Heat Transfer and Pressure Investigation of Dimple Impingement

Heat transfer and pressure results of an inline array of round jets impinging on a staggered array of dimples are reported with the consideration of various geometric and parametric effects; results are normalized against flat plate data. The heat transfer was measured by using transient wideband liquid crystal method. The geometrical configurations considered were crossflow (or spent-air exit) scheme, dimple geometries and impinging positions. Three crossflow schemes were tested such as one-way, two-way and free exits. These led to the idea of the coupling effects of impingement and channel flow depending on which one dominated. Hemispherical and cusped elliptical dimple shapes with the same wetted area were considered, and found that both dimples showed the similarity in heat transfer results. Impinging positions on dimples and on flat portions adjacent to dimples were examined. Throughout the study, the pitch of the nozzle holes was kept constant at 4 jet diameters. The investigated parameters were Reynolds number (Re_Dj) ranged from 5000 to 11 500, jet-to-plate spacing (H/D_j) varied from 1 to 12 jet diameters, dimple depths (d/D_d) of 0.15, 0.25 and 0.29, and dimple curvature (D_j/D_d) of 0.25, 0.50 and 1.15. The shallow dimples (d/D_d = 0.15) improved heat transfer significantly by 70% at H/D_j = 2 compared to that of the flat surface, while this value was 30% for the deep ones (d/D_d = 0.25). The improvement also occurred to the moderate and high D_j/D_d. The total pressure was a function of Re_Dj and H/D_j when H/D_j < 2, but it was independent of the target plate geometry. The levels of the total pressure loss of the dimpled plates were not different from those of the flat surface under the same set-up conditions. Wall static pressure was measured by using static taps located across each plate. Re_Dj and H/D_j affected the level of the static pressure while the dimple depth influenced the stagnation peaks, and the crossflow scheme affected the shape of the peaks.