(507e) Using X-Rays to Track the 3D Pathline of Individual Particles in a Double Screw Mixer

Granular flows are commonly encountered in many industries including food processing, pharmaceutical production, and energy generation. However, the ability to accurately characterize the three-dimensional (3D) granular flow structures is difficult due to the opaque nature of the flow. In this study, X-ray particle tracking velocimetry (XPTV) is used to track individual tracer particles to characterize the 3D granular flow structures inside a double screw mixer. The screw mixer features two intermeshing noncontacting screws with a screw diameter of D = 2.54 cm and a dimensionless mixing length of L/D = 10. A binary mixture of 500-6350 μm red oak chips and 300-500 μm glass beads were mechanically mixed inside the screw mixer. Individual red oak chips were tagged to preferentially absorb X-rays to allow for individual particle tracking. From two independent but temporally synced X-ray radiographic projections, the 3D coordinate of the tracer particle was obtained as a function of time by using a cone-beam compensated back-projection algorithm, and 3D particle pathlines through the screw mixer were created. It will be shown that the 3D granular flow structures inside the double screw mixer were significantly influenced by the selected operating conditions. In general, higher screw rotation speeds, longer dimensionless screw pitches, a counter-rotating down-pumping screw rotation orientation, and a material injection configuration with the red oak chips injected first, resulted in longer tracer particle pathlines and increased movement between and around the two screws. This allowed the double screw mixer to actually behave like a double screw mixer, as opposed to two single screw mixers operating in parallel, as was observed with certain operating conditions. The increased length of the tracer particle pathline is projected to increase the mixing effectiveness of the screw mixer by providing more convective mixing.