Flames propagating in obstacle-filled channels are known to achieve very high speeds of propagation that can approach the speed of sound of the burnt products. The present paper reports the results of a detailed schlieren photographic study of such high speed flames. The experiments were carried out in two rectangular channels of different cross-section equipped with arrays of periodically spaced obstacles. The mechanisms responsible for the high speed propagation are identified as those which cause intense turbulization of the flame. These include shock-flame interaction, Raleigh-Taylor instabilities in an accelerating flow and autoignition in large recirculating eddies in the wake of obstacles. The transition from deflagration to detonation (DDT) in obstacle-filled channel as well as the structure and propagation mechanisms of quasi-detonations have also been studied. The results clearly identify the propagation mechanism of quasi-detonation to be one of autoignition by shock reflections. The obstacles play a dual role: a positive role of reinitiation by providing a surface for shock reflection and a negative role of attenuation by diffraction. On a global basis, obstacles have a negative effect on the overall average propagation velocity of the detonation.
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