Numerical simulations were performed on heat transfer characteristics in an unglazed transpired collector (UTC) and a glazed flat-plate collector. The simulation results were analyzed from the viewpoint of field synergy principle. It is found that velocity and temperature gradient are almost parallel to each other over most part of the computational domain in UTC, while nearly perpendicular to each other in glazed flat-plate collector. The average intersection angle between velocity vector and temperature gradient is 42.70 degrees and 88.53 degrees in UTC and glazed flat-plate collector, respectively. Heat transfer is significantly enhanced in UTC due to good synergy between velocity vector and temperature gradient, and the thermal efficiency of UTC is therefore much higher than that of glazed flat-plate collector. Field synergy principle could explain well the mechanism of heat transfer enhancement in UTC. Heat transfer in UTC is enhanced moderately with increasing suction velocity. The average intersection angle in UTC decreases with increasing suction velocity, rapidly at suction velocities below 0.05 m/s and slightly at suction velocities above 0.05 m/s. The effect of hole pitch on heat transfer in UTC is significant, while the effect of hole diameter is less pronounced. (C) 2013 Elsevier Ltd. All rights reserved.