Effect of geometry and fill level on the transport and mixing behaviour of a co-rotating twin screw extruder

The transport and mixing properties of a co-rotating twin screw extruder with both screw elements and kneading blocks are explored using the smoothed particle hydrodynamics method. The use of a Lagrangian method enables simulation of extruders partially or fully filled with fluid and is not limited by problems associated with extreme mesh deformation due to the opposite rotation of the impellors or the small gaps between them. The cross-channel and axial fluid transport and mixing in the screw elements and kneading blocks are analysed. The effect of parametric variations in both impeller elements on the nature of the fluid flow and the mixing are explored; specifically the effects of varying the screw pitch (positively correlated to mixing rate), kneading block stagger angle (the mixing rate peaks at a block angle of 30°) and gap size between the rotating elements (positively correlated to mixing rate). Flow and mixing behaviour is also investigated for partially filled cases, with a fill level of 50% producing the highest mixing rates for the screw elements. The presence of fluid free surfaces is shown as generally positive for mixing. We demonstrate significantly improved mixing when the orientation handedness of the screw and kneading blocks leads to concentration of fluid in the kneading elements.