Laser-induced incandescence measurements of soot production in steady and flickering methane, propane, and ethylene diffusion flames

Abstract

Quantitative experimental measurements of soot concentrations and soot scattering are presented for a series of steady and flickering coflowing methane, propane, and ethylene flames burning at atmospheric pressure. Flickering diffusion flames exhibit a wide range of time-dependent, vortex-flame sheet interactions, and thus they serve as an important testing ground for assessing the applicability of chemical models derived from steady flames. Acoustic forcing of the fuel flow rate is used to phase lock the periodic flame flicker close to the natural flame flicker frequency caused by buoyancy-induced instabilities. For conditions in which flame clip-off occurs, the peak soot concentrations in the methane flickering flames are 5.5 to 6 times larger than measured in a steady flame burning with the same mean fuel flow rate, whereas the enhancement for the flickering propane and ethylene flames is only 35 to 60%, independent of the flicker intensity. Soot concentration profiles and full Mie analysis of the soot volume fraction/scattering results reveal significant differences in the structure of the soot fields and in the roles of soot inception, growth, and oxidation for the different hydrocarbon fuels. The soot concentrations have been measured using laser-induced incandescence (LII). Since this is the only technique currently available for making time- and spatially-resolved soot concentration measurements in time-varying flow fields, considerable effort has been devoted to developing LII for quantitative applications. Important considerations include (1) proper calibration measurements, (2) signal detection which minimizes interferences from C 2 Swan-band emission and broadband molecular fluorescence, (3) correction for the laser beam focus/spatial averaging effect in line image measurements, and (4) correction for LII signal extinction within the flame.