LES of a sooting ame in a pressurized swirl combustor

Large eddy simulations (LES) of a model aircraft combustor at different pressure and operating conditions are conducted. Detailed models for soot formation and evolution is used along with minimally-dissipative numerical schemes in a fully unstructured mesh simulation of this complex geometry flow. Two slightly different swirl combustors, one operated at atmospheric pressure and the other at 3 and 5 bar pressures are used that are both confined by walls and the flame is stabilized by strong swirl generated by inlet swirlers. For the atmospheric pressure case, the effect of secondary oxidation air injection, similar to that found in rich-quench-lean design of aircraft combustors, is studied. The focus is on the intermittent nature of soot formation. It is found that such intermittency comes from the trajectories traveled by the soot particles. Only a small portion of the combustor exhibit conditions suitable for soot particle growth. Due to the chaotic nature of the turbulent flow, only a small fraction of the fluid elements pass through this region, which leads to spatial and temporal intermittency. Analysis of soot trajectories show that much of the growth happens very early in the soot trajectory, with oxidation dominating at later times. It is also noted that only trajectories that start in the shear layers formed by the fuel-air jets generate appreciable soot, while fluid elements ejected from the outer recirculation zone have low probability of forming soot. Simulations at various pressures show that with increasing pressure, jet breakdown and mixing is more efficient, which somewhat curtails the generation of fuel-rich pockets needed for particle growth. On the other hand, increased kinetic rates lead to soot formation even when the probability of finding soot-favorable conditions are lower.