Vol 20, No. 3;Mar 2013 118 office@multidisciplinarywulfenia.org Natural Convection Heat Transfer from Two Parallel Horizontal Cylinders Embedded in a Porous Medium inside a Horizontal Cylindrical Enclosure Ahmed Tawfeeq Ahmed Al-Sammarraie Mechanical Engineering Department, College of Engineering, University of Tikrit Tikrit, Iraq Tel: +964 770 3769881 E-mail: ahtawfeeq@gmail.com Abstract Natural convection heat transfer from two parallel horizontal cylinders embedded in a porous medium inside a horizontal cylindrical enclosure has been experimentally studied, under constant surface temperature boundary condition. The study has investigated the effects of medium Rayleigh number, rotation angle and spacing between two cylinders on their heat loss ability. The experimental rig consists of water container, test section and two copper cylinders with 19 mm diameter. The two cylinders embedded in a porous medium (alumina granules), with a particle diameter rate (3.818 mm). The experiments were done at the range of medium Rayleigh numbers between ( 1 . 18 6 . 1   Ra ), cylinders rotation angles ( o o o 90 , 45 , 0    ) and spacing ratios (S/D= 2, 2.5, 3). The study has clearly shown that the heat loss ability is a function of medium Rayleigh number, cylinders rotation angle and spacing between them. It is noticed that this ability increased with increase of the medium Rayleigh number and reaches the maximum value at the first and second cylinder (   ,   ) at spacing ratio (S/D=3) and minimum value at spacing ratio (S/D=2) at rotation angle ( o o 90 , 45     ) for the first and ( o o 90 , 0    ) for second cylinder, respectively. The experimental results are related by two correlating equations each one explains the dimensionless relationship of natural convection heat transfer from each cylinder that represented by Nusselt number against medium Rayleigh number, rotation angle and spacing ratio. Keywords: Natural Convection Heat Transfer, Parallel Cylinders, Cylindrical Enclosure, Porous Medium. 1. Introduction The process of heat transfer from or to a horizontal cylinder has a considerable importance in most engineering applications, such as in the design of heating and cooling devices, steam pipelines, heat exchangers and etc. So it had received an ample effort of theoretical and experimental research to detect their ability to absorb or dissipate heat for all flow types and in different arrangements (Al-Thaher and Yacoub, 1992). Also, the field has headed for, in the last decades, to search for catalysts can be used in a practical way that works to increase the amount of heat transmitted from the heat exchange systems such as porous media in its two types: packed beds and fluidized beds. The utilization of the porous medium, as a catalyst for heat transfer in these systems, may increases the heat transfer