In this study, heat transfer characteristics and flow structures over protrusions in a turbulent channel flow were systematically investigated by DES numerically. Densely arranged protrusions with different depth ratios (h/D) are considered for Reynolds number Rem (based on bulk velocity and full channel height) between 3000 and 6000 while Prandtl number Pr is fixed at 0.7. It is found that larger height ratio induces higher friction factor and heat transfer. However the performance factor, on the other hand, first increases then reaches its asymptotic limit and even decreases with increasing height ratio. It is also observed that the highest skin friction, form drag and localized Nusselt number are found at the upstream portion of protrusion. Additionally, the distributions of friction factors and Nusselt number exhibit symmetrical features for the low protrusion configuration and asymmetric characteristics for the high protrusion arrangement. The asymmetric distribution of localized Nusselt number at large height ratio (h/D >= 15%) is found to be closely linked to the asymmetric vortex shedding from the back ridge of protrusions. (C) 2013 Elsevier Ltd. All rights reserved.