Three-dimensional steady-state mixed convection heat transfer experiments are conducted on five protruding discrete heat sources (aluminum) of different sizes arranged at various positions on a substrate board (bakelite), mounted on a vertical channel. The objective is to study the mixed convection and surface radiation heat transfer characteristics of these heat sources. It is seen that the temperature excess of the heat sources is largely influenced by their size, positioning on the substrate board, power generation rate, and channel aspect ratio. The aspect ratio studies are carried out to investigate the effect of the channel size on the cooling of discrete heat sources. It is seen that increasing the channel aspect ratio has a negligible effect on the heat transfer characteristic of heat sources, and a suitable aspect ratio has to be maintained to obtain the optimal temperature of heat sources. Experiments are carried out for both polished and black painted heat sources to study the contribution of surface radiation. The results suggest that radiation is significant as the heat transfer is enhanced by 10% from polished to black painted surfaces. Again, the larger heat source will have less heat dissipation rate per unit area and must be placed on the substrate bottom. © 2018 by Begell House, Inc.