Nanopowder shapes are highly effective in nanofluids properties. This study was a numerical analysis of the effect of nanopowder morphology on the natural convection and irreversibilities of water/alumina nanofluids. To this end, a square enclosure was used with constant cold (T-c) and hot (T-h) temperatures at its right and left walls, respectively. The horizontal surfaces were isolated and the enclosure was exposed to a magnetic field. Two Fins were located on the left wall sharing the same temperature as the wall. The control volume method was used to derive the algebraic form of the equations, and a SIMPLE algorithm written in Fortran was used for the simulations. To investigate the effect of nanopowder shapes, four different shapes, namely platelets, cylinders, blades, and bricks, were used. For a better comparison, nanofluids with different morphologies were compared with a Brownian motion-incorporated model. The results showed that this model predicts the highest heat transfer rate and entropy generation. As its most important result, this paper concluded that free heat convection in nanofluids is not necessarily increased by addition of nanopowder, and in some cases, the Nu number and irreversibilities can be decreased because of the significant increase in nanofluid viscosity. It was also observed that the Nu rises with Ra and falls with the magnetic field intensity. Generally speaking, the greatest and lowest heat transfer coefficients were obtained using the Angled and straight Fins, respectively. (C) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.