Flow effects on laser-induced ignition of ammonia-hydrogen mixtures: minimum ignition energy and third-lobed flame kernel dynamics

Laser-induced ignition experiments were conducted on ammonia-hydrogen mixtures under different flow velocities using an advective flow ignition platform. The effects of flow velocity and Lewis number ( Le ) on ignition probability, minimum ignition energy (MIE), flame kernel morphology, and flame development were investigated. The results show that, for mixtures with Le < 1, the MIE exhibits a non-monotonic variation with increasing flow velocity, decreasing first and then increasing as the flow evolves from the low Reynolds number to the high Reynolds number regime. This indicates that flow facilitated ignition (FFI) behavior can occur in laser-induced ignition even for Le < 1 mixtures. Mechanism analysis shows that, in the low Reynolds number regime, ignition is mainly governed by the positive and negative curvature regions induced by the characteristic third-lobed flame kernel generated during laser ignition. As the flow velocity increases, flame surface wrinkling, vortical deformation, and flame kernel fragmentation become increasingly important, while flow enhanced heat loss becomes dominant in the high Reynolds number regime and leads to a rapid increase in MIE. The early and later stage flame development results further show that moderate flow promotes flame kernel growth by increasing the projected flame area and supporting third lobe development, whereas excessive flow destabilizes the third-lobed flame kernel and weakens its ignition promoting effect. In addition, asymmetric third lobe development reduces favorable curvature effects for Le < 1 mixtures and may split the main flame kernel for Le > 1 mixtures, increasing the risk of quenching before a self-sustaining flame is established. These findings clarify the competing promoting and inhibiting effects of flow on laser-induced ignition of ammonia-hydrogen mixtures and provide guidance for optimizing laser ignition under practical flowing conditions.