Experimental and computational study on partially premixed flames in a centerbody burner

The centerbody burner was designed with the objective of understanding the coupled processes of soot formation, growth, and burnout. Fuel that issues from the center of the burner establishes two flame zones – one associated with the recirculation zone (RZ) and the other, with the trailing jet. The sooting characteristics in these two flame zones can be quite different because of variations in residence time and transport of reactants and products. Calculations performed for this burner operating under a partially premixed fuel jet suggested that soot in the RZ decreases and that soot in the trailing jet flame increases with the amount of premixing. An experimental and numerical study is performed to aid the understanding of these differences. A time-dependent, axisymmetric, detailed-chemistry computational-fluid-dynamics (CFD) model known as Unsteady Ignition and Combustion using ReactioNs (UNICORN) is used for simulating flames under different equivalence-ratio conditions. Combustion and PAH formation are modeled using the Wang–Frenklach (99 species and 1066 reactions) mechanism, and soot is simulated using a two-equation model of Lindstedt. A Lagrangian-based particle-tracking model is used for understanding the evolution of soot-like particles. Flame and recirculation-zone structures and soot in the experiments are identified using direct photographs taken with and without Mie scattering from soot particles as well as laser-induced-incandescence (LII) measurements. Calculations predict the structures of the partially premixed centerbody flames for various equivalence ratios reasonably well. Experiments confirm the predicted soot suppression in the RZs and enhancement of soot in the trailing jet flame when air is added to the fuel jet. It is found that flame movement in the RZ increases soot-particle burnout and, thereby, reduces the amount of soot within the RZ. As the flame moves closer to the fuel jet, more soot becomes entrained into the inner vortex. Motion of soot-like particles explained the spiral rings observed in the experiment. Increased particle burnout with partial premixing leads to shrinkage of soot spirals.