Abstract
Experimental and numerical results are reported on the combustion of single aluminum (Al) droplets, 187 μm diameter, that were laser ignited in quiescent environments of SF 6 and SF 6 /O 2 or Ar mixtures. Combustion measurements consisted of luminosity versus time traces, photographic observations, and electron probe microanalysis (EPMA) and electron microscope inspection of condensed-phase products of combustion surrounding burn-intercepted droplets. Planar laser-induced fluorescence (PLIF) was employed to image the gaseous intermediated reaction species ALO during droplet combustion in SF 6 /O 2 mixtures. Aluminum droplets burning in pure SF 6 burned without condensed-phase products in the surrounding envelope flames. Accordingly, no condensed-phase products were found on surfaces of droplets captured by quenching into an inert environment. In SF 6 , classical d 2 law burning behavior was observed. Droplets burned in SF 6 /Ar mixtures exhibited similar behavior, with diminished burning rates. Disruptive burning was not observed for pure SF 6 or any of the SF 6 /Ar mixtures. Condensed-phase deposits on quench plates intercepting burning droplets showed the surrouunding gaseous flame structure to consist of two zones for burning in SF 6 and three zones in SF 6 /O 2 mixtures. For each system, near the surface where the gas phase was aluminum rich, significant amounts of sulfur were found in the condensed products. PLIF showed the species profile of AlO to shift closer to the surface with increasing fluorine content in SF 6 /O 2 mixtures. Experimental results were compared with a local equilibrium combustion model. The model predicted burn rates within 20% and qualitatively predicted radial profile speciation. As the O/F ratio was increased, the final equilibrium products were predicted to shift from AlF 3 to AlF 2 O to Al 2 O 3 . The formation of AlF 2 O reduced condensed-phase products at the reaction temperature for a wide range of O/F ratios.
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