Parametric effects on heat transfer of impingement on dimpled surface

Jet impingement on a dimpled surface is investigated experimentally for Reynolds numbers in the range 5000-11500, and jet-to-plate spacing from I to 12 jet-diameters. These include spatially resolved local Nusselt numbers with impingement both on the dimpled itself and on the flat portion between dimples. Two dimple geometries are considered: hemispherical dimples and double or cusp elliptical dimples. All experiments were carried under maximum crossflow that is the spent air exits along one way. At the narrow jet-to-plate spacing such as H/D-j = 2, a vigorous recirculation occurred, which prevented the dimpled plate to enhance heat transfer The effect of impinging jet positions meant that impinging onto dimples generated more and higher energetic vortices, and this led to better heat transfer performance. Cusped elliptical dimples increase the heat transfer compared to a flat plate less than the hemispherical geometry. The influence of dimple depth was also considered, the shallower dimple, d/D-d = 0.15, improves significantly the heat transfer by 64% compared to that of the flat surface impingement at H/D-j = 4; this result was 38% higher than that for a deeper dimple of d/D-d = 0.25. The very significant increase in average heat transfer makes dimple surface impingement a candidate for cooling applications. Detailed pressure measurements will form a second part of this paper however plenum pressure measurements are illustrated here as well as a surface pressure measurement on both streamwise and spanwise directions.