Rich methane/air flames: Burning velocities, extinction limits, and flammability limit

Abstract

A theoretical investigation has been conducted to establish a reliable chemical kinetic mechanism that can determine the extinction limit of opposed-flow, strained, rich premixed, methane-air flames. In the process of developing this kinetic representation for rich methane-air flames, we found that a value of 102.5 kcal/mol for the heat of formation at 298 K for singlet methylene, which is 1 kcal/mol higher than the currently available thermochemical data, gives the best agreement with experimental data on burning velocities for equivalence ratios between 0.5 and 1.7. Using this value for Δ H f 1CH 2 in our calculations, the extinction stretch rate, K ex , was found to be K ex =2250 s −1 for Σ =1.0, K ex =2000 s −1 for Σ =1.1, and K ex =1400 s −1 for Σ =1.2. These results agree better with experiments than those using a lower heat of formation of singlet methylene. In comparison with previous calculations made by Kee et al., our predictions are basically the same, except that our extinction stretch rate is slightly higher at Σ =1.0 and that our location of the maximum extinction stretch rate is closer to that found in experiments. In addition, we establish the rich flammability limit using two different criteria to be approximately between Σ =1.62 and Σ =1.68, which agrees very well with an experimental value of Σ =1.67.