Abstract
Particles differing in size, under external mechanical activation, typically segregate rather than mix. This phenomenon, known as particle size segregation, is spontaneous and unavoidable in practice. In this work, we present an experimental investigation aiming at better understanding the segregation mechanisms occurring in industrial devices related to the storage of particulate materials. Even if storage in closed vessels is often considered a static operation, loading and unloading steps are always present and they must be considered as inherent part of the whole storage operation. These steps are clearly dynamic in nature. This work concentrates on the discharge step in which the particulate material is strongly sheared, especially close to the outlet of the storage bin giving high chances of segregation to the material. Segregation in dense sheared flows under gravity occurs mainly through the percolation of smaller particles in the voids existing between larger ones. The level of saturation of voids by small particles can strongly impact the segregation rate. It appears therefore that the relative amount of fines with respect to large particles, the level of bed dilatancy induced by shear, the interparticle friction are all variables that can affect fine particles mobility and therefore the extent of segregation. For these reasons, we carried out experiments on small-scale vessels discharging in a funnel flow regime. Binary mixtures at different fine compositions and particle size ratios have been considered. Furthermore, we carried out experiments to a better knowledge of the mixtures voidage. The correlations found in the literature fail to predict the critical fine concentration at which fine particle segregation stops. An Alternative correlation has been therefore proposed based on a micromechanical analysis of particles filling.
