CFD simulation of irreversibilities for laminar flow of a power-law nanofluid within a minichannel with chaotic perturbations: An innovative energy-efficient approach Mehdi Bahiraei a,⇑ , Khashayar Gharagozloo a , Masoud Alighardashi b , Nima Mazaheri a a Department of Mechanical Engineering, Kermanshah University of Technology, Kermanshah, Iran b Mechanical Engineering Department, Engineering Faculty, Razi University, Kermanshah, Iran article info Article history: Received 23 February 2017 Received in revised form 5 April 2017 Accepted 21 April 2017 Keywords: Non-Newtonian nanofluid Chaotic geometry Second law of thermodynamics Minichannel Entropy generation TiO 2 nanoparticles abstract The irreversibilities caused by heat transfer and friction for a power-law nanofluid in a minichannel hav- ing chaotic perturbations are examined through calculation of entropy generation rates. Chaotic advec- tion, or Lagrangian chaos, is a flow regime in which chaos is developed in the physical domain. It can intensify mixing in laminar flows and therefore, increase heat transfer. The simulations are also carried out in a straight channel. An increase in either concentration or Reynolds number augments frictional entropy generation while decreasing thermal entropy generation. By increasing concentration in the chaotic channel, total entropy generation (i.e., frictional plus thermal) decreases at low Reynolds num- bers, however, a minimum (optimal) point occurs at a high Reynolds number, which is very important based on the second law of thermodynamics. Due to intense mixing in the chaotic channel, thermal boundary layer cannot grow and consequently, thermal entropy generation in this channel is much less than that in the straight channel. Therefore, although frictional entropy generation in the chaotic channel is greater than that in the straight channel, total entropy generation in the chaotic channel is smaller, which shows a lower level of irreversibility. Moreover, compared to the straight channel, the chaotic channel is of a lower Bejan number. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Recent years have witnessed a growing interest in the applica- tion of miniature devices which have much utility in thermal engi- neering such as electronics cooling, full cells, solar energy, and so forth. Miniaturized geometries offer a high heat transfer area-to- volume ratio making them excellent choices for high heat flux applications. However, due to the small size of these devices that are commonly of millimeter or even micrometer order, the flows are laminar, and turbulent regimes would not be applicable. There- fore, due to poor flow mixing, the convective heat transfer rate is limited. For increase of flow mixing in the miniaturized devices, the lit- erature contains two categories: (1) active mixing methods which are performed utilizing moving parts, varying pressure gradients, acoustic waves, electro-magnetic fields and so forth, and (2) pas- sive mixing methods which employ no energy input except the mechanism applied to move fluid into a channel such as pressure gradient. In order to maximize mixing in flows, advection is employed to accelerate the molecular diffusion process. The classical method is through turbulence by applying high Reynolds numbers to activate the formation of a Kolmogorov energy cascade from large to small eddy scales, which creates small-scale structures that cause signif- icant molecular diffusion and flow homogenization. Chaotic advec- tion, however, is a different approach to producing small-scale structures by using the stretching and folding characteristic of chaotic flows whose Lagrangian dynamics rapidly evolves into an intricate flow pattern. Development of mixing by chaotic advection is a kinematic process which does not need great Reynolds num- bers. It has the benefit over turbulence as it does not need the high energy inputs required to keep the Kolmogorov cascade in turbu- lent mixing and can, therefore, be employed in situations in which using high Reynolds numbers is not viable. Chaotic advection intensifies the mixing in the laminar flows and consequently, increases heat transfer, which makes it attractive for use in small-scale applications. http://dx.doi.org/10.1016/j.enconman.2017.04.068 /Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: m.bahiraei@kut.ac.ir (M. Bahiraei). Energy Conversion and Management 144 (2017) 374–387 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman