Experimental Assessment of Squealer Tip Leakage Flow Structures in a Small-Core High-Speed Turbine

Abstract The unsteady tip leakage flows are accountable for aerodynamic losses that inherently hinder the performance of unshrouded turbines. Abatement of these penalties through the tightening of the operating tip clearance is highly relevant for new generations of small-core high-speed turbines, whose characteristic low aspect-ratio passages result in significant influence of secondary flow structures. A deep understanding of the development of tip leakage flows is paramount to defining a suitable transition towards tight clearance turbine configurations. In-situ experimental measurements provide insights to build such comprehension and further serve to improve the numerical resolution fidelity of these highly detached unsteady flows delivered by commercially available computational tools. This manuscript presents the experimental resolution of the unsteady pressure fields experienced in the over-tip casing of a small-core high-speed turbine. Fast response miniature pressure transducers captured at 2MHz the casing's static pressure of the TRL6 small-core turbine demonstrator “STARR”, located in the Purdue Experimental Turbine Aerothermal Laboratory. The versatility of the facility for engine-representative testing allowed identifying the independent influence of the operating pressure ratio and rotational speed on the squealer tip leakage flow structures along a 60% tip clearance reduction. Besides the pressure differential across the blade, the resulting phase-locked average fields revealed two vortical structures along the squealer tip cavity as a secondary mechanism driving the tip leakage flows, with different shifting and combination trends along the operational envelope and tip clearance reduction.