Flow dynamics and transport efficiency of coarse–fine particle mixtures in hydraulic lifting for deep-sea mining

Transport efficiency and flow stability are key challenges in hydraulic lifting systems for deep-sea mining, where solid–liquid flow dynamics strongly influence pipeline performance. This study investigates the vertical transport of coarse–fine particle mixtures using computational fluid dynamics–discrete element method. To accommodate multi-scale particles, a kernel-based volume mapping with adaptive smoothing length is employed to balance flow resolution and computational stability. Numerical results indicate that adding fine particles reduces pipeline pressure drop and increases the transport velocity of the coarse particles when the size ratio is appropriate. This improvement is accompanied by a redistribution of energy dissipation from the near-wall region to the flow core. The effect is non-linear and depends on geometric compatibility between particle sizes. The drag reduction mechanism arises from shear-induced segregation that establishes a stable core–annulus flow regime, where fine particles tend to gather near the wall and push coarse particles toward the pipe center. This segregation enhances the stability of axial transportation, tending to reduce the risks of pipe blockage and slugging. In addition, the near-wall fine particle layer suppresses the particle–wall impact, reducing pipe wall erosion. The drag force acting on fine particles per unit mass is higher than that acting on coarse particles. Particle contacts contribute directly to the axial driving force acting on coarse particles, and their relative contribution increases with fine particle fraction under appropriate flow conditions.