Experimental mass (heat) transfer in and near a circular hole in a flat plate

Experiments are performed to investigate the local heat/mass transfer characteristics for flow through a single circular hole in a thin perforated plate (modeling a combustor wall). The naphthalene sublimation technique is employed to determine the local values on the hole's inner surface and in the vicinity of the hole entrance and exit. The hole-length-to-diameter ratio varies from 0.5 to 1.5, and the ratio of the diameter of the outer boundary (active area) to the hole diameter varies from 1.5 to 4.5. The Reynolds number based on the hole diameter is between 600 and 30 000. On the windward surface, the heat/mass transfer coefficient increases rapidly as the flow approaches the hole entrance due to flow acceleration with a thin boundary layer. Inside the hole, a separation zone at the hole entrance decreases with increasing Reynolds number and then remains constant, approximately 0.56 hole diameter in depth, as the Reynolds number is increased further. The mass transfer coefficient al the reattachment point is about four times that for fully-developed tube flow. The mass transfer variations indicate a laminar separation and a turbulent reattachment flow in this Reynolds number range. The transfer coefficient on the leeward surface is small for the single hole flow because of a weak entrainment-flow velocity. The overall transfer rate is dominated by the inside hole surface (approximately 60%) in spite of its small surface area. Correlations are proposed for local/average heal transfer in short single holes as a function of Reynolds numbers and hole aspect ratios. (C) 1997 Elsevier Science Ltd. All rights reserved.