Effects of reaction mechanisms on structure and extinction of partially premixed flames

The structure and extinction behavior of partially premixed flames in a cou nterflow configuration are investigated by using five different chemistry m odels. These include the C-1 and C-2 mechanisms of Peters and Rogg (Peters, N., and Rogg, B., Reduced Kinetic Mechanisms for Applications in Combustio n Systems, Springer-Verlag, Berlin, 1993, pp. 8-12), a 12-step augmented re duced mechanism, and GRI-2.11 and GRI-3.0 mechanisms. Simulations focus on the comparison of these mechanisms in predicting the structure and extincti on of methane-air partially premixed dames over a wide range of strain rate s and equivalence ratios, including those corresponding to premixed and dif fusion dames, premixed dame speeds calculated using the C-2 and GRI-2.11 me chanisms are in good agreement with the experimental data, whereas those ob tained using the C-1 mechanism show significant differences, especially for fuel-rich conditions. The predicted flammability limits (0.5 < <phi> < 1.4 ) are found to be nearly identical for the three mechanisms. In addition, t he diffusion flame structures computed using the three mechanisms are essen tially the same, except for small differences in the peak temperature value s, Results For partially premixed dames indicate that all five mechanisms q ualitatively reproduce the double-flame structure associated with these dam es, There are, however notable quantitative differences between the predict ions of C-1, C-2, and GRI-2.11 mechanisms. For low to moderate strain rates and high levels of air premixing (<phi> < 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 cons umption and heat release rates in the premixed zone are higher, 2) the dame structure exhibits higher sensitivity to the equivalence ratio, and 3) the two reaction zones merge at a lower equivalence ratio. The extinction stra in rates for partially premixed dames are significantly higher using the C- 2 and GRI-2.11 mechanisms compared to those using the C1 mechanism, The eff ect of radiation heat transfer, computed using an optically thin model on t he partially premised flame structure, is relatively small. Also note that the premixed dame speed plays an important role in determining the stretch rate and, therefore, the structure of partially premixed flames.