The use of stepped pipeline in pneumatic conveying systems can be advantageous in reducing pipeline erosion, product degradation and flow resistance.1'2 This is due to the reduction in conveying gas velocity following an increase in pipe cross section. The benefits of this approach are obvious for high pressure dilute phase conveying systems, where a large portion of the available energy is used to overcome the friction experienced by the gas phase. It may also be appropriate for dense phase systems, for example, to prevent plugs reaching high velocity with the potential to damage system components and supports. The detail of the flow behaviour of dense phase plugs in passing through a step does not seem to be covered in the literature and some initial work is reported in this paper. 2 METHOD AND RESULTS The flow behaviour of a 500 mm long plug of material, particle size 25 um, particle density 2500 kg/m~3 passing through: an abrupt step (sudden expansion) and a gradual expansions was investigated. A 3000 mm section of conveying line with a step from 75 mm bore to 100 mm bore was modelled and meshed in 3D in Gambit 2.3.
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