Laminar flame speed of H2/CH4/air mixtures with CO2 and N2 dilution

Abstract

The laminar flame speeds of H2/CH4/air mixtures with CO2 and N2 dilution were systematic investigated experimentally and numerically over a wide range of H2 blending ratios (0–75 vol%) with CO2 (0–67 vol%) and N2 (0–67 vol%) dilution in the fuels. The experimental measurements were conducted via the Bunsen flame method incorporating the Schlieren technique under the condition of equivalence ratios from 0.8 to 2.0. To gain an insightful understanding of the experimental observations, detailed numerical simulation was carried out using Chemkin-Pro with GRI3.0-Mech. The experimental measurements were also used to validate the corresponding performance of a semiempirical correlation derived through asymptotic analysis method coupled with the reduced chemistry mechanism. The results showed that at lower H2 fraction (xH2 ≤ 0.5), the laminar flame speeds of H2/CH4/air mixtures displayed great linearly increase with the growth of H2 fractions. The combustion of mixtures with low H2 contents was dominated by CH4 conversion which was mainly controlled by the increasing OH radicals produced from the key oxidation reactions of H + O2 = O + OH. With the further increase of H2 fractions, the methane-dominated combustion gradually transformed into the methane-inhibited hydrogen combustion, resulting to the growth of laminar flame speeds was dramatical and non-linear. Due to the larger heat capacity and chemical kinetic effect, CO2 presented a stronger dilution effect on reducing the laminar flame speeds than N2. With the addition of CO2, the increasing stronger competition for H radical through CO + OH = CO2 + H with H + O2 = O + OH due to the significant reduction of H mole fractions, leading to the larger decrease of laminar flame speeds of mixtures. Besides, although the contribution of thermal effect of CO2 decreased near the equivalence ratio, the thermal effect of CO2 still preformed the dominated contribution to the total dilution effect. A comparison between the experimental data and estimated results using the semiempirical correlation showed that, the correlation using new modified coefficients provided the satisfactorily accuracy predictions on the laminar flame speeds of diluted H2/CH4/air mixtures at lower xH2 (xH2 ≤ 0.5) and lower xdilution (xdilution = 0.25). The estimated results were generally located within a deviation range of ±20% errors except for two unsatisfactory eases occurred at conditions of xH2 = 0.75 and xCO2 = 0.67. The considerably poor predictions were attributed to the significantly variation of the chemical kinetics under high H2 content and large CO2 dilution conditions.