Effects of Surface Roughness on Film Cooling

Abstract

A flat plate test section was used to study how surface roughness representative of an in-service turbine affects the film cooling adiabatic effectiveness and heat transfer coefficient for a round hole geometry. The injection was from a single row of round film cooling holes with injection angle of 30°. The density ratio of the injectant to the mainstream was 2.0 for the adiabatic effectiveness tests, and 1.0 for the heat transfer coefficient tests. A range of momentum flux ratios was examined to characterize the adiabatic effectiveness and heat transfer performance. Streamwise distributions of adiabatic effectiveness and heat transfer coefficients were obtained along the hole center line, and lateral distributions extending to halfway between hole centerlines were obtained at selected streamwise locations to 90 hole diameters downstream. Two rough surfaces with a factor of two difference in roughness levels were compared to an aerodynamically smooth surface. Both rough surfaces degraded the film cooling effectiveness with increasing degradation farther downstream. Degradation of film cooling effectiveness was greater at low momentum flux ratios. At momentum flux ratios high enough so that the cooling jets completely detached from the surface, the rough surfaces slightly increased adiabatic effectiveness in some cases. There was little difference in the effectiveness results between the two rough surfaces. Film cooling injection had little effect on heat transfer rates for any of the surfaces.