AEROTHERMAL MEASUREMENTS IN A TIGHT CLEARANCE HIGH-SPEED TURBINE

Tip leakage flows in unshrouded turbines lead to significant pressure losses and heat loads, both on the rotating blades and the adjacent casing surface. These penalties are influenced by the tip clearance size, highly pertinent to the new generation of small-core high-speed turbines. Tailored to decrease tip leakage effects, small-core turbines feature running clearances below 0.3mm, making small blade-to-blade clearance variations extremely relevant for the machine's performance. Therefore, tip clearance monitoring and assessment of the leakage flow structures are paramount to design strategies for this class of turbines. Due to the limitations of commercially available CFD tools to accurately resolve highly detached unsteady flows, in-situ empirical observations are required. Furthermore, the documentation of flow field relationships with the tip clearance is highly valuable for in-service engine applications, such as tip clearance estimations from more accessible measurements to provide feedback for clearance control systems.

The dissertation developed hereafter performs aerothermal measurements in the casing end wall of a small-core high-speed turbine at engine-representative conditions and a wide range of clearance values. A novel in-situ calibration procedure for capacitance probes is tailored to reduce the required clearance measurements and the experimental time. Its uncertainty analysis demonstrates improved prediction bands, supporting this method for tight clearance measurements. A thorough evaluation of the casing static pressure is performed with high-frequency miniature pressure transducers. Specific trends are identified with independent variations of operating pressure ratio, rotational speed, and tip clearance. The results revealed the existence of a clearance-dependent threshold rotational blade tip Reynolds, where the circumferential directionality of tip leakage flows reverses. The analysis of the convective heat flux field with varying operating parameters was achieved with Atomic Layer Thermopile sensors. The computed adiabatic parameters and unsteady contributors reveal high influence of the temperature field on the convective heat flux mechanisms. Lastly, the evaluation of the unsteady terms with tip clearance unveil the shift of thermal loads from the pressure to the suction side of the blade tip.

The achieved results have provided valuable insight into the underlying aerothermal mechanisms governing the tip clearance region, as well as connections with tip clearance size that could potentially be implemented on engine application systems.