(544a) Polydisperse Granular Flows in a Bladed Mixer: Experiments and Simulations of Cohesionless Spheres

Bladed mixers are commonly used in a variety of areas, ranging from the bulk chemical to the food and pharmaceutical industries. The appeal of these mixers comes from their ability to handle a wide variety of solids ranging from free flowing to cohesive powders and even pastes. These unit operations often involve the use of components with varying breadth and shape of the particle size distributions. Particle size differences have been shown to cause segregation in many granular systems, a phenomenon which could jeopardize product quality in industrial applications. Despite the common use of polydisperse materials, the effect of particle size distributions on flow and segregation in bladed mixers is not fully understood.

In this work, the flow and segregation of polydisperse, spherical particle mixtures in a bladed mixer was investigated using experimental and computational techniques. Discrete element simulations were able to reproduce the qualitative segregation profiles and surface velocities observed experimentally. For a binary system with a 2:1 size ratio, segregation by size occurs due to a sieving mechanism. Segregation in the binary system is fast, with a fully segregated system observed after just 5 revolutions. However, the numerical simulations showed that the extent of segregation in the bladed mixer can be reduced by introducing intermediate particle sizes in between the smallest and the largest particles. Addition of intermediate particle sizes increases convective and diffusive particle motion promoting a mixing mechanism which reduces segregation via the sieving mechanism. Void fraction within the bladed mixer increases as the degree of polydispersity is increased allowing the particles to move more freely throughout the particle bed. Higher void fractions also increase the ability of large particles to penetrate deeper into the particle bed. Normal and shear stresses are also affected by particle size distributions, with lower average values obtained for the system with the largest number of particle species. Differences in the amount of stress generated by each particle species were observed. However, the difference in stresses is reduced as the number of particle species in the system is increased.