Propagation and self-acceleration of circular expanding hydrogen/air flames at cryogenic temperature

Hydrogen is an attractive alternative fuel which may play an important role in the reduction of carbon dioxide emission. The widespread use of hydrogen raises concerns on safety issues such as fires and explosions related to the storage, transportation and utilization of liquid hydrogen. Therefore, it is necessary to understand the combustion properties of hydrogen at cryogenic temperatures. In this study, we conduct two-dimensional simulations of circular expanding hydrogen/air flame in an open space and investigate the flame morphology evolution and self-acceleration processes. Stoichiometric hydrogen/air mixtures with near-unity Lewis number at cryogenic temperature (100 K) and normal temperature (300 K) are considered so that the flame propagation is mainly affected by the Darrieus-Landau instability (DLI, or hydrodynamic instability) instead of the diffusional-thermal instability (DTI). The flame morphology, flame propagation speed pulsation, development and evolution of cellular structure, and self-acceleration are investigated. Since the thermal expansion increases greatly with the decrease in the initial temperature, the DLI at cryogenic temperature becomes much stronger than that at normal temperature, which results in significant increase in flame propagation speed, total heat release rate, acceleration exponent as well as violent cell evolution including cell growth, splitting and merging. The stronger DLI at lower initial temperature leads to larger cell number, smaller mean cell size and longer mean cell depth. Analyses on cell statistics show that the intermittent characteristic of flame propagation speed is mainly caused by cell evolution. Interestingly, the integral heat release rate and flame propagation speed at cryogenic temperature can be comparable to those at normal temperature. Such counterintuitive observation is important for fire safety control and risk assessment of cryogenic hydrogen storage and utilization.