Effects of Reaction Mechanisms on Structure and Extinction of Partially Premixed Flames

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The structure and extinction behavior of partially premixed flames in a counterflow configuration are investigated by using five different chemistry models. These include the C 1 and C 2 mechanisms of Peters and Rogg (Peters, N., and Rogg, B., Reduced Kinetic Mechanisms for Applications in Combustion Systems, Springer-Verlag, Berlin, 1993, pp. 8-12), a 12-step augmented reduced mechanism, and GRI-2.11 and GRI-3.0 mechanisms. Simulations focus on the comparison of these mechanisms in predicting the structure and extinction of methane-air partially premixed flames over a wide range of strain rates and equivalence ratios, including those corresponding to premixed and diffusion flames. Premixed flame speeds calculated using the C 2 and GRI-2.11 mechanisms are in good agreement with the experimental data, whereas those obtained using the C 1 mechanism show significant differences, especially for fuel-rich conditions. The predicted flammability limits (0.5 < O < 1.4) are found to be nearly identical for the three mechanisms. In addition, the diffusion flame structures computed using the three mechanisms are essentially the same, except for small differences in the peak temperature values. Results for partially premixed flames indicate that all five mechanisms qualitatively reproduce the double-flame structure associated with these flames. There are, however, notable quantitative differences between the predictions of C 1 , C 2 , and GRI-2.11 mechanisms. For low to moderate strain rates and high levels of air premixing (O < 2.0), the rich premixed reaction zone for the GRI-2.11 and GRI-3.0 mechanisms is located very close to the fuel nozzle. In addition, the physical separation between the two reactions zones for these mechanisms is significantly larger compared to that for C 1 and C 2 mechanisms. Important quantitative differences are also observed in the predictions of C 1 and C 2 mechanisms. Compared to the C 1 mechanism, the predictions using the C 2 mechanism indicate that 1) the methane consumption and heat release rates in the premixed zone are higher, 2) the flame structure exhibits higher sensitivity to the equivalence ratio, and 3) the two reaction zones merge at a lower equivalence ratio. The extinction strain rates for partially premixed flames are significantly higher using the C 2 and GRI-2.11 mechanisms compared to those using the C 1 mechanism. The effect of radiation heat transfer, computed using an optically thin model on the partially premixed flame structure, is relatively small. Also note that the premixed flame speed plays an important role in determining the stretch rate and, therefore, the structure of partially premixed flames.