Sizing of Soot Particles in Diesel Spray Flame - A Qualitative Comparison between TEM Analysis and LII/Scattering Laser Measurements

摘要

For better understanding of soot formation and oxidation processes in a diesel spray flame, two kinds of planar soot imaging techniques, Laser-Induced Incandescence (LII) and Laser Scattering (LS) techniques, were applied simultaneously to a diesel spray flame in a constant-volume combustion vessel under a diesel-like condition (2.5 MPa, 940 K). An analysis of LII and LS images yielded 2-dimensional distribution images of concentration, size and number density of soot particles in the spray flame, based on an assumption that LII and LS signals are proportional to the soot particle size to the power of 3 and 6, respectively. In order to obtain clearer variation trend in the soot concentration, size and number density distribution in significantly fluctuating single-shot diesel spray flames, spontaneous and time-integrated ensemble averaging of the laser-measured images were employed. The resulting spontaneous and time-integrated images exhibited different trends in which the spontaneous images showed larger particles more concentrated in the spray head than in the steady-state-like time-integrated one. The time-integrated laser measurement results of soot particle size distribution in the spray flame were qualitatively compared with Transmission Electron Microscope (TEM) analysis results of soot particles directly sampled in a diesel spray flame under the identical experimental condition. The laser-and TEM-based results showed a similar trend that young small soot particles formed in the upstream region increase their size up to the midstream due to surface growth and agglomeration, and then decrease their size in the downstream due to oxidation.