This study aims to investigate convective heat transfer inside an annular tube under a constant wall temperature as the outer boundary condition. The outer wall temperature is set to 91 °C, while the other boundaries remained unchanged. For different volume concentrations of Cu as the nanoparticle (0.011, 0.044, and 0.171), the study has been pursued. The acquired data is used to develop a correlation for Nusselt number. The correlation is valid for a Cu/Base Ethylene Glycol nanofluid flow with the volume concentrations between 0.011 and 0.171 in the hydrodynamically fully developed laminar flow regime with Re <160 which is applicable in milli and micro channel heat exchangers.