Natural convection heat transfer enhancement in horizontal concentric annuli using nanofluids ☆ E. Abu-Nada ⁎ , Z. Masoud, A. Hijazi Department of Mechanical Engineering, Hashemite University, Zarqa 13115, Jordan Available online 6 February 2008 Abstract Heat transfer enhancement in horizontal annuli using nanofluids is investigated. Water-based nanofluid containing various volume fractions of Cu, Ag, Al 2 O 3 and TiO 2 nanoparticles is used. The addition of the different types and different volume fractions of nanoparticles were found to have adverse effects on heat transfer characteristics. For high values of Rayleigh number and high L/D ratio, nanoparticles with high thermal conductivity cause significant enhancement of heat transfer characteristics. On the other hand, for intermediate values of Rayleigh number, nanoparticles with low thermal conductivity cause a reduction in heat transfer. For Ra = 10 3 and Ra=10 5 the addition of Al 2 O 3 nanoparticles improves heat transfer. However, for Ra=10 4 , the addition of nanoparticles has a very minor effect on heat transfer characteristics. © 2008 Elsevier Ltd. All rights reserved. Keywords: Nanofluids; Heat transfer enhancement; Natural convection; Concentric annulus 1. Introduction Heat transfer within horizontal annuli has many engineering applications such as heat exchangers, solar collectors, thermal storage systems, and cooling of electronic components. Several applications use natural convection as the main heat transfer mechanism. Therefore, it is important to understand the thermal behavior of such systems when only natural convection is in effect so that methods to enhance heat transfer characteristics in such systems can be devised. The geometric shape of the cylindrical annulus creates non- uniformity in heat transfer within the annulus. With a better understanding of the flow field, it is possible to devise methods for heat transfer enhancement. An innovative technique for improving heat transfer is using ultra fine solid particles in the base fluids, which has been used extensively in the past ten years. The term nanofluid refers to fluids in which nano-scale particles are suspended in the base fluid [1]. The particles are different from conventional particles (millimeter or micro-scale) in that they tend to remain suspended in the fluid and no sedimentation occur which causes no increase in pressure drop in the flow field [2]. The past decade has witnessed extensive work on convective heat transfer using nanofluids. Studies on the enhancement of heat transfer characteristics in forced convec- tion applications were conducted by a number of researchers. On the other hand, heat transfer enhancement in natural convection applications has received little attention. Examples of the work conducted on natural convection heat transfer include the work of Khanafer et al. [3] who studied Copper– Water nanofluids in a two dimensional rectangular enclosure. They found that the heat transfer rate increased by increasing the percentage of the suspended particles. Similar enhance- ment was acheived experimentally by Nnanna et al. [4] for Cu nanoparticles in ethylene glycol and by Nnanna and Routhu [5] for Alumina–Water nanofluids. However, different experi- mental findings were reported by Putra et al. [6] on the natural convection of Al 2 O 3 – and CuO–Water nanofluids inside a cylindrical enclosure heated from one side and cooled from the other. They found that the natural convection heat transfer coefficient was lower than that of pure water. Wen and Ding [7] found that the natural convection heat transfer coefficient Available online at www.sciencedirect.com International Communications in Heat and Mass Transfer 35 (2008) 657 – 665 www.elsevier.com/locate/ichmt ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: eiyad@hu.edu.jo (E. Abu-Nada). 0735-1933/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2007.11.004