Effect of CO₂ Dilution on the Laminar Burning Velocities of Premixed Methane/Air Flames at Elevated Temperature

With increased interest in reducing emissions, the staged combustion concept for gas turbine combustors is gaining in popularity. For this work, the effect of CO₂ dilution on laminar burning velocities of premixed methane/air flames was investigated at elevated temperature through both experiments and numerical simulations. Validation of the experimental setup and methodology was completed through experimental testing of methane/air mixtures at 1 bar and 298 K. Following validation, high temperature experiments were conducted in an optically accessible constant volume combustion chamber at 1 bar and 473 K. Laminar burning velocities of premixed methane/air flames with 0%, 5%, 10%, and 15% CO₂ dilution were determined using the constant pressure method enabled via schlieren visualization of the spherically propagating flame front. Results show that laminar burning velocities of methane/air mixtures at 1 bar increase by 106–145% with initial temperature increases from 298 K to 473 K. Additions of 5%, 10%, and 15% CO₂ dilution at 1 bar and 473 K cause a 30–35%, 51–54%, and 66–68% decrease in the laminar burning velocity, respectively. Numerical results were obtained with CHEMKIN (Kee et al., 1985, “PREMIX: A Fortran Program for Modeling Steady Laminar One-Dimensional Premixed Flames,”) using the GRI-Mech 3.0 (Smith et al., 2019) and the San Diego (“Chemical-Kinetic Mechanisms for Combustion Applications,” San Diego Mechanism Web Page, Mechanical and Aerospace Engineering (Combustion Research), University of California at San Diego, San Diego, CA) mechanisms. It is concluded that the GRI-Mech 3.0 (Smith et al.., 2019) better captures the general overall trend of the experimental laminar flame speeds of methane/air/CO₂ mixtures at 1 bar and 473 K. Additionally, the dilution, thermal-diffusion, and chemical effects of CO₂ on the laminar burning velocities of methane/air mixtures were investigated numerically by diluting the mixtures with both chemically active and inactive CO₂ following the determination of the most important elementary reactions on the burning rate through sensitivity analysis. Finally, it was shown that CO₂ dilution suppresses the flame instabilities during combustion, which is attributable to the increase in the burned gas Markstein length (L_b) with the addition of diluent.