Comprehensive Experimental Assessment of Unsteady Pressure and Heat Flux in a Small-Core Turbine Over-Tip Shroud

Abstract

For novel high-speed small core turbines, with tip clearance below 0.5 mm, even small blade-to-blade tip clearance variation is significant. The assessment of these complex flows is pertinent to the design of the next generation of small-core turbines. This paper provides a thorough experimental analysis of the shroud's unsteady heat flux and static pressure in a small-core squealer-tip blade turbine. Atomic layer thermopile sensors (ALTPs) and fast response pressure transducers were used to perform high-frequency acquisition at 2 MHz around the 51% axial blade chord on the shroud. Measurements were taken at engine-representative conditions at several operational conditions and tip clearances. The signals were phase-locked averaged (PLA) over the revolution period and synchronized to identify individual blade and row signature. The linear relationship between the rotational tip Reynolds and the static pressure ratio across the blade tip reveals the transition point to reverse over-tip flows. Total heat flux is decomposed into the different steady and unsteady heat flux contributions. It is demonstrated that the adiabatic wall temperature governs the unsteady heat flux and contributes to one-third of the total surface heat flux. A linear trend was observed between the unsteady heat flux and the tip clearance measured at the pressure and suction side rims. Similar trends were observed between the local heat flux and the pressure ratio across the tip. A comparison with computational fluid dynamics (CFD) predictions highlights some limitations on resolving the detached and secondary flows, evidencing the necessity of complementary experimental data.