On the relation between hydraulic losses due to bends and mixture stratification
Type:
Presented during:
CEDA Dredging Days 2007 - The Day After we Stop Dredging, Rotterdam
Authors:
Cartigny M J B, University of Utrecht; Talmon A M, Delfos R, Vlasblom W J, Delft University of Technology and Rosenbrand W F, Royal Boskalis Westminster,The Netherlands
Abstract: Hydraulic losses due to hydraulic transport trough bends in pipelines are often modelled using a ``loss coefficient ´ ´. In single-phase flow, these loss coefficients have the attractive property of being rather constant for a given geometry, provided that the Reynolds number is sufficiently high. Within the literature there are several studies available that concern the application of the loss coefficient model in solid-liquid mixture flows.
From these studies it can be concluded that the loss coefficient model is capable of giving reasonable predictions of the hydraulic losses for mixture flows, when the velocity is well above the deposition-limit velocity. It remains unclear, however, what happens when the velocity is around or below this deposition-limit velocity. Published results range from increasing to decreasing and even to negative loss coefficients, the latter implicating that the hydraulic losses due to a flow through a bended pipe are smaller than those due to a straight pipe. Within dredging applications, hydraulic transport velocities are often around or below the deposition-limit velocity. In order to quantify the relation between hydraulic losses due to sharp bends and the transport variables, scale model experiments were conducted in a transparent flow-loop. Flow rate, sand concentration and particle diameter were varied while axial pressure profiles were measured. The experiments resulted in loss coefficients which indeed vary from negative to positive depending on the transport variables. Visual observation showed the axial pressure profiles were depending on the flow regime. Each flow regime showed a typical axial pressure profile. Through visual observations the pressure profiles could be linked to several consecutive processes occurring in and downstream of the bend. In this paper the variation of the loss coefficient at velocities around and below the deposition-limit velocity are explained through considering the observed processes.
Keywords: hydraulic transport, bend, dredging, experiments