Soot aerosol properties in laminar soot-emitting microgravity nonpremixed flames

The spatial distributions and morphological properties of the soot aerosol are examined experimentally in a series of 0-g laminar gas-jet nonpremixed flames. The methodology deploys round jet diffusion flames of nitrogen-dilu ted acetylene fuel burning in quiescent air at atmospheric pressure. Full-f ield laser-light extinction is utilized to determine transient soot spatial distributions within the flames. Thermophoretic sampling is employed in co njunction with transmission electron microscopy to define soot microstructu re within the seat-emitting 0-g flames. The microgravity tests indicate tha t the 0-g flames attain a quasi-steady state roughly 0.7 s after ignition, and sustain their annular structure even beyond their luminous flame tip. T he measured peak soot volume fractions show a complex dependence on burner exit conditions, and decrease in a nonlinear fashion with decreasing charac teristic flow residence times. Fuel preheat by similar to 140 K appears to accelerate the formation of soot near the flame axis via enhanced fuel pyro lysis rates. The increased soot presence caused by the elevated fuel inject ion temperatures triggers higher flame radiative losses, which may account for the premature suppression of soot growth observed along the annular reg ion of preheated-fuel flames. Electron micrographs of soot aggregates colle cted in 0-g reveal the presence of soot precursor particles near the symmet ry axis at midflame height. The observations also verify that soot primary particle sizes are nearly uniform among aggregates present at the same flam e location, but vary considerably with radius at a fixed distance from the burner. The maximum primary size in 0-g is found to be by 40% larger than i n I-g, under the same burner exit conditions. Estimates of the number conce ntration of primary particles and surface area of soot particulate phase pe r unit volume of the combustion gases are also made for selected in-flame l ocations. (C) 1999 by The Combustion Institute.