Soot microstructure in steady and flickering laminar methane/air diffusion flames

An experimental investigation is presented to identify the mechanisms responsible for the enhanced sooting behavior of strongly flickering methane/air jet diffusion flames when compared to their steady counterparts. The work extends the implementation of thermophoretic sampling in flickering, co-flow, laminar, diffusion flames. Acoustic forcing of the fuel flow rate is used to phase lock the periodic flame flicker close to the natural flicker frequency (similar to 10 Hz for a burner diameter of similar to 1 cm). Soot primary sizes, determined as functions of flame coordinates, indicate that the largest soot primary units in strongly flickering methane/air flames are larger by similar to 60% than those measured in steady flames with the same mean reactant how rates. The primary particle size measurements, when combined with the soot volume fractions reported by other investigators, indicate that soot surface areas in the flickering flame are three to four times larger than those under steady conditions. These results, along with the fact that residence limes in the flickering flame are twice as long as those in the steady flame, suggest that specific soot surface growth rates under unsteady combustion conditions can be similar or even lower than those in the corresponding steady flames. Finally, the number densities of soot primaries in flickering flames are found to be higher by 30-50% than those in steady flames, thus suggesting stronger and/or extended soot inception mechanisms under flickering conditions. ?he combination of longer flow residence times and greater population of incipient soot particles in flickering flames appears to be primarily responsible for the higher sooting propensity of methane under laminar unsteady combustion conditions. (C) 1998 by The Combustion Institute.