Study of the soot nucleation zone of rich methane-oxygen flames

Abstract

Some aspects of the soot nucleation processes which take place in the reaction zone of an atmospheric pressure premixed flat methane/oxygen flame have been studied using a laser light scattering method and a gas chromatographic technique. A range of CH 4 /O 2 ratios of 0.9 to 1.27 was covered and the temperatures of the flames were varied by altering the inlet flow rate; in some experiments small amounts of ethylene or acetylene were added to the feeding mixtures. The maximum concentration of C 2 H 2 , C 4 H 2 and C 4 H 4 is increased when a higher R =CH 4 /O 2 ratio is used and, for the same R ratio, when the flame has a higher combustion temperature. The addition of C 2 H 2 and C 2 H 4 to the feeding mixtures increases the concentrations of all the unsaturated hydrocarbons. The angular patterns of the vertically and horizontally polarized components of the scattered light at λ 0 =514.5 nm were determined at different heights in the flame. It was found that in the reaction zone the vertical Q ˙ VV polarized components were due both to Rayleigh scattering by gas phase combustion products and to a contribution due to higher molecular weight substances. Higher combustion temperatures decrease the contribution of soot particles to the measured Q ˙ VV intensity, which is higher for richer mixtures. The depolarized Q ˙ HV component was much higher in the reaction zone than that predicted by the depolarization of the gas phase compounds. The Q ˙ VV / Q ˙ HV ratios pass through a maximum which is more pronounced and displaced toward the burner for higher CH 4 /O 2 mixtures. The extinction coefficients at λ 0 =514.5 nm in the reaction zone have also been measured and compared with the scatter results. The analysis of the experimental results shows that the formation and evolution of C 2 , C 3 and C 4 unsaturated hydrocarbons does not appear to be a process which takes place before soot nucleation. A part of the scattered signals in the first reaction zone is due to highly depolarized “anelastic” effects, whereas the depolarization in the burned gas region is due mainly to the anistropy of the soot particles. The minimum particle size observed, determined by the scatter/extinction method, is around 20 and the initial soot number concentration has a maximum of 10 13 cm −3 for the richest flame studied.