Thermophoretically-sampled soot morphology at axes of nonpremixed turbulent jet flames at atmospheric pressure

The size, morphology, and concentration of soot particles within the fuel-rich regions of two nonpremixed turbulent jet flames fueled by ethylene/air and acetylene/air, respectively, at atmospheric pressure were investigated. Experiments involved thermophoretic sampling followed by transmission electron microscopy to determine mean soot properties of principal interest, i.e., spherule diameter, aggregate size, aggregate morphology, and volume concentration at various heights along the flame axes. Similar to numerous past studies in laminar flames, soot in these turbulent flames consisted of nearly uniform spherical nano-particles that collected into aggregates of different shapes and sizes. Soot spherule (primary particle) diameters were 16-35 nm, in agreement with the narrow size range reported in many combustion environments. With fractal dimensions and prefactors in the range 1.74 ± 0.1 and 2.0 ± 0.3 respectively, aggregate morphology was almost identical to soot formed in various laminar flames and emitted from turbulent flames. These universal parameters 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 maximum soot volume fractions were estimated to be 0.8 ppm and 5.5 ppm in the ethylene and acetylene flames, respectively. The peak in soot volume fraction was much earlier than the peak in spherule diameter at the axis of the ethylene flame whereas the locations of maximum soot volume fraction and spherule diameter in the acetylene flame were almost identical. The results clearly identified various axial zones of prevailing soot nucleation, surface growth, oxidation, and agglomeration processes in the turbulent flames considered here --Abstract, page iii