Entropy generation due to natural convection in right-angled triangular enclosures filled with porous media with top angles 15 degrees and 45 degrees for various thermal boundary conditions (cases 1-4) has been studied numerically using Galerkin finite element method. Simulations are carried out for Pr = 0.025 and 1000 in the range of Darcy numbers 10(-5)-10(-3). Note that, S-theta,S-max is observed at vertex of the cavity for cases 1 and 3, near corner between left wall and bottom wall for case 2 and near lower portion of the right wall for case 4. It is found that, S-psi,S-max is observed near middle portions of the side walls for all cases and the location of S-psi,S-max mainly depends on the presence of high velocity gradients. The total entropy generation, S-total, is found to be an increasing function of Da. It is observed that decrease in Be-a nu with Da for both phi s is due to increase in fluid friction irreversibility. Analysis of the variation of Be-a nu with Da for various fluids indicates that, higher Be-a nu values are observed for Pr = 0.025 due to low frictional irreversibility. It is observed that Theta(cup) decreases with Da for all Pr due to decrease in thermal mixing. It is found that, the maximum heat transfer rate ((Nu(l)) over bar or (Nu(r)) over bar) occurs for phi = 15 degrees cavities compared to phi = 45 degrees cavities at Da = 10(-3) in both isothermal and linear heating cases for all Pr due to larger thermal gradients at phi = 15 degrees. Overall, triangular cavities with phi = 15 degrees may be the optimal top angle for right angled triangular enclosure for thermal processing of all types of fluids (Pr = 0.025 and 1000) due to its high heat transfer rate ((Nu(l)) over bar or (Nu(r)) over bar), high thermal mixing (high Theta(cup)) and less total entropy generation (S-total) for all heating strategies (cases 1-4) especially at Da = 10(-4). (C) 2012 Elsevier Ltd. All rights reserved.