Computational and experimental study of the effects of adding dimethyl ether and ethanol to nonpremixed ethylene/air flames

Two sets of axisymmetric laminar coflow flames, each consisting of ethylene/air nonpremixed flames with various amounts (up to 10%) of either dimethyl ether (CH₃-O-CH₃) or ethanol (CH₃-CH₂-OH) added to the fuel stream, have been examined both computationally and experimentally. Computationally, the local rectangular refinement method, which incorporates Newton's method, is used to solve the fully coupled nonlinear conservation equations on solution-adaptive grids for each flame in two spatial dimensions. The numerical model includes C6 chemical kinetic mechanisms with up to 59 species, detailed transport, and an optically thin radiation submodel. Experimentally, thermocouples are used to measure gas temperatures, and mass spectrometry is used to determine concentrations of over 35 species along the flame centerline. Computational results are examined throughout each flame, and validation of the model occurs through comparison with centerline measurements. Very good agreement is observed for temperature, major species, and several minor species. As the level of additive is increased, temperatures, some major species (CO₂, C₂H₂), flame lengths, and residence times are essentially unchanged. However, peak centerline concentrations of benzene (C₆H₆) increase, and this increase is largest when dimethyl ether is the additive. Computational and experimental results support the hypothesis that the dominant pathway to C₆H₆ formation begins with the oxygenates decomposing into methyl radical (CH₃), which combines with C2 species to form propargyl (C₃H₃), which reacts with itself to form C₆H₆.