Turbine Disc-Rim Cooling Design Criteria for Modern Two Stage High Pressure Turbine Aero-Engines

Modern aero-engines with two stage high pressure turbines (HPT) are attractive because they offer higher turbine polytropic efficiencies compared to the single stage. When considering the complexity associated with the secondary cooling airflow system, however, the two stage turbine is more vulnerable to excess air performance losses and requires more effort to optimize. This paper addresses the established design criteria with an emphasis on cooling air supply to the second stage turbine blades. The available cooling air is compressor delivery air fed to the front face of the rim via the first turbine stage, and lower pressure air from an earlier compressor stage is fed internally to the rear cavity. A comparison is made of the effectiveness of disc-rim cooling achieved with alternate forward and rear air feed systems to the rotor blades based on aerothermal analytical methods. Both front and rear cover-plate as well as disc feed holes are considered, and the applicable heat transfer correlation along with cavity boundary layer/mixing temperature assumptions are described. The 2-D axisymmetric finite element transient thermal model, for each design case, has been run with a square speed cycle and in this study, the aerothermal behavior at steady state maximum take-off condition is compared. Results indicate that low pressure compressor air in a cover-plate is thermally more effective than feed holes on the front or rear side of the disc, if pre-swirled compressor exit air from the first stage seals the second stage disc-rim. This is also valid if cavity boundary layer air from an earlier compressor stage sealing the rim (on either side) reaches a temperature as high as the area-weighted-mean (AWM) rim temperature due to contamination by hot gas ingestion. Although the front system offers a better overall cooling effectiveness, in comparing the front and rear feed hole concepts using cooling air from a compressor mid-stage, the rear feed hole is more effective in reducing the AWM temperature in the rotor bucket-groove, as well as the maximum rim temperatures of the second stage disc.