Particle size distribution function of incipient soot in laminar premixed ethylene flames: effect of flame temperature

Particle size distribution functions (PSDFs) of incipient soot formed in laminar premixed 24.2%, ethylene-37.9% oxygen-diluent (nitrogen and/or argon) flames with an equivalence ratio of 1.92 were studied by online sampling and scanning mobility particle sizer. Two series of flames were studied to quantify the effect of flame temperature on the characteristics of PSDFs. In the first series, the variation of the flame temperature was accomplished by varying the cold gas velocity. Temperature in the second series of flames was manipulated by the diluent composition from argon to nitrogen. The results show that for flames with the maximum temperature (T-max) around 1800 K the soot PSDFs were distinctively bimodal. As the flame temperature was increased to similar to 1850K, bimodality faded away. The distribution was unimodal for T-max > 1900 K. The variation of the characteristics of the PSDF as a function of the flame temperature is consistent with the theoretical explanation that bimodality is the result of competition between persistent particle nucleation and particle-particle coagulation in low-temperature flames. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.