The Optimization of Thermal Performance of an Air Cooler Equipped with Butterfly Inserts by the Use of Imperialist Competitive Algorithm Ehsan Rezaei, 1 Alimohammad Karami, 1 Mohsen Shahhosseni, 2 and Masood Aghakhani 3 1 Mechanical Engineering Department, Kermanshah University of Technology, Kermanshah, Iran 2 Chemical Engineering Department, Razi University, Kermanshah, Iran 3 Mechanical Engineering Department, Razi University, Kermanshah, Iran The aim of this paper is to apply a novel optimization algorithm based on the imperialist competitive algorithm (ICA) to optimize the thermal performance of an air-cooled heat exchanger equipped with butterfly inserts. Experiments included inserts inclined at angles ranging from 45 to 135°. Also, the Reynolds number varied from 4021 to 16118. After data reduction, the regression equation of thermal perform- ance was obtained as a function of the Reynolds number and the inclined angle. Then the cost function was optimized by the use of ICA. One can be sure that the thermal performance will be optimized due to the optimization of the cost function. Compu- tational results indicate that the proposed optimization-algorithm is quite effective and powerful in optimizing the cost function. According to the results, in order to obtain maximum performance, the inclined angle must be about 95°. © 2012 Wiley Peri- odicals, Inc. Heat Trans Asian Res, 41(3): 214–226, 2012; Published online 3 January 2012 in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.20412 Key words: air-cooled heat exchanger, thermal performance, butterfly insert, optimization, imperialist competitive algorithm (ICA) 1. Introduction It is commonly known that the heat transfer rate of heat exchangers, especially for single-phase flows can be improved through many enhancement techniques. In general, heat transfer enhancement (HTE) techniques can be divided into two categories: (1) active techniques which need an external power source and (2) passive techniques which do not need an external power source. Some examples of passive HTE methods include: insertion of twisted stripes and tapes [1, 2], insertion of coil wire and helical wire coil [3, 4], and mounting of turbulent decaying swirl flow devices [5, 6]. Despite the high-pressure drop caused by an insert in embedded tubes, the use of tube inserts in heat exchangers has received a lot of attention during the last two decades [2, 7]. The increase in turbulence intensity and swirling flow may be the main reasons for HTE induced by tube inserts. An experimental study was carried out on heat transfer in a round tube equipped with propeller-type swirl generators by Eiamsa-ard et al. [8]. Saha [9] investigated the heat transfer and pressure loss behaviors in rectangular and square ducts with combined internal axial corrugations and twisted-tapes with and without oblique © 2012 Wiley Periodicals, Inc. Heat Transfer—Asian Research, 41 (3), 2012 214