Influence of Impeller Configuration and Operating Parameters on Granular Mixing: A DEM Investigation

Granular mixing by a bladed mixer is common in many industries. The discrete element method (DEM) was employed to investigate the effect of different rake angles (30 (150), 45 (135), 60 (120), and 90°), different rotational speeds (25, 50, 75, 100, and 150 rpm), and different fill levels (1, 2, and 3) on the flow and the mixing performance of 2 and 3-bladed impellers. The mixer performance was evaluated using the Lacey mixing index, granular temperature, circumferential velocity, diffusion coefficient, and void percent. At rake angle ≥ 90°, the highest values of the Lacey mixing index, particle granular temperatures, and defuse coefficient were found. In contrast, the particle circumferential velocity is less at these angles. Mixing ability increased significantly when using 2-blades with rake angles of 135 and 150° and 3-blades with rake angles of 120 and 135°. The void percent increased linearly with the blade rake angle increasing. The rotational speed and fill levels strongly affected on Lacey mixing index. The recommended values of mixing rotational speed are < 100 rpm. Particles granular temperature, circumferential velocity, diffuse coefficient, and void percent increase with an increase in rotational speed. Increasing the infill level gives a better performance for the 3-bladed impeller at fill levels of H/h > 1. The average granular temperature and diffuse coefficient decreased by increasing the fill level. The granular temperature and diffusion coefficient predict the premixing performance. It is preferable to use the 3-bladed impeller at filling levels H/h > 1 and rotational speed of < 100 rpm, however using 2-blades is recommended at the low fill levels.