For better understanding of soot formation and oxidation processes in diesel combustion, effects of ambient oxygen concentration on in-flame diesel soot particle properties including concentration, size, number density and morphology were investigated in a constant volume combustion vessel via simultaneous LII (Laser-Induced Incandescence) / LS (Laser Scattering) imaging techniques and TEM (Transmission Electron Microscopy) analysis. An analysis of LII and LS images yielded 2-dimensional distribution images of concentration, size and number density of soot particles in diesel spray flame, based on a practical assumption that LII and LS signals are proportional to the soot particle size to the power of 3 and 6, respectively. The laser measurements and TEM analysis results of soot particles directly sampled in the diesel spray flame showed a consistent general trend that in the case of 21% ambient O2 concentration soot is formed earlier in the upstream region and disappears earlier due to faster oxidation, while in the case of 15% ambient O2 concentration, soot is formed later in more downstream region and disappears more slowly. In the case of 21% ambient O2 concentration, locational differences in in-flame soot processes were clearly observed in which many small young soot particles formed in the upstream grow into larger particles and the number density decreases in the downstream due to agglomeration. The lowered 15% ambient oxygen concentration slowed oxidation and disappearance of soot particles in the flame downstream and thus made continued agglomeration in the flame apparent, especially in the TEM analysis results. A comparison between the laser-measured and TEM-based sizes of soot particles in the 15% ambient O2 case showed that laser-measured soot size and TEM-based gyration radius of soot aggregates exhibit over threefold similar increase in the flame towards downstream. On the other hand, the TEM-based primary particle diameter increased in the flame towards downstream only by up to 30%. These results indicate that the optically measured soot size based on the simultaneous LII/LS technique in the present study represents more of the aggregate size than the primary particle size.
