This numerical study reports on fluid flow and heat transfer characteristics in a cooling channel with various crescent ribs mounted on one wall. Based on available experimental data, a series of simulations with various turbulence models are conducted to find the best numerical model. Three kinds of ribs, i.e., the straight rib, the crescent rib concave to the stream-wise direction, the crescent rib convex to the stream-wise direction, are considered to improve thermal performance of the cooling channel. The studied Reynolds number varies from 8000 to 24,000. Mechanisms underlying the enhanced heat transfer by ribs are clarified. It is found that the crescent ribs evidently enhance local heat transfer on the endwall downstream the ribs by generating longitudinal vortices, which intensify flow mixing. Such vortices also increase the turbulent kinetic energy and reduce thickness of the boundary layer, which lowers local temperature nearby the target surface. Numerical results show that the cases with crescent ribs significantly outperform the case with straight ribs with respect to heat transfer performance. The crescent ribbed channels provide a 21-41% higher normalized average Nusselt number relative the straight ribbed channel, while inevitably lead to a 15-80% higher pressure drop. Overall, the case with crescent ribs concave to the stream-wise direction provides the best thermal performance. (C) 2017 Elsevier Ltd. All rights reserved.