Structure of Transaxial Flow in a Powder Mixer

B.F.C. Laurent1, J. Bridgwater1 and D.J. Parker2

1 Department of Chemical Engineering, University of Cambridge, Pembroke Street Cambridge, CB2 3RA, bfcl100@cheng.cam.ac.uk, john_bridgwater@cheng.cam.ac.uk

2 Positron Imaging Centre, School of Physics and Astronomy, The University of Birmingham, Birmingham, B15 2TT, UK, d.j.parker@bham.ac.uk

ABSTRACT

Positron Emission Particle Tracking (PEPT) permitted investigation of powder flow in a horizontal cylindrical mixer of diameter 270 mm and of length 650 mm stirred by a single long flat blade for a level of fill varying between 20 and 70%. The structure of the transaxial flow was elucidated by using the concept of dispersion of blobs. The deformation of a blob was found to be strongly affected by the presence of a circulation loop situated beneath the agitator shaft, this inhibiting material dispersion. Two types of mechanism of agitation were identified as fill increased. At low fill, the blob was lifted by the blade and fell into an open space created behind the blade. The blob became elongated and the material was free to flow and to cascade on the free surface. At high fill, the blob became less elongated as the powder was channelled in the annular space between the mixer shaft and the shell before reaching the free surface. Analysis of the dispersion of blobs in different sections of the transaxial plane suggests that two regions of different topological nature coexist. The first one is situated above the region where the blade interacts with the material. The flow in this region may be regular, with blobs stretching linearly as agitation takes place. The second region is the section of the bed where the agitator blade comes in contact with the powder. This could be of chaotic nature, that is where blobs are stretched exponentially and presumably be separated from the prior region by an invariant curve.

Keywords : Positron, powder, agitation, dispersion of blobs