Measurements of local heat transfer coefficients and results of flow visualization experiments are presented for orthogonal mixed convection in a partial enclosure with uniformly heated side walls and adiabatic top and bottom walls. In this study, experiments using water as a test fluid are conducted in a small scale test section with different inlet and outlet configurations. The flow and heating conditions are chosen to simulate the high Rayleigh number (1010–1011) and low to moderate Reynolds number (0–9000) typically encountered in building heat transfer applications. As the flow rate through the enclosure increases, the enhancement of heat transfer above that for natural convection alone also increases. The local enhancement is as large as a factor of 7 for some flow situations. The variation of local heat transfer coefficient over the heated surface is found to be strongly affected by the recirculation of portions of the forced flow within the enclosure as well as the impingement on or separation of flow from the side walls in some regions. Mean heat transfer coefficients for the side walls are also reported here, which are determined by averaging the measured local values over the heated surface. The highest average enhancement is a factor of 2.6 at Ra = 1.6 × 1010 and Re = 8740 for a staggered orientation of the inlet and exit apertures. Correlations for the mean heat transfer coefficient and a flow regine map for different enclosure geometries are also proposed.