This article aims to study the heat transfer performances and the degree of thermodynamic irreversibility of two types of hybrid nanofluids, namely MWCNT + Fe3O4/water and ND + Fe(3)O(4)water used in a flattened tube. The three-dimensional numerical analysis was performed for different working conditions as: Reynolds number (within the range 250-2000), volume concentration of hybrid nanoparticles (0-0.3%) and inlet temperatures (from 313 K to 333 K). The numerical results show that the use of hybrid nanofluids in flattened tubes enhances the heat transfer at all studied Reynolds numbers. The MWCNT + Fe3O4/water hybrid nanofluids exhibited higher enhancement in heat transfer compared to ND + Fe3O4/water hybrid nanofluids. In additions, the results indicate that the increase in volume concentration of hybrid nanoparticles leads to the decrease of the total entropy generation of MWCNT + Fe3O4/water and ND + Fe3O4/water hybrid nanofluids compared to base fluid. A reduction of entropy generation of 26.483% compared to the base fluid was obtained for 0.3 vol% MWCNT + Fe3O4 hybrid nanoparticles. (C) 2017 Elsevier Ltd. All rights reserved.