Comparisons of global quench limitations between downwardly-propagating and centrally-ignited premixed ammonia/air flames by intense near-isotropic turbulence

Global quench (GQ), a complete extinction, of downwardly-propagating and centrally-ignited premixed ammonia/air flames by intensive near-isotropic turbulence generated in a central region of fan-stirred cruciform burner is explored at wide ranges of the equivalence ratio (ϕ = 0.7–1.25), the r.m.s. turbulence fluctuation velocity (u′ = 0–8.4 m/s), and the ignition energy (E_ig = 50–500 mJ) using pin-to-pin stainless-steel electrodes at 2-mm spark gap. Both central ignition flame kernels and downward propagation flames from the top vessel of the cruciform burner interacting with the central uniform intense turbulence are recorded to determine the required critical u′_c and/or Ka_c for GQ, where Ka is a turbulent Karlovitz number. We find that the low-reactivity ammonia/air laminar flames cannot propagate downwardly from the top vessel at too lean (ϕ = 0.7) and too rich (ϕ = 1.25) conditions. Results show that the large downward propagation flames are much harder to be globally quenched by turbulence than the small central ignition kernels for spherical flames. The downward flame GQ criterion is independent of E_ig, while the central ignition GQ criterion is influenced by E_ig (the higher E_ig, the larger u′_c). Contrary to the common notion that the maximum critical u′_c for flame GQ occurs near ϕ ≈ 1, we discover that the richer the downward propagation ammonia/air flame is, the larger u′_c is. Such a surprising survival capability of rich downward propagation ammonia/air flames to GQ by intense turbulence is due to the nearly linear increase of H₂ mole fraction percentage in the upstream products of the top tube of the cruciform burner from less than 1 % at ϕ = 0.9 to about 6 % at ϕ = 1.2. These turbulent downward propagation flame fronts with filiform structures prior to GQ reveal the morphology of distributed-like turbulent flames.