Investigation of early soot formation process in a diesel spray flame via excitation—emission matrix using a multi-wavelength laser source

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 the 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 (ambient temperature, Ta = 750–1130 K and ambient pressure, Pa = 2.0–3.0 MPa) at different ambient oxygen concentrations (10–21 per cent). The PAHs formed in a diesel spray flame in the combustion vessel were excited by a coherent multi-wavelength ‘rainbow’ laser light (mainly 266, 299, 342, and 398 nm, total 20 mJ) generated by converting the fourth harmonic (266 nm, 60 mJ) of a pulsed neodymium-doped yttrium aluminium garnet (Nd:YAG) laser using a Raman cell frequency converter filled with hydrogen (500 kPa). The spectra of laser-induced fluorescence from the PAHs excited by the different laser wavelengths in the flame were simultaneously captured with a spectrometer and an intensified charge-coupled device (ICCD) camera as EEM images. The EEM measured in diesel spray flames at different ambient temperatures and oxygen concentrations revealed that the timing and the region for PAH growth and soot particle formation are delayed and extended downstream at lower ambient temperatures and oxygen concentrations. At an ambient temperature of 940 K and an ambient oxygen concentration of 21 per cent, a variety of PAHs are detected around the ignition timing in the central region of the diesel spray flame and the PAHs grow into larger PAHs and soot particles as the combustion process progresses. At a lower ambient temperature of 750 K or lower ambient oxygen concentrations down to 10 per cent, the fluorescence from PAHs is detected in the central region of the diesel spray flame before the ignition timing and the intensity and the spectral characteristics of the PAH fluorescence do not change for a longer period until they are finally converted to soot particles in the downstream regions in the spray flame.