Evaluation of Filling State and Residence Time Distribution inside Plastic Mixer using Mesh-Free Simulation

Abstract

Understanding the flow state inside twin-screw extruders and continuous mixers is important in order to improve the mixing performance and production capacity of the mixers. Numerical simulations are effective in understanding the flow state, but past studies considering partially filled state in the mixer have not progressed sufficiently due to the difficulty of the partially filled simulation. In recent years, however, the development of mesh-free methods such as the particle method has led to the studies of partially filled flow in the mixers. In this study, the authors conducted partialy filled simulations using the Element-Free Galerkin Method, which is a kind of mesh-free method, and marker particle tracking and investigated the relationship between operating conditions and filling rate, average residence time, and residence time distribution of a counter-rotating continuous mixer. It was found that the filling rate had a linear relationship with the ratio of flow rate to rotational speed, Q/n, as in the case of the co-rotating twin-screw extruder. The residence time distribution normalized by the average residence time was similar for each rotor shape in the range of 1.0 ≤ Q/n ≤ 1.5, regardless of the operating conditions, while the probability density of long and short residence times became larger for Q/n < 1.0 and 1.5 < Q/n. Even though the residence time distribution gradually becomes bimodal and changes significantly for 1.5 < Q/n, the average residence time increases linearly with 1/Q as in the results for Q/n ≤ 1.5. In addition, the average residence time calculated from the mean value of the marker particles and that from the filling rate did not match when the axial filling ratio was non-uniform. Therefore, the filling ratio estimated from the average residence time in the experiment is a good estimate when the axial distribution of the filling ratio is uniform, but an overestimate when the axial filling ratio is non-uniform.