The Effect of Freestream Turbulence on Film Cooling Heat Transfer Coefficient and Adiabatic Effectiveness Using Compound Angle Holes

Abstract

The film-cooling performance of a flat plate in the presence of low and high freestream turbulence is investigated using thermochromic liquid crystal thermography. Distributions of the convective heat transfer coefficient and adiabatic effectiveness are determined over the film-cooled surface of the flat plate. Three blowing rates are investigated for a model with one hole oriented at a compound angle of 45° and with an injection angle of 30° from the flat plate surface. 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. At low blowing ratio, freestream turbulence is shown to reduce the adiabatic effectiveness due to increased mixing between the cooling air and 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. This paper also contributes high-resolution contour plots that show the wider spreading of cooling air over the film-cooled surface as a result of high turbulence, and the asymmetric regions of high heat transfer.