The Effect of Exhaust Gas Recirculation (EGR) on Fundamental Characteristics of Premixed Methane/Air Flames 2020-01-0339

Over the years, many studies have examined the natural gas flame characteristics with either CO₂, H₂O, or N₂ dilution in order to investigate the exhaust gas recirculation (EGR) effect on the performance of natural gas vehicles. However, studies analyzing the actual EGR concentration are very scarce. In the present study, spherically expanding flames were employed to investigate the EGR effect on the laminar flame speed (LFS) and the burned gas Markstein length (L_b) of premixed CH₄/air flames at 373 K and 3 bar. The EGR mixture was imitated with a mixture of 9.50% CO₂ + 19.01% H₂O + 71.49% N₂ by mole. EGR ratios of 0%, 5%, 10%, and 15% were tested. Experimental results show that LFS values are lowered by 20-23%, 38-43% and 53-54% due to 5%, 10% and 15% EGR, respectively. Additionally, it was observed that L_b values slightly increase at high equivalence and EGR ratios, where CH₄ flames are more stable and more stretched. Numerical results were obtained with the San Diego, USC Mech II, and GRI-Mech 3.0 chemical mechanisms and compared with experimental LFS values. Experimental findings are most compatible with numerical LFS values provided by the GRI-Mech 3.0. The USC Mech II slightly overpredicts LFS for very lean flames. The San Diego mechanism shows poor performance in the prediction of LFS of rich mixtures. Numerical analyses were used to determine the chemical, dilution, and thermal-diffusion effects of EGR on the LFS. Results show the most dominant effect is the dilution effect as it is responsible for 80-84% of the decrease in the LFS. The chemical effect reaches its peak around stoichiometry where the combustion temperature is highest. The thermal-diffusion effect is merely weaker than the chemical effect.