Co-rotating twin-screw extruders: Detailed analysis of conveying elements based on smoothed particle hydrodynamics. Part 2: Mixing

A novel approach for the simulation of the flow in co-rotating twin-screw extruders based on smoothed particle hydrodynamics (SPH) was presented in Part 1. Specifically, we showed detailed results for the flow field in a completely filled conveying element, which are in excellent agreement with data from the literature obtained with computational fluid dynamics (CFD). Moreover, we studied the flow in the partially filled conveying element, facilitated by the inherent capabilities of SPH for modeling free-surface flows. In Part 2, we show a detailed analysis of the mixing effects based on the presented SPH simulations. We studied the mixing using tracer particles for the completely and partially filled states, evaluated the time evolution of the intensity of segregation and fitted kinetic laws in order to determine mixing rates. We conducted this separately for overall mixing and axial mixing and analyzed the contributions of axial and cross mixing to the overall mixing rates. We showed these results for various operation states and finally, presented a case study highlighting the effect of the residence time on mixing together with the determined mixing rates per screw revolution. This confirms that SPH is a very promising tool for the investigation of mixing in complex geometries in both, completely filled and partially filled states. The presented results provide an excellent basis for the further improvement of simplified models of entire extrusion processes, including a quantification of mixing. (C) 2015 Elsevier Ltd. All rights reserved.