*Due to their high load flexibility and air-quality benefits, axial (sequential) stage combustion systems have become more popular among ground-based power gas turbine combustors. However, inert post combustion products passing from the first stage to the second stage affects the flame reactivity and stability in the second stage of the combustor. The present study investigates laminar flame characteristics of the second stage combustion of a sequential combustor. The constant pressure method for spherically expanding flames was employed in an optically accessible constant volume combustion chamber to obtain laminar burning velocities and burned gas Markstein lengths of premixed methane/air mixtures diluted with flue gas at 3 bar and 423 K. Although laminar flame characteristics of methane/oxidizer mixtures diluted with either CO~2~, N~2~ or H~2~O have been studied widely, experimental results for true combustion exhaust gas content (CO~2~+N~2~+H~2~O) are very rare. In the current study, the post combustion products passing from the first stage to the second stage was simulated with a 9.50% CO~2~+71.49% N~2~+19.01% H~2~O mixture by volume and the tested dilution ratios were 0%, 5%, 10%, and 15%. Experimental results showed that laminar burning velocities of CH~4~/air mixtures at 3 bar and 423 K were decreased by 18-23%, 36-42% and 50-52% with additions of 5%, 10% and 15% combustion products, respectively. As the equivalence and dilution ratios increased, the burned gas Markstein lengths at these initial conditions increased slightly, suggesting the methane/air flames are more stable and stretched at these higher ratio conditions. Numerical results were obtained from CHEMKIN using the GRI-Mech 3.0, USC Mech II, and San Diego mechanisms. Numerical laminar burning velocity results were compared with experimental findings to evaluate the performances of these mechanisms for methane/air flames at high temperature, high pressure, and high dilution ratios. The GRI-Mech 3.0 numerical data matched well with the experimental data, with no more than a 9% difference between the two sets of results at any given time. For very lean methane/air flames, the USC Mech II produced numerical laminar flame speeds slightly higher than the experimental values. For rich methane/air flames, the San Diego mechanism agreed poorly with the experimental data. The dilution, thermal-diffusion, and chemical effects of inert post combustion gasses on the laminar burning velocities of methane/air mixtures at 3 bar and 423 K were found using numerical results. The dilution effect was primarily responsible, accounting for 79-84% of the decrease in the laminar burning velocity due to the addition of combustion products. The chemical effect slightly outweighed the thermal diffusion effect and was most important near stoichiometry where combustion temperatures are highest.*


Laminar Flame Characteristics of Sequential Two-Stage Combustion of Premixed Methane/Air Flames

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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