Probing gas-to-particle transition in a moderately sooting atmospheric pressure ethylene/air laminar premixed flame. Part II: Molecular clusters and nascent soot particle size distributions

A comprehensive investigation was conducted on a sooting ethylene laminar premixed flame. The present contribution focuses on measurements of soot nuclei by: a) thermophoretic sampling on fine wires followed by microscopy analysis and b) high-dilution probing followed by High-Resolution Differential Mobility Analysis (HR-DMA). The results complement Part I that was focused on the characterization of the gas composition and of soot properties by optical measurements, and highlighted significant artifacts induced by the heat loss associated with the presence of a stagnation plate. Significant novelties in the diagnostics include: the ability to perform spatially resolved thermophoretic measurements on fine wires straddling a critical temperature range that rules out artifacts brought about by either condensation of nonvolatiles or surface kinetics, as demonstrated by the invariance of particle size with the wire residence time in the flame; and the high-resolution capability of the DMA revealing the presence of small (D < 3 nm) nanoparticles both at lower heights in the flame, where younger soot is present, and further downstream at the same location where more mature soot is present. At the location at which both thermophoretically collected particles and HR-DMA data are available, the size distribution functions are in good agreement, except for the smallest particles that are quantifiable only by HR-DMA. Comparison of soot volume fraction and particle sizes from multiple techniques reveals good self-consistency, especially in the region where somewhat more mature soot particles are present (HAB >= 12.5 mm). A simplified three-dimensional model of the HR-DMA sampling procedure using a finite element solver package reveals the perturbation introduced by the cold probe to the temperature-time history experienced by the sampled aerosol and provides the input parameters for a one-dimensional streamtube model of the perturbed flame that included detailed chemistry. Even though additional work is needed to make the HR-DMA determination of the small particle peak fully quantitative, the developed database provides a useful benchmark for experimental validation of soot nucleation models, casting a shadow on established measurements that rely on commercial instrumentation with lower resolution. (C) 2017 Published by Elsevier Inc. on behalf of The Combustion Institute.