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 using a novel vertical micro flow reactor with a controlled temperature profile were conducted. The performance of several chemical mechanisms was evaluated at equivalence ratios 0.3, 0.5, and 0.7, and at different dilution ratios by observing the weak flames. Comparisons between CH4/O2/Xe and CH4/O2/N2 flame locations indicated that CH4/O2/Xe mixtures have a higher reactivity than CH4/O2/N2 flames both experimentally and computationally. Furthermore, both experimental and computational results indicated that lower dilution and lower equivalence ratios increase reactivity. Comparisons between experimental and computational weak flame positions showed that computational results obtained with San Diego mechanism showed the best agreement to the experimental results compared to GRI-Mech 3.0 and AramcoMech 1.3. Comparison between CH4/O2/Xe and CH4/O2/N2 flame structures indicated that observable differences are seen between 1000 and 1100 K. Sensitivity analysis showed that OH radicals are important in determining the weak flame location. Reaction path and rate of production analysis showed that OH radicals are largely produced by (R1) H + O2 ⇔ OH + H, and are inhibited by the third-body related reactions represented by (R10) H + O2 + M ⇔ HO2 + M. Since the third-body collision efficiency is larger for N2 than Xe, it was shown that the reactivity becomes 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.