Experimental and numerical studies were conducted to investigate the forced convection and flow friction of a turbulent airflow in a horizontal air-cooled rectangular duct, with square-sectioned cross-ribs mounted on its bottom surface. Cross-sectional dimensions of the cross-ribs were 6.37 mm x 6.37 mm. Reynolds number of the fully turbulent flow was maintained constant at 12,380. Heat was supplied uniformly to the airflow via bottom surface of the duct only. Effects of varying the angle formed by the cross-ribs between 30degrees and 120degrees on the forced convection and flow friction were studied. It was found that an optimum angle corresponding to the highest heat transfer coefficient occurred between 60degrees and 70degrees.Computational predictions of forced convection and flow friction of the same rectangular duct mounted with cross-ribs were performed with the zonal k-epsilon. model, the stability-guaranteed second-order difference scheme and the block implicit method. A comparison with the experimental results indicated that a reasonably good agreement had been achieved. Existence of an optimum angle formed by the cross-ribs between 60degrees and 70degrees was also indicated, except the numerical predictions were lower than the experimental findings by approximately 2-10%. However, the flow friction had been slightly over-estimated by 2-11% with the numerical model.It was shown that forced convection could be enhanced sufficiently by mounting cross-ribs on the internal surfaces of a rectangular duct, especially when the optimum angle was used. (C) 2003 Elsevier Ltd. All rights reserved.