Effect of endwall boundary conditions on turbine blade mass transfer in the presence of high freestream turbulence

The naphthalene sublimation technique is used to investigate the effect of two endwall mass-transfer boundary conditions, constant concentration (equal to that on the blade surface) and impermeable wall, on turbine-blade surface mass (heat) transfer with the influence of high freestream turbulence. In the upstream region near the leading edge of the blade, no measurable difference has been found with these two boundary conditions. Farther downstream, especially after the point where the suction and pressure legs of the horseshoe vortex merge (at about a quarter of the surface distance along the suction surface), a lower mass transfer rate is found on the blade suction surface very close to the endwall with the constant concentration boundary condition compared with the impermeable wall boundary condition. The effect was reduced in the presence of higher freestream turbulence (Tu > 8.5%). In the case of very high freestream turbulence (Tu = 18%) with large length scale (L-x = 8 cm), the effect of endwall boundary conditions can be ignored. The extent of the area affected by the secondary flow on the suction surface is the same for the different boundary conditions. In the area far away from the endwall, the flow is two-dimensional and the endwall boundary conditions have no effect on the blade surface mass transfer. The results also show that the pressure surface is not affected by the endwall boundary conditions over its entire extent.