A comprehensive understanding of energy flow and entropy generation is needed for an optimal process design via reducing irreversibilities in terms of 'entropy generation'. In this study, analysis on entropy generation during natural convection in a trapezoidal cavity with various inclination angles (phi = 45 degrees, 60 degrees and 90 degrees) have been carried out for an efficient thermal processing of various fluids of industrial importance (Pr = 0.015, 0.7 and 1000) in the range of Rayleigh number (10(3) - 10(5)). The total entropy generation (S-total), average Bejan number (Be-av) and average heat transfer rate ((Nu(b)) over bar and (Nu(t)) over bar) have been computed. The comparison of magnitudes of S-theta and S-psi indicates that maximum entropy generation due to heat transfer (S-theta,S- max) is identical for both Ra = 10(3) and Ra = 10(5) whereas maximum entropy generation due to fluid friction (S-psi,S- max) is lower for Ra = 10(3) and that is higher for Ra = 10(5) due to enhanced fluid flow at higher Ra irrespective of phi and Pr. The total entropy generation (S-total) is found to increase with Pr due to increase in S-psi with Pr. The non-isothermal heating strategy (case 2) is found to be an energy efficient due to less total entropy generation (S-total) values despite its lower heat transfer rate ((Nu(b)) over bar) based on lesser heating effect than isothermal heating (case 1) for all phi s. (C) 2011 Elsevier Ltd. All rights reserved.