The Propagation of Turbulent Flames and Detonations in Tubes

Abstract

By artificially increasing the roughness of the wall of the tube using a wire spiral or a sequence of equally spaced orifice plates, it is possible to generate very intense large scale turbulence. Under these conditions it is possible to observe five different propagation regimes of combustion waves. The self quenching regime corresponds to a flame accelerating initially to a high velocity before quenching itself when the turbulent mixing rate exceeds the chemical reaction rate of the combustible mixture. The weak turbulent deflagration regime corresponds to flame speeds of the order of few tens of meters per second. The steady state velocity of this regime is achieved by the balance of the positive and negative effects of turbulence on the burning rate (ie., enhancement of mass and energy transport versus quenching due to mixing and flame stretch). In the sonic or choking regime, the flame speed corresponds closely to the sound speed in the burnt gases. The gasdynamic choking is brought about by the combined effect of friction and heat addition in a compressible pipe flow. The quasi-detonation regime corresponds to the low velocity detonation phenomenon in which the severe momentum losses give detonation velocity significantly below the normal Chapman-Jouguet value. The existence of this regime is based on the criterion λ/d ≤ 1 where “λ” and “d” denote the detonation cell size and the orifice diameter respectively. The fifth regime of normal Chapman-Jouguet detonation occurs when d/ λ≥ 13 in accord with the result of the critical tube diameter problem. Qualitative discussions of the turbulent flame structure according to the ideas of Chomiak are given. A unified concept is advanced in that it is postulated that shear and turbulence play the essential roles not only in the propagation of deflagration, but in detonation as well. Auto-ignition by shock heating is assigned a lesser role, while the transverse turbulent shear layers generated by the triple shock configuration in the front of a cellular detonation are assumed to play the key role in the enhancement of rapid chemical reactions necessary for the propagation of a detonation wave. The principle argument being that since free radicals are in abundance in the reaction zone, it is more efficient to induce chemical reactions in the unburned gases by rapid turbulent mixing rather than to generate the free radicals by thermal dissociation in the shock. Thus the role of the shock front in detonation is to preheat the mixture leading to higher local diffusion rates and more important, to generate turbulent shear layers via triple shock collisions.