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

The structure andextinctionbehaviorofpartiallypremixed  ames ina counter owconŽ gurationare investigated by using Ž ve different chemistry models. These include the C1 and C2 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, andGRI-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  ames over a wide range of strain rates and equivalence ratios, including those corresponding to premixed and diffusion  ames. Premixed  ame speeds calculated using the C2 and GRI-2.11 mechanisms are in good agreement with the experimental data,whereas those obtainedusing the C1 mechanismshow signiŽ cantdifferences, especially for fuel-rich conditions. The predicted  ammability limits (0:5 <Á < 1:4) are found to be nearly identical for the three mechanisms. In addition, the diffusion  ame structures computed using the three mechanismsare essentially the same, except for small differences in thepeak temperature values.Results for partiallypremixed  ames indicate that all Ž vemechanismsqualitativelyreproduce thedouble- ame structure associatedwith these  ames. There are, however, notablequantitativedifferences between the predictions of C1, C2, and GRI-2.11 mechanisms. For low to moderate strain rates and high levels of air premixing (Á < 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 signiŽ cantly larger compared to that for C1 and C2 mechanisms. Important quantitative differences are also observed in the predictions of C1 and C2 mechanisms. Compared to the C1 mechanism, the predictions using the C2 mechanism indicate that 1) the methane consumption and heat release rates in the premixed zone are higher, 2) the  ame 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  ames are signiŽ cantly higher using the C2 and GRI-2.11 mechanisms compared to those using the C1 mechanism.The effect of radiationheat transfer, computed using an optically thinmodel on the partially premixed  ame structure, is relatively small. Also note that the premixed  ame speed plays an important role in determining the stretch rate and, therefore, the structure of partially premixed  ames.