The influence of particle size on the flow of initially fluidised powders O. Roche a,1 , M.A. Gilbertson b, ⁎ , J.C. Phillips a , R.S.J. Sparks a a Centre for Environmental and Geophysical Flows, Department of Earth Sciences, Wills Memorial Building, University of Bristol, Queens Road, Bristol, BS8 1RJ, Great Britain, UK b Centre for Environmental and Geophysical Flows, Department of Mechanical Engineering, University of Bristol, University Walk, Bristol, BS8 1TR, Great Britain, UK Received 6 November 2003; received in revised form 12 April 2006 Available online 14 July 2006 Abstract When particles are allowed to move over a horizontal surface, the effect of gas flow through them is to increase the distance over which they move, termed their mobility. This has already been shown for cases when gas is continuously passed through a current of particles, but this investigation shows that this is also true when the gas flow is only initially present. Experiments were conducted on a column of fluidised particles that were released into an enclosed channel by the removal of a wall, and the distance travelled by the particles was measured. The behaviour of fine particles (group A in the Geldart classification of fluidised particles) was distinct from that of larger particles. The mobility was modified when they were mixtures of different-sized particles. In particular, when there was no gas flow, the mobility was a maximum when the proportion of fine particles was 30% and the magnitude of this effect increased with the size of the coarser component of the mixture. All the different mixtures of particles acted in a similar manner with increasing mobility for a given gas flow rate with proportion of fine particles until roughly half the mixture was composed of fine particles, and there was then no further increase. © 2006 Elsevier B.V. All rights reserved. Keywords: Powder handling; Fluidisation; Particle size; Currents; Mobility; Particle mixtures 1. Introduction An important feature of granular material is its mobility or the degree to which it is capable of moving freely from its source. This is exploited in industrial processes as a mechanism by which solid material can be transported from one place to another. It is also important in many environmental flows, such as the pyroclastic flows produced by some explosive volcanic eruptions, where large, dangerous and destructive currents of material can travel up to several tens of kilometres [1]. The extent and behaviour of granular flows depends to a large extent on the role of friction within them. In systems where there is no gas flow when a stream of cohesionless particles is poured onto a horizontal surface, it is well known that a wedge-shaped pile forms with a surface angle equal to the internal angle of friction [2]. The behaviour of the particles is dominated by inter- particle friction and movement is confined to a thin layer close to the top of the pile [3]. When particles are introduced into a system and gas is passed vertically through them at a high enough rate that the drag it exerts on the particles balances their weight, then the effects of friction become negligible. As a result, rather than a pile, a thin and mobile current is formed in which nearly all the particles participate [4,5]. In many situations, the friction between the particles that subsequently form a current are negligible because of a gas flow passed through them or because they are in free fall, but the current that is formed moves into an environment where this is no longer the case. When this happens, it is not known the extent to which the initial conditions affect the subsequent motion of a granular current: once the air flow through the particles ceases, does friction immediately dominate the motion of the current irrespective of initial conditions, or do the initial conditions largely determine the subsequent motion of the particles? The picture is complicated by the presence of physical Powder Technology 166 (2006) 167 – 174 www.elsevier.com/locate/powtec ⁎ Corresponding author. Tel.: +44 117 9289732; fax: +44 117 9294423. E-mail addresses: O.Roche@opgc.univ-bpclermont.fr (O. Roche), m.gilbertson@bristol.ac.uk (M.A. Gilbertson). 1 Current address: Laboratoire Magmas and Volcans, UMR Universit´e Blaise Pascal-CNRS-IRD, 5 rue Kessler, 63038 Clermont-Ferrand, France. 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.05.010