Pressurization and heat transfer during twin screw extrusion processing of concentrated suspensions

Extrusion processing of concentrated suspensions is commonly employed in various industries including composites, magnetics, ceramics, energetics, food, and personal care products. The objectives of this study were to collect experimental data from the twin screw extrusion process using an instrumented and industrial-scale co-rotating extruder in conjunction with a well-characterized concentrated suspension, which exhibits viscoplasticity and wall slip and to compare the data with the results of numerical simulation using the Finite Element Method. It was discovered that as the mass flow rate is successively reduced under constant screw speed, boundary, and inlet temperatures, the backflow becomes more pronounced and gives rise to numerical instabilities in the solution of the conservation equations. Here a Streamline-Upwind/Petrov-Galerkin method is used to prevent numerical instabilities associated with spurious node-to-node oscillations due to the presence of convection terms. However, the adequacy of SUPG to suppress the numerical instabilities is diminished as the backflow increases with decreasing flow rate. Noting both experimentally and numerically that the viscous energy dissipation also decreases with decreasing flow rate, the calculations were carried out under isothermal conditions at relatively low rates where SUPG is no longer adequate to suppress the instabilities. It was found necessary to carry out successive non-isothermal calculations at higher rates, where SUPG still can alleviate instabilities, and to extrapolate the findings to determine the bulk temperature condition at the relatively low flow rates. Thus, in the isothermal calculations at the low flow rates, the shear viscosity of the suspension at the extrapolated bulk temperature was used. Overall, excellent agreement was found between the experimental and numerical simulation results. The study also further demonstrated the importance of the use of the correct interfacial, i.e., flow boundary condition at the wall, by properly accounting for the wall slip behavior of concentrated suspensions.