Natural convection of nanofluids around several pairs of hot and cold cylinders in an adiabatic enclosure is investigated numerically. Hot and cold cylinders are maintained at the different constant temperatures (T-h > Tc) while the walls of the enclosure are thermally insulated. A parametric study is undertaken to explore the effects of the pertinent parameters, such as; Rayleigh number, size and type of the nano particles, shape of the enclosure, orientation and number of the hot and cold cylinders on the fluid flow and heat transfer characteristic. The simulations show that at low Ra, by changing shape of the enclosure from square to triangular one, the heat transfer rate decreases. It is also found that at each Ra, there is an optimum volume fraction of nanoparticles (phi(opt)) where the heat transfer rate within the enclosure has a maximum value. Moreover, the results of this study showed by altering orientation of the hot and" cold cylindeis from horizontal to vertical mode, the heat transfer rate enhances. Finally, the results indicated that, by decreasing the size of the nanoparticles, the heat transfer rate and optimal particle loading (phi opt) enhances. (C) 2016 Elsevier Ltd. All rights reserved.