Numerical study on stretch extinction mechanism of NH3/H2/air laminar counterflow premixed flames

Ammonia and hydrogen are two most promising carbon-free fuels emerging in recent years, and their co-combustion is well recognized as an efficient approach to solve the issues associated with ammonia's poor combustion behaviors. This study emphasizes fundamentally the combustion properties, particularly the stretch-induced extinction limit as well as the underlying physical mechanism of the NH₃/H₂/air laminar counterflow premixed flames by carrying out simulations with detailed fuel chemistry and transport models. The results demonstrate that hydrogen addition significantly extends the ammonia flame extinction strain rate, with the equivalence ratio corresponding to the maximum extinction strain rate shifting toward leaner stoichiometry as hydrogen addition increases. The combination of thermal, chemical, and transport effects of hydrogen enhances the NH₃/H₂ premixed flame stability. More specifically, the contribution of thermal effect to extinction prevails under the fuel-rich condition, decreasing with the decrement of equivalence ratio. The effective Lewis number of the premixture is responsible for the distinct thermal effect response behaviors in fuel-lean condition compared with the stoichiometric and rich conditions. By comparatively analyzing chemical kinetics and flame structure between the strongly-stable and near-extinction flames, it elucidates the governing chemical pathways and critical radical interactions responsible for the NH₃/H₂ stretched premixed flame extinction.