In this study, natural convection inside a square cavity filled with nanofluids with several pairs of heaters and coolers (HACs) inside is investigated numerically in the range of Rayleigh numbers between 10(4) and 10(7). Walls of the cavity are insulated and heaters and coolers walls are isothermal with temperatures of T-h and T-c (T-h > T-c). Two-dimensional Navier-Stokes and energy equations are solved using finite volume discretization method. Effects of various design parameters on the heat transfer rate are investigated. Design parameters considered in this study are: position, surface area, shape and orientation of HACs, volume fraction and types of nanoparticles. The results show that the highest and the lowest impacts of design parameters, on the enhancement of heat transfer rate are caused by changing the HAC position and types of nanoparticles, respectively. Moreover, it is found that for a constant surface area of the HAC at the entire range of Rayleigh number, rate of the heat transfer increases with changing orientation of the HAC from horizontal to vertical. Our simulations indicate that the heat transfer rate at all Rayleigh numbers can be enhanced more efficiently by increasing number of HACs than increasing the HAC size. The optimum value of volume fraction of nanoparticles which result in the highest rate of heat transfer in most cases found to be equal to 1%, and beyond that the heat transfer rate decreases. (C) 2013 Elsevier Ltd. All rights reserved.