Evaluating distributive mixing potential by mean distance of streamlines and a passive scalar

In single-screw extrusion, a uniform melt temperature distribution as well as a sufficient material homogenization are essential for an excellent quality of the final product. Therefore, mixing elements are used at the end of the extruder screw to generate the needed shearing and deformation of the polymer melt. CFD-simulations (Computational Fluid Dynamics) are the most common tool for the design and optimization of mixing elements, but the modeling and evaluation of distributive mixing processes in numerical simulations is a well-known problem. In this study, two different approaches for the prediction of the distributive mixing quality were evaluated: the change of the mean distance of streamlines on pre-defined cross-sections and a passive scalar integrated into the numerical solver. For this purpose, various flow simulations of a Block-Head-Mixer were performed and compared. The results show that the passive scalar has advantages over the evaluation by mean distance of streamline positions, especially regarding the integration into the simulation environment and when used for longer mixing sections.