The structure of fast turbulent flames in very rough, obstacle-filled channels

Flames propagating in very rough confined tubes or channels are known to acheve very high speeds of propagation that can approach the speed of sound of the burnt products. The mechanisms underlying the propagation of such flames are not understood. The present paper reports the results of a detailed photographic study of such high speed flames. The experiments are carried out in rectangular channels equipped with arrays of periodically spaced obstacles to generate controlled roughness. Observations are via high speed schlieren photography. The mechanisms responsible for the high speed propagation are identified as those which cause intense turbulization of the flame. These include shock-flame interaction, Rayleigh-Taylor instabilities in an accelerating flow and auto-ignition in large recirculating eddies in the wake of obstacles.