Evaluation of Mixing Performance in Baffled Screw Channel Using Lagrangian Particle Calculations

Abstract

Different kind of discontinuous baffles, which had the larger geometric ratios and higher height than the previous ones, were inserted in the unwound channel of a single-screw extruder to generate chaotic mixing in the screw channel. The periodic unit of the flow channel was modeled as a dynamic system of complex duct flow. The finite-volume method was used to solve the three-dimensional flow of purely viscous, non-Newtonian fluid obeying the power law constitutive model. Lagrangian particle calculations along with the statistical methods were performed by a fourth-order Runge–Kutta scheme. The effect of the baffle's geometric ratio on the mixing kinematics was investigated numerically. Poincare sections were applied to reveal the different patterns and sizes of the KAM tori. Period points were analyzed on the lines of the perturbative theory. Distributive mixing was then visualized by the evolution of passive tracer particles initially located at different positions. Shannon entropy index and cumulative residence time distribution (RTD) were then used to evaluate the statistical results. The growth of the tracer's interface with time was also obtained to reveal the intensity of the chaotic mixing. The cases with the geometric ratios being 0.5, 0.6, and 0.8 had the similar RTD performance, with shorter mean residence time and the narrower broadening of RTD. Among all the test cases, case B with a geometric ratio equal to 0.5 had the best mixing ability. The results also imply there is likely an optimal baffle geometric ratio between 0.5 and 0.6 that can achieve more efficient mixing.