Combustion of magnesium powders in products of an air/acetylene flame

Abstract

This paper is aimed at characterizing combustion dynamics of fine magnesium powders. Two spherical, micron-sized Mg powders with different particle size distributions were introduced into an air–acetylene flame. Particles were observed to burn in laminar flames as well as in flames with turbulence induced by an auxiliary swirling air flow. Particle emission was detected above the flame emission background and emission pulses for individual particles were recorded using an array of three filtered photomultiplier tubes. Particle size distributions were correlated with their emission times, interpreted as burn times, for different turbulence levels. Ratios of the recorded filtered emission signals were used to obtain temperatures of the burning particles. Partially burned particles were collected and examined. Particle burn times were approximately proportional to d0.8, where d is the particle diameter. The particle spherical shapes were not preserved. For the coarser powder, halos of condensed ultrafine MgO crystals were observed around quenched particles. Presence of slightly greater amounts of MgO on the surface of as received particles of the finer powder resulted in their longer burn times and elevated temperatures. These observations are interpreted assuming that the initial small MgO particles adhered to the metal surface result in the formation of solid MgO islands and inclusions on surface of the burning Mg droplets. Such islands block evaporation of magnesium and thus reduce the burn rates. In addition, they serve as condensation centers for the combustion products and grow rapidly during combustion. As a result, a relatively small number of the initial fine MgO particles can cause substantial disruption in the burning particle shape, surface morphology, and burn rate. Larger MgO inclusions can be heated above the Mg boiling point resulting in an increased measured temperature.