Experimental Assessment of Tip Loss Variation in LPT Blades With Tip Height, Squealer Geometry, and Coolant Flow Rate

Abstract This study presents an extensive experimental campaign aimed at finely sampling the loss generated by leakage flow in a highly-loaded Low-Pressure Turbine (LPT) rotor blade. A total of 20 different combinations of the most influencing parameters affecting tip leakage flow generation were investigated. Specifically, tests were conducted for both flat and squealer tip configurations with varying tip gaps τ/C = 0.71%, 2.14%, 3.57%). For the squealer configuration, different Reynolds numbers (Rex = 100k and Rex = 200k) and coolant flow ratios (mc/mo = 0%, 0.45%, 0.9%) were also tested. Additionally, tests were performed for both Reynolds numbers without tip gap to provide the reference spanwise loss distribution characterizing the passage vortex effect alone. By comparing these gap-free distributions with results from experiments with tip gap, the contribution to loss production due to the tip leakage flow was isolated. Five-hole pressure probe data acquired in a downstream tangential plane provided a clear view of the effects of flow parameter variations on the tip leakage vortex core, the spanwise distribution of losses, and flow angles downstream of the blade. Tip leakage losses estimated for the different cases were compared with the flat tip case, allowing us to highlight the delta losses with respect to the reference condition. This data were used to tune a tip loss model generalizing current observations. The results presented in this paper are expected to provide useful guidance for designers regarding loss expectations as a function of the analyzed parameters.