Flame shape transitions and precursor events near lean blowout

Extensive research is on-going to understand the flame holding criteria for the combustor near its lean blow out limit. Sudden blow out of combustor is not preferable, and thus blowout process in a laminar flame needs to be understood thoroughly. In this work, blow out process of the stagnation point stabilized flame is discussed in detail and effect of various velocity profiles on blowout process is analyzed. To understand the dynamics of the flame, high speed visualization was performed using direct chemiluminescence and third mirror schlieren techniques. The experiments were conducted for two nozzles having different length (L) to diameter (D) ratios; two velocity profiles associated with these two nozzles are top-hat velocity profile and fully developed velocity profile. The flame shape appears to be a flat circular disk and the flame edges were found to be flapping in case of top-hat velocity profile burner while the flapping motion was absent in the case of the burner having fully developed velocity profile. In both the cases, a 'hole' inside the flame was found prior to its blow out and it grows in size with time and eventually causes the flame to blow out. These precursor events (local extinction of the flame followed by its recovery) prior to blowout were not observed in the nozzle having fully developed velocity profile while this phenomenon was found to be existing in the nozzle having top-hat velocity profile. © 2012 by the American Institute of Aeronautics and Astronautics, Inc.