Evaluation of effect of paddle element stagger angle on the local velocity profiles in a twin-screw continuous mixer with viscous flow using Finite Element Method simulations

The effect of paddle element geometry, specifically a systematic change in stagger angle, on the velocity distribution of a Newtonian corn syrup was evaluated in the mixing region of a 2″ Readco continuous processor using 3D FEM simulations. Local velocities and regions of backflow were compared for three configurations of the paddle elements in the mixing region consisting of nine pairs of paddle elements with the central three being in a neutral (FLAT), staggered 45° forward (45F) or staggered 45° reverse (45R) configuration. The total material flow rate through the mixer was independent of the paddle element stagger but increased with screw speed when the mixer was operated with the barrel fully filled. The stagger angle variation caused only local disturbances in axial flow. The overall magnitudes of velocity were highest for the FLAT configuration followed by 45F and 45R. The local X and Y velocity components in the region of stagger showed no significant variation with paddle element stagger while the Z velocity component varied significantly in this region. Increased forward flow was seen for the 45F configuration while significant local backflow was seen for the 45R configuration at all positions of the paddle element rotation. The FLAT configuration had greater levels of pressure in the intermeshing region, suggesting a squeeze flow while there were not significant variations in pressure for the 45F and 45R configurations, suggesting a predominantly conveying/leakage flow in the axial direction. Variation in local flows is critical to good mixing.