Session: 40-04: Turbine Cavity Flows and Flowpath Geometry Effects I

Paper Number: 153754

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

The inherently unsteady tip leakage flows in unshrouded turbines are accountable for pressure loss and high thermal loads on the blades and adjacent shroud. The tip leakage characteristics are highly dependent on the tip clearance dimensions and the blade tip geometry. Tighter tip clearances lessen tip leakage flows from the achieved reduction in cross sectional leakage area. However, the area constraint additionally results in faster leakage flow speeds, therefore intensifying the experienced pressure and thermal burdens. Squealer tip blades diminish the leakage effects by forcing the development and interaction of vortical structures between the rims added to the blade tip. Both design features are pertinent to new generation of small-core high-speed turbines, characterized by high aspect ratio passages with significant presence of secondary flow structures. Tailored to to decrease tip leakage effects, small-core turbines feature running clearances below 0.3mm. Small clearance variations then become extremely relevant for the machine performance, such as those found from blade-to-blade or set by the operational speed envelope.  Therefore, the assessment of the leakage flow structures is paramount to define novel design strategies for these class of turbines. The limitations of commercially available CFD tools to accurately resolve highly detached unsteady flows structures in tight clearance configurations evindece the requirement of in-situ empirical observations. This manuscript performs a thorough experimental examination of a small-core squealer tip leakage flows, assessing the presence of vortical structures as well as the variations resulting from tip clearance and operational condition changes. To that end, fast response miniature pressure transducers acquiring at 2MHz are instrumented onto the STARR test rig in the Purdue Experimental Turbine Aerothermal Lab. Engine representative tests were performed in this small-core high-speed turbine test rig, and the versatility of the facility allowed isolating pressure, rotational speed and tip clearance variations. The phase-locked averaged measurements allow retrieving the over-blade pressure field. Further analysis reveals the influence of the vortical structures inherent to the squealer blade tip. The identification of these structures throughout the tested envelope resulted in a clear visualization of the shifting trends with increasing rotational speed at constant pressure ratio and vice versa. Finally, the modularity of the test section allowed the experimental evaluation of a tight clearance configuration 60% tighter than the baseline geometry. The impact of clearance on the tip leakage structures is evidenced in the retrieved pressure fields, as well as the newly observed behaviors with varying operational condition.

Presenting Author: Antonio Castillo Purdue University

Presenting Author Biography: Antonio Castillo is a PhD candidate from Purdue University. He has been working in the PETAL group as a research assistant since 2020, conducting aerothermal measurements and performance assessment of turbine geometries. His research is focused on the characterization of the unsteady flows and heat transfer processes in the tip clearance region.

Authors:

Antonio Castillo Purdue University
Guillermo Paniagua Perez Purdue University