Pipe flows of solid-liquid mixtures in the form of slurries are frequently encountered in many engineering applications. The pressure gradient is perhaps the most serious concern of designers, as it dictates the selection of pump capacity. Due to their versatility, in the sense of applicability to large scale systems and more complex flows, CFD models have been an attractive tool in recent years. The fully-suspended flow of solid-liquid slurries in horizontal pipes is simulated by means of a two-fluid model. In particular, the effect of the wall boundary condition for the solid phase on the pressure gradient is discussed, considering different alternatives. The numerical predictions were compared to experimental measurements from open literature over a wide range of operating conditions, in terms of pipe diameter (53.2 to 103 mm), grain material (glass beads and sand), particle size (90 to 370 μm), slurry velocity (1.5 to 8 m/s), and mean solids concentration (up to 40%). The equilibrium wall function of Launder and Spalding for smooth walls proved adequate for fully-suspended flows, but overestimates the losses if the particles are large compared to the boundary layer thickness. Guidelines are provided for defining the numerical set-up and evaluating the uncertainty of the numerical results.

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