Experimental studies have been conducted to examine the forced convection and flow friction characteristics of air-cooled horizontal equilateral triangular ducts whose internal surfaces have been fabricated with uniformly spaced square ribs. Effects of duct geometry (i.e. relative rib height (H/D) and relative rib-to-rib spacing (S/W)) as well as the hydraulic-diameter based Reynolds number (Re-D) on heat transfer coefficient and friction factor of a fully developed turbulent air flow in a horizontal triangular duct with ribbed internal surfaces have been fully investigated. The ranges of experimental parameters under consideration are: HID from 0.11 to 0.21, S/W from 3.41 to 13.93, and ReD from 4000 to 23,000. Optimum relative rib height and relative rib-to-rib spacing corresponding to maximum thermal performance of this system have been determined, which are equal to 0.18 and 7.22, respectively. Flow friction in the triangular duct increases rather linearly with the relative rib height, but a maximum flow friction factor is obtained at the relative rib-to-rib spacing of 7.22. Non-dimensional expressions for prediction of average Nusselt number and friction factor in terms of ReD, HID and S/W have been developed correspondingly, which correlate well with the experimental data with maximum deviations of +/-3.5% and +/-8.7%, respectively. (C) 2004 Elsevier Ltd. All rights reserved.