| The energy required for initiation and propagation of detonations in tubes of finite diameter depends on the structure of the reaction zone, and especially upon the length of the induction zone, which is detemined by the ignition delay time of the fuel/oxidizer mixture. To establish the length of the induction zone in dust detonations, the shock wave ignition of a variety of dust particles, including coal, graphite, oats, diamond, and high explosive, RDX, was investigated experimentally and theoretically.;In the theoretical model, the particle acceleration, subsequent convective heating by the hot gas flow, and chemical exothermic surface reaction in pores as well as on the surface of the particle were taken into consideration. In the numerical calculation, it was observed that only the thin layer adjacent to surface in contact with the surrounding hot gas experienced a significant temperature change. Based on this observation, two simplified methods of computing the ignition delay were devised by judicious choices of the definition of ignition, and by defining three characteristic times (acceleration, thermal, and chemical). Finally, an asymptotic analysis for the limit of large activation energy was carried out for spherical geometry with constant average values of gas recovery temperature (T(,f)) and convective heat transfer coefficient (h). And under the assumption that the exothermic reaction of Arrhenius type occurs only at the surface of a semi-infinite body, the asymptotic analysis was applied to assess the effects of varying values of T(,f) and h. It was found that the ignition delay times of dust particles behind incident shock waves could be reasonably predicted with the use of suitably chosen values of T(,f) and h.;Dust particles with average particle sizes ranging from 2 to 74 microns were introduced into the test section using inertial and air injectors. Incident shock waves in the Mach number range of 4.1-4.9 were generated to initiate dust ignition in pure oxygen. The radiation from the dust cloud was monitored by a photomultiplier. This technique provided reliable comparative measurements of the ignition delays of the dusts. Much shorter ignition delays were observed for oats dust than for any of the other dusts. It was also noted that RDX-A (mean diameter, 37 micron) had a shorter ignition delay than RDX-E (mean diameter, 2 micron). |
