On the role and the origin of the gas pressure gradient in the discharge of fine solids from hoppers

The Brown and Richards (Principles of Powder Mechanics, Pergamon Press, Oxford, UK, 1970) correlation for the discharge rate of fine powders from a hopper was modified to account for the gas pressure gradient near the outlet. According to Donsi et al. (Chem. Eng. Sci. 52 (1997) 4291) there is a transition between a granular flow region and a suspended flow region near the hopper outlet. Brown and Richards (1970) stated that the particle discharge rate depends on the flow conditions just above this transition surface. In the modified equation that is developed to account for the gas pressure, a term including the gas pressure gradient at this surface appears. The gas pressure gradient is evaluated from the literature experimental results by considering the Donsi et al. (1997) finding that a significant part of the gas pressure gradient near the hopper outlet is due to the suspended motion. Furthermore, a simplified analysis is carried out to evaluate from the experimental results the voidage variation within the solids phase that is responsible for the onset of the gas pressure gradient. (C) 2003 Elsevier Ltd. All rights reserved.