(24a) Experimental Techniques to Study Non-Spherical Particles in Granular Flows

Modelling of gas-solid flows has long been restricted to spherical particles. As a result the translational velocity has gained most interest as this determines the overall solids flow. Recently a lot of effort is being put into the simulation of non-spherical particles and as such the particle orientation and rotation becomes more and more important. This in turn sets challenges to develop novel experimental techniques as well. In this paper an introduction is given to two new techniques that are expected to have a large impact in the study of non-spherical particulate flows.

Most experimental techniques are not so well suited to study the orientation of particles. Magnetic Particle Tracking (MPT) however is a technique that tracks a single magnetic marker that acts as a dipole introducing a small magnetic field that can be tracked with a set of Anisotropic MagnetoResistive (AMR) sensors. Because the dipole/marker has a position and the magnetic field a direction, both position and orientation of the markers can be tracked over time. Experiments in a 3D cylindrical fluidized bed have been performed with MPT for three different types of particles with similar mass and volume but different aspect ratios: spheres and two cylinders with aspect ratio of 2.25 and 4.5. Experiments show distinctly different flow patterns for the cylinders with respect to the spheres. The orientation of the cylinders is largely determined by their interaction with the walls., whereas the spheres have no preferred orientation as expected.

Because MPT can determine the orientation as an intrinsic property of the technique, it is a very powerful tool in studying particulate flows involving non-spherical particles. It can however not determine position and orientation of multiple particles at the same time and as such can also not determine the mutual alignment of particles. Image analysis techniques can do this but are restricted to visually accessible systems, i.e.; pseudo 2D fluidized beds. A Digital Image Analysis (DIA) technique has been developed that can determine position and orientation of cylinders of particles in plane with the wall of the pseudo 2D fluidized bed. This technique was used to study the segregation and mixing of a mixture of spheres and cylinders of aspect ratio 10 with equal mass and volume. Orientation and position distributions as well as mutual alignment, determined with a nematic order parameter, are used to show that segregation occurs through formation of semi-aligned clusters of particles. These clusters are seen by the flow as larger particles, segregating from the bulk of the system.

These two techniques, MPT and DIA, are capable of determining two very important parameters to characterize non-spherical particles in granular flow, that previously could not be studied; orientation and mutual alignment. Acknowledgements

This research was funded by the European Research Council, under the Advanced Investigator Grant Scheme, contract no. 247298 (Multiscale flows), and the 3TU Centre of Excellence â?? multiscale phenomena