Tracking the evolution of soot particles and precursors in turbulent flames using laser-induced emission

A point-wise, time-resolved optical technique is employed to track simultaneously the evolution of soot particles and soot precursors in a turbulent flame of ethylene/nitrogen (1/1 by vol.). A picosecond laser, 80 ps@266 nm is used to measure scattering and laser-induced fluorescence (LIF) in selected UV and visible bands while Laser-induced incandescence (LII) is excited using a pulsed laser with 8 ns@1064 nm. Fast photomultiplier tubes are used to ensure time-resolved measurements and LIF decay times of the order of 3.5 ns are detected. The LIF signals are attributed to nanoparticles or condensed phase species made up of PAHs, whereas LII was correlated with soot. Data conditioning based on the noise level was made to delineate the potentially large influence of intermittency on the statistics of the detected signals. It was found that LIF signals with long decay times were detected well before the onset of soot particles, as seen by LII, and these continue well into the maximum soot formation region. The correlation between the LII and LIF signals confirmed the co-presence of small nanoparticles together with high concentrations of mature soot. Analysis of conditioned mean soot concentration suggests that the intermittency occurring at the edge and tip of the flame does not locally decrease the soot concentration since large LII signals were still detected. These simultaneous, time-resolved measurements are extremely useful to provide insights on the evolution of combustion-generated particles in turbulent sooting flames.