Two-dimensional soot aggregate sizing by multi-angle light scattering

摘要

This paper demonstrates two-dimensional soot aggregate size measurement in sooting coflow laminar diffusion flames using multi-angle light scattering. The experiment employed a pulsed laser at 532 nm that was focused into a sheet 7 mm high. The scattered light was imaged onto a cooled EMCCD camera with optics set to maximize spatial resolution while maintaining sufficient depth of field. The detection angle was varied from 10 to 90 degrees with respect to the laser beam. The distance between the camera and flame was kept constant to ensure the same collection efficiency. The Guinier analysis involves taking signal ratios of a near-zero angle to other angles. In order to obtain information in two dimensions, the images at different angles were geometrically warped to ensure spatial coincidence. The spatially transformed images at all angles were then correctly re-sampled. A set of eight ratio images was obtained and Guinier analysis was applied for each set of corresponding pixels. The normalized ratios at the eight angles were found to have good linear relations with respect to the square of the wave vectors, q, at these angles. The radius of gyration was then determined on a pixel-by-pixel basis from the slope of the ratio vs. q~2 plot. A full two-dimensional map of soot radius of gyration was obtained by sequential measurements at different flame heights. Measurements were performed on C_2H_4/N_2 coflow laminar diffusion flames at different dilution levels and, hence, soot loadings.