The mechanism of detonation attenuation by a porous medium and its subsequent re-initiation

The attenuation and re-initiation mechanism of detonations transmitted through a porous section consisting of a two-dimensional array of staggered cylinders was investigated experimentally and numerically for acetylene oxygen mixtures. It was found that the leading order attenuation mechanism is the wave diffraction around the cylinders. The local re-amplification permitting the self-propagation of the wave was due to wave reflections from adjacent obstacles. The critical conditions for transmittance of a detonation wave were found to correspond approximately to a pore size equal to approximately 30-60 detonation induction lengths, or one to two cell sizes. For quenched detonations, the re-initiation mechanism was found to rely on wave reflections from neighbouring pores. Depending on the mixture sensitivity, one or several shock reflections may be necessary to re-amplify the attenuated detonation wave back to a self-sustained wave. For the latter case, a novel mechanism was identified, where each shock reflection gives rise to a significant enhancement of the gas reactivity and burnout of large portions of unreacted gas. This leads to a slow acceleration of the leading front, punctuated by small-scale local sudden re-accelerations. The resulting wave interactions give rise to a topologically complex reaction zone structure consisting of alternating layers of reacted and unreacted gas. The role of turbulent diffusive burning during this transient is discussed.