The thermal performances of solid and perforated rectangular blocks attached on a flat surface in a rectangular duct were determined with respect to the heat transfer from the same plate without blocks. The data used in the performance analyses were obtained experimentally for varying flow and geometrical conditions. It was found that the solid blocks generated a net energy loss despite significantly enhanced heat transfer due to the increased heat transfer surface area. When the blocks were perforated, the loss in the net energy was recovered and depending on the geometrical and flow conditions, a net gain in energy, up to 20%, was achieved. For both the solid and the perforated blocks, increases in Reynolds number led to decreases in the performance. (C) 2000 Elsevier Science Ltd. All rights reserved.