(121f) Transition between “Clusters” and “Stratified” Flow Regimes in the Pneumatic Conveying System – Gas Flow Driven Granular Jump

Fluid-mechanical mechanisms governing the pneumatic conveying processes have been studied extensively, both experimentally and theoretically, due to their significance for efficient transportation of particulate materials. Comprehensive reviews are available. The current paper arises from the earlier work of the authors, however, unlike the previous work which focused on the identification of the flow features and the measurement of solids flow rate in the dense phase pneumatic conveying using electrical capacitance tomography, the current paper is devoted to a new and unreported flow feature which observed for very low solids flow rates and relatively low superficial gas velocities ? on the borderline between ?clusters? and ?stratified? flow regimes.

This new phenomenon, which to our knowledge has not been reported until now, is termed here the ?granular jump? to express its apparent similarity to hydraulic jumps in shallow waters and similar phenomena described in relation to granular flows in chutes, silos and avalanches. However, its main distinguishing feature is that the flow of solids is imposed by the gas flow (due to the nature of pneumatic conveying processes) in a horizontal flow. Therefore the aerodynamic forces seem to interplay with gravity forces normally responsible for the formation and behaviour of granular jumps.

The main aim of the presentation will be to report this new type of granular jump to the research community to include a unique footage obtained by means of a high speed camera. Some similarities are drawn between the observed phenomenon and the broader class of problems exhibiting transition between super- and sub-critical flows. Similarly, comparisons with the already established models of granular jumps and the order-of-magnitude calculations are provided to open further discussions and encourage further research into this interesting phenomenon.