Ignition and Incendivity of Laser Irradiated Single Micron-Size Magnesium Particles

Abstract

The ignition of single magnesium particles (28 to 120 micron diameter) suspended in cold oxidant atmospheres by means of an electrostatic levitation device, and the incendivity of these particles when suspended in a stoichiometric methane-air mixture, were investigated using pulsed laser radiation to heat the particles. Highspeed Schlieren and direct photographic techniques were used in the study. The critical radiant power density required for particle ignition was found to increase with particle size. Approximately equal radiant power densities were required for ignition of identical particles suspended in air and pure oxygen, although an increase in radiant power density was necessary for particle ignition in a helium-20 percent oxygen mixture. Results indicate that particle ignition occurred in the magnesium vapor shroud following surface vaporization. The incendivity experiments indicate that the critical radiant power densities necessary for the particle to ignite methane-air mixtures increases with decreasing particle diameter. Incendivity depends on the amount and rate of energy released by the irradiated particle. The incendive mechanism is believed to constitute a three-stage process.