Aerodynamic Impact of Secondary Air Injection Flowrates on the Main Annulus Flow Field for a Transonic High-Pressure Turbine Stage

In this study, the influence of secondary air blowing on the main annulus flow field of a transonic high-pressure turbine (HPT) was investigated experimentally and numerically. The experimental setup was a single-stage, unshrouded turbine with a blading consistent with modern HPTs. The entire testing campaign was conducted in a full annular, rotating, continuous turbine test rig at the Japan Aerospace Exploration Agency (JAXA), which realized the most faithful matching unrivalled of both the primary and secondary stream similarity parameters to reality. An elaborate secondary air system implemented in the hardware enabled it to mimic all the dominant coolant/purge streams representative of advanced hot sections: the full coverage film-cooling on stator and rotor airfoils, disk wheelspace purge air blown forward and aft of the rotor, and coolant injection through the over-tip casing. Detailed three-dimensional flow field measurements for various secondary air flowrates were performed by traversing a pneumatic thermometric combination probe downstream of the rotor. A complete set of numerical simulations was run concurrently with the testing to determine how well they captured the flow physics, particularly the interaction of the ejected secondary streams with the primary flow. The JAXA in-house code, UPACS, and its best practice settings, based on past verification and validation efforts, were employed and not intentionally tuned to better fit the data. Given the study's primary goal, important features such as coolant holes, internal plenums, and wheelspace cavities that affect the interactions were all directly resolved in the calculation rather than simplified by the boundary conditions and/or source-term modeling. The rotor exit survey in the experiment suggested that the both disk forward hub purge and the blowing from the over-tip casing intensified the passage vortex in the cascade. In contrast, it was indicated that a positive effect also resulted from ejection from the tip casing, diminishing the tip leakage vortex. Additionally, the data suggested that coolant blown off the airfoil has an impact also on the near-endwall zone in particular and can boost (by vane coolant) or weaken (by blade coolant) the secondary flow.