Title: Discharge and clogging of grains from a hopper, and the influence of interstitial fluid

Author (Invited): Douglas Durian, University of Pennsylvania

Abstract:

The flow of grains from a hopper can be suddenly arrested by formation of a stable arch or dome over the outlet, which is trouble for applications. This problem is related to jamming, but is distinct due to presence of boundaries and gradients. We argue how the fraction F of pre-clogged configurations may be deduced from the average mass discharged between clogs. And we construct a simple model to account for the observation that F decays exponentially in hole width to the power of dimensionality. Contrary to much recent work, we thus conclude that the clogging transition is not sharp but rather is defined by observation limits similar to the glass transition. To alter the dynamics of clog formation, we are now studying behavior for submerged hoppers and have found an unexpected surprise: There is a terminal surge in which the discharge rate increases just before the hopper empties. This is caused by a pressure head that pumps water through the packing, faster than the grains, at a rate proportional to the reciprocal of the packing height. The same effect happens in air but is smaller and previously unnoticed.

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