Direct Experimental Measurements of Heat Transfer Coefficient Augmentation Due to Approach Flow Effects

Film cooling can have a significant effect on the heat transfer coefficient (HTC) between the overflowing freestream gas and the underlying surface. This study investigated the influence of approach flow characteristics, including the boundary layer thickness and character (laminar and turbulent), as well as the approach flow Reynolds number, on the HTC. The figure of merit for this study was the HTC augmentation, that is, the ratio of HTCs for a cooled versus uncooled surface. A heated foil surface provided a known heat flux, allowing direct measurement of HTC and augmentation. The foil was placed both upstream and downstream of the film cooling holes, in order to generate an approaching thermal boundary layer, as representative of actual engine conditions. High-resolution IR thermography provided spatially resolved HTC augmentation data. An open-literature shaped-hole design was used, known as the 7-7-7 hole, in order to compare with existing results in the literature. A variety of blowing conditions were tested from M = 0.5 to 3.0. Two elevated density ratios of DR = 1.20 and DR = 1.80 were used. The results indicated that turbulent boundary layer thickness had a modest effect on HTC augmentation, whereas a very high level of augmentation was observed for a laminar approach boundary layer. The presence of upstream heating greatly increased the HTC augmentation in the near-hole region, although these effects died out by 10-15 diameters from the holes.