Deflagration-to-detonation transition in end gases

Abstract

Flame acceleration and deflagration-to-detonation transition (DDT) experiments were performed in a 28-cm-diameter, 6,4-m-long combustion duct filled with obstacles. The run-up distances (the distance between the ignition point and the location of onset of detonation) for DDT and the flame speeds just before onset of detonation were measured for detonable H 2 -airsteam mixtures. For less-sensitive mixtures, flame acceleration also occurred and choked velocities were reached within the length of the duct, but DDT events were not observed. Because of reflection of the precursor shock off the end plate, the gas mixture in the end gas region was further pressurized. Transition to detonation occurred in the end gas region for some of these less-sensitive mixtures. As a result of higher initial pressures, DDT in the end gas region produced very high overpressures at the end plate. Because of the high pressure and temperature created by the reflection of the precursor shock, the end gas region provides the most favorable conditions for DDT. If DDT did not occur in the end gas region, the resulting overpressures were found to be much lower. The pressure traces measured at the end gas region clearly indicated the limiting mixture composition for transition from deflagration to detonation. The limiting mixture composition, therefore, can be used to represent the transition limits for choked flames. However, being limited by the size of the end gas region, the present study does not reveal the effects of scale, particularly related to turbulence in the transition process. Thus, it is not certain whether the limiting mixture composition will be the same in situations in which turbulence also plays a key role in the transition process.