Effect of a physical inhomogeneity on steady-state detonation velocity

A set of experiments has been performed which demonstrate that ’’hot spots’’ in an explosive control some aspects of the steady-state detonation process. It is known, from experiments, that the relationship between explosive charge size and steady-state detonation velocity (the ’’diameter-effect curve’’) is qualitatively different for homogeneous and heterogeneous explosives. The experiments to be described strongly indicate that this qualitative difference is due to localized hot regions arising from flow modifications due to the presence of physical inhomogeneities. These local hot regions apparently produce an effective chemical reaction rate which allows a stable steady-state flow at a much smaller charge size than is possible in an analogous homogeneous material. Detonation-velocity measurements were carried out on cylindrical rate sticks at a number of radii thus generating diameter-effect curves. The explosives studied were composed primarily of commercial-grade nitromethane confined in Pyrex tubes. Silica impurities, with a known particle-size distribution, were added to the nitromethane to produce a heterogeneous material. The silica impurities were held in suspension with a guar-gum gelling agent. An attempt is made to interpret the new results by use of earlier numerical and experimental results concerning initiation of detonation at hot-spots.