Impact of the convex pattern placed on the bottom part of the pipe on mixture flow assurance and behavior

Abstract

Pipelines are widely recognized for their efficiency in material transport, and dredging operations rely heavily on them to convey sediment. In pursuit of more sustainable dredging practices, reducing the flow resistance within the dredging pipelines presents a promising approach, as the lower resistance directly decreases the energy consumption of the pump.

A previous study by Yun Peng Yan investigated the effect of the convex pattern on the wear rate in dry bulk material transport. The finding showed that the convex pattern reduced wear by inducing dilation in the bed layer, temporarily creating free space that allowed the particles to roll more easily. This rolling mechanism, observed under dry conditions, is particularly interesting to examine in wet environments. If the same mechanism also occurs inside the pipe in wet conditions, it could influence the flow resistance and potentially improve the efficiency of the dredging pipe. Since no prior research has tested convex patterns under wet conditions, the present study investigates how a convex pattern plate installed at the bottom of a pipe affects flow resistance and wear distribution during the transport of a gravel–water mixture.

In this study, experiments were carried out using gravel with an average particle diameter of d_50=3 mm. The convex pattern plate was evaluated against a plain plate, which served as a reference. The findings revealed that, consistent with previous dry-condition studies, the convex pattern induced particle rolling along the bed within the sliding-bed regime. However, this rolling mechanism did not lead to a reduction in flow resistance. Instead, the convex pattern configuration consistently exhibited higher resistance in both the stationary-bed and sliding-bed regimes. This increase in resistance also affected the deposition limit velocity, which increased from 0.9 m/s for the plain plate to 1 m/s for the convex pattern plate.

In the sliding-bed regime at lower flow velocities, abrasive wear was observed in both configurations. However, the wear behavior changed at higher velocities within the same regime. For the plain plate, the most severe wear was concentrated in the joints between the pipe segments, where the surface irregularities intensified abrasion. In contrast, the convex plate showed reduced wear at the joints and top surfaces, as the stationary particles in the bed provided a protective layer. However, at higher velocities, the upper layers of the bed began to move more rapidly and collided with the crests of the convex structures. This transition shifted the wear mechanism from abrasion to impact wear, causing the most severe damage at the tops of the convex patterns. In both types of plates, the segment directly exposed to the incoming flow experienced the heaviest wear.

In general, the anticipated reduction in flow resistance from the convex pattern was not achieved. The installation of convex structures at the bottom of the dredging pipes does not improve the transport efficiency and, in fact, may increase resistance, making the approach unsuitable as a general wear-reduction strategy. Interpreted through Newitt’s theory and the Darcy–Weisbach relation, the higher resistance observed in the 40 mm pipe suggests that this effect could be amplified in larger pipe diameters. This is because the water resistance decreases with increasing diameter, and then the solid effect increases to compensate for the higher mechanical friction introduced by the presence of convex structures. However, the additional resistance measured in this study was relatively minor and the deposition limit velocity increased by only 0.1 m/s. Since this increase is small, the improvement may still be advantageous in critical pipeline sections where wear protection is required. Therefore, a localized application of convex patterns could remain a practical solution.