External irradiation effect on the growth and evolution of in-flame soot species

Abstract

External irradiation-soot species interaction is a subject that is of great interest due to its existing and future use in research and industrial sectors. An assessment of the impact of broadband radiation on the evolution of soot species within a laminar ethylene-air flame is therefore performed to improve the fundamental understanding of the interaction process. Radiation at an average flux value of 120 kW/m^2 is provided by a solid-state plasma light to the lower region of the flame. Soot samples, thermophoretically collected at flame positions that are close and downstream from the irradiation location, are imaged using normal-resolution and high-resolution transmission electron microscopes (TEMs). The results, derived from processing the TEM images, reveal that the application of an external irradiation is found to increase the soot loading of the flame, and have a pronounced impact on the soot morphology, and influence the in-flame soot growth processes/mechanisms. The effects are also found to persist downstream from the irradiation location. The effects are mainly attributed to coupling of the broadband irradiation with the soot precursors, for the configuration used. Thermophoretic sampling, coupled with TEM imaging, is used to derive the required soot morphological data. The particulates characterization, however, can be complex when the images are of low contrast, noisy and have non-uniform background, or the samples collected have large variability in shape and size and have some degree of overlapping. A processing method that permits time-efficient automated characterization of particulates from the TEM grids, is therefore developed. The parameters required to be set to facilitate the automated process are identified and assessed. The proposed method is first applied to TEM images of samples acquired from the non-irradiated flame. The automated result is then compared with that derived via manual assessment, for validation purpose. The same analysis is also applied to samples extracted from the irradiated flame, which were previously observed to have different geometrical characteristics, to assess the morphological dependence of the proposed method. Using the optimised setting, the largest discrepancies associated with the automated results of primary particle diameter, fractal dimension and prefactor value of the aggregates for the tested cases, are approximately 3, 1 and 10%, respectively, when compared with the manual assessments.