EFFECTS OF SURFACE ROUGHNESS ON THREE-DIMENSIONAL FLOW STRUCTURE WITHIN SHAPED FILM COOLING HOLES

Additive manufacturing (AM) techniques are expanding what is possible for designing and constructing gas turbine components that can handle increasingly harsh operating conditions. However, AM techniques can also introduce surface roughness that is more prominent than conventional manufacturing methods. The influence of this roughness could have an important effect on the performance of film cooling holes. However, little is known experimentally on what happens inside of a film cooling hole since this region is challenging to access with traditional measurement techniques. This study uses magnetic resonance velocimetry (MRV) to observe the in-hole flow structure of a scaled version of a baseline diffuser-shaped hole configuration with and without surface roughness at two blowing ratio conditions. The roughness geometry is derived from computed tomography (CT) scans of a metal-AM diffuser hole. The three-component, three-dimensional, time-averaged velocity field is measured by MRV and includes the flow from the plenum, within the hole, and in the vicinity of the hole exit. The momentum distribution within the diffuser differs between the smooth and rough holes, with peak velocities and flow asymmetry influenced by the surface roughness. Flow along the leeward wall of the diffuser is nearly separated, and the size of this separated flow region is smaller in the roughened cases. These effects are accentuated at larger blowing ratio. These data provide explanatory evidence of how the momentum distribution within smooth and rough holes may impact surface effectiveness results downstream of the shaped holes. CT scans of the surface roughness coupled with the MRV measurements provide non-optical means to characterize the hole geometry and as-built performance of additively manufactured test coupons with important implications for the field.