In the present study, natural convection of Cu-water nanofluid in a cavity with an array of porous fins on its hot wall has been numerically analyzed using two-phase approach. Use of porous fins, instead of solid ones, improves conduction while could have negligible effect on convection as flow can pass through them. Therefore, the effects of the number of fins and their length on heat transfer enhancement and entropy generation are scrutinized. The study has been conducted for the certain pertinent parameters of Rayleigh number (Ra = 10(4) to 10(6)), Darcy number (Da = 10(-1) to 10(-4)), and the nanoparticle volume fraction (phi = 0 to 0: 04) and results are investigated in terms of heat transfer, entropy generation and performance coefficient (PEC). Numerical results indicate that adding porous fins with a high Darcy number improves heat transfer while fins with a low Darcy number can weaken the convection and decline Nusselt number. In strong flow fields an increase in either the length or the number of fins has insignificant effect on Nu. Also, low concentration of nanoparticles enhances the heat transfer more than high values of nanoparticles. On the other hand, entropy generation decreases by increasing the number of fins and PEC enhances by using porous fins in most of the studied cases. PEC of pure fluid is higher than the nanofluid at low Ra numbers, while opposite fact is observed for high Ra values. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.