CFD Calculations of the Effect of Rotor Speed Ratio in Rubber Mixing Processes
The processing of polymer materials or rubber compounds deals with the conversion from raw materials into finished products and the steps include compounding and chemical reactions, which in turn change the molecular structure and morphology. An internal mixer is the type of equipment that is typically used for such processing. For instance, it combines the polymer with the carbon black, silica and other ingredients to achieve the desired product. The internal mixer is typically equipped with two rotors which counter-rotate thereby mixing the materials. Two of the most important features of such a mixer include the narrow clearance zone between the rotor tips and chamber wall, and the much large space between the two rotors. The former aids in the breaking of agglomerates into the finer particles, which is critical for dispersive mixing, whereas the latter provides the extensive area for the mixing to occur, which affects the distributive mixing behaviour. Three-dimensional (3D), transient, computational fluid dynamics (CFD) calculations of a non-isothermal nature are carried out for rubber mixing in a two-wing rotor-equipped chamber that is partially-filled with rubber. The objective in this study is to analyze the mixing efficiency for three different rotor speed ratios of 1, 1.125 and 1.5. The motion of the rotors is incorporated using the moving mesh technique. Governing equations include the continuity, momentum and energy equations and in addition, an Eulerian-based volume of fluid method is used for the multiphase calculations involving rubber and air. The highly viscous and non-Newtonian nature of the rubber is handled using a Carreau-Yasuda model along with an Arrhenius formulation for the temperature dependence. Results presented include comparisons of velocity magnitude and temperature for the different speed ratios. Also included is a comprehensive assessment of the dispersive and distributive mixing characteristics through Lagrangian statistics such as length of stretch and cluster distribution index. Comparisons showed that the speed ratio of 1.5 displayed the best dispersive and distributive mixing behavior.
Keywords - CFD, non-Newtonian, polymer processing, dispersive mixing, distributive mixing