Matrix cooling has opened new possibilities for enhancing the convective heat transfer coefficients without compromising upon the structural rigidity and the life of the gas turbine blade at elevated temperatures. However, the dense structure of the matrix significantly increases the flow resistance and imposes the limitation to its usage. Recently, a matrix with a gap on the sidewalls called open matrix has been proposed by few researchers to reduce the associated pressure penalties. This detailed experimental investigation aims to study the open matrix channel flow and presents the effects of varying sidewall gaps on heat transfer characteristics and friction factor in the open matrixes having rib angle 45 deg for three different subchannel aspect ratios 1.2, 0.8, and 0.4. Liquid crystal thermography has been utilized to discern the detailed heat transfer characteristics. The results have been evaluated in terms of augmentation Nusselt number, friction factor ratio, and overall thermal performance factor over the Reynolds numbers 5800–14,000. The closed matrixes provided the highest augmentation in Nusselt number, and the gaps on the sidewall have shown an overall reduction in augmentation Nusselt number in most cases. However, the suitable sidewall gaps show the effective reduction in pressure penalties for the smaller subchannel aspect ratios. The highest augmentation Nusselt number among the open matrixes has been found as 3.83 with a reduced friction factor ratio for the matrix with a 4-mm gap in subchannel aspect ratio = 0.8 (i.e., 4 subchannels) at Re = 8100.