Characterization of a Coflowing Methane/air Non-premixed Flame with Computer Modeling, Rayleigh-raman Imaging, and On-line Mass Spectrometry

A non-sooting lifted methane/air coflowing non-premixed flame has been studied experimentally and computationally. The flame structure was computed using a model that solves the fully elliptic governing equations, utilizes detailed transport coefficients and a chemical kinetic mechanism with C1 to C6 hydrocarbons, and includes an optically thin radiation submodel. Gas temperature, major species mole fractions, and non-fuel hydrocarbon concentrations were experimentally mapped in two dimensions with both probe techniques (thermocouples and on-line mass spectrometry) and optical diagnostics (Rayleigh and Raman scattering). A differential polarization strategy was used to remove C2 and polycyclic hydrocarbon fluorescence interferences from the Raman scattering signals, which dramatically improved the quality of the laser diagnostic images over what had previously been possible. Good agreement was observed between the probe and laser images; this validates the Rayleigh-Raman data processing procedure, and it shows that the probes produce negligible perturbations to the flame structure. The spatial precision of the data and range of measured quantities provides a sensitive test of the computations. Nonetheless, the model reproduces most of the experimental observations, including the overall flame height and liftoff height, the peak concentrations and spatial distributions of major species, and the peak concentrations of oxygenated hydrocarbon intermediates such as ketene and soot precursors such as benzene and acetylene.