Study on the combustion limit, near-limit extinction boundary, and flame regimes of low-Lewis-number CH₄/O₂/CO₂ counterflow flames under microgravity

To obtain an accurate and comprehensive understanding on the fuel-lean combustion limit, near-limit extinction boundary and flame regimes, experimental and numerical investigations using premixed counterflow flames were conducted for low-Lewis-number mixtures. Counterflow flame experiments were conducted under microgravity (2.2 ≦ a ≦ 12.9 s⁻¹) and under normal gravity (35 ≦ a ≦ 150 s⁻¹) using fuel-lean CH₄/O₂/CO₂ mixtures (Le = 0.75) where a is the stretch rate. The mole fraction ratio of O₂ to CO₂ in the mixtures was 0.40. Microgravity experiments at a = 2.7–3.2 s⁻¹ showed that transitions from planar flames to ball-like flames occurred near extinction for the investigated mixture. In addition to planar flames and ball-like flames, multi-dimensional flames such as cellular flames and twin-curved flames were also observed at a = 2.2–5.5 s⁻¹ in microgravity experiments. In conjunction with experimental results under normal gravity, experimental flame regimes for overall stretch rates were obtained for the first time and the region where ball-like flames were observed showed a qualitative agreement with our previous study based on the 3-D transient computations with the diffusive-thermal model. Extinction points obtained by microgravity experiments were found to scatter at very low stretch rates where multi-dimensional flames such as ball-like flames and cellular flames were observed, indicating the existence of a flame regime at lower stretch rates leaner than the planar counterflow flame extinction boundary. In addition, flame bifurcation at low stretch rates and at ϕ = 0.58 and 0.60 were experimentally observed for the present low-Lewis-number mixture, indicating the validity of the previous computational and theoretical studies on the G-shaped extinction curve of planar counterflow flames.