Ultra-lean combustion characteristics of premixed methane flames in a micro flow reactor with a controlled temperature profile

Experiments and computations for CH₄/O₂/Xe and CH₄/O₂/N₂ mixtures at ultra-lean conditions were conducted in a novel vertical micro flow reactor with a controlled temperature profile. The performance of several chemical mechanisms was evaluated at 0.3 0.5 and 0.7 equivalence ratios and at different dilution ratios by observing the weak flames. CH₄/O₂/Xe mixtures displayed a higher reactivity than CH₄/O₂/N₂ flames both experimentally and computationally. Also lower dilution and lower equivalence ratios led to increased reactivity. Comparison between CH₄/O₂/Xe and CH₄/O₂/N₂ flame structures showed observable differences between 1000 K and 1100 K. Reaction path and rate of production analysis showed that OH radicals are largely produced by (R1) H + O₂ ⇔ OH + H and were inhibited by the third-body related reactions represented by (R10) H + O₂ + M ⇔ HO₂ + M. Since the third-body collision efficiency was larger for N₂ than Xe the reactivity became lower for N₂ mixtures than for Xe due to the larger effect of third-body reactions which explained the difference in reactivity for Xe diluted and N₂ diluted flames.