Study of the shock to detonation transition by time-resolved emission spectroscopy
This study is part of a research program on the detonation of high explosives. The objective of this work is to clarify the mechanism of shock initiation of nitromethane (NM), by spectral analysis in the range 0.3-0.85 µm. NM is a liquid explosive, transparent in the visible and near-infrared range. The experiments consist in plane shock impact on explosive targets at 8.6 GPa. Time-resolved radiance profiles and intensity spectra show that chemical species have several optical characteristics depending on the phases of the shock to detonation transition. Shocked NM remains transparent and its temperature is higher than 2500 K because of local chemical reactions. The reactions products produced during the superdetonation formation are semi-transparent: they are optically thick for wavelengths higher than 0.6 µm. The study shows that detonation products behind the detonation wave do not behave like a blackbody in the visible range. Detonation products are semi-transparent and optically thick. A model for the absorption coefficient is proposed for a water vapour and carbon clusters mixture. It is based on a Rayleigh scattering regime and equation of radiative tranfer (ERT) for an emitting and absorbing medium. The resolution of the ERT for semi-transparent gases by an inversion method enables to determine the temperature profiles in the explosive. The method is tested on simple cases of detonation.