Heat transfer coefficient, flow field, and wall static pressure distributions were measured in a rotating two-pass square duct with detachment of 90 degrees ribs from the first pass leading wall and second pass trailing wall as well as attachment of 90 degrees ribs onto the other two opposite walls. Laser-Doppler velocimetry was used to measure the local flow velocity. The ribs were square in cross-section and their detached-distance/height ratio was 0.38. The rib-height/duct-height ratio and the pitch/rib-height ratio were 0.136 and 10, respectively. Duct Reynolds number was fixed at 1 x 10(4) and rotating number ranged from 0 to 0.2. Results are documented in terms of the main flow development, cross-stream secondary flow structure, the distributions of the pressure coefficient, the variation of friction factor with Ro, and passage averaged Nusselt number ratios under constant flow rate and constant pumping power conditions. For CFD reference, the periodic fully developed rotating flow condition is attained after the 6th rib pair in the first pass. In addition, the relationships between the regional averaged Nusselt number, transverse and streamwise mean velocity components, and turbulent kinetic energy are addressed. Using these relationships the general superiority of heat transfer enhancement of the attached-detached 90 degrees ribs arrangement over the attached-attached one can be reasonably illustrated. Simple expressions are obtained to correlate the friction factor with Ro, which are lacking in the published literature. The respective contributions of the ribs and passage rotation on the passage friction loss are identified. (c) 2007 Elsevier Ltd. All rights reserved.