Sooting Characteristics of Partially Premixed Flames in Centerbody Burner

摘要

The centerbody burner was designed with an objective of understanding the coupled processes of soot formation, growth, and burnout through decoupling them using recirculation zones. Calculations performed for the centerbody burner by partially premixing the fuel jet with air suggested that soot in the recirculation zone decreases and that in the trailing jet flame increases with the amount of premixing. The latter effect was surprising and contradicted the understanding gained on partially premixed coaxial jet flames, in which soot decreased with premixing. An experimental and numerical study was undertaken for resolving this anomaly. A time-dependent, axisymmetric, detailed-chemistry CFD model (UNICORN) is used. Combustion and PAH formation are modeled using Wang-Frenklach (99 species and 1066 reactions) mechanism and soot is simulated using a two-equation model of Lindstedt. Flame and recirculation-zone structures and soot in the experiments are identified using direct photographs and Laser-Induced-Incandescence (LII) technique. Calculations have reasonably predicted the structure of the partially premixed, centerbody flame for various equivalence ratios. Experiments have confirmed the counterintuitive soot enhancement in the trailing jet flame with the addition of air to the fuel jet. It is found that flame movement in the recirculation zone increased soot-particle burnout and reduced the amount of soot within the recirculation zone. As the flame moved closer to the fuel-side vortex more acetylene and soot formed in the recirculation zone got transported into the trailing jet flame and, thereby, increased soot in the latter.