| Combustible dust layers are frequently present in the workplace, posing a great potential hazard to personnel and property. The roles which dust layers play in severe dust explosions were investigated in a 70 m long and 30 cm i.d. horizontal flame acceleration tube (FAT) with one end closed and the other end open to the atmosphere. A variety of dusts such as corn dust, cornstarch, Mira Gel starch, wheat dust and wood flour were layered on the bottom half of the FAT. To initiate the combustion process, a detonation tube filled with a stoichiometric H{dollar}sb2{dollar}/O{dollar}sb2{dollar} mixture at room temperature and 1 atm pressure was used to ignite a pre-suspended dust cloud with a dust concentration of 500-600 g/m{dollar}sp3.{dollar} Combustion waves generated by the dust cloud travel toward the open end of the FAT and are continuously fueled by the dust/air mixtures. Flame propagation processes in the FAT were closely monitored by a variety of measuring instruments at different locations.; The study demonstrates that, under certain favorable conditions, deflagration to detonation transition (DDT) can be achieved in pre-layered dust/air mixtures. Among the favorable conditions, the moisture content of the dust, the exposed area of the dust layers to the convective flow, and the physical appearance of the dust are the most influential factors, which indicates that the pre-layered dust combustion is dominated by the dust/air mixing process. Unlike pre-suspended dust/oxidizer detonations, it is found that DDT requires a much longer distance in pre-layered dust/air mixtures due to the slow fuel/oxidizer mixing process. Stable quasi-detonation were reached in some runs, but self-sustained Chapman-Jouguet detonations were not observed possibly due to the limitation of the tube length. Attempts were made to determine the structure of dust detonations fueled by a dust layer. Preliminary evidence indicates that for Mira Gel starch the leading shock is essentially a triple shock configuration which involves a Mach stem and for wheat and wood dusts there possibly exists a multi-headed spin structure. |
