Recent advances in understanding of flammability characteristics of hydrogen

The current increasing interest in hydrogen utilization and increasing understanding of hydrogen combustion motivate this review of flammability characteristics of hydrogen. The intent is to present a thorough and self-contained tutorial that covers the existing fundamental knowledge in a uniform and concise manner. The presentation begins with an up-dated exposition of the elementary chemical mechanism of hydrogen oxidation, including the latest chemical-kinetic results, with evaluated selections of reaction-rate parameters. Understanding of the mechanism is emphasized through presentation of systematically reduced overall steps and their associated rates. Useful simplifications of the chemistry are thereby exposed and appraised, identifying applicable quasi-steady-state approximations. The status of our knowledge of the fundamental transport properties for hydrogen combustion is then summarized, with indication of the relevance of thermal diffusion for hydrogen. Hydrogen–oxygen autoignition processes are next analyzed, including the important differences found under conditions above and below the crossover temperature at which the rates of the branching and recombination steps are equal, with an explanation of the classical explosion diagram that exhibits three explosion limits. Time-dependent and counter-flow mixing layers are addressed in the context of ignition processes. Knowledge of hydrogen deflagrations is reviewed, including their flame structures, burning velocities, and flammability limits, with special emphasis on peculiarities and simplification that occur in the vicinity of the lean limit. Deflagration instabilities and effects of strain and curvature on deflagrations are described, resulting under appropriate circumstances in flame balls, the structures, characteristics, and importance of which are analyzed. The structures and stabilization mechanisms of hydrogen diffusion flames are reviewed, pointing out the current state of knowledge and current uncertainties in their extinction conditions. Hydrogen detonations also are considered, with explanations given of their detonation velocities, structures, and instabilities, including cellular detonations and emphasizing the importance of future studies of vibrational relaxation effects in these detonations. Finally, some comments and observations on the applications and future prospects for hydrogen usage are offered from viewpoints of safety and energy production.