The Combined Effects of Freestream Turbulence, Pressure Gradients, and Surface Roughness on Turbine Aerodynamics

A significant amount of research has been aimed toward turbine blade surface roughness, freestream turbulence, and pressure gradients and their associated efficiency losses. Typically, roughness studies use artificially created surfaces that are easily characterized by statistical parameters such as average centerline roughness, Ra, which is, in turn, often correlated to the well-defined equivalent sandgrain roughness, ks. This research differs in that it uses scaled facsimiles of real turbine blade surfaces to characterize correlations between blade roughness, with the combined effects of freestream turbulence and pressure gradients, and skin friction coefficient (Cf). The models tested yielded Ra values ranging from 0.1 to 1.2 mm and were representative of eroded, fuel deposited, pitted, and thermal barrier coated (TBC) surfaces with spallation. For the eroded surfaces, the addition of roughness caused Cf increases up to 300% when compared to flat plate data. Addition of freestream turbulence caused increases up to 125%. The combined effects of roughness and turbulence yielded increases up to 380%. This is 55% larger than simply summing the two independent effects. Though other surfaces typically offered less dramatic results, it was concluded that the Cf increases due to combined effects were consistently higher than the corresponding sum of the parts. The results presented for the combined effects of roughness and pressure gradients were inconclusive due to errors in measurement. However, limited observations seem to corroborate the trends seen with zero pressure gradient.