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Drop Length at Vertical Drops

Author(s): Robert H. A. Janssen

Linked Author(s): Robert Janssen

Keywords: Vertical drops; Free-falling nappe; Super-critical flow

Abstract: Drop boxes are used in hydraulic engineering to convey flow through a vertical drop within a small area. Drop boxes need to dissipate the fluid energy from one or more incoming channels or pipes, and discharge it in a controlled manner into an outlet. Where the flow transports sediment, the drop box also needs to be designed to limit the deposition of sediment within the box. For slurry conveyance systems, such as those in mineral processing plants, drop boxes are used not only for a change in elevation, but also to collect flow upstream of flow distributors and/or changes in directions of open channels conveying the slurry. In these systems, an additional consideration arises in that the drop boxes need to be designed to reduce the wear on the box walls and base caused by the abrasive sediment. Therefore, the box needs to be large enough to allow the energy from the incoming jets to be sufficiently dissipated, and avoid regions of high turbulence at the face of either the walls or base of the box. Existing available design tables for slurry boxes were developed for flow rates considerably lower than those present in many modern mineral processing plants, and hence a review of these design tables was required. The objective of this study was to investigate the validity of using equations of motion for defining the trajectory of a free-falling nappe into a drop box, for both sub- and super-critical flow. By using the commonly adopted ratio of depth at the brink to critical depth of 0. 715, the trajectory equation was able to satisfactorily match the results presented by othersfor sub-critical approach flow conditions. For super-critical approach flow, it was found that higher values of the ratio between brink depth and critical depth are required for the trajectory equation to match the observed conditions. Since this ratio varies with the approach Froude number, super-critical approach flow profiles presented by others were used to derive two empirical relations for the depth of flow and the slope of the free surfaceat the brink of the drop. These parameters were used as the starting conditions for the trajectory calculations, resulting in satisfactory agreement with the measured trajectory profiles.


Year: 2013

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