Computational modeling, temperature measurements, and flow visualizations were conducted to investigate the enhancement of combined natural convection, conduction, and radiation heat transfer from a 25.4 mm x 25.4 mm discrete heat source in a 127 mm x 127 mm and 41.3 mm high enclosure, using pin-fin heat sinks. The pin-fin array was attached to the heated component, which was flush mounted on the enclosure base. Effects of flow confinement by enclosure walls on heat transfer from the pin-fin heat sink were found prominent. The existing natural convection heat transfer correlations for pin-fin arrays in free space showed large deviations from present measurements. Radiation was also found to contribute significantly to the overall heat transfer. Experimental results show that enhancement of heat transfer using pin-fin heat sinks was significantly different for horizontally and vertically heated orientations of the enclosure.