Demarcation of reaction effects on laminar burning velocities of diluted syngas–air mixtures at elevated temperatures

Laminar burning velocities of CO₂/N₂ (60%, 70%) diluted H₂/CO/air mixtures were measured at elevated temperatures using the externally heated diverging channel method. The computed burning velocities using the two reaction mechanisms (Davis et al., Proc. Combust. Inst. 2005;30(1):1283–1292; FFCM-1, http://nanoenergy.stanford.edu/ffcm1 ) are compared with the experimentally determined burning velocities. The accuracy of the chemical kinetic mechanisms at high dilution rates and elevated temperatures was investigated for various hydrogen fractions in the fuel composition. The burning velocity is observed to increase at high temperatures due to higher mixture enthalpy. The dilution effect on the variation of laminar burning velocity was stronger for the CO₂ dilution case compared to N₂ dilution. A comparison between the FFCM-1 mechanism and experimental measurements shows an accurate depiction of the reaction chemistry regarding the prediction of laminar burning velocities. The role of third-body reactions and direct inhibiting effect of N₂ and CO₂ molecules on burning velocity of diluted syngas–air mixtures is analyzed in detail. Detailed kinetic analysis revealed that the use of GRI 3.0 collision efficiency factors in the Davis mechanism helps in accurately predicting the burning velocities at elevated temperatures and high CO₂ dilution rates. The thermal effect dominates the reduction in laminar burning velocity for N₂ dilution case. The FFCM-1 mechanism agrees well with the experiments for syngas flames diluted with N₂ compared to the Davis mechanism. The addition of third-body efficiency of N₂ in the FFCM-1 mechanism improved the predictions of laminar burning velocities for the N₂ dilution case.