The present numerical investigation, based on the finite volume method, deals with the characterization of flow and thermal fields inside differentially heated square enclosures filled with Al2O3-water nanofluid. The study focuses on the effect of shapes and aspect ratios of nanoparticles (NPs), depicted by Rayleigh number (Ra), solid volume fraction (phi), and enclosure on both flow and heat transfer enhancement. Streamlines, isotherms contours, and velocity profiles as well as the average Nusselt number are considered. Results found show that the heat transfer rate increases with Rayleigh number as well as with nanofluid volume fraction. For the six different examined cases of NPs' aspect ratios, nanofluid with oblate spheroids NPs (d(p) = 0.13) was found to engender a significant enhancement in the overall heat transfer. In addition, heat transfer rate was more pronounced at great values of aspect ratios of NPs for prolate spheroids. Results also showed that heat transfer enhancement decreases as the Rayleigh number increases independently of the considered enclosure, shapes, and aspect ratios of NPs.