This work focuses on the numerical study of natural convection fluid flow and heat transfer of (Cu, Al2O3 and TiO2)-water nanofluids in the annuli of two partially-heated square ducts. The left and right vertical walls of the outer duct have constant but different temperatures Th and Tc respectively, while the inner duct is maintained at the same and opposite temperatures. The ducts top and bottom walls are kept insulated. The governing equations are solved numerically using the finite-volume method. The SIMPLEC algorithm was employed to couple velocity and pressure fields. Using the developed in house code, the effects of pertinent parameters such as the Rayleigh number, volume fraction of nanoparticles and aspect ratio on the fluid flow and heat transfer inside the cavity were investigated. It is observed from the results that the average Nusselt number increases when both the Rayleigh number and the volume fraction of the nanoparticles increase. Moreover, heat transfer in nanofluid with Cu nanoparticles is much better than Al2O3 and TiO2 nanoparticles.

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