air (t ≥ 120 μs or r ≥ 30 cm), combustion and emission occur throughout, and we find no significant differences between the spectra collected from the interior of the fireball versus those from its surface.