The flow and heat transfer in ribbed coolant passages of aspect ratios (AR) 1: 1, 4: 1, and 1:4 are numerically studied through the solution of the unsteady Reynolds averaged Navier-Stokes (URANS) equations. The URANS procedure, which utilizes a two equation k-epsilon model for the turbulent stresses, is shown to resolve large-scale bulk unsteadiness. The computations are carried out for a fixed Reynolds number of 25,000 and density ratio of 0.13, while the Rotation number is varied between 0.12 and 0.50. At higher rotation numbers (>= 0.5) at least three inter-rib modules are required to ensure periodicity in the stream-wise direction. The flow exhibits unsteadiness in the Coriolis-driven secondary flow and in the separated shear layer. The average duct heat transfer is the highest for the 4:1 AR case. For this case, the secondary flow structures consist of multiple roll cells that direct flow both to the trailing and leading surfaces. The 1:4 AR duct shows flow reversal along the leading surface at high rotation numbers. For this AR, the potential for conduction-limited heat transfer along the leading surface is identified. The friction factor reveals an increase with the rotation number, and shows a significant increase at higher rotation numbers (similar to Ro = 0.5). (c) 2005 Published by Elsevier Ltd.