The evolution of particulates across the sooting limit in turbulent premixed opposed jet flames

Abstract

Soot formation in combustors is a complex process comprising highly intermittent interactions between physical and chemical processes across a wide range of time-scales. The influence of turbulence on the molecular pathways initiating particulate formation remains unquantified. Controlling soot emissions to the atmosphere will require overcoming large gaps in the understanding of soot formation/oxidation especially in turbulent combustion. The complexities of soot formation in turbulent flames suggests that the use of a flexible compact burner configuration with well–defined boundary conditions and precise control of flow characteristics is of significant advantage. The novel back–to–burnt opposed jet configuration features fractal grid generated turbulence and provides accurate control of flow parameters. The study includes the analyses of the overall flame structure of turbulent premixed ethylene/air flames, the relative concentrations of PAHs associated with soot inception and particle size distributions. The experiments covered a series of sooting flame conditions with variations in the equivalence ratio (1.7 ≤ \phi_{UN} ≤ 2.2), the total rate of strain (255 ≤ a_{T} [s−1] ≤ 610) and burnt gas temperature (1400 ≤ T_{LN} [K] ≤ 1700). The conditions traverse the soot inception limit, e.g. the transition from lightly to heavily sooting flames, with non- intrusive ELS and PAH–PLIF combined probe sampling to quantify gaseous and PAH species using GC–TCD and GC–MS, respectively. The probe sampling features comprehensive sampling steps used to provide accurate concentrations of major gaseous, PAH species and particles with minimum losses. It is shown that the rate of strain exerts a substantial influence on both PAH concentrations and soot formation. Hence, it is likely that soot formation in turbulent flames becomes dominated by contributions from low strain regions. It is also found that the stoichiometry of the mixture controls the concentrations of PAHs associated with soot inception. The results obtained clearly show that benzo(a)pyrene is prevalent in flame structures and that relatively large amounts are condensed onto soot particles. A transition between bimodal and unimodal shapes of the particle size distributions shows strong competitions between oxidation, aggregation and surface growth processes in the turbulent flames.