Experimental study was carried out concerning the effects of a rod in the free convection boundary layer to the heat transfer by using Mach-Zender interferometer. The rod, whose diameter is from 10 to 18 mm, is made of a wood and is set over the width of the heated plate. The normal position of the rod from the heated plate is changed from 0 to 20 mm. For the upfacing horizontal heated plate set in the ambient air, the rod seems to obstruct the air flow, and also interferogram behaves as if rising flow is generated at the settling position of the rod. Heat transfer tends to be improved when the rod is set on the center section of the plate. For the vertical heated plate, the isotherms are separated into two sections, over and under the rod. The local heat transfer decreases near the settling position. However, it increases at the downstream region of the rod since the isotherms are closely spaced near the plate.

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