Detecting mixing barriers in Twin-Screw extruder elements via Lagrangian Coherent Structures

Twin-screw extruders (TSEs) are known for their good mixing performance. Although global mixing performance has been the subject of many computational fluid dynamics studies, the actual mixing mechanism remains largely unexplored, probably due to the complexity of chaotic flow patterns caused by the complex screw geometry. In this work, we aim to understand laminar mixing in various twin-screw extruder elements via Lagrangian Coherent Structures (LCS). An LCS computation requires fluid element trajectories, which can be a limiting factor in 3D applications. Bypassing this potential problem, we evaluated LCS within a Smoothed Particle Hydrodynamics (SPH) framework and established that, unlike conventional methods, this methodology is efficient in complexly shaped deforming fluid domains. Mixing barriers in realistic conveying, kneading and mixing elements are computed, compared, and discussed. Repelling and attracting LCS reveal the stretching and folding events necessary for efficient laminar mixing and offer a novel viewpoint for geometry optimization.