Effects of Soret diffusion and thermal stratification on ignition and flame propagation in cryogenic hydrogen/air mixtures

This study investigates ignition and flame propagation in cryogenic hydrogen/air mixtures through one-dimensional numerical simulations with detailed chemistry and transport. The influences of cryogenic temperature, Soret diffusion, and thermal stratification on minimum ignition energy (MIE), flame kernel development, and flame propagation are examined. The results show that the MIE increases significantly as the temperature decreases, resulting in a narrower flammability range for cryogenic hydrogen/air mixtures compared to those at normal temperature. Soret diffusion increases/decreases MIE for hydrogen/air mixtures near rich/lean flammability limits, with these effects amplified under cryogenic conditions. In thermally stratified mixtures, the MIE decreases as the stratification radius increases until it reaches a constant value when the stratification radius exceeds a critical value. This highlights potential safety hazards in cryogenic hydrogen leakage scenarios where thermal stratification can reduce ignition energy requirements. Cryogenic temperature substantially reduces flame propagation speed and exhibits opposing effects on Markstein length under lean and rich conditions, causing it to become more negative/positive for lean/rich mixtures. A distinctive thermal stratification-induced transition regime emerges for thermally stratified cases. Flame structure analyses reveal that cryogenic temperature significantly lowers flame temperature, heat release rate, and radical concentrations, while amplifying the effects of flame stretch. For thermally stratified cases, the flame structure undergoes a transition during propagation through the stratification layer. Based on these analyses, insights for flamelet modeling of cryogenic hydrogen combustion are provided, emphasizing the need to incorporate flame stretch, cryogenic temperature, and thermal stratification effects through appropriate flamelet libraries and controlling variables. • Cryogenic temperatures amplify Soret diffusion effects on ignition. • Thermal stratification reduces ignition energy in cryogenic H 2 leakage. • Cryogenic temperatures exhibit opposing effects on Markstein length. • Flame structure analyses reveal flame stretch and thermal stratification effects. • Insights for flamelet modeling of cryogenic H 2 combustion are provided.