A Flamelet Model for Premixed Methane-Air Flames

Abstract

The structure of premixed methane-air flames is analyzed using the “laminar flamelet concept”. A new model based on one-dimensional set of transformed coupled second order differential conservation equations describing the species concentrations of CO₂, CO, O₂, CH₄, H₂O, H₂ and N₂ and the sensible enthalpy are presented in the present work. The equations are rigorously derived and solved numerically. In these equations, a reaction progress variable (c) is taken as the independent variable that varies from zero to one. A three-step chemical kinetic mechanism is adopted. This was deduced in a systematic way from a detailed chemical kinetic mechanism. The rates for the three steps are related to the rates of the elementary reactions of the full reaction mechanism. Calculations are made for different fixed values of the scalar dissipation rate (x) until the flamelet eventually reaches the extinction limit at different levels of pressure. Moreover, simultaneous and instantaneous l-D measurements of CO₂, O₂, CO, N₂, CH₄, H₂O, H₂, OH and temperature have been carried out in a premixed laminar methane-air flame. A one-dimensional UV Raman-Raylcigh and Laser Induced Predissociation (LIPF) technique has been applied in the present work. The spatial and temporal resolutions are limited to the signal-to-noise ratio and the laser pulse duration. The results of the calculations are assessed against the measurements and previous predictions based on the asymptotic approach of CI mechanism and a 4-step reduced mechanism. The burning velocity at different equivalence ratios was also deduced from the flamelet properties and assessed against available data.