Int. J. Nanoparticles, Vol. 2, Nos. 1/2/3/4/5/6, 2009 173 Copyright © 2009 Inderscience Enterprises Ltd. Nanofluids natural convection heat transfer in horizontal annulus O. Zeitoun and Mohamed E. Ali* Mechanical Engineering Department, College of Engineering, King Saud University, Riyadh, 11421, Saudi Arabia E-mail: ozeitoun@ksu.edu.sa E-mail: mali@ksu.edu.sa *Corresponding author Abstract: The effect of using nanoparticles on natural convection heat transfer in horizontal cylindrical annuli was investigated numerically using finite volume technique together with SIMPLE algorithm for D o /D i = 2 and 1.25. Aluminium oxide (Al 2 O 3 ) nanoparticles in water as base fluid were used in the current investigation. Laminar conditions up to Rayleigh number Ra of 10 9 were investigated for 0%, 1% and 4% nanoparticle concentrations. The results indicated that nanoparticles can increase heat transfer in the conduction dominated regime while it decreases it in the convection dominated regime. Keywords: nanofluids; natural convection; heat transfer; laminar; horizontal; cylindrical annulus. Reference to this paper should be made as follows: Zeitoun, O. and Ali, M.E. (2009) ‘Nanofluids natural convection heat transfer in horizontal annulus’, Int. J. Nanoparticles, Vol. 2, Nos. 1/2/3/4/5/6, pp.173–181. Biographical notes: O. Zeitoun is an Associate Professor in Mechanical Engineering Department at King Saud University, Saudi Arabia. He received his BSc and MSc degrees in Mechanical Engineering at Alexandria University, Egypt, and PhD in Mechanical Engineering at McMaster University, Canada. His current research interests include heat transfer, two-phase flow and energy. M. Ali is a Professor in Mechanical Engineering Department, Thermal Fluid Sciences section at King Saud University. He obtained his MSc and PhD from the University of Colorado at Boulder, Colorado, USA. His main research interests involve fluid mechanics thermodynamics and heat transfer. He has published more than 50 papers in well recognised journals and conferences. He is a Reviewer for most of the international journals in the field. He was a Visiting Professor at both University of Colorado at Boulder Colorado and The Northwestern University at Evanston, Illinois at USA.