Influence of impeller configuration and operating parameters on granular mixing: a DEM investigation

Granular mixing with bladed mixers is widely used across various industries. This study employed the Discrete Element Method (DEM) to explore the effects of rake angles (30° (150°), 45° (135°), 60° (120°), and 90°), rotational speeds (25, 50, 75, 100, and 150 rpm), and fill levels (1, 2 and 3) on the flow and mixing efficiency of 2- and 3-bladed impellers. The Lacey mixing index, granular temperature, circumferential velocity, diffusion coefficient, and void percentage were used to evaluate mixer performance. Results showed that for rake angles ≥ 90°, the highest values for the Lacey mixing index, granular temperature, and diffusion coefficient were achieved, although particle circumferential velocity decreased. The mixing efficiency improved significantly with 2-bladed impellers at rake angles of 135° and 150° and 3-bladed impellers at 120° and 135° rake angles. The void percentage increased linearly with rake angle. Rotational speed and fill levels were found to strongly affected the Lacey mixing index, with optimal rotational speeds < 100 rpm. Increasing rotational speed raised particle granular temperature, circumferential velocity, diffusion coefficient, and void percentage. For 3-bladed impellers, performance improved at fill levels where H/h > 1, although granular temperature and diffusion coefficient decreased with increasing fill levels. Granular temperature and diffusion coefficient were found to be reliable indicators of pre-mixing performance. It is recommended to use 3-bladed impellers at H/h > 1 and rotational speeds < 100 rpm, while 2-bladed impellers perform better at lower fill levels.