Surface morphology and self-acceleration of expanding spherical flames

This paper reports results on the cellular and pulsating instabilities developed over the surface of expanding spherical premixed flames. Regarding the cellular instabilities, experimental data from time-resolved images of hydrogen-air flames exhibiting hydrodynamic and diffusional-thermal cells were used to determine the flame acceleration exponent α, defined through R(t)=R₀+At^α. where A and R₀ are constants. Results show that α seems to be bounded by the maximum value of 1.33 ± 0.03, which is approached regardless of the nature of the cells, and therefore suggesting the existence of a self-similar mode of propagation. Regarding pulsating instability, spiral waves and target patterns were experimentally observed over the flame surface in rich hydrogen-air, rich hydrogen-oxygen, and lean butane-oxygen-helium mixtures at elevated pressures. The transition boundary to instability was found to agree well with the Sivashinsky criterion, Ze(Le-1)>4(1+3^1/2) ≈ 10.9, provided local values are used in the evaluation of the Zel'dovich number, Ze, and the Lewis number, Le.