IJST, Transactions of Mechanical Engineering, Vol. 36, No. M2, pp 165-180 Printed in The Islamic Republic of Iran, 2012 © Shiraz University NUMERICAL STUDY OF MIXED CONVECTION IN AN ANNULUS BETWEEN CONCENTRIC ROTATING CYLINDERS WITH TIME-DEPENDENT ANGULAR VELOCITY * A. ABEDINI AND A. B. RAHIMI ** Faculty of Engineering, Ferdowsi University of Mashhad, P.O. Box No. 91775-1111, Mashhad, I. R. of Iran Email: rahimiab@yahoo.com Abstract– Numerical study of mixed convection in an annulus between concentric rotating horizontal cylinders is presented in this work by using finite volume method. The mixed convection is due to natural convection because of the temperature difference of the cylinders, and forced convection is a result of cylinder rotation with different time functions and in different directions. Here, the effect of different cylinders motions are investigated on flow parameters such as streamlines, and heat transfer parameters such as temperature contours, Nusselt numbers, along with required torque for rotation. The non-dimensionalization of the governing equations is different in this study such that the non-dimensional Reynolds number does not appear in the equations and the velocity magnitudes are Reynolds numbers themselves that can be used locally for analyzing flow in solution field. The obtained results are compared with the existing analytical and numerical of simpler cases which prove good agreements. Keywords– Mixed convection, annulus, rotating concentric cylinders, time-dependent angular velocity 1. INTRODUCTION Mixed convection problem in an annulus between concentric cylinders has attracted special attention because of its wide engineering applications such as mixtures, thermal energy storage systems, cooling of electrical components, double pipe heat exchangers designed for chemical processes and food industries, and nuclear reactors. Because of small dimensions and low velocities, flow in these devices is assumed laminar. Such laminar flow is often influenced by body forces which tend to produce secondary flow in the annular cross section. A good overview on concentric and eccentric annuli has been carried out by Kuehn and Goldstein [1, 2]. They measured the heat transfer coefficients in air and water in concentric and eccentric horizontal annuli. Their experimental data is commonly used to validate most of the recent numerical studies. Some years later Shahraki [3] demonstrated the effect of temperature-dependent properties on the streamlines and temperature distributions in a concentric annulus. The varying viscosity had the strongest effect on the flow fields, while the thermal conductivity had the strongest effect on temperature profiles. An unsteady natural convection in a horizontal annulus was investigated by Mizushima et al. [4]. Instability analyses for natural convection in horizontal annuli are extensively addressed in literature. Numerical results of the turbulent flow in a two dimensional domain were presented by Farouk and Guceri [5]. In their work, the natural convection was considered in the annulus between two horizontal concentric cylinders. The stream-vorticity equation was discretized by finite difference technique and turbulence was modeled by the κ-ε approach for Rayleigh numbers above 105. Padilla et al. [5] investigated laminar and unsteady natural convection at low Rayleigh number in a  Received by the editors October 27, 2011; Accepted March 13, 2012.  Corresponding author