Novel flame dynamics in rich mixture of premixed propane-air in a planar microcombustor

This paper reports the first experimental observations of various novel unsteady flame propagation modes in a two-dimensional, high-aspect ratio rectangular quartz channel with a positive wall temperature gradient for rich premixed propane-air mixtures. Various flame propagation modes are observed on progressively increasing the mixture velocity, while keeping the equivalence ratio fixed: FREI (Flame with Repetitive Extinction and Ignition) mode, oscillating FREI mode, oscillating flame mode, and wavy flame mode. The FREI mode resembles the classical FREI flame propagation reported earlier in the literature. In the oscillating FREI mode, the flame front oscillates in the transverse direction between the upper and lower walls of the channel, while propagating upstream as in the classical FREI mode. A sudden peak in flame intensity is observed in this mode before its extinction at an upstream location. In the oscillating flame propagation mode, the flame front anchors itself at an axial location and exhibits periodic oscillations in the transverse direction without extinction. In the wavy flame mode the flame anchoring happens at a location close to the downstream end of the channel. The flame front exhibits visibly irregular fluctuations, while anchored at this axial location. A Fast Fourier transform analysis of the flame intensity data shows that FREI and oscillating FREI modes consist of a single dominant frequency of similar to 100 Hz, whereas multiple dominant frequencies are present for oscillating and wavy flame modes. The appearance of these multiple oscillating and propagating flame modes is attributed to flame bifurcation behavior due to thermal-wall coupling in the channel.