Bubble-assisted multiphase flushing of settled solids in pipelines: Insights from integrated experiments and computational fluid dynamics simulations

Abstract

Water flushing during slurry transportation in pipelines requires a large amount of water to ensure operational efficiencies and system safety. To develop effective flushing strategy with reduced water consumption, in this study we combined experimental measurements with numerical simulations to correlate removal dynamics of settled sandbeds in horizontal pipelines under clean water and bubble-laden flows. Experiments were conducted in a labscale pipeline under varying solid deposit height, Reynolds number (Re) (12,070-18,020), and particle sizes (45–$425\mu m$), alongside a novel bubble-assisted approach to enhance flushing efficiency. Key findings reveal that an increase in bed level height prolongs the flushing period, due to the interparticle friction at a dense solid bed in redispersing particles. The flow condition critically governs removal efficiency: increasing Reynolds number from 12,070 to 15,810 boosts particle mobilization and shortens flushing time by more than 50%. However, a trivial improvement occurs at the higher range of Re (15,810-18,020), possibly due to turbulence saturation. Particle size impacts solid mobilization, with fine particles ($<100\mu m$) flushing faster than coarse counterparts ($>300\mu m$), attributed to their higher surface area-to-volume ratio, amplifying frictional drag. Notably, introducing bubbles as a secondary phase increases the flushing rate by 10%–50%, with micro-nanobubbles (MNBs) ($<100\mu m$) demonstrating superior efficacy. This study highlights the potential of bubble-assisted flushing as an effective and clean strategy for reducing water consumption and mitigate downtime in industrial pipeline flushing process.