Heat transfer of an air jet impinging on a rough surface

The effect of surface roughness of a uniformly heated plate on the average heat transfer characteristics of an impinging air jet was experimentally investigated. Two aluminum plates, one with a flat surface and the second with some roughness added to the surface were fabricated. The roughness took the shape of a circular array of protrusions of 0.5mm base and 0.5mm height. A circular Kapton heater of the same diameter as the plates (70mm) was used to supply the necessary power. The surfaces of the plates were polished to reduce radiation heat losses and the back and sides insulated to reduce conduction heat losses. Target plate-surface temperatures were measured over a Reynolds number ranging from 9,625-38,500 based on flow rate through a 6.85mm diameter nozzle. The same measurements were repeated for nozzle exit-to-plate spacing, z/d, ranging from 1 to 10. The average Nusselt number for both cases was plotted versus the Reynolds number and then versus nozzle exit-to-plate spacing, z/d, and their correlations were determined. The results indicate an increase of up to 6.0% of the average Nusselt number due to surface roughness. This increase although modest provides an encouraging incentive to further investigate the surface roughness as a parameter for promoting heat transfer.