Examination of the mixing ability of single and twin screw mixers using 2D finite element method simulation with particle tracking

Abstract

The mixing in model 2D single screw and co-rotating twin screw dough mixers is evaluated using the commercial CFD package Polyflow with a Carreau flow model for dough to generate velocity profiles and particle trajectories. The mixed Galerkin FEM simulations use a rotating reference frame technique with the single paddle mixer and a mesh superposition technique in the twin paddle mixer. Differences in the velocity profiles are noted. Distributive mixing is evaluated visually as well as using the segregation scale and cluster distribution index; stretching is evaluated using the length of stretch and mixing efficiency; and dispersive mixing is evaluated using the mixing index in combination with the shear stress. Since material points stay on their streamlines in the 2D single screw mixer, the 2D twin screw mixer is found to be generally more effective and efficient, although there are still areas of poor mixing.