14th European Conference on Turbomachinery Fluid dynamics & Thermodynamics
The inlet conditions of modern low pressure turbine (LPT) present significant flow non-homogeneities, which can have direct consequences on both its aerodynamic performance and vibrations. The rise in turbine entry temperatures and pressure ratios needed to achieve higher efficiencies and power output requires an increase of cooling and purge flow injections, leading to the generation of strong temperature distortions at the inlet of the LPT. Additionally, the stronger interaction between engine components due to the drastic decrease of axial spacing and to the reduction of the stage count facilitates the propagation of circumferential distortions of total pressure and temperature. This paper reports on the results of an experimental investigation focused on total temperature distortions at the inlet of a modern low pressure turbine stage and on the propagation of such inflow disturbances through the stage, with a particular focus on both the aerodynamic and aero-elastic performance of the turbine. The measurement activity was carried out in a one and a half stage subsonic turbine test facility at the Institute of Thermal Turbomachinery and Machine Dynamics at Graz University of Technology at a stable engine relevant operating condition and during transient operation. Total temperature distortions with different intensities were generated upstream of the stage through the localised injection of air at different mass-flow and temperature, ranging from 30% to 130% of the nominal inlet temperature, in one azimuthal position, aligned to the stator vane leading edge. A setup with clean inflow was additionally used as reference. Both steady and unsteady aerodynamic measurements were taken upstream and downstream of the investigated stage by means of a five-hole-probe (5HP) and a fast response aerodynamic pressure probe (FRAPP) respectively. The rotor blade vibration data was acquired with Strain gauges applied on different blades in combination with a telemetry system. The circumferential temperature perturbation caused modifications on the main aerodynamic structures constituting the flow field and on the interaction between the turbine stator and rotor rows depending on their temperature and intensity. The rotor vibrations were also influenced by the alterations created by the distortion in the flow. In particular, the temperature of the distortion was found to influence directly the amplitude of the rotor blade forced response. Therefore, important aerodynamic and aero-elastic effects were linked to the presence of total temperature distortions in the inlet flow field of a LPT stage.