Measurements and modeling of laser-induced incandescence of soot at different heights in a flat premixed flame

Laser-induced incandescence (LII) measurements were conducted in a laminar flat rich premixed CH4/O-2/N-2 flame at atmospheric pressure using a Nd:YAG laser operated at 1,064 nm. Time-resolved LII signals were measured at 610 nm (20 nm FWHM) at different heights above the burner exit surface and at different laser fluences. The averaged soot temperature over 10 ns around the laser peak was determined from spectrally resolved LII signals over the spectral range of 500-630 nm. Flame temperature and soot primary particle diameter and aggregate size at several flame heights were obtained using the NO-LIF thermometry and from image analyses of sampled soot. The time-resolved LII signal and the averaged soot temperature were modeled using two LII models. To achieve reasonable agreement between the modeled and experimental LII signal and soot temperature at low laser fluences, it is necessary to use flame height-dependent soot absorption functions. Such practice can be attributed to the continuous variation of soot thermal and/or radiative properties during the surface growth process after inception. At high laser fluences, both LII models are unable to reproduce the overall shape of the normalized time-resolved LII signal profiles and the averaged soot temperatures.