Optical determination of hydrocarbon particle sizes produced by pyrolysis of acetylene

Optical determination of particle sizes produced by flames is complicated by uncertainties in the properties of the produced solid particulates which is a result of the complex reaction pathways which lead to their formation and growth. This work reports on the design and construction of an apparatus for the generation of hydro-carbon particles by thermal decomposition of acetylene in the drift-tube environment and the results of optical measurements leading to the determination of particle sizes and particle number densities. The transmission measurements were performed in the ultraviolet portion of the electromagnetic spectrum and the applicability of the optical constants for soot which were obtained from the theoretical extrapolations has been verified for the wavelengths in the range from 260 to 400 nm. The determination of particle sizes and number densities was obtained by applying a novel method for the deconvolution of the results of multiwavelength transmission measurements. For pressures of approximately 290 Torr, the temperature of 1350 C, and growth-time in the vicinity of 50 milliseconds, particle sizes were determined to be in the vicinity of 60 nanometers from the transmission-measurement results, while the diffusion-broadening measurements yielded values in the range from 35 to 40 nanometers in diameter. The agreement between the results of the two different optical particle-sizing techniques is acceptable considering the assumptions which entered the determination of particle sizes in each of the two measurement techniques. The growth of particles was modeled by mapping of the essential features of the evolution of soot particles onto the two fractal-growth models. The applicability of the scaling theory to the solution of the coagulation (Smoluchowski) equation under the conditions which existed in the apparatus was tested by evaluating the reaction-rate constants in the coagulation equation.