Pulsating instability in H₂-air partially premixed flames

Steady/unsteady hydrogen-air partially premixed flames (PPFs) in a counterflow configuration were computationally studied with detailed chemistry and transport, and the pulsating instability of PPFs was observed. During PPFs oscillation, the pulsating instability for the premixed flame is caused by a large overall activation energy (Ea) and a slow O₂ diffusion process (Le >1). The nonpremixed flame undergoes forced oscillation caused by oscillated H₂ diffusion flux from the premixed flame zone. In H₂-air PPFs, premixed/nonpremixed flame structures and the interactions between them are controlled by the equivalence ratio (φ) and global strain rate (a). These factors also control the stability boundary. Large φ and/or a values retard pulsating instability, because increases in φ and/or a cause the two flames to move closer to each other, thereby intensifying the conduction heat flux from nonpremixed flame to premixed flame. The temperature in strengthened premixed flame is insensitive to the perturbation of chemical heat release, on the other hand, the competition between H + O₂ → OH + O (R1) and H + O₂ + M → HO₂ + M (R9) decreases, which leads to smaller Ea. Thus, the chemical reactions are also insensitive to the perturbation of flame temperature.