Flooding (‘‘flushing’’) of powder through a small orifice by an external load

Loosely packed cohesive powders, through which air is difficult to permeate, can be caused to flow through small orifices if the interstitial air pressure is increased by an external load. We call this phenomenon ‘‘flushing.’’ Noncohesive free‐flowing granular materials do not give this phenomenon. The experimental apparatus consisted of a cylindrical vessel having a small side orifice near the bottom, and a piston that presses the powder of a given initial voidage in the vessel with a constant load. Threshold conditions are required to give rise to the flushing, and these are defined by the initial voidage and the piston pressure. The efflux rate from the orifice depends on the initial voidage, density, mean size of the particles, external load, and orifice diameter, but is independent of the powder head.