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Optical Measurements of Soot Size and Number Density in a Complex Flow, Swirl-Stabilized Combustor,

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In-flame optical measurements of soot particulates in a turbulent, recirculating i.e., complex flow model laboratory combustor are described. A nonintrusive optical probe based on large angle 60, 20 degs intensity ratio scattering was used to yield a point measurement of particulate in the size range of 0.08 to 0.38 microns. Performance of the optical technique was evaluated, and an exploratory assessment of the spatial distribution of soot was conducted with attention to fuel molecular structure, fuel loading, and a smoke-suppressant additive ferrocene. Isooctane and mixtures of isooctane with various ring and aromatic compounds blended to yield the smoke point of a JP-8 stock were prevaporized and introduced through a hollow cone nozzle. Addition of ring compounds to the base isooctane substantially changed the distribution of soot and increased the overall emission by 300. Production of soot was substantially reduced by a decrease in fuel loading, and marginally reduced or not affected by the additive depending on fuel structure. The optical technique is a potentially powerful tool for providing the experimental evidence necessary to understand the processes of soot formation and burnout in complex flows typical of gas turbine combustors. However, scanning electron micrographs of extracted samples established that the technique is limited to the large particle wing of the soot size distribution, and optical and electronic processing can induce biasing and uncertainties which must be understood and controlled before the potential of the technique can be fulfilled.

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