Stability of nanofluids: Molecular dynamic approach and experimental study H. Farzaneh a , A. Behzadmehr a,⇑ , M. Yaghoubi b , A. Samimi c , S.M.H. Sarvari d a Mechanical Engineering Department, University of Sistan and Baluchestan, Islamic Republic of Iran b School of Mechanical Engineering, Shiraz University, Islamic Republic of Iran c Chemical Engineering Department, University of Sistan and Baluchestan, Islamic Republic of Iran d Mechanical Engineering Department, Shahid Bahonar University of Kerman, Islamic Republic of Iran article info Article history: Received 25 September 2015 Accepted 16 December 2015 Keywords: Nanofluid Dispersion stability DLVO Molecular approach Experimental study abstract Nanofluids as volumetric absorbent in solar energy conversion devices or as working fluid in different heat exchangers have been proposed by various researchers. However, dispersion stability of nanofluids is an important issue that must be well addressed before any industrial applications. Conditions such as severe temperature gradient, high temperature of heat transfer fluid, nanoparticle mean diameters and types of nanoparticles and base fluid are among the most effective parameters on the stability of nano- fluid. A molecular dynamic approach, considering kinetic energy of nanoparticles and DLVO potential energy between nanoparticles, is adopted to study the nanofluid stability for different nanofluids at dif- ferent working conditions. Different forces such as Brownian, thermophoresis, drag and DLVO are consid- ered to introduce the stability diagrams. The latter presents the conditions for which a nanofluid can be stable. In addition an experimental investigation is carried out to find a stable nanofluid and to show the validity of the theoretical approach. There is a good agreement between the experimental and theoretical results that confirms the validity of our theoretical approach. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction An overview on the rate of energy consumptions in different industrial installations indicates that improving thermal efficiency of the processes is very important. Various methods have been introduced to enhance heat transfer. Improving thermal character- istic of working fluid is one of the attractive methods for augment- ing the performance of different thermal devices. Nanofluid as a volumetric absorption system in solar thermal energy or as a work- ing fluid in different heat transfer equipments has been proposed by different researchers (for instance [1–4]) because of their inter- esting thermophysical properties such as thermal conductivity and absorption coefficient. However, there are many serious problems that must be over- come prior to any application of nanofluids. Among them nanopar- ticles’ aggregation and sedimentation or physicochemical stability of nanofluids, are the most important issues that must be well addressed. Sedimentation of nanoparticles causes severe problems on the hydrothermal behaviors of a process. However, various methods have been proposed for stabilization of nanofluid such as applying ultrasonic waves for breaking nanoparticle aggregates, coating nanoparticles with polymeric surfactants to prevent aggre- gation, external force field employment on nanofluid and changing of electrostatic properties of nanoparticles’ surfaces by variation of PH. Although, some of these methods are effective on the stabiliza- tion of nanofluids to some extent but none of them can perfectly solve the problem of the nanofluid stability. It is well known that the interparticle forces in nanofluid play an important role on the nanofluid stability. In 1917, Smoluchowski [5] made the first attempt to estimate the effect of direct motion of particles on coagulation. Fuchs [6] introduced a critical diameter for particles, above which aggrega- tion dominates (about 1 lm). Xinfang et al. [7] studied the influ- ence of hexadecyltrimethyl ammonium bromide (CATB) dispersant on the stability of copper nano-suspensions at different PH numbers. They introduced an optimum PH number in which maximum stability is achieved. Dongsheng et al. [8] investigates stability of alumina–water nanofluid at different PH numbers and various concentrations of sodium dodecylbenzenesulfonate (SDBS). They presented optimum values for PH number and SDBS concentration in which the stability of the nanofluid would be maximized. Xinfang et al. [9] also studied stability of copper water based nanofluid using CATB, TX-10 and SDBS surfactants. They http://dx.doi.org/10.1016/j.enconman.2015.12.044 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: amin.behzadmehr@eng.usb.ac.ir (A. Behzadmehr). Energy Conversion and Management 111 (2016) 1–14 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman