| Two experiments were performed to investigate the interaction of a combustion wave with porous media. The first experiment was performed in a 1.22m long, 76mm wide, and 152mm high horizontal channel with a nitrogen-diluted stoichiometric methane-oxygen mixture at initial pressures of 20-50kPa. A layer of 12.7mm diameter ceramic-oxide spheres was placed along the bottom to partially obstruct the channel, leaving a gap of free space above. For a fixed gap height the bead layer thickness had very little effect on explosion propagation. For a fixed bead layer thickness the explosion propagation was strongly influenced by the gap height. For example, a 31% nitrogen diluted mixture at room temperature resulted in DDT for a gap height of 38mm at initial pressures of 30-50 kPa, but not for 109mm over the same pressures. The gap above the bead layer permits DDT as long as the gap height is larger than one detonation cell width. Propagation of the detonation wave over the bead layer is possible if the gap height can accommodate at least two detonation cells. For a 38mm gap, velocity measurements and sooted foil imprints indicate that the detonation undergoes successive failure and re-initiation, referred to as "galloping" in the literature.;In the second experiment, the head-on collision of a combustion front with a layer of 3 and 12.7mm diameter ceramic-oxide spheres was investigated in a 61cm long, 76.2mm diameter vertical tube for a nitrogen-diluted stoichiometric ethylene-oxygen mixture at initial pressures of 10-100kPa. Four orifice plates were placed at the ignition end to accelerate the premixed flame to a "fast-flame" or a detonation wave. For fast-flames pressures recorded at the bead layer face were up to five times the reflected CJ detonation pressure. This explosion developed by two distinct mechanisms: (a) shock reflection off the bead layer face and (b) shock transmission into the bead layer and subsequent explosion therein. The measured explosion delay time (time after shock reflection from the bead layer face) was found to be independent of the incident shock velocity. Thus, it was shown that explosion initiation is not the direct result of shock reflection but is more likely due to the interaction of the reflected shock with the trailing flame. The bead layer was found to be very effective in attenuating the explosion and isolating the tube endplate. |
