Broken C-shaped extinction curve and near-limit flame behaviors of low Lewis number counterflow flames under microgravity

To examine the effect of Lewis number on the extinction boundary, flame regimes, and the formation of sporadic flames, microgravity experiments on counterflow flames for CH4/O2/Kr (Le ≈ 0.7-0.8) and CH4/O2/Xe (Le ≈ 0.5) mixtures, and three types of computations, which are one-dimensional computations with a PREMIX-based code using detailed chemistry, and three- and one-dimensional computations with the thermal-diffusion model using an overall one-step reaction were conducted. In the microgravity experiments, planar flames, planar flames with propagating edges, planar flames with receding edges, star-shaped flames, cellular flames, and sporadic flames were identified, and their regions of existence in the equivalence ratio-stretch rate plane were obtained. Sporadic flames were formed for Xe mixtures but not for Kr mixtures in the experiments. Similarly, sporadic flames were formed at Le = 0.50 but not at Le = 0.75 in the three-dimensional computations with the thermal-diffusion model. Also, the flame regime of sporadic flames extended far beyond the extinction boundaries obtained in the one-dimensional computations in both experiments and the three-dimensional computations. Furthermore, a comparison of the sporadic flames and flame balls in the three-dimensional computations showed that sporadic flames are intermediate combustion modes that segue flame balls to propagating flames.