Direct numerical simulation of circular expanding premixed flames in a lean quiescent hydrogen-air mixture: Phenomenology and detailed flame front analysis

The transition to cellularity and the dynamics of lean premixed hydrogen/air flames propagating outwards in 2D circular domains under the combined influence of the hydrodynamic and thermodiffusive instabilities is investigated computationally using detailed chemistry and transport. In response to monochromatic (single wavelength) and polychromatic perturbations imposed initially on the flame, the non-monotonic rate of increase of the surface area reflects the transitions of the perturbed front dynamics. The relation of the wavelength of the cellular structures with the growth rate of their amplitude is investigated separately during the initial interval of accelerated growth marking the onset of cellularity as well as during later times when the wrinkled front undergoes a continuous process of cell creation and annihilation. As the flame expands, the local minima of the initial perturbation determine the primary troughs which have a dominant effect on the long term evolution since they constrain the waveangle over which secondary cells can form and interact and define the periodicity of the problem. During the time interval of propagation considered in this study, it is found that the temporal evolution of the mean flame radius does not follow a power law, but varies almost linearly in time.