Mach Number/Surface Roughness Effects on Symmetric Transonic Turbine Airfoil Aerodynamic Losses

Abstract

The effects of surface roughness and Mach number on the aerodynamic performance of turbine airfoils are investigated in compressible, high-speed flows with exit freestream Mach numbers of 0.6, 0.8, and 0.9 and three different inlet turbulence intensity levels of 0.9, 5.5, and 16.2%. Three symmetric airfoils, each with the same shape and exterior dimensions, are employed with different rough surfaces. The nonuniform, irregular, three-dimensional roughness is characterized using the equivalent sand grain roughness size. Increased surface roughness size and increasing freestream Mach number produce larger magnitudes of nondimensional total pressure loss coefficients. Magnitudes of integrated aerodynamic losses change by a much larger amount as either the freestream Mach number or turbulence intensity are altered, when the airfoil is roughened (compared to smooth airfoil results). This is partially a result of the thicker boundary layers, which develop over the roughened surfaces giving greater blockage and less expansion of the flow through the airfoil passage. As a result, total pressure losses indicate that oblique shock waves are present at the trailing edge of the airfoil (with transonic passage flow) when the airfoil is smooth, which are not present when the airfoil surfaces become roughened.