The present study investigates laminar flame characteristics of the combustion within the second stage of a sequential combustor. The method of constant pressure for spherically expanding flames was employed to obtain laminar burning velocities (LBV) and burned gas Markstein lengths (Lb) of premixed methane/air mixtures diluted using flue gas at 3 bar and 423 K. Combustion residuals were imitated using a 19.01% H2O + 9.50% CO2 +71.49% N2 mixture by volume, while tested dilution ratios were 0%, 5%, 10%, and 15%. Experimental results showed that the LBV was decreased by 18-23%, 36-42%, and 50-52% with additions of 5%, 10%, and 15% combustion products, respectively. As the dilution and equivalence ratios increased, the Lb values increased slightly, suggesting that the stability and stretch of the CH4/air flames increased at these conditions. Numerical results were obtained from CHEMKIN using the GRI-Mech 3.0, USC Mech II, San Diego, HP-Mech, NUI Galway, and AramcoMech 1.3 mechanisms. The GRI-Mech 3.0 and HP-Mech performed best, with an average of 2% and 3% difference between numerical and experimental LBVs, respectively. The thermal-diffusion (TD), dilution (D), and the chemical (C) effects of inert post-combustion gases on the LBV were found using numerical results. The dilution effect was primarily responsible, accounting for 79-84% of the LBV reduction.

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