Spectral signatures (0.2–5 μm) of soot initiation in the pyrolysis of toluene near 2000 K

Abstract

The spectral and temporal distributions for optical absorption and radiation occurring in the formation of soot have been investigated for pyrolysis of 1.5% toluene/98.5% argon near 2000 K and 0.8 atm in a shock tube. The experiments employed four concurrent spectrally resolved optical diagnostics: ultraviolet absorption (200–400 nm); absolute emission in the visible (400–700 nm); HeNe laser extinction (632.8 nm); and absolute infrared emission (2–5 μm). The measurements show clearly the progression from parent fuel to increasingly complex species and finally to soot particles. In the earlier phase of the reaction, the measurements reflect molecular absorption and emission signatures of species formed prior to the appearance of soot particles, and exhibit nearly continuous spectral behavior due to extensive overlap between bands. At later times, the spectral signature in all channels is controlled by soot particles. In particular, infared emission at 3–5 μm provides a reliable quantitative monitor of soot particles in pyrolysis. This technique has been employed to determine a working value for the soot extinction cross section at the HeNe laser wavelength (632.8 nm), thus quantifying that diagnostic for a limited set of experimental conditions.