A heat exchanger is a device that facilitates the transfer of energy between two fluids through a solid barrier. Simulations were performed in a turbulent flow regime to investigate the two-dimensional forced convective heat transfer of the nanofluid water / Al2O3 within a counter-flow heat exchanger. This study is numerical and was conducted using a single-phase approach with constant thermophysical properties. Conduction through the interface was taken into account in the computations. The results unequivocally showed an improvement in the overall coefficient of heat transfer depending on the Reynolds number along with the type of fluid. The use of nanofluid significantly increases total heat transfer in contrast to the pure base fluid; however, this is accompanied by an increase in friction coefficients, leading to higher pumping costs.