In gas turbine engines, film cooling holes are often fed by an internal crossflow, with flow normal to the direction of the external flow around the airfoil. Many experimental studies have used a quiescent plenum to feed model film cooling holes and thus do not account for the effects of internal crossflow. In this study, an experimental flat plate facility was constructed to study the effects of internal crossflow on a row of cylindrical compound angle film cooling holes. There are relatively few studies available in literature that focus on the effects of crossflow on film cooling performance, with no studies examining the effects of internal crossflow on film cooling with round, compound angled holes. A crossflow channel allowed for coolant to flow alternately in either direction perpendicular to the mainstream flow. Experimental conditions were scaled to match realistic turbine engine conditions at low speeds. Cylindrical compound angle film cooling holes were operated at blowing ratios ranging from 0.5 to 2.0 and at a density ratio (DR) of 1.5. The results from the crossflow experiments were compared to a baseline plenum-fed configuration. This study showed that significantly greater adiabatic effectiveness was achieved for crossflow counter to the direction of coolant injection.
Surface curvature is known to have significant effects on film cooling performance, with convex curvature inducing increased film effectiveness and concave curvature causing decreased film effectiveness. Generally, these curvature effects have been presumed to scale with 2r/d at the film cooling hole location, where r is the radius of curvature and d is coolant hole diameter. In this study, the validity of this scaling of curvature effects are examined by performing experiments in regions of large and low curvature on a model vane. Single rows of cylindrical holes were placed at various locations along the high curvature section of the suction side of the vane. For the first series of experiments, a single row of holes was placed at two locations with different local surface curvature. The coolant hole diameters were then adjusted to match 2r/d values. Results from these experiments showed that there was better correspondence of film performance when using the 2r/d scaling, but there was not an exact matching of performance. A second series of experiments focused on evaluating the effects of curvature downstream of the coolant holes. One row of holes was placed at a position upstream of the highest curvature, while another row was placed at a downstream position such that the radius of curvature was equivalent for the two rows of holes. Results indicated that the local radius of curvature is not sufficient in understanding the performance of film cooling. Instead, the curvature envelope downstream of the coolant holes plays a significant role on the performance of film cooling for cylindrical holes.
Many analyses in the literature have assessed the appropriate manner in which to scale an experimental test rig to represent film-cooled engine components. For conventional testing using low conductivity models, the key parameters are the mainstream Reynolds number, scaled coolant flow rate, and the adiabatic film effectiveness. The few studies that have sought scaling parameters for conjugate testing have identified that one must additionally match the heat transfer coefficient ratio between the internal and external surfaces and external Biot number. However, these analyses have focused on blade or nozzle regions with single or sparse film rows. The validity of this scaling approach to regions or components with substantial bore cooling contributions is unclear — for example the showerhead and/or platform of a blade or nozzle, or a component like a shroud. The present analysis outlines the drivers for potential departure from the accepted scaling. A numerical study is performed to assess potential errors due to the traditional scaling. The results of the analysis demonstrate that the additional parameter, the ratio of bore cooling to external heat transfer coefficient, is more appropriate in the near hole region especially in cases where film cooling is not significant.
Surface curvature is known to have significant effects on film cooling performance, with convex curvature inducing increased film effectiveness and concave curvature causing decreased film effectiveness. Generally, these curvature effects have been presumed to scale with 2r/d at the film cooling hole location, where r is the radius of curvature and d is coolant hole diameter. In this study, the validity of this scaling of curvature effects are examined by performing experiments in regions of large and low curvature on a model vane. Single rows of cylindrical holes were placed at various locations along the high curvature section of the suction side of the vane. For the first series of experiments, a single row of holes was placed at two locations with different local surface curvature. The coolant hole diameters were then adjusted to match 2r/d values. Results from these experiments showed that there was better correspondence of film performance when using the 2r/d scaling, but there was not an exact matching of performance. A second series of experiments focused on evaluating the effects of curvature downstream of the coolant holes. One row of holes was placed at a position upstream of the highest curvature, while another row was placed at a downstream position such that the radius of curvature was equivalent for the two rows of holes. Results indicated that the local radius of curvature is not sufficient in understanding the performance of film cooling. Instead, the curvature envelope downstream of the coolant holes plays a significant role on the performance of film cooling for cylindrical holes.
The leading edge of a turbine vane is subject to some of the highest temperature loading within an engine, and an accurate understanding of leading edge film coolant behavior is essential for modern engine design. Although there have been many investigations of the adiabatic effectiveness for showerhead film cooling of a vane leading edge region, there have been no previous studies in which individual rows of the showerhead were tested with the explicit intent of validating superposition models. For the current investigation, a series of adiabatic effectiveness experiments were performed with a five-row and threerow showerhead. The experiments were repeated separately with each individual row of holes active. This allowed evaluation of superposition methods on both the suction side of the vane, which was moderately convex, and the pressure side of the vane, which was mildly concave. Superposition was found to accurately predict performance on the suction side of the vane at lower momentum flux ratios, but not at higher momentum flux ratios. On the pressure side of the vane, the superposition predictions were consistently lower than measured values, with significant errors occurring at the higher momentum flux ratios. Reasons for the underprediction by superposition analysis are presented.
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