This paper reports an experimental study on the natural convection heat transfer of CuO/Turbine oil nanofluids in a square enclosure with dimensions, lengh × width × height (cm), 10 × 10 × 10. One of the walls is cooled at a constant temperature bath (water) and the opposite wall of that is heated by a constant flux heat source, while the remaining walls of the enclosure is adiabatic. In the present experiment oil-based copper oxide nanofluids of various concentrations from 0 wt.% to 0.8 wt.% are formulated. The averaged particle size of copper oxide is about 60 nm. The results are presented in terms of the Nusselt number as a function of Rayleigh numbers, and based on the changes in thermophysical properties of the nanofluid under study, shows that applying the nanofluid for natural convection heat transfer enhancement in enclosure is not appropriate, and with increasing the concentration of nanoparticles, showing systematic heat transfer enhancement over the entire range of the Rayleigh number considered. There is an optimum concentration for CuO above 0.8% wt, that the presence of nanoparticles is suitable.

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