Co-rotating twin-screw extruders: Detailed analysis of conveying elements based on smoothed particle hydrodynamics. Part 1: Hydrodynamics

Due to the complex geometry of the rotating screws and, typically, free surface flows in partially filled screw sections, first principles simulations of the flow in co-rotating intermeshing twin-screw extruders using the well-established, mesh-based CFD (computational fluid dynamics) approaches are highly challenging. These issues can be resolved via the smoothed particle hydrodynamics (SPH) method thanks to its meshless nature and the inherent capability to simulate free surface flows. In our previous work, we developed a novel method for modeling the boundary conditions with complex wall geometries, under which SPH could be efficiently applied to complex surfaces of typical screw geometries of extruders. In this work, we employed SPH and our boundary method to study the flow in a conveying element in detail. To address unresolved clearances, we developed a new model that is coupled to SPH and can correctly account for the flow through unresolved clearances. A validation of our approach using CFD data from the literature for a completely filled conveying element indicated excellent agreement. Consequently, we studied the flow in a partially filled conveying element and obtained results for the flow rate, the power input and the axial force with variable filling ratio. A detailed analysis of the corresponding mixing phenomena is presented in Part 2. Our results show that the proposed method is a comprehensive approach to study the flow in different types of screw elements in detail, providing an excellent basis for further development of simplified models of entire extrusion processes.