The present paper reports heat-transfer enhancement and the corresponding pressure drop over a flat surface in a channel flow due to perforated rectangular cross-sectional blocks attached on its surface. The channel had a cross-sectional area of 80 x 160 mm(2) with blocks 10 x 25 mm(2). The experiments covered the following range: Reynolds number (Re) 6670-40 000, the hole inclination angle (0) 0 45 degrees, the perforation open-area ratio (phi) 0.05-0.15, the diameter of the perforations (D) 2.5-8.0 mm, and the number of the blocks (N-b) 2-7 (giving the ratio of the distance between the blocks to the channel hydraulic diameter (S-x/D-e) 1.407-0.309). The blocks were transverse to the main flow. It was observed that the heat-transfer enhancement increased with increasing theta, phi and D and decreasing S-x/D-e and Re. The pressure drop was not affected by theta while it decreased with increasing D, Re, S-x/D-e, and phi. Correlation equations were developed for the average Nusselt number ((N) over baru) and the friction factor (f). Performance analysis indicated that the solid blocks could lead to energy loses up to 20% despite significantly enhanced heat-transfer due to the increased surface area. The energy lost was recovered by perforations opened in the blocks by which means it was possible to achieve energy gains up to 40%. (C) 2001 Elsevier Science Inc. All rights reserved.