Local mass/heat transfer on turbine blade near-tip surfaces

Local mass transfer measurements on a simulated high-pressure turbine blade are conducted in a linear cascade with tip clearance, using a naphthalene sublimation technique. The effects of tip clearance (0.86-6.90% of chord) are investigated at an exit Reynolds number of 5.8 x 10⁵ and a low turbulence intensity of 0.2%. The effects of the exit Reynolds number (4-7 x 10⁵) and the turbulence intensity (0.2 and 12.0%) are also measured for the smallest tip clearance. The effect of tip clearance on the mass transfer on the pressure surface is limited to 10% of the blade height from the tip at smaller tip clearances. At the largest tip clearance high mass transfer rates are induced at 15% of curvilinear distance (S_p/C) by the strong acceleration of the fluid on the pressure side into the clearance. The effect of tip clearance on the mass transfer is not very evident on the suction surface for curvilinear distance of S_s/C<0.21. However, much higher mass transfer rates are caused downstream of S_s/C≈ 0.50 by the tip leakage vortex at the smallest tip clearance, while at the largest tip clearance, the average mass transfer is lower than that with zero tip clearance, probably because the strong leakage vortex pushes the passage vortex away from the suction surface. High mainstream turbulence level (12.0%) increases the local mass transfer rates on the pressure surface, while a higher mainstream Reynolds number generates higher local mass transfer rates on both near-tip surfaces.