Diffused-exit film cooling holes fed by an internal crossflow
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Film cooling is an essential technology to the operation of modern gas turbine engines, allowing for greater efficiency and part durability. Due to film cooling’s complexity, laboratory studies of film cooling isolate various effects by intentionally simplifying or neglecting various aspects of the film cooling problem. One such aspect that had been consistently neglected by film cooling studies is how the internal flow within the turbine blade affects film cooling performance. Studies have found that feeding the holes with an internal crossflow, directed perpendicular to the mainstream flow, can cause up to a 50% reduction in film cooling effectiveness. This result is of concern because internal crossflow is a common internal flow condition in gas turbine engines. However, none of the former studies have made a concerted effort to examine the important scaling parameters governing this effect. Nor have they provided experimental evidence showing the cause of this reduction in effectiveness due to internal crossflow. In this study, a wide range of flow conditions was studied for two common film cooling hole geometry types: axial and compound angle diffused-exit film cooling holes. Internal crossflow-to-mainstream velocity ratios of VR [subscript c] = 0.2-0.6 were tested along with jet-to-mainstream velocity ratios of VR = 0.2-1.7. Film cooling effectiveness and discharge coefficients were measured for this full range of flow conditions for both geometries in order to produce a sufficiently large data set to observe important trends in the data. It was found that the discharge coefficients, centerline effectiveness, and centerline location all scaled with the crossflow-to-jet velocity ratio, VR [subscript i] for the axial holes. Temperature and velocity fields showed that VR [subscript i] also scaled the in-hole temperature and velocity fields. A swirling flow within the hole was shown to cause ingestion of mainstream into the diffused exit of the hole and biasing of the issuing jet in the outlet diffuser, which reduced film cooling effectiveness. The direction of bias at the exit resulted from the direction of the internal crossflow and was critical for compound angle holes. Crossflow directed counter to the lateral direction of coolant injection caused improved film cooling effectiveness relative to the in-line crossflow direction.