| Summary: | <p>Hydrogen is a promising alternative fuel due to its zero-carbon nature. However, safety issues associated with hydrogen storage at cryogenic conditions are a key constrain to its industrial use. Understanding hydrogen flame propagation at cryogenic conditions is essential for ensuring safe hydrogen utilization. Since cryogenic hydrogen flame is prone to both Darrieus-Landau instability (DLI) and diffusional-thermal instability (DTI), it is necessary to evaluate the effects of cryogenic temperature on unstable hydrogen flame propagation and acceleration. In this study, two-dimensional numerical simulations are performed to assess the effects of cryogenic temperature on the nonlinear evolution and acceleration of fuel-lean hydrogen flames. By changing the equivalence ratio, initial temperature and channel size, the intensity of DLI and DTI varies, resulting in various regimes of flame evolution. It is found that cryogenic flames are more unstable than normal flames. For fuel-lean hydrogen flames, DTI leads to chaotic evolution of cellular flame front. In contrast, for stoichiometric or fuel-rich flames without DTI, flames propagate in a steady single-cusp regime. Analyses show that long-term dynamics for fuel-lean flames including cell splitting, merging and lateral movement are caused by flow-flame-chemistry interaction. Moreover, cryogenic temperature leads to profound increase in the local reaction rate for fuel-lean flame, while it shows minimal impact for stoichiometric flame, demonstrating the strong influence of DTI on cryogenic flame structure and acceleration. Accelerative propagation of fuel-lean flames is caused by the combined effects of flame surface area increase and local reaction rate enhancement. Enhancement of flame consumption speed is primarily caused by strong DTI at cryogenic temperature, with a secondary contribution from DLI. Furthermore, results show that in narrow channels flame front evolution and acceleration are sensitive to the domain size. Flame consumption speed changes non-monotonically with channel width and evolves in an oscillatory manner. Results also show that the domain size affects the flame acceleration primarily by constraining the flame surface area increase rather altering the local reaction rate. The present study provides insights to cryogenic hydrogen flame propagation as well as hydrogen safety control.</p>
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