Effects of water vapor on auto-ignition characteristics and laminar flame speed of methane/air mixture under engine-relevant conditions

In this paper, the effects of water vapor on auto-ignition characteristics and laminar flame speed of the methane/ air mixture were numerically simulated by using the Chemkin code under the engine-relevant conditions. Different methane combustion mechanisms were separately simulated under the same boundary conditions and compared to the experimental data obtained from the literature. Then, the detailed methane combustion mechanism was selected to investigate the effects of water vapor on the auto-ignition characteristics of the methane/air mixture and methane/hydrogen/air mixture under the relative high pressure condition. In addition, the effects of water vapor on the laminar flame speed of methane/air and methane/hydrogen/air mixture was also studied. Furthermore, the chemical, thermal and dilution effects of the water vapor on the laminar flame speed of methane/air were quantitatively evaluated. Last, the effects of initial temperature and pressure on the laminar flame speed of methane/air were also analyzed based on the detailed methane combustion mechanism. The results indicated that the ignition delay time of the methane/air was increased with increasing water vapor ratio. In addition, the dilution and thermal effects of the water vapor played the decisive roles in the laminar flame speed of the methane/air mixture compared to its chemical effect. Furthermore, the laminar flame speed and adiabatic combustion temperature of the methane/air were decreased with increasing water vapor ratio. Moreover, the formation and oxidation rates of the H, O and OH radicals during the methane/air combustion were prohibited with adding water vapor.