Numerical study on local characteristics of turbulent hydrogen/air flames at ambient and cryogenic temperatures

Hydrogen is a promising carbon-free fuel for achieving net zero. However, cryogenic conditions used for hydrogen storage introduce additional combustion safety concerns. Understanding the behavior of turbulent hydrogen/air flames under such conditions is therefore essential. In this study, three-dimensional direct numerical simulations (DNS) are conducted to investigate the local characteristics of turbulent stoichiometric hydrogen/air flames at ambient (300 K) and cryogenic (100 K) temperatures. The results show that cryogenic temperature suppresses small-scale flame wrinkling, leading to a smoother flame front and weaker flame acceleration than in the ambient case. This behavior is attributed to the increased curvature Markstein number at lower temperatures, which enhances the sensitivity of flame stretch to negative curvature and promotes the local destruction of flame surface area in highly curved regions. Both consumption and displacement speeds are negatively correlated with curvature, with the larger Markstein number arising from the greater gas expansion ratio under cryogenic conditions. In addition, both flames are thickened in a similar manner, as the large positive normal strain rate in the cryogenic flame is offset by negative kinematic restoration, resulting in a similar effective normal strain rate to that of the ambient flame. However, a narrower distribution of normalized surface density function indicates that turbulence has a weaker influence on the flame structure at cryogenic temperature. Moreover, due to reduced suppression of a key elementary reaction, the decorrelation between heat release rate (HRR) and fuel consumption rate (FCR) is more pronounced at cryogenic temperature. Furthermore, statistical analyses show that turbulence primarily affects reactions and mixing in the low-temperature zone, and its influence becomes weaker at lower initial temperature. This study provides new insights into turbulent hydrogen/air flames under cryogenic conditions and contributes to the prevention of combustion hazards during liquid hydrogen storage. Novelty and Significance Statement The novelty of this work lies in it providing the first detailed analysis of the local characteristics of turbulent hydrogen/air flames under ambient and cryogenic conditions. The study systematically quantifies how cryogenic temperature alters flame response to turbulence, identifies mechanisms governing the distinct flame morphology under cryogenic conditions, and clarifies the origin of flame thickening and its differences from the ambient flame. Additionally, it reveals how turbulence modifies chemistry and mixing in cryogenic flames, particularly in the low-temperature zone. This study fills a critical gap in the understanding of turbulent hydrogen/air flames under cryogenic conditions by revealing their local dynamics, structure, and chemistry. The findings are significant to hydrogen safety, as they provide the physical basis needed to mitigate combustion hazards associated with liquid hydrogen storage. The results are especially valuable for hydrogen combustion in cryogenic systems, where novel insights different from ambient-temperature scenarios are detailed.