Effect of aspect ratio on entropy generation in a rectangular cavity with differentially heated vertical walls ☆ Gamze Gediz Ilis a , Moghtada Mobedi a , Bengt Sunden b, ⁎ a Department of Mechanical Engineering, Izmir Institute of Technology, 35430, Urla, Izmir, Turkey b Division of Heat Transfer, Lund Institute of Technology, Lund, Sweden Available online 26 March 2008 Abstract In the present study, entropy generation in rectangular cavities with the same area but different aspect ratios is numerically investigated. The vertical walls of the cavities are at different constant temperatures while the horizontal walls are adiabatic. Heat transfer between vertical walls occurs by laminar natural convection. Based on the obtained dimensionless velocity and temperature values, the distributions of local entropy generation due to heat transfer and fluid friction, the local Bejan number and local entropy generation number are determined and related maps are plotted. The variation of the total entropy generation and average Bejan number for the whole cavity volume at different aspect ratios for different values of the Rayleigh number and irreversibility distribution ratio are also evaluated. It is found that for a cavity with high value of Rayleigh number (i.e., Ra =10 5 ), the total entropy generation due to fluid friction and total entropy generation number increase with increasing aspect ratio, attain a maximum and then decrease. The present results are compared with reported solutions and excellent agreement is observed. The study is performed for 10 2 b Ra b 10 5 , 10 - 4 b ϕ b 10 - 2 , and Pr = 0.7. © 2008 Elsevier Ltd. All rights reserved. Keywords: Natural convection; Entropy generation; Bejan number 1. Introduction Natural convection heat transfer in enclosures has recently been an important topic due to its wide applications in energy storage systems, electronic cooling devices, heating and cooling of buildings etc. Entropy is employed as a key parameter for evaluation of quality in engineering applications. The second law of thermodynamics has been applied to cavity problems to determine entropy generations due to heat and flow transport in the cavity and consequently minimize the entropy generation. Dagtekin et al. [1] dealt with the prediction of entropy generation of natural convection in a Γ-shaped enclosure. They found that the main entropy generation is formed due to heat transfer for Ra b 10 5 , the contribution due to fluid friction becomes stronger for Ra N 10 5 . The transient state of entropy generation for laminar natural convection in a square cavity with heated vertical walls was numerically solved by Magherbi et al. [2]. Yilbas et al. [3] studied the natural convection and entropy generation in a square cavity with differential top and bottom wall temperatures. For the considered cavity, the total entropy generation increases with increasing wall temperature which means it becomes almost optimum for a certain Rayleigh number. Erbay et al. [4, 5] studied entropy generation during transient laminar natural convection in a square enclosure being heated either completely or partially from the left side wall and cooled from the opposite side wall. It is found that the active sides in the completely heated case are at the left bottom corner of the heated wall and right top corner of the cooled wall at the same magnitude. The optimization in an inclined square enclosure for minimum entropy generation was analyzed by Baytas [6]. Based on that study, the local heat transfer irreversibility and the local fluid friction irreversibility change by the inclination angle and the minimum entropy generation depends considerably on the inclination. Numerical prediction of local and total entropy generation rates for natural convection of air in a vertical channel symmetrically heated with a uniform heat flux was studied by Andreozzi et al. [7]. Mahmud and Islam [8] numerically investigated laminar free convection and entropy generation Available online at www.sciencedirect.com International Communications in Heat and Mass Transfer 35 (2008) 696 – 703 www.elsevier.com/locate/ichmt ☆ Communicated by J.W. Rose and A. Briggs. ⁎ Corresponding author. E-mail address: bengt.sunden@energy.lth.se (B. Sunden). 0735-1933/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2008.02.002