Effects of Surface Roughness and Turbulence Intensity on the Aerodynamics Losses Produced by a Suction Surface of a Simulated Turbine Airfoil

Abstract

The effects of surface roughness on the aerodynamic performance of turbine airfoils are investigated with different inlet turbulence intensity levels of 0.9 percent, 5.5 percent and 16.2 percent using the University of Utah Transonic Wind Tunnel. Three symmetric airfoils, each with the same shape and exterior dimensions, are employed with different rough surfaces created to match the roughness which exists on operating turbine vanes and blades subject to extended operating times and significant surface particulate deposition. The non-uniform, irregular, three-dimensional roughness is characterized using the equivalent sand grain roughness size. Mach numbers along the airfoil range from 0.4 to 0.7. Chord Reynolds numbers based on inlet and exit flow conditions are 0.54×106 and 1.02×106, respectively. The contributions of varying surface roughness and turbulence intensity level to aerodynamic losses, Mach number profiles, normalized kinetic energy profiles, and Integrated Aerodynamics Losses (IAL) are quantified. Results show that effects of changing the surface roughness condition on IAL values are substantial, whereas the effects of different inlet turbulence intensity levels are generally relatively small. Relative to smooth airfoils, these variations are due to: (i) augmentations of mixing and turbulent transport in the boundary layers which develop along the roughened airfoils, (ii) thicker boundary layers at the trailing edges of roughened airfoils, (iii) separation of flow streamlines at airfoil trailing edges, and (iv) increased turbulent diffusion in the transverse direction within the wakes of roughened airfoils as they advect downstream.