Extinction characteristics of CH4/O2/Xe radiative counterflow planar premixed flames and their transition to ball-like flames

Extinction characteristics of CH4/O2/Xe radiative counterflow premixed flames and their transition to ball-like flames were examined by computations and microgravity experiments. First, the flammability limit of flame ball for the mixture was estimated to be leaner than that of counterflow premixed flame by one-dimensional computations with detailed chemistry. Extinction experiments under microgravity showed that there was a ball-like flame prior to total extinction in the vicinity of the stagnation-plane in the low-speed counterflow field at stretch rate of 1.6–3.2 s−1. Formation of such a ball-like flame occurred subsequent to the extinction of counterflow flames and the ball-like flame was finally extinguished when the equivalence ratio was further decreased. Two-dimensional computations indicated that the temperature of the ball-like flame increased with the decrease of equivalence ratio in the near-limit condition when it approached extinction. The temperature distribution of the computational ball-like flame was in qualitative agreement with that of the flame ball. The ball-like flame in the counterflow field was considered to be stable based on steady-state two-dimensional computation at an equivalence ratio slightly richer than the limit of a transient ball-like flame. Such a stable computational ball-like flame is not perfectly spherical. The ball-like flame observed in this study is thought to have a close correlation with the ideal flame ball which is generally established in a quiescent mixture.