The Many Faces of Turbine Surface Roughness

Results are presented for contact stylus measurements of surface roughness on in-service turbine blades and vanes. Nearly 100 turbine components were assembled from four land-based turbine manufacturers. Both coated and uncoated, cooled and uncooled components were measured, with part sizes varying from 2 to 20 cm. Spanwise and chordwise two-dimensional roughness profiles were taken on both pressure and suction surfaces. Statistical computations were performed on each trace to determine centerline averaged roughness, rms roughness, and peak to-valley height. In addition, skewness and kurtosis were calculated; as well as the autocorrelation length and dominant harmonics in each trace. Extensive three-dimensional surface maps made of deposits, pitting, erosion, and coating spallation expose unique features for each roughness type. Significant spatial variations are evidenced and transitions from rough to smooth surface conditions are shown to be remarkably abrupt in some cases. Film cooling sites are shown to be particularly prone to surface degradation.