The flow mixing devices on a grid spacer are designed to enhance the turbulence and heat transfer in sub-channels. The present study evaluates the effects of different mixing vane configurations on flow pattern and heat transfer in the downstream of the mixing vanes in the sub-channels of fuel assembly. This is done by obtaining velocity and pressure fields, turbulent intensity and the heat transfer coefficient using a three-dimensional CFD analysis. Two blade groups, two-dimensional and three-dimensional, were modeled. The two-dimensional blades are classified in ring type, diamond type, square type, homographic type and multiply type. Three-dimensional blades containing split vane, ripped open, trumpet shaped and split trumpet shaped are applied in a 2 x 2 rod bundle array. Also, a 3 x 3 rod bundle array is modeled to evaluate the neighborhood effects on the thermo-hydraulic parameters. The latter model shows a good comparison with the available experimental data by 5-12% difference. A standard K-epsilon model is used as a turbulence model and the symmetry condition is set as boundary conditions. It was confirmed that the turbulence in the sub-channel was significantly promoted by spacer and mixing devices. However, their effects rapidly decreased to a fully developed level after passing approximately 10 times the hydraulic diameter downstream of the spacer. The CFD results showed good agreement with the measurements. The static pressure of the fluid in the flow direction drops rapidly, then in a very short distance rise up, followed by a decrease near to the linear slope down stream. The split trumpet vane was the best with a 14% increase in the heat transfer coefficient, however regarding the manufacturing possibilities, the split vane spacers are expected to significantly enhance the overall heat transfer of a nuclear fuel assembly about 9.82% with a reasonable increase in the pumping cost. (C) 2007 Elsevier Ltd. All rights reserved.