Formation and evolution of flame kernels in autoignition of a turbulent hydrogen/air mixing layer at 50 atm

摘要

Autoignition of a turbulent stratified mixing layer between nitrogen-diluted hydrogen and hot air under an elevated pressure of 50 atm is studied using direct numerical simulations (DNS) in this work. Homogeneous isotropic turbulence is superimposed on the flow field. Reduced chemical mechanisms and a detailed multi-component diffusion model are employed. In addition to turbulent mixing ignition (TMI), homogeneous mixing ignition (HMI) and laminar mixing ignition (LMI) are also investigated for comparison. Autoignition chemistry over a wide range of pressures is studied in HMI and LMI, which shows different behaviors at elevated pressures versus low pressures. The importance of H2O2 and HO2 in TMI is highlighted as radical sinks during the ignition process and can also be used as an indicator for locating the ignition spots. Moreover, OH radicals can be used as a marker variable for the transition of autoignition to flame propagation under high pressures. According to the present study, turbulence has some influence on the radical explosion stage especially for the conservation of H2O2 under the elevated pressure of 50 atm. Autoignition kernels forming away from the most reactive mixture fraction iso-surface are identified for the first time, which is a hybrid of autoignition and diffusive-ignition.