Dual-pulse Laser-Induced Incandescence to Quantify Carbon Nanostructure and Related Soot Particle Properties in Transient Flows – Concept and Exploratory Study

In this work a dual-pulse laser-induced incandescence method is presented, which is potentially suitable for the in-situ determination of E(m,λUV)/E(m,λNIR) in transient flows. The dual-pulse LII is irradiating a particle ensemble by two subsequent laser pulses of different wavelengths with a time delay long enough to ensure cooling of the particles after the first pulse, but short enough to prevent the particles from moving out of the probe volume. In the experiment, the first pulse is kept at low and constant fluence, whereas the fluence of the second pulse is periodically modulated. If coincidence of the generated two LII signals is repeatedly achieved, E(m,λi)/E(m,λj) can be evaluated. The synchronous temperature determination of the laser-pulse heated particles even enables the quantification of the absolute values of E(m,λi) and E(m,λj). In an exploratory study using model soot particles generated in a spark discharge generator, the applicability of the method is investigated in detail. The nanostructural and optical properties of the model particles, the influence of the first pulse and other boundary conditions are studied comprehensively. Both, E(m,λ) and E(m,λi)/E(m,λj) are in excellent agreement with ex-situ analyses of the nanostructure and data reported in the literature. In the low-fluence regime of the first pulse, nanostructural modifications of the particles can be ruled out. It is finally shown that the dual-pulse LII not only offers a rapid, non-intrusive diagnostic method of averaged E(m,λi)/E(m,λj) and absolute E(m,λ) values as well as primary particle sizes, but also enables the determination of their statistical distributions, which in turn provides inferences about the nano structure and properties such as graphene-like layer lengths distribution and characteristics inferred thereof.