The frequency of periodic structures in vertical pneumatic conveying of large particles

•Slug frequency in vertical gas-liquid flow is fitted by a dimensionless equation.•The limited data for slugs in pneumatic conveying fit a similar equation.•In a pipeline/bend/riser system, the horizontal section frequency persists.•A simple equation is presented for the apparent viscosity of solids beds.

The transport of larger particles, defined as type D in the classification of Geldart (1973), by pneumatic conveying, is characterised by periodic structures, namely slugs. The frequency of these slugs in vertical pipes shows a trend similar to that seen for equivalent structures in gas-liquid flows. A simple equation has been derived to specify the apparent viscosity of the dense particle areas. From the non-dimensional relationship describing the frequency, its dependence on different variables can be identified. It can be inferred that pressure will not have a strong effect on frequency. There is a complication when the piping consists of a horizontal pipe followed by a bend and a vertical section. If there is slug flow in the horizontal pipe, the frequency in the vertical pipe could be the same as that in the horizontal section.

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