CFD simulation of transient particle mixing in a high shear mixer

Particle mixing is one of the key operations in pharmaceutical processing. In this work, an Eulerian–Eulerian multiphase framework has been employed to model and simulate particulate flow and mixing behaviour in the blending of dry powders for inhalation. The kinetic theory of granular flow and the frictional stress model are used to close the transport equations of dense particulate flow in a high shear mixer. The transient mixing dynamics, including start-up, within the mixer are captured by adding a scalar transport equation as a tracer. The solid velocity profile at the wall is experimentally determined by using a high speed camera and particle image velocimetry (PIV) evaluation. The evolution of a tracer movement is experimentally tracked using an imaging technique that is processed in the Matlab image toolbox to obtain the local particle concentration. The model can capture the main features in granular flow motion, e.g. bed height and the dominating flow direction. The mixing mechanism is found to be a combination of azimuthal, axial and radial mixing at the same order of magnitude. Rapid mixing is captured in the simulation and is in agreement with experimental data. Even though the continuum-based model can predict well some flow features and the transient mixing time, there is a need for further development of the continuum description of dense particulate flows.