(56b) A Study of Powder Flow Patterns (Dispersion vs. Convection) in a Continuous Annular Couette Cell Using Tracers

We studied the flow fields in a continuous Couette device obtained by superimposing a slow axial flow on radial shearing of particles thereby allowing the granular matter to move from the quasi-static to the intermediate regime of flow upon a slight decrease in solid volume fraction. This increase in porosity provides enough space for random collisions among particles so that the shear stress becomes dependent on the shear rate. In order to ?see? the flow patterns we used colored particles and inserted them into the continuous flow. The insertion is made in such a way as to produce a narrow ?pulse? of colored particles in an otherwise white background. The outflow of the mixture was collected at the bottom of the Couette cell at given time intervals of approximately one minute. Samples of approximately 80 grams were collected at a time and analyzed with a colorimetry technique.

Colorimetry entails the discharge of the sample mixture at very slow rate in front of a color-sensitive camera that assigns ?colored? or ?white? to each particle and gives the results in percentages. Residence time distributions of colored particles inside the bed show a clear peak and we were able to find the dispersion vs. convection pattern by transforming the RTD data. Another interesting trend is seen in the particle size of the tracer, when shown relative to the particle size of the white ?base?. We found that the coarser tracers are travelling axially faster compared to the finer tracer particles on the fine end of the distribution. The picture that emerges is that in the region where the bed is highly sheared, the axial velocity is somewhat higher (the solid fraction is lower) and the larger particles use this space to flow axially somewhat faster as compared to fine particles. Shearing extends into the bed to approximately 10 particle sizes depending on shape and size while beyond that, the bed is packed and moves axially very slowly. Diffusion is only important in the shearing zone and especially near the rotating wall where the shear is highest.