Recent interest in developing Al-Mg alloys as reactive materials prompted studies of combustion mechanisms for particles of such alloys with different Al/Mg ratios. Reference experiments with pure Al and Mg powders are also desired to better understand and model combustion of the respective alloys. While combustion of pure Al powders has been addressed in many recent studies, combustion of magnesium explored mostly coarse, 50 µm, and larger particles. This effort is focused to characterize and understand combustion dynamics for fine Mg powders. Spherical, micron-sized magnesium particles were introduced in an air-acetylene flame using a custom-made screw feeder. Particles were observed to burn in laminar flames as well as in the flames with turbulence induced by a swirling air flow. Particle emission was well detectable above the flame emission background and emission pulses for individual particles were recorded using an array of three filtered photomultiplier tubes. Particle size distribution was correlated with particle emission times (and thus, burn times) for different turbulence levels. The effect of turbulence on the measured burn times was stronger for the finer particles. Partially burned particles were collected and examined using an electron microscope. The particle spherical shapes were not preserved, with greater discrepancies from spherical shapes observed for finer particles. It was also observed that the presence of greater amounts of MgO on surface of metal particles results in a noticeable increase in the particle burn time. This effect is difficult to interpret considering that the initial MgO amount is negligible compared to the MgO formed on the particle surface during its combustion. It is proposed that the initial MgO present in the shape of small particles adhered to the metal surface results in the formation of 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 thus tend to 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. The measured color temperatures inferred from the burning particle emission exceed significantly the boiling point of Mg, but are much lower than the adiabatic flame temperatures. It is also observed that the temperatures decrease for the particles burning in turbulent flows. It is suggested that the measured temperatures represent those of the MgO inclusions embedded in the boiling Mg; their reduction in the higher turbulence flow is associated with an accelerated rate of heat transfer.
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