Entropy generation minimization in turbulent mixed convection flows

Abstract This paper reports the results of a numerical investigation of turbulent mixed convection from a symmetrically heated vertical channel, bathed by a steady upward flow of cold air. The computations have been performed using FLUENT 6.2 by employing the k–e model for turbulence with enhanced wall treatment. The entropy generation rates due to (i) heat transfer across a finite temperature difference and (ii) irreversibility due to fluid friction have been calculated as post-processed quantities with the computed velocity and temperature profiles. Optimal inlet velocities at which the total entropy generation rate reaches a minimum value are found to exist, for every set of heat flux and aspect ratio. Further, this optimum velocity turns out to be independent of the aspect ratio and increases linearly with the heat flux. Simple and easy to use correlations for the optimum Reynolds number and the dimensionless average wall temperatures corresponding to the optima are developed. Plots of total entropy generation rate against the velocity clearly demonstrate that near the optimum conditions, buoyancy does not have a significant role to play in deciding the optimum. For the range of parameters considered in this study, it is seen that for optimum conditions, the ratio of the entropy generation due to fluid friction to total entropy generation rate, known in literature as the Bejan number, varies within a narrow band (0.14–0.22).

[1]  B. Abu-Hijleh.  NATURAL CONVECTION AND ENTROPY GENERATION FROM A CYLINDER WITH HIGH CONDUCTIVITY FINS , 2001 .

[2]  A. Bejan.  A Study of Entropy Generation in Fundamental Convective Heat Transfer , 1979 .

[3]  Dibakar Rakshit,et al.  Thermodynamic optimization of conjugate convection from a finned channel using genetic algorithms , 2005 .

[4]  C. Balaji,et al.  A Holistic Optimization of Convecting-Radiating Fin Systems , 2002 .

[5]  A. Baytaş.  Entropy generation for natural convection in an inclined porous cavity , 2000 .

[6]  Roydon Andrew Fraser,et al.  Analysis of mixed convection—Radiation interaction in a vertical channel: Entropy generation , 2002 .

[7]  Haecheon Choi,et al.  Direct numerical simulation of heated vertical air flows in fully developed turbulent mixed convection , 2003 .

[8]  Naresh Kumar,et al.  Second law optimization of convective heat transfer through a duct with constant heat flux , 1989 .

[9]  A. Baytaş,et al.  Optimization in an Inclined Enclosure for Minimum Entropy Generation in Natural Convection , 1997 .

[10]  Chin-Hsiang Cheng,et al.  Numerical predictions of entropy generations for mixed convective flows in a vertical channel with transverse fin array , 1994 .

[11]  P. Nag,et al.  Thermodynamic optimization of convective heat transfer through a duct with constant wall temperature , 1987 .

[12]  Mourad Magherbi,et al.  Entropy generation at the onset of natural convection , 2003 .

[13]  Adrian Bejan,et al.  Thermodynamic optimization of cooling techniques for electronic packages , 1996 .

[14]  Roydon Andrew Fraser,et al.  FREE CONVECTION AND ENTROPY GENERATION INSIDE A VERTICAL INPHASE WAVY CAVITY , 2004 .

[15]  Antonio Auletta,et al.  Entropy generation in natural convection in a symmetrically and uniformly heated vertical channel , 2006 .

[16]  M. Sasikumar,et al.  Optimization of convective fin systems: a holistic approach , 2002 .

[17]  H. Herwig,et al.  Entropy production calculation for turbulent shear flows and their implementation in cfd codes , 2005 .

[18]  Duu-Jong Lee,et al.  Second-law analysis on a pin-fin array under crossflow , 1997 .