RESTRATIFICATION IN HYDRAULIC TRANSPORT: IS IT A BEND EFFECT?

摘要

The availability of reliable calculation models for hydraulic losses in dredge pipelines is important to project calculations for dredging work. In the development of such type of models the availability of validation material is crucial. Within the wealth of validation material from hydraulic circuits, peculiar results are found in some flow loops for 0.5 mm solids and greater at flow velocities just beyond the deposit limit velocity. Here the hydraulic gradient may increase two-fold in comparison when only the velocity is increased by 10% .This phenomenon is not described by existing calculation models. This effect, originally encountered in the Delft O 150 mm slurry loop, has been identified as a "restratification effect" characterised by a persistent two-layered concentration distribution. Electrical Resistance Tomography (ERT) measurements of concentration distributions indicate that the phenomenon also occurs in the Delft O 40 mm slurry loop. Therefor it is embarked on a combination of systematic ERT and hydraulic gradient measurements in order to verify whether the course of the hydraulic gradient is similar in this circuit and, if so, to create a basis for unraveling the underlying physical processes. A series of systematic tests is conducted for two different lengths of the upstream pipe-section. In both situations the "restratification effect" is found in the course of the hydraulic gradient and in the ERT measurements. Comparison of the results of both configurations show that the "restratification effect" is influenced by upstream conditions (in this case a 180° bend), but yet, it can not be established whether the "restratification effect" is initiated by the very presence of the bend.