Large angle intensity ratioing for the measurement of particle size in a combustion environment

An optical technique, based upon large angle intensity ratio scattering, is described for the nonintrusive point measurement of soot particle size and size distribution in a swirl-stabilized model laboratory combustor. This technique makes use of Mie scattering theory in the interpretation of the measured intensity of light scattered at angles of 60° and 20° off of the forward axis by a soot particle traversing a focused laser beam. Assuming a homogeneous, spherical scattering center, the ratio of the two peak intensities is directly correlated with the diameter of the particle within a size range encompassing approximately 0.1 to 0.4 μm. Results of Mie scattering theory are discussed for particle diameters up to 1.0 μm, and an instrument package and experimental layout are described in detail based upon the results of these calculations. Areas demanding special attention are addressed, such as (1) the effect of particle nonsphericity, (2) errors incurred due to an uncertainty in the soot index of refraction, (3) the correction of a particle size dependent optical measurement volume, and (4) technique calibration using spherical latex particles of known diameter and index of refraction. Finally, practical limitations to the application of the intensity ratioing technique to a combusting environment are discussed, including interpretational challenges in understanding measurements arising from multiple particle scattering.