Effects of Intrinsic Instabilities on the Response of Premixed Hydrogen/Air Conical Flames to Inlet Flow Perturbations

As a zero-carbon fuel, hydrogen is considered a promising alternative fuel. Hydrogen flames can be greatly affected by intrinsic instabilities including the diffusional-thermal instability (DTI) and Darrieus-Landau instability (DLI). Therefore, it is important to understand their properties, especially for cryogenic flames that are related to the safe utilization of liquid hydrogen. In this work, we conduct two-dimensional simulations of unsteady hydrogen/air conical flames to assess the effects of intrinsic instabilities, DTI and DLI, on the response of premixed hydrogen/air conical flames to inlet flow perturbations. The equivalence ratio and initial temperature are changed to respectively achieve different Lewis numbers (related to DTI) and expansion ratios (related to DLI). It is found that under certain conditions flame pinch-off occurs, during which a separated flame pocket is formed by the strong amplification of flame wrinkles generated by the inlet flow perturbations. The underlying mechanism of flame pinch-off enhancement due to DTI and DLI is different. For fuel-lean hydrogen/air at normal temperature, the flame front wrinkling is enhanced by strong DTI and it is the stretch-chemistry interaction that leads to flame pinch-off. However, for stoichiometric hydrogen/air at cryogenic temperature, there is a strong effect of DLI and flame pinch-off is mainly induced by flame-flow interaction. Moreover, downstream flow and flame speed near the separated flame pocket for flames exhibiting strong DTI and DLI are compared and the difference is analyzed. The findings indicate that intrinsic flame instability can amplify flame wrinkling and fluctuations in heat release rate, thereby contributing to flame pinch-off.