Aeromechanical Challenges of Shrouded Low Pressure Turbines for Geared Turbofan Engines

The primary scope of this thesis is to contribute to meeting the challenges associated to the integration of shrouded blades into high speed low pressure turbines of geared turbo-fan engines. Doubling the rotational speed of the low pressure shaft bears considerable advantages in terms of the fuel consumption, the noise emissions, the length as well as the part count compared to conventional multi-spool engine designs. However, the quadratic increase of the centrifugal blade stress with rotational speed has impeded the application of shrouded low pressure turbine blades, which are aerodynamically more favorable and have a higher robustness against flutter instabilities, in current designs due to high cycle fatigue concerns. As a consequence of the reduced error margin in the fatigue design of high speed blade rows, two immediately arising questions for the design of the next gener-ation of geared turbofan engines are addressed in this work: How accurately can current industrial methods predict the unsteady aerodynamic forces in a shrouded low pressure turbine? And: At what performance cost can the mass of a shroud be reduced by plat-form cutbacks to mitigate fatigue issues? The current work was carried out within the scope of a joint academic and industrial re-search effort to elucidate the aerodynamic forcing in a fully shrouded low pressure turbine experimentally and numerically as well as to determine the performance deterioration of different shroud platform geometries. The latest, optimized and tested design of a 1.5-stage fully shrouded low pressure turbine with non-axisymmetric enwalls operating with rim seal purge flow injection represents the baseline for both campaigns. A modular, high resolution and low noise, multi-purpose telemetry system has been developed and inte-grated into the axial research turbine facility “LISA” at ETH Zurich. A unique experi-mental data set comprised of fast response traverse and wall pressure measurements has been acquired. The unsteady surface pressure of both the rotor as well as the second sta-tor has been probed using a total of 60 flush-mounted, ultra-miniature size, piezo-resistive pressure transducers. The aerodynamic forcing functions and the turbine performance have been characterized using a combination of multi-hole pneumatic probe and fast re-sponse aerodynamic probe (FRAP) measurements on the inter-row planes. Leakage flows have been tracked using the unsteady entropy probe (FENT). The comparison of the ex-perimental data with time-accurate RANS simulations highlights the capabilities and lim-itations of commercial solvers and enables the quantification of the prediction error in the aerodynamically induced blade root stress. The unsteady rotor surface pressure in a shrouded, subsonic low pressure turbine is driv-en by convective perturbations and acoustic interaction. Five main forcing functions have been identified: Potential interaction with upstream and downstream rows, convective inflow perturbations and incidence variation in the main flow, secondary flow interaction at the endwalls, inter-blade interaction and leakage and cooling flow interaction. The suc-tion side of the rotor blades is dominated in the front by the relative motion of the first stator’s potential field. The superposition with wake-induced effects results in the peak pressure amplitudes in the order of 13% percent of the rotor pressure drop around the crown at the tip of the blade. The modulation by the potential interaction with the sec-ond stator, whose vane count is changed to differ from the first stator’s for the forcing measurements, results in significant amplitudes at difference and sum frequencies down to the ninth engine order. Contrary to the convective perturbation propagation on the major area of the suction side, the wake-induced surface pressure deficit on the pressure side is reinforced by the acoustic interaction with the adjacent suction side and leads to significantly higher propagation speeds than the flow velocity. Three-dimensional forcing effects have been observed at the hub of the rotor due to the purge air vortex, which is modulated by the first stator potential, and its interaction with the rotor hub passage vortex. Since the phase of local thrust and lift fluctuations are highly sensitive to the purge flow rate, the functionality of the purge flow could potentially be extended from cooling to blade excitation control. Numerical predictions show excellent agreement in vast areas of the turbine flow field. The amplitude and phase characteristic of the dominant rotor surface pressure fluctua-tions are generally captured well. The measurement based correction of the unsteady ro-tor surface pressure enables the quantification of the prediction error of the alternating blade root stresses using fluid-structure-interaction analyses to be 2% to 5%. Turbulence intensity measurements at turbine inlet and transition modelling show potential to fur-ther improve the prediction accuracy. The buildup of non-physical duct modes at turbine exit challenges the standard boundary condition setup in terms of periodicity and fixed exit mass flow. Three different partial tip shroud platform designs have been tested experimentally and compared to a full-shroud baseline. A detailed analysis of the fluid dynamics is carried out and complemented by numerical predictions to provide a starting point for shroud opti-mizations. The leading and trailing edge platforms are cut back separately by 11.6% and 5.3% to isolate the effect of each modification. The final rotor then features a combined partial shroud. The performance reduction for both isolated cutbacks equals 0.7% relative to the baseline, while the detrimental effects add up to 1.1% for the combined cutback. The main loss mechanism for all cutbacks is the intensification of fluid exchange with the shroud cavities and the re-entry into the main flow. This results in increased blockage by the tip passage vortex for the leading edge cutback, while the injection of leakage fluid in-to the suction side boundary layer of the rotor results in a flow underturning for the trail-ing edge cutbacks. The usage of shroud cutbacks proves to be an effective measure to at-tenuate the noise emission and excitation potential of non-synchronous, low frequency cavity modes by up to 15dB at the exit of the turbine