MEASUREMENTS OF SOOT VOLUME FRACTIONS IN A 7.1 CM METHANE POOL FIRE USING PLANAR LASER-INDUCED INCANDESCENCE

摘要

Soot volume fractions were measured using planar laser-induced incandescence (PLII) technique in a 7.1 cm methane pool fire at different flame heights up to 24 cm above burner exit. Flat field of detection system was collected using a blackbody to correct variations in pixel response. The signals were further corrected for the uneven planar laser energy distribution resulting from the Gaussian beam profile using a co-flow laminar flame via a single point translation method. This procedure also yielded a calibration of the LII signals. Based on instantaneous soot images of the methane pool fire, there is very little soot within a height of 3 cm of the burner exit. Further away from the burner exit, the soot particles are distributed in twisted and stretched streaks. Instantaneous peak soot volume fraction is an order of magnitude higher than an average over 700 frames because of the high intermittency. Statistical quantities including RMS, probability distribution function (PDF) and spatial correlation coefficient of soot volume fractions were obtained. The results show that: (1) soot volume fractions strongly fluctuate so that the average and RMS is one the same order of magnitude, (2) the PDF of soot volume fractions has a spike at very low values followed by a long tail at high values, (3) the soot streaks in the methane fires are less contiguous than those in more luminous fires, and (4) the soot distribution is non-isotropic with the buoyant flow leading to relatively large vertical length scales.