A water channel study was conducted on a cylindrical leading edge model of a film-cooled turbine blade to assess the effects of surface modifications on film spreading. A single radial coolant hole located 21.5° from the stagnation line, angled 20° to the surface and 90° to the flow direction supplied dyed coolant flow. Surface modifications included a variety of dimples upstream and downstream of the coolant hole and transverse trenches milled coincident with the coolant hole. Compared to the unmodified surface, a single row of small cylindrical or spherical dimples upstream of the coolant hole steadies the jet at blowing ratios up to M=0.75. Medium and large spherical dimples downstream of the coolant hole have a similar effect, but none of the dimple geometries studied affect the coolant jet above M=0.75. A single-depth, square-edged transverse trench spreads the coolant spanwise, increasing the coverage of a single coolant hole more than two times. This trench suffers from coolant blow-out above M=0.50, but a deeper, tapered-depth trench entrains and spreads the coolant very effectively at blowing ratios above M=0.50. The tapered trench prevents jet liftoff and is the only geometry studied that holds the coolant closer to the surface than the unmodified coolant hole.
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