Dynamics of an inverse diffusion flame and its role in polycyclic-aromatic-hydrocarbon and soot formation

Residence time and thermo-chemical environment are important factors in the soot-formation process in flames. Recent studies have revealed that the soot generated in an inverse diffusion flame (IDF) is not fully carbonized as it is in a normal diffusion flame. For understanding the chemical and physical structure of the partially carbonized soot formed in inverse diffusion flames, knowledge of the flow dynamics of these flames is required. A time-dependent, detailed-chemistry computational-fluid-dynamics (CID) model is developed for simulation of an ethylene-air inverse jet-diffusion flame that has been studied experimentally. Steady-state simulations show that all of the polycyclic-aromatic-hydrocarbon (PAH) species are produced outside the flame surface on the fuel side. Unsteady simulations reveal that buoyancy-induced vortices establish outside the flame because of the low fuel jet velocity (similar to 40 cm/s) employed. These vortices in inverse diffusion flames, as opposed to those in normal diffusion flames, appear primarily in the exhaust jet. The advection of these vortices at 17.2 Hz increases mixing and causes PAH species to be more uniformly distributed in downstream locations. While the concentrations of rapidly formed radical and product species are not altered appreciably by the flame oscillation, concentrations of certain slowly formed PAH species are significantly changed. The dynamics of 20-nm tracer particles injected from the 1200 K fuel-side contour line suggest that soot particles are reheated and cooled alternately while being entrained into and advected by the buoyancy-induced vortices. This flow pattern could explain the experimentally observed large size and slight carbonization of IDF soot particles. Published by Elsevier Inc. on behalf of The Combustion Institute.