Laser Spectroscopy of Early Soot Formation Process in a Transient Spray Flame(Imaging and Spectral Measurements of Laser-induced Emission at Two Different Excitation Wavelengthes)

Abstract

In order to investigate early soot formation process in a diesel spray flame, 2 D imaging and spectral measurements of laser-induced emission from soot precursors and soot particles in a transient spray flame achieved in a rapid compression machine (2.8 MPa, 710 K) were conducted. 3rd harmonic (355 nm) and 4th harmonic (266 nm) Nd : YAG laser pulses were used as the light source for laser-induced fluorescence (LIF) from soot precursors and laser induced incandescence (LII) from soot particles in the spray flame. The 2-D imaging covered an area between 30 mm and 55 mm downstream from the nozzle orifice. The results of 2-D imaging showed that strong laser induced emission excited at 266 nm appears only on the laser incident side of the spray flame, in contrast to an entire cross-sectional distribution of the emission excited at 355 nm, indicating that 266 nm-excited emitters are stronger absorbers and more abundant than 355 mil excited emitters in the spray flame. The spectral measurments were conducted at three different positions, 35, 45 and 55 mm downstream from the nozzle orifice, along the central axis of the spray, where LIF from soot precursors was observed in a previous 2-D imaging study. The spectra measured in upstream positions showed broad emission peaked around 400 to 500 nm, which is attributable to LIF from polycyclic aromatic hydrocarbons (PAHs). The spectra measured in downstream positions appeared very much like grey-body emission from soot particles.