Mixing experiments in 3D-printed silos; the role of wall friction and flow correcting inserts

The mixing of particles during silo discharge is of high industrial significance, yet quantitative studies of this process in literature are few. Here we present experimental measurement of particle mixing in 3D-printed silos during complete discharge, accounting for hopper half angle, the wall condition, and the addition of flow correcting inserts. Mono-sized mustard seed are dyed one of two colours (yellow or blue) to enable them to be distinguished during a step-change experiment. The silo is partially filled with yellow particles, followed by an equal mass of blue, then discharged onto a rotating table apparatus which automatically collects particle samples for proportion analysis. The experiments are repeated for six hopper half angles, and for three different wall conditions of different smoothness. Finally, three flow correcting inserts are added to a silo to quantify their influence. It is found that, while the wall condition has a large influence on the mixing and the size of the stagnant "pocket" region (the non-flowing region in a funnel-flow silo), the addition of flow correcting inserts has the greatest effect in reducing mixing and pocket region size. Our method of completely discharging the silo has the advantage that the pocket region mass can be easily quantified, in contrast to classical continuous flow residence-time experiments. (C) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.