This study experimentally investigated the fluid flow and heat transfer characteristics of the porous heat sink situated in a rectangular channel with bypass spaces. The novel heat sink, with a size of 60 mm (L) x 60 mm (L) x 24 mm (H), was manufactured by inserting a copper conductive cylinder into the aluminum foam block. Three heat sinks were employed as test specimens: (Mode A) the pure Al-foam heat sink, (Mode B) the Al-foam heat sink with a 20 mm-diameter copper cylinder, and (Mode C) the Al-foam heat sink with a 30 mm-diameter copper cylinder. Air was employed as the coolant. The relevant parameters were the Reynolds number (Re) and the bypass ratio (R-A = A/(L x H), A was the cross-section area of the rectangular channel). The results of flow visualization depicted that the vortices appeared behind the heat sink and more near the heat sink as increasing the bypass ratio or the diameter of the copper cylinder. Heat transfer experiments indicated that the Mode B heat sink had the maximum average Nusselt number for various values of RA and Re. Additionally, the heat transfer enhancement by the conductive cylinder ((Nu) over bar/(Nu(A)) over bar,(Nu(A)) over bar was the average Nusselt number of Mode A) increased with increasing the Re at R-A < 1.6. However, as R-A > 1.6, smaller Re would have a bigger (Nu) over bar/(Nu(A)) over bar. Finally, according to all the experimental data, this work provided the empirical formula for the average Nusselt number. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.