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The film-cooling performance of a flat plate in the presence of low and high freestream turbulence is investigated using liquid crystal thermography. High-resolution distributions of the adiabatic effectiveness are determined over the film-cooled surface of the flat plate using the hue method and image processing. Three blowing rates are investigated for a model with three straight holes spaced three diameters apart, with density ratio near unity. High freestream turbulence is shown to increase the area-averaged effectiveness at high blowing rates, but decrease it at low blowing rates. At low blowing ratio, freestream turbulence clearly reduces the coverage area of the cooling air due to increased mixing with the main flow. However, at high blowing ratio, when much of the jet has lifted off in the low turbulence case, high freestream turbulence turns its increased mixing into an asset, entraining some of the coolant that penetrates into the main flow and mixing it with the air near the surface.

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The film-cooling performance of a flat plate in the presence of low and high freestream turbulence is investigated using thermochromic liquid crystal thermography. Full-surface distributions of the convective heat transfer coefficient are determined for three blowing rates on a model with three straight holes spaced three diameters apart. An increase in heat transfer coefficient due to mass injection is clearly observed in the images and is quantitatively determined for both the low and high freestream turbulence cases. The increase in heat transfer coefficient is greater than in previously published research, possibly due to the use of different, more representative thermal boundary conditions upstream of the injection location. These boundary conditions, along with high resolution images, may account for the appearance of “fork tine” patterns of high heat transfer due to the presence of these vortices, not previously seen. Although the driving potential for heat transfer is less, it is observed that in some instances film cooling may cause an increase in overall heat transfer due to the increase in heat transfer coefficient.