The study of natural convection in rhombic enclosure has been a subject of interest in the recent past. In the current work, the effect of various shapes of the rhombic enclosure (phi=45 degrees, 60 degrees, and 75 degrees) is investigated for fixed areas (A=0.5, 1, and 1.5) over an extensive range of parameters (Pr-m=0.015-1000, Da(m)=10(-5)-10(-2), and Ra-m=10(6)) for an optimal thermal configuration. The rhombic enclosure is subjected to the isothermal heating of the bottom wall along with the cold side and top walls. The results are shown in the form of entropy generation maps (S and S) along with the heatlines (), streamlines (), and isotherms (). It is observed that S decreases near the left wall whereas S near the top and right walls increases with phi, irrespective of A. A large regime near the bottom portion of the left wall and right portion of the bottom wall corresponds to the higher magnitude of S and that extends over a wider region for the higher phi. The results in terms of total entropy generation (S-total), average Bejan number (Be-av), and average Nusselt number (Nub) are compared with those within the square cavity (phi=90 degrees). Overall, it may be concluded that the rhombic enclosure with phi=45 degrees at A=0.5 is the optimal configuration for the thermal processing of the fluids based on the moderate