Abstract

The present study aims to investigate the film cooling effect on the local mass transfer distributions on both suction and pressure surfaces near the endwall region of a showerhead film-cooled turbine blade by employing the naphthalene sublimation technique. The mass transfer measurements are also conducted with a smooth turbine blade for comparison. For film-cooled blade, there are three rows of injection holes staggered at the leading edge of test blade. Each row consists of seven equally spaced injection holes at a ratio of pitch to diameter equals 3. The inclined angles of injection hole are 90 degrees to the streamwise direction and 30 degrees to spanwise direction. The closest distance from the center of injection hole located on the front stagnation line to the endwall is just one pitch of hole. Experiments are conducted over a six-blade planar cascade in an open-type wind tunnel with mainstream turbulence level of 0.4%, inlet boundary layer momentum thickness of 0.955 and shape factor of 1.355. For all measurements, the exit Reynolds number is kept at a constant value of 397,000 and the blowing ratio is 0.8.

Close to the endwall the local mass transfer rate over both surfaces of the tested smooth turbine blade is much higher than that in the two-dimensional flow region. Measured results indicate that the vortex system on the suction side of tested smooth blade consists of a leading edge corner vortex, a suction leg of horseshoe vortex, a suction side corner vortex, a passage vortex and the wall vortex induced by the strong passage vortex. Downstream of S/C ≈ 0.431, the passage vortex and the wall vortex become dominant. On the pressure side of smooth turbine blade, the leading edge corner vortex, the pressure leg of horseshoe vortex and the pressure corner vortex are still visible but they all are swept out from the blade surface by the secondary flows at Sp/C ≈ 0.256.

The presence of film cooling has little influence on the leading edge corner vortex and suction side corner vortex but results in a later induction of comer vortex on pressure side compared to that without film cooling. Instead of a three-peak vortex system, the impact location of the passage vortex occurs far downstream. Therefore, the wall vortices induced by passage vortex that might be observed from smooth blade vanished in the mid-curvature portions of suction side of film-cooled blade. In addition, the ejected flows hold the leading edge comer vortex and the pressure side corner vortex on the pressure side of film-cooled blade.

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