On extinction limits and polycyclic aromatic hydrocarbon formation in strained counterflow diffusion flames from 1 to 6 bar

Abstract

The extinction limits of methane/nitrogen/air diffusion flames in counterflow configuration were investigated by measurements and computations from 0.1 to 0.6 MPa. The numerical predictions were found to be in satisfactory agreement with the experiments for these conditions. The accuracy of the extinction limits at 0.1 MPa was better than 10%: at 0.3 MPa, it was better than 20%. Different reaction pathways leading to polycyclic aromatic hydrocarbon (PAH) formation close to the extinction limits and in fuel-rich conditions were examined computationally in order to contribute to the understanding of the process of soot precursor formation at high pressures in more detail. UV laser fluorescence measurements were performed to measure relative PAH concentrations at high pressures and a constant global strain rate of 200 s −1 . Profiles of selected species were computed. In the experiments and computations, the concentrations of PAHs, leading candidates for soot precursor formation, increased with increasing pressure. From the reaction flux analysis with respect to PAHs, it can be concluded that the strong incline of the aromatics with pressure is caused by the influence of combinative reaction steps. The calculated maximum concentration of the PAH C 32 H x indicates non-sooting conditions for a methane/air flame burning at 0.3 MPa and a global strain rate of 200 s −1 , in agreement with the experiments.