Investigation of dilution effect with N2/CO2 on laminar burning velocity of premixed methane/oxygen mixtures using freely expanding spherical flames

The main objective of this work was to measure the dilution effect of nitrogen and carbon dioxide on the unstretched laminar burning velocities and differential diffusional stability of premixed methane-oxygen mixtures at ambient conditions and different equivalence ratios. Freely expanding spherical flame method was used. The combustible mixture was prepared by following (100-Z)%CH4+2O2ϕ+Z%(N2/CO2) where Z corresponds to the percentage of diluent in the mixture by volume and ϕ refers to equivalence ratio. The percentage of N2/CO2 in the mixture was varied between 50-70%/30-50%. The equivalence ratio was varied from 0.6 to 1.4. Necessary stretch correction to the measured data was done following weak stretch theory. PREMIX with GRIMech3.0 was used to simulate all the experimental operating conditions. The addition of diluents has reduced the burning velocity of the mixture considerably due to the decrease in energy content of the mixture, increase in specific heat capacity and the corresponding reduction in the flame temperature and decrease in thermal diffusivity. At 50%N2/CO2 in the premixed methane-oxygen mixtures, the burning velocity measured with CO2 was 3.6 (average value estimated for all studied equivalence ratios) times smaller than the mixtures diluted with N2. Burning velocity was reduced by 88% due to the addition of 50%CO2 as compared to 0%CO2 in the combustible mixture and out of which reduction due to thermal effect was 81.5% and chemical effect (participation of added CO2 in the chemical reactions) was 6.5% (average value estimated for all studied equivalence ratios). For all the studied dilution cases, the burned gas Markstein length remains positive indicating that the mixtures were stable towards the differential diffusion effects. Agreement of burning velocities obtained between PREMIX with GRIMech3.0 and measurements was not good at lower levels of diluents. Based on the literature, a new species, and few elementary reactions were added to original GRIMech3.0. By doing sensitivity analysis, kinetic parameters of some elementary reactions were also modified. Simulations with the modified GRIMech3.0 showed good agreement with most of the experimental data.