Effects of variable viscosity and thermal conductivity of Al 2 O 3 –water nanofluid on heat transfer enhancement in natural convection Eiyad Abu-Nada * Department of Mechanical Engineering, Hashemite University, Zarqa 13115, Jordan article info Article history: Received 18 November 2008 Received in revised form 9 February 2009 Accepted 10 February 2009 Available online 28 March 2009 Keywords: Nanofluid Viscosity Thermal conductivity Natural convection Annulus abstract Heat transfer enhancement in horizontal annuli using variable properties of Al 2 O 3 –water nanofluid is investigated. Different viscosity and thermal conductivity models are used to evaluate heat transfer enhancement in the annulus. The base case uses the Chon et al. expression for conductivity and the Ngu- yen et al. experimental data for viscosity which take into account the dependence of these properties on temperature and nanoparticle volume fraction. It was observed that for Ra P 10 4 , the average Nusselt number was reduced by increasing the volume fraction of nanoparticles. However, for Ra = 10 3 , the aver- age Nusselt number increased by increasing the volume fraction of nanoparticles. For Ra P 10 4 , the Nus- selt number was deteriorated every where around the cylinder surface especially at high expansion ratio. However, this reduction is only restricted to certain regions around the cylinder surface at Ra = 10 3 . For Ra P 10 4 , the difference in Nusselt number between the Maxwell Garnett and Chon et al. model predic- tion is small. But, there was a deviation in prediction at Ra = 10 3 and this deviation becomes more signif- icant at high volume fraction of nanoparticles. The Nguyen et al. data and Brinkman model gives completely different predictions for Ra P 10 4 where the difference in prediction of Nusselt number reached 30%. However, this difference was less than 10% at Ra = 10 3 . Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Natural convection heat transfer is an important phenomenon in engineering systems due to its wide applications in electronic cooling, heat exchangers, and thermal systems. Enhancement of heat transfer in such systems is very essential from the industrial and energy saving perspectives. The low thermal conductivity of conventional heat transfer fluids, such as water, is considered a pri- mary limitation in enhancing the performance and the compact- ness of such thermal systems. An innovative technique for improvement of heat transfer using nano-scale particle dispersed in a base fluid, known as nanofluid (Choi, 1995), has been studied extensively in recent years (Daungthongsuk and Wongwises, 2007; Trisaksri and Wongwises, 2007) mainly for forced convection applications. However, natural convection heat transfer research using nanofluids has received very little attention and there is still a debate on the effect of nanoparticles on heat transfer enhance- ment in natural convection applications. Examples of these controversial results are the results reported by Khanafer et al. (2003) who studied Cu–water nanofluids in a two dimensional rectangular enclosure. They reported an increase in heat transfer with the increase in percentage of the suspended nanoparticles at any given Grashof number. Oztop and Abu-Nada (2008) showed similar results, where an enhancement in heat transfers was registered by the additions of nanoparticles. How- ever, contrary experimental findings were reported by Putra et al. (2003) using Al 2 O 3 and CuO water nanofluids. They reported that the natural convection heat transfer coefficient was lower than that of clear flow. Additionally, another experimental work, in natural convection, by Wen and Ding (2006) reported deteriora- tion in heat transfer by the addition of nanoparticles. Most re- cently, Abu-Nada et al. (2008) showed that the enhancement of heat transfer in natural convection depends mainly on Rayleigh number and for certain Rayleigh numbers, Ra = 10 4 , the heat trans- fer was not sensitive to nanoparticles concentration whereas at higher values of Rayleigh number an enhancement in heat transfer was taking place. Therefore, there is still a controversy on the effect of nanofluids on heat transfer in natural convection and the numerical simulations seem to over estimate the enhancement of heat transfer in natural convection. In fact, convective heat transfer is affected by the thermophys- ical properties of the nanofluid such as viscosity and thermal con- ductivity. A recent nanofluid heat transfer study on forced convection conducted by Ben Mansour et al. (2007) revealed that for forced convection different expressions for the thermophysical properties of nanofluids lead to totally different predictions for the performance of system. All of the previous mentioned numerical 0142-727X/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.ijheatfluidflow.2009.02.003 * Tel.: +962 390 3333; fax: +962 382 6613. E-mail address: eiyad@hu.edu.jo International Journal of Heat and Fluid Flow 30 (2009) 679–690 Contents lists available at ScienceDirect International Journal of Heat and Fluid Flow journal homepage: www.elsevier.com/locate/ijhff