Enhancement of the heat transfer from a flat surface in a channel flow by attachment of rectangular cross-sectional blocks has been investigated as a function of Reynolds number (Re), arrangement of the blocks with respect to the main flow direction as well as each other, and the numbers (spacing) of the blocks. The channel had a cross-sectional area of 80 x 160 mm(2) (i.e. an aspect (width-to-height) ratio of 2). Re, based on the hydraulic diameter of the channel (D-e) and bulk mean velocity (u), changed in the range of 6670-40000. The blocks were positioned both transverse and parallel with respect to the main flow direction. The parallel blocks were arranged in both in-line and staggered orientation with respect to each other. The effect of the blocks on the flow pressure drop was also measured. The results indicated that the heat transfer could be enhanced or reduced depending on the spacing between the blocks and their positioning and arrangement. For a given pressure drop, the best heat transfer enhancement by the blocks over that from a smooth surface (without blocks) was obtained when the blocks were positioned parallel to the flow and arranged in a staggered manner with respect to each other. Copyright (C) 2001 John Wiley & Sons, Ltd.