Effect of sandpaper-like small wall roughness on deflagration-to-detonation transition in a hydrogen-oxygen mixture

Deflagration-to-detonation transition (DDT) in a stoichiometric hydrogen-oxygen mixture was experimentally investigated using a channel with a sandpaper-like rough wall as a small blockage. The combustion channel was 486 mm long, 12 mm wide, and 10 mm high in the inner cross-section, and the top and bottom walls were covered with a sand cloth with surface roughness of 1000 mu m by Rz and 100 mu m by Ra (rough wall condition). The channel wall without the sand cloth (polished wall condition) was also tested for comparison. The entire process from the flame propagation following spark ignition to the detonation transition was visualized through optical windows on the side walls by high-speed schlieren photography. Although only the slow subsonic flame was observed in the polished wall condition, the wall roughness greatly enhanced flame acceleration and the detonation transition occurred at 120 mm downstream from the ignition. In the rough wall condition, the reaction front along the channel wall, which might propagate in the unreacted gas in the many cavities between the flame edge and the rough wall, was observed. This reaction front finally developed to form the high-speed tulip flame. The prominent reaction front near the channel wall and the accumulation of compression waves (the precompression zone), which increased the pressure up to 10-15 times the initial pressure, were observed immediately ahead the tulip flame, and this triggered the detonation onset. The estimated temperature in the precompression zone of 600-700 K was not high enough to induce instantaneous self-ignition. The present observation might indicate the experimental evidence for the possible mechanism of the final detonation onset, which was local spontaneous flame acceleration coupled with the compression wave immediately ahead of the flame front; which was suggested for highly reactive mixtures (Liberman et al. 2010). (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.