The flow behavior in a hydrocyclone is a highly swirling and turbulent multiphase structure. A multiphase CFD model with sub modules for the air-core, turbulence, and particle classification with a suitable slurry viscosity model was used to simulate performance of hydrocyclones. The predicted velocity field from the LES, DRSM turbulence models is compared with the LDA measurements data for a 75 mm cyclone. The multiphase CFD model is used to understand the particle size segregation inside a 6 in. hydrocyclone. The predictions are validated against the Renner (1976) data, which was originally measured using high-speed sampling probe at different precisely controlled positions. The overall classification curve predicts close to the experimental data. It is observed that the predicted position sample size distributions are in good agreement with the experimental data, at most of the cyclone sampling positions. Close to the forced vortex (inner position), the predicted size distributions slightly deviate from the measured data. The discrepancy may be an effect of sampling turbulence due to probing close to the unstable forced vortex. Simulations are also carried out using two different CFD models, with and without the viscosity correction due to the fines fraction. The predictions are improved with respect to Renner's data with the fines viscosity corrected CFD model. © 2012 Elsevier Ltd. All rights reserved.