Numerical investigation on the intensification of coaxial slurry mixing systems

•PropellerU + Anchor was easier to make solids suspend than PropellerD + Anchor.•The introduction of the outer anchor had both positive and negative effects.•Flow pattern of PropellerD + Anchor was transformed from axial to radial flows.•A larger off-bottom spacing was recommended for the PropellerU + Anchor. The dense solid concentration brought difficulties to solid suspension, and the mixing intensification of the coaxial mixer, which comprised a close-clearance anchor and a center propeller, was researched numerically in the challenging system. The results indicated that by employing the propeller in an up-pumping direction rather than a down-pumping direction, the state of solid accumulation at the center bottom could be significantly improved, whereas the uniformity of axial solid distribution was also relatively poor. The quality of the suspension deteriorated at the dense solid concentrations, and energy loss would be more significant, particularly near the location of the center propeller. In addition, a larger outer impeller speed was necessary to prevent solid deposition, but it also led to a decrease in cloud height. A larger off-bottom spacing was advised for the up-pumping propeller, and the structure could achieve superior intensification performance for dense solid-liquid mixing. [Display omitted]