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Local anomalies in slip for slurry flow within pipes near the deposit limit velocity

Presented during:

CEDA Dredging Days 2012 - Virtue, Venture and Vision in the Coastal Zone

Authors:

van Grunsven F - Delft University of Technology, the Netherlands; Talmon AM - Delft University of Technology Deltares, the Netherlands

Abstract: The findings described in this paper are a result of measurements from multiple sand-water mixtures within a closed 40 mm diameter Perspex pipe circuit taken at the dredging laboratory from the Technical University Delft between April and June 2011. The average in-situ volumetric concentration in horizontal pipes is different from the delivered volumetric concentration due to non-uniformity of velocity and concentration profiles. The ratio of in-situ concentration and delivered concentration is defined as the "slip factor". The main goal of the measurements was to determine the slip factor as a function of the average slurry velocity for multiple mixtures at various densities using fine, medium and coarse sand. The slip factor was determined by measuring the average in-situ concentration by the use of Electrical Resistance Tomography (ERT) and the delivered concentration by two differential pressure meters connected to a vertical U-loop.

The in-situ concentration is expected to consistently decrease for an increasing average slurry velocity as the bed erodes by turbulent flow. Our measurements show a distinctive increase in average in-situ concentration near the deposit limit velocity for medium and coarse sand. This effect increased for increasing concentration and particle size. Within these flow conditions, the ERT measurements reveal an increased area of high solids concentration in the lower portion of the pipe. These findings corroborate with previous, larger scale laboratory measurements at TUD, where the hydraulic friction showed a local maximum in the sliding bed regime for coarse particles, which is at odds with all existing models for hydraulic gradients.

Measurements revealed an anomaly in solids concentration within slurry flow presumably coinciding with an associated local maximum in the hydraulic gradient. This has to be checked to verify that this is not an artifact of the TUD circuit.