Mixing and segregation of spheres of three different sizes on a batch stoker grate: Experiments and discrete element simulation

Abstract

Mixing and segregation in a tri-disperse granular assembly of polyoxymethylene (POM) spheres induced by the moveable stoking bars of a generic batch grate system are examined. Each particle size class features a separate colour. Stroke bar velocity and stroke length are varied. Different moving modes of the bars are analysed. Optically transparent walls of the grate allow for the localization of the visible particles. Based on the visible particle positions a segregation index is calculated. The initial arrangement of the particles in the experiments, which exhibits small statistical differences introduced by the grate filling procedure, has an influence on the progression of the segregation index. The experiments are compared with discrete element (DEM) simulations employing an in-house DEM code.Experiments are in good general agreement with the simulations. The particle rearrangement during bar movement is characterized by an initially fast mixing on short time-scales and a slow process to reach a final state of segregation. These two processes are influenced by the penetration depth of the bars into the bed and the specific movement mode.Three modes predominantly showed segregation in the direction of bar movement, whereas two modes showed large-scale spatial particle rearrangement. Two moving modes show bridging at the beginning of the experiments, an effect that could be reproduced by the DEM simulations. The influence of the modes and their specific parameters on segregation indices, a mixing rate and a segregation efficiency are discussed in detail.