Low-frequency oscillations in ammonia/hydrogen/nitrogen and methane flames approaching lean blow off

A self-sustained low-frequency oscillation in premixed bluff-body stabilised flames approaching lean blow-off (LBO) was investigated experimentally. Simultaneous high-speed PIV and OH${}^{\ast }$-chemiluminescence images were obtained to provide time-averaged and time-resolved flame dynamics and flow fields. The LBO limits were determined for both methane and decomposed ammonia fuel blends with different ammonia volume fractions using long or short enclosures. Flames in the long enclosure were more stable due to the longer flow residence time. For NH₃/H₂/N₂ fuel blends, higher percentages of H₂ result in wider LBO limits due to the higher extinction strain rate. It was found that close to LBO both CH₄ and NH₃/H₂/N₂ flames in the long enclosure resulted in periodic oscillations, characterised by large-scale reignition and extinction. The oscillation frequency, between 0.5 to 10 Hz, was shown to vary linearly with bulk inlet velocity (10-110 m/s) with a constant Strouhal number. This periodic reignition phenomena was attributed to the accumulation of unburnt fuel in the outer recirculation zones, which is periodically recharged by unburnt fuel issuing from the injector as lean blow-off is approached, and may also extend the LBO limits in the long confinement.

Novelty and Significance Statement

Lean blow-off is an important phenomena in many combustion applications, and understanding this with both hydrocarbon and carbon-free fuels is important. The lean blow-off behaviours of methane and ammonia/hydrogen/nitrogen flames in two different length confinements were presented and compared. A low-frequency oscillation was observed in the longer confinement near lean blow-off under various equivalence ratios and bulk velocities. While some previous studies have briefly reported similar observations, they lack detail on frequency scaling or the underlying physical process. A linear relationship between the oscillation frequency and bulk velocity was observed for the first time. The flame structures and flow fields during one oscillation cycle were investigated, demonstrating that the oscillation was caused by periodic reignition and extinction of the lifted flames. Other important time scales, such as the reaction time, and chamber fill time were also characterised for the first time to improve our understanding of the phenomena.