In this paper, results of a thermodynamic investigation using the entropy generation minimization method for fully developed turbulent forced convection in a circular tube using water-Al2O3 nanofluid are presented. Circular tube cross section areas in the range 0.0000025 <= A(c) <= 0.05 m(2) were considered. Nanofluid volume fractions in the range 0% <= phi <= 6% and Reynolds numbers in the range 5000 <= Re <= 180,000 were used for each of the circular tube cross section areas considered. The entropy generation rates were determined numerically from the temperature and velocity fields obtained during the computational fluid dynamics analysis. From the study, it is shown that there is an optimal cross section area at each Reynolds number for which the entropy generation in the tube is a minimum. The optimal cross section area increases as the Reynolds number increases. The study also shows presence of a Reynolds number at each nanofluid volume fraction and cross section area beyond which using nanofluids does not make thermodynamic sense. Correlations for the optimal Reynolds number and Reynolds number beyond which using nanofluids makes no thermodynamics sense were derived and presented. (C) 2015 Elsevier Ltd. All rights reserved.