Structure of Overfire Soot in Buoyant Turbulent Diffusion Flames at Long Residence Times

The structure of soot was investigated within the fuel-lean (overfire) region of overventilated buoyant turbulent diffusion flames burning in still air. The study was limited to the long residence time regime where characteristic flame residence times are roughly more than an order of magnitude longer than the laminar smoke point residence time and soot generation factors (the mass of soot emitted per unit mass of fuel carbon burned) are relatively independent of flame residence times, Both gaseous and liquid fuels were used to provide a range of H/C ratios (1-2.7) and fuel types (alkynes, alkenes, alkanes, aromatics, and alcohols) as follows: toluene, acetylene, benzene, propylene, ethylene, n-heptane, propane, and isopropanol. Measurements included transmission electron microscopy to find primary particle diameters, the number of primary particles per aggregate and aggregate geometrical and fractal dimensions. The results show that the structure of soot varies with fuel type but is relatively independent of both position in the overfire region and flame residence time for the long residence time regime. Mean primary particle diameters were 30-51 nm and the mean number of primary particles per aggregate were 255-552, with the larger values associated with the more heavily sooting fuels. Aggregate fractal dimensions, however, were less dependent on fuel type, only varying in the range 1.70-1.79. The structure measurements are used to estimate the optical properties of overfire soot, based on a recent approximate theory for polydisperse aggregates, finding significant differences between aggregate and Rayleigh scattering properties in the visible and near-infrared portions of the spectrum, even though the primary particles are well within the Rayleigh scattering regime.