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Magnetic Resonance Imaging (MRI) experiments have been performed inconjunction with Direct Numerical Simulations (DNS) to study neutrally buoyantparticle-laden pipe flows. The flows are characterized by the suspension liquidReynolds number ($Re_s$), based on the bulk liquid velocity and suspensionviscosity obtained from Eilers' correlation, the bulk solid volume fraction($\phi_b$) and the particle-to-pipe diameter ratio ($d/D$). Six different caseshave been studied, each with a unique combination of $Re_s$ and $\phi$, while$d/D$ is kept constant at 0.058. These cases ensure that the comparison isperformed across different flow regimes, each exhibiting characteristicbehavior. In general, an excellent agreement is found between experiment andsimulation for the average liquid velocity and solid volume fraction profiles.This study presents, for the first time, accurate and quantitative velocity andvolume fraction profiles of semi-dilute up to dense suspension flows using bothexperimental and numerical methods. The discrepancy between the experiments andsimulations can be explained by various reasons, including a difference inparticle size distribution, uncertainty in experimental parameters used as aninput for the DNS, slight variations in particle roughness and frictionalcollisions in the simulations. Eventually, three different flow regimes areidentified. For low bulk solid volume fractions a drag increase is observed.For moderate $\phi_b$ the drag is found to decrease, due to particleaccumulation at the pipe centre. For high volume fractions the drag is found todecrease further. For solid volume fractions of 0.4 a drag reduction higherthan 25% is found. This drag reduction is linked to the strong viscositygradient in the radial direction, where the relatively low viscosity near thepipe wall acts as a lubrication layer between the pipe wall and the dense core.

From nearly homogeneous to core-peaking suspensions: insight in suspension pipe flows using MRI and DNS