Detonation initiation in unconfined fuel-air mixtures

Abstract

The critical energy threshold for detonation initiation in unconfined gaseous fuel-air mixtures has been determined as a function of fuel concentration in air for a hydrocarbon fuel, which consists of methylacetylene, propadiene, propane, and propylene. A similar function was found for 6% by volume n-propyl nitrate and 94% hydrocarbon fuel mixture. Detonation limits of the hydrocarbon-air mixture were found to be 3.3 and 9.8%. The minimum critical energy threshold for high explosive blast wave initiation of the hydrocarbon-air mixtures was found to be 19 kcals at stoichiometric. The n-propyl nitrate/hydrocarbon fuel was detonable over the range 2.8–10.1% with a minimum critical energy threshold of 12 kcals. Propylene oxide-air mixtures were detonable with a minimum critical energy threshold of 4 kcals, and the propylene oxide was detonable in a much broader mixture range than the hydrocarbon or hydrocarbon/ n-propyl nitrate fuels. Detonation velocities were measured to be 1830 ± 43 m/sec in 5.0% hydrocarbon fuel-air mixtures, which is in good agreement with calculated Chapman-Jouguet velocities. Measured detonation pressures of 250 ± 20 psi were lower than the calculated pressures of 270 psi. The experimentally measured radii at which wave velocities from spherical initiating charges decayed to Chapman-Jouguet velocities were approximately the same as the calculated critical radii r∗. Several interesting detonation wave initiation and propagation phenomena were observed on high-speed film and recorded by pressure and impulse measurements.