Due to its simplicity and low cost, air cooling is still the most attractive method for computer systems and other electronic equipments. In this field, achieving the best possible performance of air cooling was the primary goal for researchers in the past several years. Mixed convection heat transfer has been studied in horizontal channels, open at left and right sides with protruding discrete heaters installed at the bottom side. The flow is assumed to be steady and two dimensional. Non-dimensional equations of mass momentum and energy with Boussinseq approximation are solved using finite volume technique. The fluid flow and heat transfer characteristics in the channel and on the three faces of the heat sources are studied. The effect of mixed convection parameter Gr/Re2 (Ar) on the flow (stream function) and the heater maximum temperature is also investigated. The model predictions were verified by comparing them with experimental data obtained from test done on Hampden Model H6882 device.