The IAEA is organizing a Coordinated Research Project (CRP) on the application of Computational Fluid Dynamics (CFD) for nuclear reactor design. Within this CRP, an open benchmark is organized on fluid mixing in rod bundles. KAERI has offered precise experimental data of the velocity field downstream of a mixed twist-split type mixing grid. Additionally, temperature measurements in various axial and azimuthal locations of a partly heated rod were made available by KAERI. The experimental set up consists of a 2000 mm long bundle of 4x4 rods with pitch to diameter ratio of 1.35, placed in a square housing of 142x142 mm. Each rod has a diameter of 25.4 mm. The mixing grid is located at 2-3 height of the channel with 5 spacer grids located upstream and 3 downstream of the mixing grid. These experiments are analysed with the TrioCFD code. Turbulence is treated with the RANS approach by using a non-linear eddy viscosity model. The comparison of measured and calculated mean velocity profiles is presented. Measured profiles of velocity mean values were calculated correctly for the central sub-channel and the rod to rod gap both close and far from the mixing grid. The measured temperature is underestimated by the calculations, most probably due to the localizations of the thermocouples inside of the wall of the rod. A correction based on the estimation of the temperature difference between surface and wall leads to satisfying comparison between measurement and calculation.