Sand suspension deposition in horizontal low-concentration slurry pipe flows

Detailed observations in a slurry transparent horizontal loop of the sand transport deposition regime, ranging from fully suspended particles to bed motion as a train of dunes, are reported. Deposition, or scouring, is known as a deleterious sand transport regime in view of pipeline erosion. The two velocity thresholds limiting the deposition regime: the critical sand-carrying velocity threshold or deposition threshold below which particles begin to deposit, and the minimum conveying velocity threshold, below which particles cannot be suspended anymore and self-organize into dunes, span about 20 % of the bulk velocity such that detection of the beginning of the deposition provides a margin of safety before accumulation risks. Observed moving, snaking structures are likely to be the actors of the reckoned erosion on pipelines bottom in this scouring regime. From a digital treatment of the videos of the deposited particles, the deposition regime presents a salient characteristic: a consistent increase in the number of detected entities as the bulk velocity decreases down to a brutal trend inversion once all particles are eventually deposited and self-organise and hide into dunes. The findings are, in a first approximation, independent of the sand size and concentration, for the cases considered, and provide a basis for the design of monitoring methods for systematic detection of the deposition regime.