Roughness and Secondary Flow Effects on Turbine Vane External Heat Transfer

Detailed external heat transfer measurements on a highly loaded turbine vane with varying surface roughness are presented. The investigation concentrates on the area close to the end wall, where secondary flow effects dominate the heat transfer distribution. A total of eight different surface topographies including a smooth reference surface are considered. The rough surfaces consist of evenly spaced truncated cones with varying height, diameter, and distance, thus covering the full range of roughness Reynolds numbers m the transitional and fully rough regimes. For each surface configuration, measurements are conducted at two different inlet turbulence levels (Tu1 = 4% and Tu1 = 8.8 %) and a Reynolds number of Re1, = 2.5 x 105. The results show a strong influence of roughness on the onset of transition and the heat transfer coefficients in the region with turbulent boundary layer. In areas affected by secondary flow, the effect of roughness on heat transfer is most pronounced, reaching two times the augmentation rates found in the 2-D turbulent boundary layer. In a series of supplemental studies, a trip wire is placed on the suction side to fix the transition location close to the stagnation line. Avoiding the dominating effect of different transition locations, a deeper insight into the effect of roughness on the turbulent boundary and on the complex flowfleld close to the end wall is achieved. Complementary results to be presented include oil paint visualizations and pressure distribution measurements.