Computational Fluid Dynamics Simulations of an In-line Slot and Tooth Rotor-Stator Mixer

Title of Thesis: COMPUTATIONAL FLUID DYNAMICS SIMULATIONS OF AN IN-LINE SLOT AND TOOTH ROTOR-STATOR MIXER Author: Derrick I. Ko Master of Science, 2013 Directed by: Professor Richard V. Calabrese Department of Chemical and Biomolecular Engineering Unlike conventional stirred tanks, rotor-stator mixers provide high deformation rates to a relatively limited volume, resulting in a region in which intensive mixing, milling, and/or dispersion operations can occur. FLUENT was used to conduct threedimensional CFD simulations of the IKA prototype mixer, an in-line slot and tooth rotorstator device, for a low-speed low-flow condition and a high-speed high-flow condition. The working fluid was water. Turbulence was modelled with the RANS equations, realizable k-ε turbulence model, and non-equilibrium wall functions. The main objective of this project was to develop an enhanced CFD model of the IKA prototype mixer with the necessary refinement in the shear gap to accurately resolve these high shear values. The grid independence study showed that the required mesh depended on the operating scenario. For the low operating scenario (N = 300 rpm and Q = 0.315 L/s), grid independence was demonstrated for the 10 million cell mesh (with 20 cells across the shear gap). With this mesh, flow field convergence in the shear gap only is reached with about 12 revolutions of the rotor. About 20 revolutions would be needed to also reach convergence in the stator slots. It was demonstrated that the convergence of flow in the larger, more open stator slots was slower than in the highly-confined shear gap regions, thus accounting for the need for additional revolutions to reach convergence in the stator slots. For the high operating scenario (N = 1800 rpm and Q = 2.54 L/s), grid independence was not demonstrated at the most refined mesh tested (24 cells across the shear gap and 24 million cells overall). The model could not be further refined due to limits in the available computational resources. Flow field convergence in the shear gap requires about 16 revolutions, while convergence of the flow in the stator slots requires about 24 revolutions. A higher number of revolutions are required for the high operating scenario because of the greater intensity of turbulence. Velocity and total deformation fields were investigated in the stator slots and the shear gap for both operating scenarios. Some qualitative differences were observed in the flow solutions. When a stator slot is blocked by a rotor tooth, the high operating scenario showed a jetting phenomenon near the volute cover, while the flow was a circulation loop in the low operating scenario. Within the shear gap, the high operating scenario showed greater axial and radial velocities (normalized to rotor tip speed). The variation in total deformation rate on the stator teeth was also greater in the high operating scenario. It is recommended that the model be validated using PIV experiments. In addition, if further work with the low operating scenario is desired, additional study will be needed to determine whether the flow within the shear gap is laminar. If laminar flow is found, a technique for accounting for laminar flow in a specific region of a model should be developed. COMPUTATIONAL FLUID DYNAMICS SIMULATIONS OF AN IN-LINE SLOT AND TOOTH ROTOR-STATOR MIXER