Combustion of magnesium particles in oxygen-inert atmospheres

The combustion mechanism of laser-ignited magnesium particles in the 100 ν-size range was studied by cinemicrography of burning particles and by scanning electron micrography of quenched samples. Burning was investigated in room-temperature, atmospheric-pressure, oxygen-argon atmospheres, with oxygen mass fractions ranging from 0.03 to unity. Certain observations also were made for burning in air, for burning at pressures between 1 5 and 2 atm, for burning at ambient temperatures up to roughly 500°C, for burning in atmospheres with water concentrations of roughly 2%, and for burning in carbon dioxide. Prompt and delayed types of ignition were identified. Combustion was found to involve an extended gas-phase reaction zone, surface accumulation of solid oxide, jetting, spinning, and fragmentation. Regular burning, with little accumulation of oxide within the particle, also was observed but only in dilute atmospheres. Except in very dilute atmospheres, buildup of surface oxide was found to produce extinction earlier than expected, either by enhancing the burning rate or by preventing combustion from going to completion. A number of different extinction modes were observed. Burning time was measured as a function of initial flame diameter. Ratios of flame to particle diameters also were estimated. A simplified theoretical model was developed for describing the quasi-steady combustion. Theory and experiment agree in most but not all respects. The theory suggests the occurrence of a homogenous magnesium-oxygen reaction followed later by condensation of the oxide. The fact that the oxide which condenses on the particle surface is solid rather than liquid, is reasoned to influence the combustion mechanism significantly.