Characterization and Laboratory Simulation of Turbine Airfoil Surface Roughness and Associated Heat Transfer

The physical characteristics of surface roughness observed on 1st stage high pressure turbine vanes which had been in service for a long period were investigated in this study. Profilometry measurements were utilized to provide details of the surface roughness formed by deposits of foreign materials on different parts of the turbine vane. Typical measures of surface roughness such as centerline average roughness values were shown to be inadequate for characterizing roughness effects. Using a roughness shape parameter originally derived from regular roughness arrays, the turbine airfoil roughness was characterized in terms of equivalent sandgrain roughness in order to develop an appropriate simulation of the surface for laboratory experiments. Two rough surface test plates were designed and fabricated and these test plates were evaluated experimentally to quantify the heat transfer rate for flow conditions similar to that which occur on the turbine airfoil. Although the roughness levels on the two test plates were different by a factor of two, both surfaces caused similar 50% increases in heat transfer rates relative to a smooth surface. The effects of high free-stream turbulence, with turbulence levels from 10% to 17%, were also investigated. Combined free-stream turbulence and surface roughness effects were found to be additive, resulting in as much as a 100% increase in heat transfer rate.Copyright © 1996 by ASME