Permeability of granular mixtures under shear

Lipiejko, Natalia and Jones, Thomas (2024) Permeability of granular mixtures under shear. Powder Technology, 444: 120064. ISSN 0032-5910

Abstract

Fluidised granular flows are gas–particle mixtures that occur in a wide range of industrial applications and geophysical flows. One factor governing the fluidisation behaviour of a granular flow is its permeability — the ability of the gas to move through the particle column. Although extensive research has been carried out on the gas–particle permeability under static conditions, most of the industrial and geophysical processes are dynamic phenomena, where parameters such as the shear rate change with both time and space. Here, the effects of shear rate on the permeability of gas–particle mixtures were studied through novel experiments where a granular column comprising particles of diameter 63–250 Um was sheared at shear rates between 0 and 213 s−1 while an increasing flux of compressed air was introduced into the base of the column. Regardless of the shear rate, the granular column starts to expand upon increasing the air velocity, however, columns sheared at higher rates require greater air velocities to exhibit bubbling (and column expansion). We also instrumented the column with a pressure sensor. For the static column, as the air velocity increases, the pressure gradient across the granular column increases linearly until it reaches a maximum value, slightly decreases, and then plateaus. Conversely, for high shear rates, the pressure gradient continuously increases with increased air velocity, never reaching a maximum value or a plateau. The pressure gradient data were analysed alongside the videography in terms of the minimum fluidisation velocity and the minimum bubbling velocity, and both were found to be greater for higher shear rates. Consequently, our results show that increased shear increases the permeability of granular columns and the experimental data further suggests that in order to accurately describe the evolution of the pressure gradient across a sheared granular column, processes in the vertical as well as radial plane should be taken into account.