An empirical means of predicting the discharge coefficients of film cooling holes in an operating engine has been developed. The method quantifies the influence of the major dimensionless parameters, namely hole geometry, pressure ratio across the hole, coolant Reynolds number, and the freestream Mach number. The method utilizes discharge coefficient data measured on both a first-stage high-pressure nozzle guide vane from a modern aero-engine and a scale (1.4 times) replica of the vane. The vane has over 300 film cooling holes, arranged in 14 rows. Data was collected for both vanes in the absence of external flow. These noncrossflow experiments were conducted in a pressurized vessel in order to cover the wide range of pressure ratios and coolant Reynolds numbers found in the engine. Regrettably, the proprietary nature of the data collected on the engine vane prevents its publication, although its input to the derived correlation is discussed. Experiments were also conducted using the replica vanes in an annular blowdown cascade which models the external flow patterns found in the engine. The coolant system used a heavy foreign gas (SF6 /Ar mixture) at ambient temperatures which allowed the coolant-to-mainstream density ratio and blowing parameters to be matched to engine values. These experiments matched the mainstream Reynolds and Mach numbers and the coolant Mach number to engine values, but the coolant Reynolds number was not engine representative (Rowbury, D. A., Oldfield, M. L. G., and Lock, G. D., 1997, “Engine-Representative Discharge Coefficients Measured in an Annular Nozzle Guide Vane Cascade,” ASME Paper No. 97-GT-99, International Gas Turbine and Aero-Engine Congress & Exhibition, Orlando, Florida, June 1997; Rowbury, D. A., Oldfield, M. L. G., Lock, G. D., and Dancer, S. N., 1998, “Scaling of Film Cooling Discharge Coefficient Measurements to Engine Conditions,” ASME Paper No. 98-GT-79, International Gas Turbine and Aero-Engine Congress & Exhibition, Stockholm, Sweden, June 1998). A correlation for discharge coefficients in the absence of external crossflow has been derived from this data and other published data. An additive loss coefficient method is subsequently applied to the cascade data in order to assess the effect of the external crossflow. The correlation is used successfully to reconstruct the experimental data. It is further validated by successfully predicting data published by other researchers. The work presented is of considerable value to gas turbine design engineers as it provides an improved means of predicting the discharge coefficients of engine film cooling holes.
This paper discusses measurements of the discharge coefficients of gas turbine nozzle guide vane film cooling holes under fully engine representative conditions. These unique experiments were carried out in a large scale annular blowdown cascade which models the three-dimensional external, flow patterns found in modern aero-engines, including all secondary flow phenomena. Furthermore, the coolant system design allows the coolant-to-mainstream density ratio and blowing parameter to be matched to engine values, although they can be independently varied. The results confirm that the discharge coefficients of film cooling holes are significantly altered by external crossflow. The discharge coefficient is usually reduced by external crossflow, but under certain external flow conditions it can be increased over the non-crossflow case. This previously unhightighted phenomenon has been termed ‘the crossover effect’, and, although an initially surprising result, is of importance to aero-engine designers as taking account of it should lead to improved predictions of coolant consumption. As a consequence, more uniform blade cooling should be achieved and, in turn, the attainment of greater component durability will be possible.
This paper discusses large-scale, low-speed experiments that explain unexpected flow-interaction phenomena witnessed during annular cascade studies into the influence of external crossflow on film cooling hole discharge coefficients. More specifically, the experiments throw light on the crossover phenomenon, where the presence of the external crossflow can, under certain circumstances, increase the discharge coefficient. This is contrary to most situations, where the external flow results in a decrease in discharge coefficient. The large-scale testing reported helps to explain this phenomenon through an increased understanding of the interaction between the emerging coolant jet and the free-stream flow. The crossover phenomenon came to light during an investigation into the influence of external crossflow on the discharge coefficients of nozzle guide vane film cooling holes. These experiments were performed in the Cold Heat Transfer Tunnel (CHTT), an annular blowdown cascade of film cooled vanes that models the three-dimensional external flow patterns found in modern aero-engines. (Rowbury et al., 1997, 1998). The variation in static pressure around the exit of film cooling holes under different flow conditions was investigated in the large-scale tests. The study centered on three holes whose geometries were based on those found in the leading edge region of the CHTT vanes, as the crossover phenomenon was witnessed for these rows during the initial testing. The experiments were carried out in a low-speed wind tunnel, with the tunnel free-stream flow velocity set to match the free-stream Reynolds number (based on the local radius of curvature) and the “coolant” flow velocity set to replicate the engine coolant-to-free-stream momentum flux ratio. It was found that the apparent enhancement of film cooling hole discharge coefficients with external crossflow was caused by a reduction in the static pressure around the hole exit, associated with the local acceleration of the free-stream around the emerging coolant jet. When these measured static pressures (rather than the free-stream static pressure) were used to calculate the discharge coefficient, the crossover effect was absent. The improved understanding of the crossover phenomenon and coolant-to-free-stream interactions that has been gained will be valuable in aiding the formulation of predictive discharge coefficient schemes.
This paper discusses the need and the procedure for scaling discharge coefficient measurements made in an ambient temperature experiment to render them applicable to the engine situation. Among the dimensionless parameters affecting the discharge coefficients of film cooling holes are the Reynolds number and the coolant Mach number. Experiments have been conducted in a large scale annular blowdown cascade of film cooled nozzle guide vanes. The coolant system design, using a heavy ‘foreign gas’ (an SF6/Ar mixture) at ambient temperatures, allows the coolant-to-mainstream density ratio and blowing parameters to be matched to engine values. By using elevated pressure tests, the effect of varying the coolant Reynolds number without external flow is observed experimentally and these results are then used to correct the discharge coefficients measured on the vane with external crossflow. Data is presented and discussed for two cooling hole geometries, namely cylindrical and fanshaped holes.
This paper discusses large-scale, low-speed experiments that explain unexpected flow-interaction phenomena witnessed during annular cascade studies into the influence of external crossflow on film cooling hole discharge coefficients. More specifically, the experiments throw light on the crossover phenomenon, where the presence of the external crossflow can, under certain circumstances, increase the discharge coefficient. This is contrary to most situations, where the external flow results in a decrease in discharge coefficient. The large-scale testing reported helps to explain this phenomena through an increased understanding of the interaction between the emerging coolant jet and the freestream flow. The crossover phenomenon came to light during an investigation into the influence of external crossflow on the discharge coefficients of nozzle guide vane film cooling holes. These experiments were performed in the Cold Heat Transfer Tunnel (CHTT), an annular blowdown cascade of film cooled vanes which models the three-dimensional external flow patterns found in modern aero-engines. (Rowbury et al., 1997 and 1998). The variation in static pressure around the exit of film cooling holes under different flow conditions was investigated in the large-scale tests. The study centred on three holes whose geometries were based on those found in the leading edge region of the CHTT vanes, as the crossover phenomenon was witnessed for these rows during the initial testing. The experiments were carried out in a low speed wind tunnel, with the tunnel freestream flow velocity set to match the freestream Reynolds number (as based on the local radius of curvature) and the ‘coolant’ flow velocity set to replicate the engine coolant-to-freestream momentum flux ratio. It was found that the apparent enhancement of film cooling hole discharge coefficients with external crossflow was caused by a reduction in the static pressure around the hole exit, associated with the local acceleration of the freestream around the emerging coolant jet. When these measured static pressures (rather than the freestream static pressure) were used to calculate the discharge coefficient, the crossover effect was absent. The improved understanding of the crossover phenomenon and coolant-to-freestream interactions that has been gained will be valuable in aiding the formulation of predictive discharge coefficient schemes.
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