The Effect of Temperature on the Sooting Behavior of Laminar Diffusion Flames

The effect of temperature on soot formation in a laminar propane diffusion flame was investigated. The temperature of the flame on a Parker Wolf hard burner was varied by adjusting the inert nitrogen content in the oxidizer flow. Local soot aggregate properties, such as aggregate diameters and their number densities as well as soot volume fractions were determined using simultaneous laser light scattering and extinction methods, while spherule diameters were obtained using sample extraction and transmission electron microscopy. Soot volume fractions and aggregate diameters were seen to increase with height above the burner, while the opposite trend was noted for aggregate number densities for the three flame temperatures investigated. Spherule diameters were seen to increase with height particularly in the early part of the flame. An increase in temperature resulted in a considerable increase in local soot volume fractions and aggregate diameters. Spherule size and degree of agglomeration also varied somewhat with temperature. Soot aggregate number densities, at least in the regions of noticeable soot loading, were found to be largely insensitive to changes in temperature. Heights above the burner were converted to residence times in the flame allowing for buoyancy. The mean rate of growth of the largest soot aggregates was found to be essentially independent of flame temperature, although the absolute soot particle diameters were larger in the hotter flames, resulting in an increase in sooting with temperature. An "effective growth residence time" was introduced to take account of the effects of temperature such that both total soot loadings and maximum soot diameter were the same for any given normalized time for all flame temperatures investigated.