The Influence of In-Hole Roughness on Leading Edge Overall Effectiveness

Abstract The impact of surface roughness on gas turbine heat transfer has been of interest for many years, but relatively little attention has been paid to the effects of in-hole roughness. Even then, most efforts have been aimed at characterizing the influence of internal roughness on the heat transfer coefficient or the resulting impact on adiabatic effectiveness. That important previous work has documented roughness’s deleterious effects on the adiabatic effectiveness, but these are coupled with improved internal heat transfer coefficients. Characterization of these competing effects of internal roughness on the overall effectiveness has received little attention. In the present work, overall effectiveness experiments were conducted using a model of a leading edge showerhead. The approximately 10x scale model was made of hydrodynamically smooth conducting material to provide a Biot number representative of typical turbine components and allow determination of the overall effectiveness distribution for a baseline case of a smooth surface finish. Various combinations of rough surfaces were used to examine their influence on overall effectiveness — one with the exterior surface roughened, one with the internal surfaces (both holes and plenum) roughened, and one with both the internal and the external surfaces roughened. Then, the internal roughness was decomposed to investigate specifically in-hole roughness and plenum roughness individually. Surface roughness was provided by adhering filtered sand to the surfaces of interest, providing a root mean square roughness to hole diameter ratio of Rq/d = 0.025 and arithmetic mean roughness to hole diameter ratio of Ra/d = 0.020. This unique combination of roughness experiments allowed for segregation of the effects of interior surface roughness from the effects of the rough external surface. The interior hole roughness has competing effects on the overall effectiveness through a reduction in external film cooling performance, but at the same time, improved internal cooling. These competing effects were clear with overall effectiveness improvements occurring in regions where internal cooling dominates and degradations occurring where film cooling plays a larger role in overall effectiveness distributions.