Effect of diffusive thermal instability on intrinsic limits of counterflow flame, unstrained planar flame and flame ball

To clarify the effect of Lewis number (Le) on near lean-limit flame behaviors, counterflow flame experiments under microgravity and one-dimensional computations with a detailed reaction mechanism for unstrained planar flame, counterflow flame and flame ball were conducted using CH4/O2/Kr (Le = 0.7-0.8), CH4/O2/Xe (Le = 0.5) and CH4/H2/O2/Xe (Le = 0.3) mixtures. In microgravity experiments, rich varieties of flames were observed. It was also confirmed that the locations of flame regime on stretch rate-equivalence ratio plane were changed with the value of Le. In the computation, a new parameter, J, representing fuel flux, was introduced to support discussing comprehensive combustion limit theory which can cover intrinsic limits of unstrained planar flame, counterflow flame and flame ball. The J parameter for unstrained planar flames and flame ball were presented. Flame ball exhibited smaller value of J at an intrinsic lean limit than unstrained planar flame. Intrinsic limits of flame ball, unstrained planar flame and counterflow flame were summarized and it was confirmed that flame ball has the widest lean limits and the difference between the limits of flame ball and those of counterflow flame became larger with the decrease of Lewis number.