Investigation of laminar partially premixed methane-air counterflow flames

The behavior of atmospheric laminar methane-air partially premixed counterflow flames is investigated. A one-dimensional computer code is used to solve for the velocity, temperature and species concentrations in the flames utilizing the GRI-Mech 2.11 chemical reaction mechanism. The inflow axial velocities and the stoichiometry of both the rich and lean inlet streams are parametrically varied, and the consequent effects on NOx production are discussed. The reaction mechanism is found to provide unsatisfactory results for the rich side of partially premixed flames. The flammability limit is overpredicted by this mechanism for rich mixtures, leading to a separated rich flame that cannot physically exist. A kinetic analysis of the chemical mechanism is performed using a sensitivity code in order to identify the discrepancies in the flame chemistry on the rich side, and a few reactions that might be problematic are identified. This work is in progress, and our objective is to determine the kinetic cause of the overprediction of the rich flammability limit. 1. Introduction: The combustion modes in which industrial burners have been classically categorized include those of nonpremixed (diffusion) and premixed burning. However, partially premixed combustion is known to occur in both highly turbulent diffusion flames as well as in technical realizations of premixed burners. Consequently, in these systems the fuel stream is enriched with air and vice-versa to levels at which the mixture is barely or not at all ignitable. Under certain conditions, the subsequent combustion occurs under lean premixed conditions that include the corresponding NOx reduction potential, however, without the safety concerns associated with fuel-air premixing. Such flames are considered herein.