Secondary dust explosions

Industrial dust explosions typically occur in a sequence-a primary explosion in a predispersed dust cloud followed by one or more secondary explosions. The convective flow induced by the primary explosion disperses the surrounding layered dust, resulting in a dust cloud into which the flame continues to propagate. Continual repetition of this process results in a secondary explosion- a rapidly accelerating flame accompanied by substantial overpressures, that causes most of the damage. Such secondary explosions have been studied in the laboratory in a flame acceleration tube (FAT), a 36.6-m-long tube with an internal diameter of 0.3 m. Tests have been conducted with flow-enhanced cornstarch, small size cornstarch, baghouse dust, floor sweepings, wheat dust, and navy bean dust. The development of the secondary explosions in these dusts is compared and discussed in terms of the observed pressures, fame velocities, gas velocities, gas temperatures, and dispersed dust concentrations. The presence of turbulence-generating grids in the flame path is found to result in flame acceleration, quite analogous to gaseous flames. This observation confirms the findings of numerical models. Experimental results from a study of dust entrainment using high-speed motion photography are presented.