This study is a numerical investigation of a heat exchanger under external flow. Temperature variable properties are used to solve the governing equations. Air is selected as an external fluid and water and MgO-MWCNTs/EG hybrid nanofluid are selected as radiator fluid. In this paper, the efficiency of the horizontal and vertical radiator are studied using experimental data for thermo-physical properties of MgO-MWCNTs/EG Nanofluid. Then, tubes with circular and elliptic sections are investigated for different flows, inlet temperatures and concentrations. The pressure drop and Nusselt number for each simulation are analyzed. Results are presented in terms of temperature distribution, pressure drop, Nusselt number and their sensitivity analysis for different states. The results indicate that radiators with vertical tubes have better efficiency up to 10% than radiators with horizontal tubes. In addition, heat exchangers with circular pipes have 25% lower pressure drop and those with elliptical tubes had 10% higher Nusselt number. The finding shows that an increase in the Nano-fluid concentration improves the Nusselt number and increases the pressure drop.