The effect of array size and substrate thermal conditions upon heat transfer characteristics of an array of heated modules subjected to buoyancy assisted convection cooling in air is investigated. The comparative analysis of numerical and experimental data for multimodular array of heated modules mounted on a printed circuit board (PCB) forming one of the walls of a vertical channel is presented. Numerical investigation involved turbulent flow analysis based on a realizable model. Experimental efforts involved development of a test rig and use of state-of-the-art data acquisition system for simulating the heat transfer phenomenon from the module surfaces under various test conditions. Moderate to high flow velocities in the channel and the heat flux values near those occurring in electronic cooling applications using air as coolant are considered. Initially, the data for mean Nusselt number occurring at various rows in the array under consideration are presented, and in order to validate the numerical model, variation pattern and deviation between numerical and experimental results are analyzed keeping the substrate (PCB) as the insulated surface. The same numerical model was extended to study the cases of varying substrate thermal conditions. An empirical relation is proposed for evaluating Nu under different substrate thermal conditions for array geometries considered and flow rates within the parametric ranges discussed.