Ozone-affected auto-ignitive hydrogen-air flames: Transitions near critical temperatures

Abstract

This study investigates the effects of ozone addition on autoignition-assisted hydrogen-air flames with detailed kinetics and transport. For homogeneous ignition, a critical temperature ($T_c$) was identified that significantly influences the reaction pathways and ignition characteristics. It is shown that below $T_c$, the system exhibits a distinct two-stage reaction process during autoignition, characterized by initial ozone decomposition followed by high-temperature hydrogen-oxygen reactions. Above $T_c$, the two ignition stages merge, leading to drastically reduced ignition delay time—a small temperature difference near $T_c$ can result in a hundredfold reduction. For the auto-ignitive flames, similar transition in terms of flame speeds occurs near the critical temperature, for which the proposed scaling law based on Damköhler number holds for both conditions below and above $T_c$. Comparative analysis of zero-dimensional (0D) and one-dimensional (1D) simulations reveals pronounced differences in the evolution of key species such as H₂, H, HO₂ and O₃. In 1D flames, transport processes lead to more efficient radical buildup and earlier ozone consumption compared to the 0D case. The spatial coupling of the H₂ diffusion zones with O₃ consumption zones above $T_c$ was found to enhance the overall combustion process. The effects of elevated pressure have also been illustrated. These findings underscore the critical influence of transport effects and subtle temperature variations on radical accumulation, reaction pathways, and flame dynamics in ozone-assisted hydrogen combustion.