The effect of low swirl configurations on stability limits and emission characteristics in premixed ammonia-methane-air swirling flames

Ammonia has recently gained significant attention as a zero-carbon fuel and a carrier of hydrogen. However, its flame instability and emissions remain major challenges. In this study, the effect of low swirl configuration on stability limits and emissions of premixed ammonia-methane-air swirling flames at various equivalence ratios and ammonia blending ratios were investigated. The low swirl configuration was achieved by using three center through-hole swirlers with different mass fluxes ratio (R_m) for non-swirling and swirling flows. The results showed that the flames exhibited thermoacoustic instability at R_m = 0 % and detected higher emissions for all conditions where the ammonia mixing ratios are less than 60 %. By contrast, the swirlers with R_m = 25 % and R_m = 49 % ensured flames stabilization under all operating conditions. The swirler with R_m = 25 % effectively reduced NO_x emissions about 30 % while slightly narrowing the stability limits. CO and unburned NH₃ emissions also decreased significantly, but N₂O emissions increased. This was due to the non-swirling flow reduced the flame temperature, which inhibited the thermal cracking of N₂O. As R_m = 49 %, the stability limits became very narrow, although the emissions decreased further. In conjunction, it is found that increasing R_m caused the flame structure to shift from the V-shape to the W-shape, with the flame obviously lifting and the recirculation zone in the flow field shrinking. These changes are responsible for the transformation of the stability limits and emission with different low swirl configurations, which provide a feasible idea for use of ammonia as a gas turbine fuel to improve combustion efficiency and reduce emissions.