Evolution of properties for aging soot in premixed flat flames studied by laser-induced incandescence and elastic light scattering

Abstract

A detailed study has been made of soot growth in two premixed flat ethylene/air flames, at Φ = 2.1 and Φ = 2.3, where focus has been on following the change in optical properties from nascent to more mature soot, and the importance of these properties for laser-induced incandescence (LII). A combination of two-color LII (2C-LII) and elastic light scattering was utilized for studies of soot absorption and sublimation for a range of laser fluences in a pump-probe experiment, and the experimental results were compared with LII model predictions. Both flames show similar trends, indicating that the soot becomes less transparent during the growth process until some level of maturity is reached at higher flame heights, where the measured properties reach almost constant values. A sublimation fluence threshold of ~0.14 J/cm2 (at 1064 nm for a flame temperature around 1700 K) was evaluated for mature soot, corresponding to a sublimation temperature of ~3400 K. Soot peak temperatures from 2C-LII were evaluated both using a constant E(m) and a wavelength dependence for E(m) extracted from extinction measurements, leading to a discussion on how the sublimation temperature relates to the maturity of soot.