Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts Second law analysis and multi-criteria optimization of turbulent heat transfer in a tube with inserted single and double twisted tape Shoaib Khanmohammadi * , Nima Mazaheri Department of Mechanical Engineering, Kermanshah University of Technology, Iran ARTICLE INFO Keywords: Entropy generation Multi-objective optimization Second law Convective heat transfer ABSTRACT Present paper deals with numerical modeling of heat transfer in a tube with two types of twisted tape inserted in a circular tube. Single twisted tape and double twisted (coaxial) tape as two enhancing elements are added to the circular tube. A numerical investigation conducted to explore the effects of these elements on the pressure drop (ΔP), convective heat transfer coefficient (h) and entropy generation (S). The results demonstrate that increasing the Reynolds number has a positive effect on the convective heat transfer coefficient in all cases due to enhance the flow swirling which improves the heat transfer on the tube wall. The results represent that the coaxial tape in the lower twisted ratio (TR) has a better performance than single tape while the opposite is true for a high twisted ratio. The results of pressure drop for two studied cases reveal that pressure drop in the coaxial tape is higher than single tape in all TRs and with increasing TRs the pressure drop experience a decrement. It can be concluded that the lower TR in two types of inserts leads to the lower total entropy generation. Also, the results of exergy destruction rate and second law efficiency demonstrate coaxial tape exhibit better performance than single tape. A genetic algorithm based multi-objective optimization for coaxial tape implemented. The results of opti- mization displays, the best performance can be achieved in the case TR, Reynolds number and heat flux are 3.46, 9008.93 and 97894.1 W/m 2 , respectively. 1. Introduction These days, owing to great demands and growing the consumption of energy, efficient use of energy has led to the development and em- ployment of many techniques termed as Heat Transfer Enhancement (HTE) methods. The main purpose of applying HTE methods is to in- crease the heat transfer rate and decrease the size and cost of the thermal systems. Applying these approaches in many engineering and industrial applications such as heat recovery processes, cooling of thermal devices, refrigeration, steam generation and air conditioning is an effective way for efficient use of energy. Generally, heat transfer enhancement methods can be classified into two different groups, namely passive and active. Active methods need an external energy source, while the passive ones do not require any extra energy. The passive methods are consisting of the modification in the geometries of devices that results in generating the severe flow mixing that causes greater heat transfer rates. One of the most popular passive methods which has recently be- come attractive to increase the heat transfer rate and increase the ef- ficiency of thermal devices for various applications is utilizing the swirl flow generators. The swirl flow generators create high flow mixing with the low cost and are effective and economical way to use as heat transfer improvement. Furthermore, employing the twisted tapes can augment the pressure drop and cause higher pumping power values. Therefore, designing a twisted tape with the reasonable pressure drop augmentation is one of the main concerns to increase the heat transfer with low pumping power. So far, different designs of twisted tapes have been studied by many investigators [1–14]. Chang et al. [15] experimentally investigated the influences of using serrated twisted tape on heat transfer enhancement in a circular tube. They found that by a decrease in twisted ratio, the Nusselt number and friction factor increase. The heat transfer en- hancement of tube fitted with the serrated twisted tape was 1.25–1.67 times greater than that of the tube fitted with the smooth twisted tape. Saha et al. [16] experimentally studied the features of heat transfer and pressure drop in a circular tube fitted with regularly spaced twisted tape. The results showed that the tape with a higher width able to improve heat transfer more than that with smaller width but lead to more friction loss. They showed that the classical twisted tape leads to higher pressure drop and heat transfer rate than the regularly spaced https://doi.org/10.1016/j.ijthermalsci.2019.105998 Received 19 October 2018; Received in revised form 4 January 2019; Accepted 25 June 2019 * Corresponding author. Department of Mechanical Engineering, Faculty of Energy, Kermanshah University of Technology, Kermanshah, Iran. E-mail address: sh.khanmohammadi@kut.ac.ir (S. Khanmohammadi). International Journal of Thermal Sciences 145 (2019) 105998 1290-0729/ © 2019 Elsevier Masson SAS. All rights reserved. T