Over the last decades, intensive studies are performed on nanofluids behavior in many real-life situations as well as in simulated conditions. A lot of nanofluids types and enclosures are present in the open literature and one can acknowledge the intensive efforts in describing especially alumina or CNT based fluids and circular configurations (i.e. pipes, tubes, microtubes). Nevertheless, the enhanced thermophysical properties of the nanofluids are strongly recommending these new fluids as an extremely interesting option to classical heat transfer fluids. Thus, this article aims to study a less common heat transfer fluid, namely ZnO-water nanofluid and the chosen physical situation is natural convection in a closed rectangular enclosure. The numerical analysis was performed for different parameters as: heating wall position, nanoparticle volume concentration and Rayleigh number. Plus, an entropy generation analysis was conducted and a decrease was noticed while the Rayleigh number increases. In conclusion, research outlined a significant increase in entropy generation as well as a better behavior at heat transfer. (C) 2017 Elsevier Ltd. All rights reserved.