Optimization of Pin-Fins for a Heat Exchanger by Entropy Generation Minimization and Constructal Law

P.O. Box 552, Xi’an 710072, Shaanxi, China Masoud Asadi Department of Mechanical Engineering, Azad Islamic University Science and Research Branch, Tehran 1615918683, Iran e-mail: masoud2471@gmail.com Giulio Lorenzini Full Professor Department of Industrial Engineering, University of Parma, Parco Area delle Scienze, 181A, Parma 43124, Italy 1 Introduction Employing pin-fins on a heated surface promotes heat transfer performance. Given their inexpensive and simple structure, pin- fins have extensive applications in cooling ranges from electronic equipment to the automobile industry. In diesel engines, about two-thirds of the input energy is wasted through the exhaust gas and cooling water. In this sense, it is important that a serious effort should be launched for conserving this energy. A pin-fin heat exchanger is an excellent choice for recovering waste energy in an automobile with diesel engine. Traditional methods for design- ing the exchanger are not very applicable because of relatively high pressure drop. Effective optimization methods are therefore necessary to enhance heat transfer performance with low pressure drop. Although there exist various methods for optimization of designs of heat exchangers, such optimizations were considered as manual designs by using different optimization algorithm rather than based on nature design from the standpoint of thermodynam- ics. This study focuses on the optimization of pin-fin geometry for a new heat exchanger by using EGM and CL. EGM is widely used to evaluate thermal and energy systems in view of thermodynamic imperfection [1,2]. For example, Saffari- pour and Culham [3] presented a new nonintrusive method for the measurement of entropy production in microscale thermal-fluid devices. The entropy generation map was also obtained by post- processing the velocity and temperature distribution data. They used microparticle image velocimetry and laser-induced fluores- cence methods to measure data. Li and Kleinstreuer [4] analyzed the entropy generation in trapezoidal microchannels. They found that there existed an optimal Reynolds number range in order to 1Corresponding author. Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received March 22, 2014; final manuscript received June 25, 2014; published online March 17, 2015. Assoc. Editor: Cesare Biserni. Optimization of Pin-Fins for a Heat Exchanger by Entropy Generation Minimization and Constructal Law Pin-fins are considered as one of the best elements for heat transfer enhancement in heat exchangers. In this study, the topology of pin-fins (length, diameter, and shape) is opti- mized based on the entropy generation minimization (EGM) theory coupled with the con- structal law (CL). Such pin-fins are employed in a heat exchanger in a sensible thermal energy storage (TES) system so as to enhance the rate of heat transfer. First, the EGM method is used to obtain the optimal length of pin-fins, and then the CL is applied to get the optimal diameter and shape of pin-fins. Reliable computational fluid dynamics (CFD) simulations of various constructal pin-fin models are performed, and detailed flow and heat transfer characteristics are presented. The results show that by using the proposed system with optimized pin-fin heat exchanger the stored thermal energy can be increased by 10.2%.