Entropy generation analysis of turbulent convection flow of Al 2 O 3 –water nanofluid in a circular tube subjected to constant wall heat flux Vincenzo Bianco a,⇑ , Oronzio Manca b , Sergio Nardini b a University of Genoa – DIME/TEC, Division of Thermal Energy and Environmental Conditioning, Via All’Opera Pia 15/A, 16145 Genova, Italy b Seconda Università degli Studi di Napoli, Dipartimento di Ingegneria Industriale e dell’Informazione, Via Roma 29, 81031 Aversa, CE, Italy article info Article history: Received 1 June 2013 Accepted 24 September 2013 Keywords: Nanofluids Entropy generation Convection Constant heat flux abstract A parametric investigation of entropy generation of nanofluid turbulent forced convection inside a circu- lar section tube subjected to constant wall heat flux is presented. The analysis is developed for different inlet conditions (i.e. constant Re, fixed mass flow rate and constant velocity) and in a concentration range from 0% up to 6%. The impact of the dispersed nanoparticles on total, thermal and frictional entropy generation is investigated and optimal working conditions are highlighted. The study shows that at the increase of Re, the optimal particles concentration to minimize entropy generation decreases. The impact of nanoparticles dimension is also taken into account, showing that its effect depends on the flow conditions. When working with constant mass flow rate, an optimal concentration is determined to minimize the frictional entropy generation, whereas thermal entropy generation results to increase. The study shows that to minimize total entropy generation when velocity is kept constant, a low concen- tration of nanoparticles is necessary. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The aim of the present paper is to develop a parametric investi- gation on the entropy generation of nanofluid turbulent convection flow inside a circular section tube subjected to constant wall heat flux, in order to determine optimal working conditions for the system under investigation. Nanofluids consist in the suspension of nanoparticles inside a base fluid, in order to increase its thermal conductivity. This idea is not new, in fact it was proposed about one hundred years ago by Maxwell [1], but, due to the limited manufacturing capabilities of that time, there was the possibility to utilize only micrometer particles, which have the inconvenience to cause a relevant in- crease of shear stress, sedimentation and abrasion. This technolog- ical constraint limited the diffusion of solid–liquid mixture as heat transfer fluid. In the early 90s, the concept proposed by Maxwell was developed at Argonne laboratory by using nanoparticles and, for the first time, the word ‘‘nanofluid’’ was introduced. Nanofluids present an increase of thermal conductivity and do not show an ex- treme penalty on the shear stress. For these unique features, in the last years, they have attracted the attention of many researchers all around the world, as reviewed in [2]. In the present paper, a methodology is proposed to address the problem of the optimal design when, nanofluids are utilized. Entropy generation analysis is considered an effective tool to investigate thermal design optimization [3] and by minimizing it, better working conditions can be achieved. Entropy generation represents the irreversibility of a system, therefore by reducing it, a more efficient system is obtained. Entropy generation minimi- zation is a well-established approach to pursue the optimization of thermal system, as reported in [3–6]. The aim of the present work is to apply the entropy generation analysis to nanofluids flow, in order to establish optimal working conditions for this particular category of heat transfer fluid. Specif- ically, when working with nanofluids, it is of great importance to determine the optimal concentration to use and the most conve- nient particles dimension to consider. In our opinion, entropy gen- eration analysis offers a rigorous physical framework to solve the above mentioned problems. The present work provides a fast and simple methodology to determine the main nanofluid parameters (i.e. concentration and dimensions) to guarantee the optimal working condition of a given device. On the contrary, to perform the optimization of a new de- vice (i.e. the shape) a detailed CFD analysis is necessary, in order to study the interaction between the fluid flow and the considered object. To the best of authors’ knowledge, in literature there are not available works dealing with entropy generation analysis of nanofluids forced turbulent convection in circular tube subjected to constant wall heat flux, which take into account, at the same time, the concentration and dimension of nanoparticles. Thus, it 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.09.049 ⇑ Corresponding author. Tel.: +39 010 353 2872. E-mail addresses: vincenzo.bianco@unige.it, vbianco@libero.it (V. Bianco). Energy Conversion and Management 77 (2014) 306–314 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman