WALL ROUGHNESS EFFECTS ON STAGNATION-POINT HEAT-TRANSFER BENEATH AN IMPINGING LIQUID JET

Jet impingement cooling applications often involve rough surfaces, yet few studies have examined the role of wall roughness. Surface protrusions can pierce the thermal sublayer in the stagnation region and increase the heat transfer. In this paper, the effect of surface roughness on the stagnation-point heat transfer of an impinging unsubmerged liquid jet is investigated. Experiments were performed in which a fully developed turbulent water jet struck a uniformly heated rough surface. Heat transfer measurements were made for jets of diameters 4.4-9.0 mm over a Reynolds number range of 20,000-84, 000. The Prandtl number was held nearly constant at 8.2-9.1. Results are presented for nine well-characterized rough surfaces with root-mean-square average roughness heights ranging from 4.7 to 28.2 mum. Measured values of the local Nusselt number for the rough plates are compared with those for a smooth wall, and increases of as much as 50 percent are observed. Heat transfer in the stagnation zone is scaled with Reynolds number and a roughness parameter. For a given roughness height and jet diameter, the minimum Reynolds number required to increase heat transfer above that of a smooth plate is established. A correlation for smooth wall heat transfer is also given.