Effect of in-hole roughness on film cooling from a shaped hole

While much is known about how macro-geometry of shaped holes affects their ability to successfully cool gas turbine components, little is known about the influence of surface roughness on cooling hole interior walls. For this study a baseline shaped hole was tested with various configurations of in-hole roughness. Adiabatic effectiveness measurements at blowing ratios up to three showed that in-hole roughness caused decreased adiabatic effectiveness relative to smooth holes. Decreases in area-Averaged effectiveness grew more severe with larger roughness size and with higher blowing ratios for a given roughness. Decreases of more than 60% were measured at a blowing ratio of three for the largest roughness values. Thermal field and flowfield measurements showed that in-hole roughness caused increased velocity of core flow through the hole, which increased the jet penetration height and turbulence intensity resulting in increased mixing between coolant and the mainstream. Effectiveness reductions due to roughness were also observed when roughness was isolated to only the diffused outlet of holes, and when the mainstream was highly turbulent.