Study on the ignition characteristics of CH4/H2/air mixtures in a micro flow reactor with a controlled temperature profile

Abstract

The ignition characteristics of stoichiometric CH4/H2/air mixtures with different H2 dilution ratios were studied by using a micro flow reactor (MFR) with a controlled temperature profile. The weak flame positions of the mixtures were determined by the luminosity of CH∗, and the ignition temperatures (defined as the corresponding wall temperatures) were obtained. The ignition temperature of the mixture is smoothly decreased with a small amount of H2 addition, while the decrease of the ignition temperature is more rapid with increasing H2 dilution. One-dimensional computation with a detailed reaction mechanism was also conducted to study the ignition characteristics of the CH4/H2/air mixtures. The computational ignition temperature which is defined as the wall temperature at the peak of the heat release rate (HRR) peak is also obtained. Both the experimental and computational results show that the weak flame shifts to the lower temperature region with an increase of the H2 mole fraction. And the non-linear decrease of the ignition temperature of the CH4/H2/air weak flame versus increasing H2 dilution was also well reproduced by the numerical computation. The flame structures, primary exothermic and endothermic reactions of the CH4/H2/air weak flames were studied in detail. The importance of H and OH in the ignition process of the mixtures was analyzed, and the kinetic effects of H2 on the ignition of the mixture in the intermediate-temperature region (1000–1200 K) and high-temperature region (1200–1300 K) were elucidated. The analysis of the computational results reveals that the OH production routes in the intermediate-temperature region are substantially changed with different H2 dilutions, while the OH production routes at the peak of the heat release rate are not obviously influenced. In the end, the primary mechanism for the non-linear effects of H2 addition on the ignition temperatures of the CH4/H2 mixtures was clarified.