Experimental and kinetic study on ignition delay times of methane/hydrogen/oxygen/nitrogen mixtures by shock tube

Ignition delay times of methane/hydrogen/oxygen/nitrogen mixtures with hydrogen amount-of-substance fractions ranging from 0–20% were measured in a shock tube facility. The ambient temperature varied from 1422 to 1877 K and the pressure was maintained at 0.4 MPa behind the reflected shock wave. The experiments were conducted at an equivalence ratio of 2.0. The fuel mixtures were diluted with nitrogen gas so that the nitrogen amount-of-substance fraction was 95%. The experimental ignition delay time of the CH₄/H₂ mixture decreased as the hydrogen amount-of-substance fraction increased. The enhancement of ignition by hydrogen addition was weak when the ambient temperature was >1750 K, and strong when the temperature was <1725 K. The ignition delay time of 20% H₂/80% CH₄ was only one-third that of 100% CH₄ at 1500 K. A modified model based on GRI-Mech 3.0 was proposed and used to calculate the ignition delay times of test mixtures. The calculated results agreed with the experimental ignition delay times. Normalized sensitivity analysis showed that HO·+H₂ →H·+H₂O was the main reaction for the formation of the H· at 1400 K. As the hydrogen amount-of-substance fraction increased, chain branching was enhanced through the reaction H·+O₂→O·+HO·, and this reduced the ignition delay time. At 1800 K, the methyl radical (H₃C·) became the key species that influenced the ignition of the CH₄/H₂/O₂/N₂ mixtures, and sensitivity coefficients of the chain termination reaction 2H₃C·(+M)→C₂H₆(+M), and chain propagation reaction HO₂+H₃C·→HO·+CH₃O decreased, which reduced the influence of hydrogen addition on the ignition of the CH₄/H₂ mixtures.