EXPERIMENTAL INVESTIGATION OF SECONDARY FLOW CONTROL DEVICES IN TURBINE BLADE CASCADES FOR AERO-ENGINE APPLICATIONS

The present thesis focuses on the experimental investigation of the performance of flow control devices designed to mitigate the strong secondary flows that arise in modern, compact aero-engine turbine blade cascades. Specifically, the study investigates the effectiveness of suction side shelf-like fences, innovative endwall ridging designs and their combined use, together with squealer and squealer–winglet tip configurations under different coolant mass flow ratios, in mitigating secondary vortex structures. To address this challenge, the analysis is conducted on two representative turbine blade cascades developed for lightweight, high specific thrust aero-engine applications: a nozzle guide vane (NGV) cascade characterized by long chord, low aspect ratio vanes operating in a highly diffusive channel environment, and a highly loaded unshrouded low pressure turbine (LPT) rotor cascade featuring tip clearance. Due to their geometrical characteristics, both cascades exhibit intense vortex structures—a strong passage vortex in the NGV and an extended tip leakage vortex in the unshrouded LPT rotor. These vortical structures generate secondary losses comparable to profile losses, hence providing an excellent framework in which mitigation devices are required and their effectiveness can be evaluated under strongly three-dimensional flow conditions. The performances of fences and ridges in attenuating the passage vortex in the NGV cascade, and of squealer and squealer–winglet tip configurations combined with coolant ejection in reducing the tip leakage vortex in the LPT cascade, are assessed through dedicated experimental campaigns in the large-scale, low-speed wind tunnels of the Aerodynamics and Turbomachinery Laboratory at the University of Genova. Flow measurements in tangential planes were performed both upstream and downstream of the baseline (without secondary flow devices applied on) and controlled configurations of both cascades. Total pressure losses were evaluated using a Kiel probe, while exit velocity and flow angle distributions were captured with a five-hole probe. This experimental approach provides a comprehensive overview of how the devices act in reducing secondary flows and, consequently, influence the cascade efficiency with respect to the baseline cases. Different operating conditions were also investigated, including Reynolds number variations for both cascades, as well as tip clearance and coolant mass flow ratio variations for the unshrouded LPT rotor, enabling a detailed analysis of leakage flow behavior. The experimental results show that, compared to the baseline configurations, fences limit the penetration of the passage vortex in the endwall normal direction, while ridges effectively suppress the cross-passage motion induced by the vortex. An optimized ridging design yields further reductions in secondary losses, and consistent benefits are also observed under combined fence–ridge application. These passive control strategies enhance flow uniformity at the turbine cascade exit, generating a favorable carry-over effect and a consequent improvement in stage efficiency. For tip vortex mitigation, squealer cavity reduces losses nearly proportionally to tip gap size with respect to a flat tip configuration, while squealer–winglet configuration provides additional reductions compared to both flat and squealer tips. Coolant ejection at low mass flow rates produces negligible effects on losses and downstream flow patterns, whereas higher injection rates confine the leakage vortex closer to the endwall but increase the associated losses. These findings are subsequently incorporated into a refined tip-loss model to generalize the observed trends. The experimental database also supports validation activities for both low- and high-fidelity simulations performed by Morfo Design. The simulations successfully reproduce the main features of losses and flow deviations, with LES providing deeper physical insight. In particular, the analyses clarify the mechanisms through which fences and ridges mitigate passage vortex effects inside the vane channel, and those by which squealer and squealer–winglet tips weaken the tip leakage vortex and modulate the influence of coolant ejection. Overall, the results demonstrate the capability of these flow control devices to effectively suppress secondary flow structures and improve turbine efficiency, highlighting their potential for integration in next-generation compact aero-engines.