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The flow of fluidised particles

Hogg, AJ; Gilbertson, MA; Jessop, DE (Bristol)
Monday 05 January 2009, 11:45-12:10

Seminar Room 1, Newton Institute


A gas flow through an ensemble of relatively heavy particles has a significant effect on their collective properties. When the vertical gas flow is sufficiently strong, it is possible for the weight of the particles to be supported fully and the granular material is fluidised. At this transition, there is a reduction in frictional effects generated by collisions and longer sustained contacts between the particles as the drag between the solids and the gas becomes the dominant physical process. Although there has been much recent progress in understanding aspects of dry granular flows down slopes, little attention has been paid to the interactions between the grains and the interstitial gas, but in the scenario investigated by this study, this process dominates the dynamics. There are many applications in which the interaction between the phases is central to the bulk motion, ranging from pneumatic conveying of industrial materials to the runout of volcanic particulate flows, in which the fluidising process substantially reduces the drag and increases the mobility of the material. In this work we present new experimental results for the propagation of relatively fine glass particles along a sloping, rigid, but porous surface, through which gas flows and fluidises the particles. We introduce the particles at a constant rate and measure their propagation through the apparatus. Typically they flow as a relatively thin, but highly mobile layer. For a range of angles of inclination, we measure the bulk characteristics of the motion, such as the temporal development of the depth of the layer and the flow speed, and from high-speed videography and particle tracking, we determine the velocity profiles of the grains. These feature slip at the base, a region of shear and then a plug-flow throughout the rest of the layer. We analyse these results in terms of a two-phase mathematical model. A key component of this model is the inter-phase drag that leads to the support of the excess weight of the granular layer. However the results indicate that there is also a drag that balances the gravitationally-related force driving the motion parallel to the underlying boundary. We present a mathematical model for this force, which is due to granular interactions, and demonstrate that it may account for both the fully-developed velocity profiles and the temporally evolving behaviour measured in the experiments.


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