Soot measurements at the axis of an ethylene/air nonpremixed turbulent jet flame

The size, morphology, and volume fraction of soot particles within the fuel-rich regions of a non-premixed turbulent jet flame fueled by ethylene/air at atmospheric pressure were investigated. Experiments involved thermophoretic sampling followed by transmission electron microscopy and laser extinction techniques to determine mean soot properties of principal interest at various heights along the flame axis. Similar to numerous past studies, soot in this turbulent flame consisted of nearly uniform spherical particles that collected into aggregates of different shapes and sizes. Soot spherule (primary particle) diameters were 19 to 35 nm, in agreement with the narrow size range reported in many combustion environments. With fractal dimensions and prefactors in the range 1.74 +/- 0.11 and 2.2 +/- 0.4, respectively, aggregate morphology was also found to be almost identical to soot formed in various laminar flames and emitted from turbulent flames. These universal parameters apparently resulted from the dominant cluster-cluster agglomeration mechanism, which was also responsible for the monotonic increase in the average aggregate size with height above the burner. The mean soot volume fraction reached a maximum of 1.1 ppm at a height lower than the peak axial location of the spherule size, while the particle number density varied in the range of 10(10)-10(11) particles/cm(3) with a similar trend. The peak value of the actual specific surface area was 2.5 cm(2)/cm(3), which was about a factor of three larger than the value estimated by a volume-equivalent spherical model. The results clearly identified various axial zones of prevailing soot nucleation, surface growth, oxidation, and agglomeration processes in the non-premixed turbulent flame considered here. (C) 2003 The Combustion Institute. All rights reserved.