Laser-Induced Incandescence and Complementary Diagnostics for Flame Soot Characterization

Abstract: This work has been aimed at developing and applying laser-induced incandescence (LII) for flame soot characterization. The basic principle of LII is rapid heating of the soot particles to temperatures of 3500-4000 K by short laser pulses. Thereby the intensity of the soot incandescence is increased. By detection of this increased incandescence and analysis of the detected signal, the volume fraction, particle size and optical properties of the soot can be evaluated. Additionally, both optical and probing techniques have been utilized in combination with LII for studies of specific soot properties. LII has been applied for characterization of different laboratory flames of interest for soot studies. The soot distribution in flat premixed ethylene/air flames on McKenna burners was found to deviate somewhat from the predicted one-dimensional behavior, where no variation is supposed to be seen radially. A variation was also seen depending on the choice of co-flow gas. Additionally, partially premixed flames burning vaporized n-heptane and n-decane and diluted flat unstrained CH4/O2 diffusion flames were characterized in terms of soot volume fraction distributions. As the ageing process of soot particles can be followed as height above burner (HAB) in flat premixed flames, these have been utilized as measurement targets for studies of soot formation. Significant differences have been found between newly formed nascent soot and more mature soot. The LII signal response of newly nucleated nascent soot particles in low-sooting flames was found to deviate from what is commonly seen. Instead of displaying an S-shaped fluence curve (signal vs. laser energy) the fluence curve of the nascent soot showed an almost linear trend. Even though these results are challenging to interpret, they show potential for LII as a diagnostic technique for investigations of these newly nucleated soot particles. Evaluation of the absorption function, E(m), showed a significant increase with soot maturity, approaching a nearly constant value of ~0.35 for mature soot. A similar trend was found when combining LII and elastic light scattering for measuring the sublimation threshold i.e. the onset of sublimation. The evaluated sublimation threshold temperature was found to increase with maturity and reach an essentially constant temperature at ~3400 K for mature soot. When studying processes affecting the decay time of time-resolved LII signals, an increasing level of aggregation of the soot particles was found to increase the decay time. A plausible explanation is an aggregate shielding effect, effectively decreasing the heat conduction rate of the soot. Additionally, by combination of LII and rotational coherent anti-Stokes Raman spectroscopy, a local gas heating effect could be measured. The gas temperature was found to increase ~100 K in a flame with 4 ppm of soot when heating the soot by ~2000 K, effectively increasing the decay time of the LII signal. If not accounting for effects increasing the decay time of LII signals in e.g. soot particle size evaluations, this will lead to an over prediction of the sizes.