Soot inception in a methane/air diffusion flame as characterized by detailed species profiles

• Published in: Combustion and flame 1985-01, Vol.62 (2), p.157-181
• Main Authors: Smyth, Kermit C., Miller, J.Houston, Dorfman, Robert C., Mallard, W.Gary, Santoro, Robert J.
• Format: Article
• Language: English
• Subjects: burners; chemical variables measurement; combustion; flames
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/0010-2180(85)90143-9

Detailed species concentration profiles have been measured using optical and mass spectrometric methods in an atmospheric pressure methane air diffusion flame burning on a Wolfhard-Parker slot burner. Relative concentrations have been determined for OH by laser-induced fluorescence and, in addition, laser-induced production of C 2 has been monitored by fluorescence measurements. Broadband ultraviolet and visible fluorescence have been observed, and both are attributed to PAH, although other molecules may be responsible for these emissions at elevated temperatures. Small soot particles were detected by laser-induced ionization. Using a direct sampling mass spectrometer, absolute concentrations have been measured for methane, oxygen, nitrogen, carbon dioxide, water, hydrogen, acetylene, butadiene, and toluene. Profile measurements of several additional intermediate hydrocarbons have also been made, including methylacetylene (and/or allene), vinylacetylene, diacetylene, triacetylene, benzene, and naphthalene. These profiles are combined with velocity, temperature, and Rayleigh scattering measurements to characterize the region of chemical growth in a luminous diffusion flame. The soot inception zone occurs at the high temperature edge of a region in which intermediate hydrocarbons are abundant. The positions of the peak concentrations for these species follow isothermal contours as a function of height above the burner. This result illustrates the dominant role of chemical steps in the growth processes which lead to the formation of the earliest soot particles. An analysis of the species concentration profiles in terms of the local equivalence ratio successfully correlates the data for the major species and the temperature, but is not adequate for any of the profiles of the intermediate hydrocarbons or for the small soot particles.