Experiments were performed to measure the heat transfer coefficient on the surface of a square flush heat source mounted at the center of an FR-4 plate in a small horizontal enclosure. Cases with the plate facing upwards and downwards and with the backside either insulated or convecting were considered. The heat transfer coefficients exhibited distinct behavior at high aspect ratio in which the dominant length scales were related to the source. At intermediate aspect ratios length scales of both the source and the enclosure were relevant, and at small aspect ratios a conduction limit was observed. The data were in significant disagreement with commonly available correlations for horizontal plates in infinite media and for enclosures with differentially heated walls.
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