Many researches and developments are carrying out to improve the heat transfer properties of different fluids. In this project CFD calculations will be done for a seven vertical rod bundle with a hexagonal configuration to see and determine the improvement of heat transfer properties while using Nano fluid with different concentrations in compare of traditional fluids which in this project, water is used as the basic fluid and Zr is represented as Nanofluid. The numerical simulation is done using ANSYS FLUENT 14.5 assuming the fluids are single phase. The hexagonal arrangement of seven rods is designed and meshed by GAMBIT 2.3.16. Calculations have been done for different flow rates starting from 5 liter per minute up to 15 liter per minute and for each flow rate three different powers have been applied from 100W to 700W. At the end the results has been studied for a sub-channel between the heaters, by determining heat transfer coefficient, Reynolds number and calculating the correlation equations for Nusselt number. Keywords: CFD calculations, Nanofluid, Forced convection, Nusselt number, Correlation equations