Heat transfer correlations of perpendicularly impinging jets on a hemispherical-dimpled surface

Heat transfer results of an inline array of round jets impinging on a staggered array of hemispherical dimples are reported with the consideration of various parametric effects such as Reynolds number (ReDj), jet-to-plate spacing (H/Dj), dimple depth (d/Dd) and ratio of jet diameter to dimple projected diameter (Dj/Dd) for both impinging on dimples and impinging on flat portions. The results were normalized against those from a flat plate. The heat transfer was measured by using transient wideband liquid crystal method. Our previous work (Kanokjaruvijit and Martinez-Botas (2005) [1]) on the effect of crossflow scheme suggested that jet impingement coupled with channel-like flow formed by the crossflow helped enhance heat transfer on a dimpled surface; hence three sidewalls were installed to constrain the spent air to leave in one direction. Throughout the study, the pitch of the nozzle holes was kept constant at 4 jet diameters. The Reynolds number (ReDj) ranging from 5000 to 11,500, jet-to-plate spacing (H/Dj) varying from 1 to 12 jet diameters, dimple depths (d/Dd) of 0.15, 0.25 and 0.29, and dimple curvature (Dj/Dd) of 0.25, 0.50 and 1.15 were examined. The shallow dimples (d/Dd = 0.15) improved heat transfer significantly by 70% at H/Dj = 2 compared to that of the flat surface, while this value was 30% for the deep ones (d/Dd = 0.25). The improvement also occurred to the moderate and high Dj/Dd. Thereafter, the heat transfer results were correlated in dimensionless form by using logarithmic multiple regression. The correlations were reported with necessary statistics.