Fluorescence anisotropy in a diffusion flame to shed light in the 'dark region'

In this study, we used advanced spectroscopic methods to study an ethylene laminar non-smoking diffusion flame. The objective of the work was to study precursor particles in the flame region separating the PAH- and soot-zone of the flame, named the 'dark zone' of the flame following the definition given by Vander Wal (Combust. Flame 109 (1997) 399-414). Precursor species were investigated by static and dynamic fluorescence, excited in the far and near UV with a picosecond laser source. In order to demonstrate the attribution of fluorescence signals to particles, rotational motion of fluorescing compounds was investigated by in situ time resolved fluorescence anisotropy (TRFA). From the analysis of the decay time of the fluorescence anisotropy, we estimated the diameter of the photo-selected species that can absorb excitation pulses and emit fluorescence. Measurements were performed as a function of the flame radius and height form the fuel nozzle. Particles with molecular-like spectroscopic properties, with average sizes ranging from 10 to 20 nm were detected in the center of the flame. The results of the present study demonstrate the validity and the potential of TRFA as an innovative and advanced diagnostic tool for soot formation studies in combustion. © 2012 The Combustion Institute.