Depletion of Fuel Aromatic Components and Formation of Aromatic Species in a Spray Flame as Characterized by Fluorescence Spectroscopy

In-flame sampling followed by chemical and spectroscopic analysis coupled with laser-induced fluorescence (LIF) measurements excited in the UV region (266 nm) were performed on a spray combustion system burning a diesel oil and a vegetable aromatic-free fuel like a rapeseed oil. A large fluorescence emission peaked in the UV was found in the spray region of the diesel oil flame attributed to the unburned aromatic species of the fuel. In the rapeseed oil flame, the absence of aromatic species in the fuel justify the lack of the fluorescence emission in the UV region. The comparison of LIF spectra measured in the PAH-formation region with the fluorescence spectra of the high molecular weight species sampled both in the diesel oil flame and in the rapeseed oil flame suggests a relationship between the broad and unstructured fluorescence measured in the visible and these high molecular weight species formed in the pyrolytic regions of spray flames. The visible emission feature could be assigned to flame-formed PAH species contained in the high molecular weight species if their fluorescence spectra are shifted toward the visible for effect of the high temperature flame environment. Alternatively, responsible for visible emission could be the unidentified heavier part of the high molecular weight species that are know to fluoresce mainly in the visible range of the emission spectrum.