Impingement jet array heat transfer with small-scale cylinder target surface roughness arrays

•The first physical effect is increased local mixing, vorticity fluctuations, and turbulent transport.•The second physical effect is increased thermal resistance and more substantial thermal insulation barrier.•The third physical effect responsible is increased wetted surface area with added roughness, relative to a smooth target plate.•Nusselt number ratios in the present study range from just above 1.0 to values as high as 2.25.•Nusselt number variations for the small cylinder roughness show different trends compared to target plates with other shapes of small roughness. The present investigation considers special small-scale roughness patterns on impingement target surfaces to increase surface heat transfer augmentation levels of impingement jet array cooling. Utilized are three different heights of cylinder small roughness elements, which are mounted on three different test surfaces. The cylinder-shaped roughness is small-scale because diameter is 7.5 percent and maximum height is 25 percent of impingement hole diameter. Associated results are compared with spatially-averaged Nusselt number ratio distributions measured on target surfaces with triangle small roughness, or rectangle small roughness. Data are obtained for impingement jet Reynolds numbers of 900, 1500, 5000, and 11,000. Nusselt number variations for the small cylinder roughness show different trends with streamwise development and changing roughness height, compared to target plates with small rectangle roughness and small triangle roughness. In general, this is because roughness elements which contain surface shapes with sharp edges generate increased magnitudes of vorticity with length scales of the order of the roughness element diameter. Such generation is not always present in an abundant fashion with the small cylinder roughness because of the smooth contours around each roughness element periphery. Such effects are illustrated by several data sets, including Nusselt numbers associated with the small cylinder roughness with a height of 0.250D at a turbulent Reynolds number of 11,000.