Green transition minerals: Flow self-organization and passive drag reduction mechanisms

Vertical hydraulic lifting of deep-ocean polymetallic nodules is a critical but energy-intensive process essential for the supply of green transition materials. Current transport models, which often assume uniform particle morphology, fail to capture the complex energy dissipation mechanisms inherent in realistic multi-dispersed flows. This study employs a high-fidelity Large Eddy Simulation–Computational Fluid Dynamics–Discrete Element Method (LES-CFD-DEM) coupling approach to investigate the energy performance and cross-scale transport characteristics of coarse particle mixtures with wide size ratios (Bsl = 0.20–0.80). The analysis reveals that optimizing the particle size distribution significantly alters the flow regime, shifting the system from a state dominated by particle-contact energy loss to one controlled by fluid forces. Results indicate that while coarse particles form chain-like clusters in the core region—reducing particle velocity from 0.39 m/s to 0.23 m/s—the presence of smaller particles provides a void-filling effect. This mechanism creates a particle-sparse region near the wall, acting as a lubricant to reduce frictional energy losses. Notably, the system achieves the minimum specific pressure drop optimal energy efficiency at a size ratio of 0.5. Based on these findings, a generalized prediction model incorporating size and density ratios is proposed to correct discrepancies in traditional energy consumption estimations. This work shows that particle-size gradation can induce a self-organized transport state in which coarse-particle core accumulation, fines-assisted interstitial filling, near-wall particle depletion, and force-partition redistribution collectively reduce hydraulic losses. Within the present binary-mixture framework and reference operating condition, these findings provide a mechanistic basis for the design of lower-resistance deep-sea lifting systems.