Soot precursor material: Visualization via simultaneous lIF-LII and characterization via tem

Abstract

Simultaneous combined laser-induced fluorescence and laser-induced incandescence (LIF-LII) images are presented for both a normal and inverse diffusion flame. The excitation wavelength dependence distinguishes the LIF and LII signals in images from the normal diffusion flame while the temporal, decay distinguishes the signals in images of the inverse diffusion flame. Each flame presents a minimum in the combined LIF-LII intensity in a region separating the fuel pyrolysis and soot containing regions. Opacity, geometric definition, and extent of crystallinity measured through both bright and dark field transmission electron microscopy (TEM) characterizes the thermophoretically sampled material from within this minimal LIF-LII intensity region. TEM analysis reveals rather different soot processes occurring within the normal and inverse diffusion flame. In the normal diffusion flame, rapid chemical and physical coalescence of PAHs results in initial formation of soot precursor particles that are highly crystalline and evolve toward fully formed soot. In the inverse diffusion flame, rapid coalescence of pyrolysis products occurs, producing tarlike, globular structures equivalent in size to fully formed soot aggregates but with markedly less crystallinity than normal-appearing soot. These different material properties are interpreted as reflecting different relative rates of chemical and physical coalescence of fuel pyrolysis products versus carbonization. Significantly, these TEM images support qualitative photophysical arguments suggesting that, in general, this “dark” region observed in the LIF-LII images demarcates a transitional region in which a fundamental change in the material chemical/physical properties occurs between solid carbonaceous soot and condensed or gaseous molecular growth material.