Flowability and failure

There is difficulty, however, in discussing powder flowability in terms of rheological phenomena because although the powder mass consists of discrete particles there is an absence of continuity, as found in liquids. Flowability, although ill defined, can be used to describe the ease with which powders flow out of chutes or hoppers. 

A direct way to assess the rate of flow is to time the passage of a standard amount of powder discharged from a specified funnel, opening or hopper (ISO 4490 2000 ASTM B855-06). 

At the microscopical particle level, the contribution to sliding and flowing is due to the interplay of particle surfaces and surface energies achieved by rolling particles. These modes of movement may, however, be co-incidental because a rolling particle wiU eventually be transposed, in an assembly of particles, by inter-particle sliding and therefore failure. 

For rough walls, Stephen and Bridgwater (1978) found that the failure zone area, for the internal failure of powders, was twice the number of particle diameters reported by Nedderman and Laohakul. These failure zones can be related to the voidage fluctuations seen in randomly packed beds of spheres adjacent to container walls (Ridgway Tarbuck 1968). It is generally found that the internal angle of friction is greater than the wall friction. 

Contact surface roughness is therefore a significant factor in the frictional behaviour of solid surfaces and thus applicable to any moving particulate system. Particle movement can be conceptualised either as a model of smooth contact between particles and walls (Hertzian Hertz 1882) or as a model which has rough contact area between the particulate system and wall (Archardian Archard 1953, 1957). Both models have been used to predict the physical behaviour between particles and particles or between particles and a wall. 

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