Local mixing time distributions in a flat, horizontally rotating cylinder

Mixing is a crucial aspect in the design and operation of reactors as well as the optimization of an integrated process. Typically, the mixing behavior is assessed and modeled by considering the integral mixing time. Since the mixing time is a scalar quantity, insights into the interactions between the mixing process and local flow phenomena can only be provided by determining local mixing time distributions. A computer vision algorithm was developed to calculate mixing time distributions using a colorimetric approach. The method was applied to the mixing process in a flat, horizontally rotating cylinder. Challenges specific to the experimental setup were considered by extending the functionality of conventional algorithm approaches. The challenges arise from the moving phase interface and a pronounced circular flow pattern, which are addressed by a smoothing functionality and alternative assignment logics for the mixing time. The obtained distributions revealed changes in the flow state and the existence of a characteristic trailing vortex within the filling volume. The vortex occurred only in a limited part of the volume and showed a significant influence on the mixing process. Its course exhibited a speed dependent angle to the phase boundary and led to the formation of a recirculation zone with pronounced segregation. Increasing the width of the reactor reduced the formation of the trailing vortex, demonstrating the influence of the axial walls on flow characteristics and mixing in the cylinder geometry. • Algorithm for calculating local mixing times in the presence of a challenging dynamic phase boundary and image perturbations due to particles or bubbles. • Disclosure of flow structures that strongly define mixing and a dominant trailing vortex in a partially filled, horizontally rotating cylinder. • Effect of speed, filling level, width and dosing position on the mixing process. • Significant influence of the axial walls on the flow and mixing process, revealed by variations in the cylinder width.