The present study evaluates an innovative approach for enhancement of surface heat transfer in a channel using concavities, rather than protruding elements. Serving as a vortex generator, a concavity is expected to promote turbulent mixing in the flow bulk and enhance the heat transfer. Using a transient liquid crystal imaging system, local heat transfer distribution on the surface roughened by an staggered array based on two different shapes of concavities, i.e. hemispheric and tear-drop shaped, have been obtained, analyzed and compared. The results reveal that both concavity configurations induce a heat transfer enhancement similar to that of continuous rib turbulators, about 2.5 times their smooth counterparts 10,000 ≤ Re ≤ 50,000. In addition, both concavity arrays reveal remarkably low pressure losses that are nearly one-half the magnitudes incurred with protruding elements. In turbine cooling applications, the concavity approach is particularly attractive in reducing system weight and ease of manufacturing.

The effects of array configuration and pin-endwall fillet on the heat transfer and pressure drop of short pin-fin arrays are investigated experimentally. The pin-fin element with endwall-fillet, typical in actual turbine cooling applications is modeled by a spool-like cylinder. The arrays studied include an in-line and a staggered array, each having 7 rows of 5 pins. These arrays have the same geometric parameters, i.e. H/D = 1, S/D = X/D = 2.5, and the Reynolds number ranging from 5 × 103 to 3 × 10. One of the present results shows that the staggered array always has a higher array-averaged mass transfer coefficient than its in-line counterpart. However, the pressure drop for the staggered array is higher compared to the in-line configuration. These trends are unaffected by the existence of the pin-endwall fillet. Another significant finding is that an array with pin-endwall fillet generally produces lower heat transfer coefficient and higher pressure drop than that without endwall-fillet. This leads to the conclusion that pin-endwall fillet is undesirable for heat transfer augmentation. In addition, naive use of the heat transfer results obtained with perfectly circular cylinders tends to overestimate the pin-fin cooling capability in the actual turbine. The effects of endwall-fillet on the array heat transfer and pressure drop are much more pronounced for the staggered array than for the in-line array; however, they diminish as the Reynolds number increases.

Measurements are presented of local convection heat transfer for the case of flow through a narrow slot-type channel where one of the bounding walls contains a transverse rectangular cavity. The experimental situation is a stationary modeling of some salient features of flow through the clearance gap at the grooved tips of axial turbine blades. Cavity depth-to-width ratios of 0.1, 0.2, and 0.5 are included for each of clearance-to-width ratios of 0.05, 0.10, and 0.15. Overall heat transfer on the cavity floor is in general reduced as cavity depth is increased, but reduction with the deepest cavity tested is essentially the same as that of the intermediate depth cavity. Resistance to flow through the gap is increased as cavity depth is increased, but again the change between the deepest and intermediate depth cavities is small. In addition to the stationary experiments, heat transfer in the cavity with a moving as well as stationary shroud is modeled with a finite-difference method. The numerical results indicate that, within the range of parameters considered, heat transfer characteristics in the cavity are virtually unaffected by the shroud movement. This is in agreement with a previous finding for heat transfer on ungrooved blade tips.

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