Experimental and computational study of the flame ball regime at normal gravity

Ultra lean flames propagating in a tube of 12.2 mm internal diameter in mixtures of (40 mol% H2 + 60 mol% CH4) fuel gas with air have been studied experimentally, using OH Planar Lased Induced Fluorescence (PLIF), and by numerical simulations, for equivalence ratios between 0.30 and 0.50. Hypothetical OH LIF signals were reconstructed from the calculated data and compared with measurements. The simulated OH LIF intensity distributions are found to be similar to the measured ones in the experimental flames for equivalence ratios 0.31 f 0.44. Within this range, when the equivalence ratio was decreased down to the lean limit value, the shapes of the simulated and experimental flame fronts changed from relatively large egg-shaped to small, about 3 mm diameter, nearly spherical flame balls. Simulations showed the presence of a recirculating flow in near-limit flames. The front of the lean limit flame balls resides entirely within the volume of this recirculating flow. The flame front was nearly uniform in lean limit flame balls and became increasingly non-uniform with the departure from the limit, with an eventually vanishing bottom front. The experimental and simulated flammability limits lie far below the theoretical limit for a planar flame, and simulated flame temperatures exceed adiabatic temperatures, indicating strong preferential diffusion (Lewis number) effects in these flames. These effects are qualitatively analyzed based on the behavior of measured and simulated flame parameters. It is suggested that preferential diffusion in non-uniform flames, well-removed from the lean flammability limit, is primarily controlled by flame stretch, while in near-uniform limit flame balls it is dominated by a curvature effect similar to that in microgravity flame balls.