CHARACTERIZATION AND LABORATORY SIMULATION OF TURBINE AIRFOIL SURFACE-ROUGHNESS AND ASSOCIATED HEAT-TRANSFER

The physical characteristics of surface roughness observed on first-st age high-pressure turbine vanes that had been in service for a long pe riod were investigated in this study. Profilometry measurements were u tilized 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 val ues were shown to be inadequate for characterizing roughness effects. Using a roughness shape parameter originally derived from regular roug hness arrays, the turbine airfoil roughness was characterized in terms of equivalent sand-grain roughness in order to develop an appropriate simulation of the surface for laboratory experiments. Two rough surfa ce test plates were designed and fabricated. These test plates were ev aluated experimentally to quantify the heat transfer rate for flow con ditions similar to that which occurs 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 percent increases in heat tra nsfer rates relative to a smooth surface. The effects of high free-str eam turbulence, with turbulence levels from 10 to 17 percent, were als o investigated. Combined free-stream turbulence ans surface roughness effects were found to be additive, resulting in as much as a 100 perce nt increase in heat transfer rate.