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Abstract: In this paper, the natural convection in a square cavity filled with Al2O3/water nanofluid has been investigated. The flow and heat transfer characteristics of the nanofluid in the cavity are documented when bottom wall is under heterogeneous heating, the right wall is considered cold while the top and left walls are adiabatic. Nine different cases are considered for the non-uniform heat flux where the total heat flux applied to the cavity is the same for all the cases in magnitude but the profile is different. For different values of Rayleigh numbers (103–106), nanoparticle volume fractions (0–9%), and aspect ratios, the optimal profile of heat flux is determined in which Nusselt number is maximized. It is found that the trend of Nusselt number is different for the nine cases at Ra =103 where the conduction is the dominant phenomenon. The results also reveal that for high values of Rayleigh number, i.e. 106, the Nusselt number is minimized where the heat flux in the vicinity of adiabatic wall is smallest while the heat flux near the cold wall is biggest. [Copyright &y& Elsevier]