Condensed-phase modifications in magnesium particle combustion in air

Free-falling individual magnesium particles were produced and ignited using a pulsed micro-are and burned in air. Particle combustion temperatures were monitored in real time using a three-color pyrometer. Black-body and MgO radiation histories were separated using interference filters and compared with each other. Particles were rapidly quenched at different combustion rimes and their elemental compositions were studied using energy and wavelength dispersive spectroscopy techniques. Particle combustion times and radiation histories were consistent with those reported previously. Measurements of the elemental compositions of quenched particles showed substantial amounts of dissolved oxygen, consistent with recent metal particle combustion studies using the same experimental technique. An estimate based on the rate constants reported in the literature for the magnesium vapor-phase burning, showed that the reaction rate is insufficiently fast to prevent oxygen gas diffusion through the vapor-phase stand-off flame to the particle surface. A combustion mechanism is discussed in which oxygen approaching the surface of a burning particle is dissolved in magnesium producing a new liquid-phase Mg-O solution. The heterogeneous reaction of oxygen dissolution begins simultaneously with the vapor-phase combustion and later on the heterogeneous reaction becomes the primary combustion mechanism. After the solution becomes saturated, an exothermic phase transformation, Mg-O-solution --> Mg + MgO, occurs that contributes to the production of large amounts of oxide detected on the surfaces of partially burned particles and causes the observed oscillations and peaks in particle radiation. (C) 2000 by The Combustion Institute.