Nickel solids concentration distribution in a stirred tank was investigated using computational fluid dynamics (CFD) and experimental methods. The laser Doppler velocimetry (LDV) method was used to measure the velocity field for the liquid-only system and an optical technique was employed to determine the axial solids concentration distribution. The poly-disperse multiphase simulation approach was used to investigate the influence of the particle size on the solids concentration distribution. The effect of the solids loading on the liquid flow field was investigated and a good agreement was obtained between the experimental and simulation results. It was shown that the turbulent dispersion force is important for solids suspension, especially for high solids loading. Regions of inhomogeneity in the tank were identified. It was found that, for a given solids loading, the solids concentration distribution depended on both particle size and particle size distribution. Solids suspension theories are discussed and a correlation between these theories and scale-up rules is proposed. (c) 2005 Elsevier Ltd. All rights reserved.