Natural convection of air over a constant heat-flux heated plate is studied experimentally for the case of rib elements located in a laminar boundary layer. Temperature measurements are performed using the IR thermography technique. Experimental results are presented with respect to the influence of the conductivity of rib elements and the geometrical parameter w/s, the ratio of the distance between two ribs to the rib width. A commercial finite-volume code is employed in order to understand the flow features around the ribs responsible for the heat exchange in the boundary layer. The places where minimum heat transfer and flow reattachment occur are determined experimentally and compared with the numerical predictions. (C) 1997 Elsevier Science Ltd.