Particle scale investigation of the mechanisms of wet particle flow in rotating drums

Abstract

Granular flow in rotating drums shows various phenomena important to both science and engineering. The presence of moisture further complicates the flow behaviour due to interparticle liquid bridges. Understanding the mechanisms of wet particle flow requires the investigation at particle scale.

This project studied the flow of wet particles in rotating drums through numerical simulations based on the discrete element method (DEM). The capillary force and liquid distribution among particles were explicitly considered. Physical experiments were performed to validate the model, showing agreement in terms of flow patterns, repose angle and avalanche frequency.

Particle flows were studied in two states. In quasi-static state, avalanches were observed. The relationship between maximum avalanche angle and capillary force was comparable with theories based on Mohr-Coulomb criterion and force balance. In dynamic state, particles showed continuous flow at weak cohesions but transited to avalanches at high cohesion. The energy and frequency of collisions decreased with cohesion and more collisions were observed below the flow surface.

The capillary force in general reduced mixing performance. However, mixing of dry particles in drum transverse plane was poorest at 64% filling, and increasing cohesion at that filling improved mixing. A mathematical model based on particle circulation period was proposed to predict the mixing performance. The mixing in the axial direction follows normal diffusion. The diffusivity increased with rotation speed, decreased with cohesion and filling. The granular temperature was related to diffusivity, which however did not follow the theory for thermal systems.

The segregation of binary mixtures of wet particles was insensitive or slightly enhanced at low surface tensions but reduced significantly at high surface tension, which is due to the competing effects of cohesion limiting both segregation and mixing. For radial segregation in drum transverse plane, the interaction between particle size and density was captured by an existing theory which predicted the optimum mixing performance. On the other hand, anomalous transport of larger particles was found during band formation along the drum axis. By separating the displacements caused by different interaction forces, a definite driving force for segregation was identified for dry systems but not for cohesive systems.