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

Abstract

Abstract 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 [1] using the GRI-Mech 3.0 [2] and the San Diego [3] mechanisms. Excellent agreement was observed between the GRI-Mech 3.0 [2] and experimental data at ϕ ≤ 1.2 for methane/air mixtures at 1 bar and 298 K. However, for mixtures at ϕ < 1.3, 1 bar and 473 K, mixtures at ϕ < 1.2 for 5% and 10% dilutions, and for mixtures at ϕ < 0.9 for 15% dilution, laminar burning velocities predicted by the GRI-Mech 3.0 mechanism [2] were slightly higher than experimental results. The San Diego mechanism [3] showed good agreement with experimental data at ϕ ≤ 0.9 for methane/air mixtures at 1 bar and 298 K. However, for mixtures at ϕ > 0.9, 1 bar and 298 K, mixtures at ϕ > 1.2, 1 bar and 473 K, and mixtures at ϕ > 1.1 with 5%, 10% and 15% dilution, the San Diego mechanism [3] predicted slower laminar flame speeds than the experimental results. On the other hand, the laminar burning velocities predicted by the San Diego mechanism [3] were slightly faster than the experimental results for leaner mixtures. 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. Lastly, it was shown that CO2 dilution suppresses the flame instabilities during combustion, which is attributable to the increase in the burned gas Markstein length (Lb) with the addition of diluent.