Laser Spectroscopy of Soot Formation Process in a Disel Spray Flame via Excitation-Emission Matrix Technique

Abstract

In order to investigate the early soot formation processes in diesel combustion, spectral analysis of polycyclic aromatic hydrocarbons (PAHs) formed in the early soot formation region in a diesel spray flame was conducted via a newly developed Excitation-Emission Matrix (EEM) technique using a multi-wavelength laser source. The experiments were conducted using an optically accessible constant volume combustion vessel under diesel-like conditions (Ta =750 to 1 130 K and Pa=2.0 to 3.0 MPa). The PAHS formed in a diesel spray flame were excited by a coherent multiwavelength “rainbow” laser light (mainly 266, 299, 342 and 398 nm, total 20 mJ) generated by converting the 4th harmonic (266 nm, 60 mJ) of a pulsed Nd : YAG laser using a Raman cell frequency converter filled with hydrogen (500 kPa). The laser-induced fluorescence (LIF) from the PAHs excited by the different laser wavelengths in the flame was simultaneously captured using a spectrometer and an ICCD camera as EEM images. The EEM measured in diesel spray flames at different ambient temperatures and oxygen concentrations revealed that at ambient temperature of 940 K and oxygen concentration of 21%, a wide varieties of PAHs are detected around the ignition timing in the central fuel rich region of diesel spray flame and the PAHs are converted to larger PAHs and soot particles as the combustion process progresses. In contrast, at lower ambient temperature of 750 K or lower oxygen concentrations down to 10%, the fluorescence from PAHs are detected in the central fuel rich region before ignition and the intensity and the spectral characteristics of the PAH fluorescence remain unchanged until they are finally converted to soot particles in the downstream regions.