Experimental Investigation of Rim Seal Instabilities and Annulus-Flow Interactions in Two Transonic Turbine Stages

Abstract Rim cavities in gas turbines are critical to both thermodynamic efficiency and structural integrity throughout the engine’s lifecycle. As efforts to improve performance continue, understanding the unsteady flow phenomena at the interface between the purge cavity and main annulus flows becomes increasingly important. Such instabilities are strongly influenced by cavity geometry and operating conditions, underscoring the need for precise measurements under engine-representative conditions. This study investigates rim-seal instabilities in two high-speed axial turbine stages tested in the von Karman Institute’s short-duration rotating facility CT-3. The first configuration is a scaled-up high-pressure turbine (HPT) stage, while the second represents a low-pressure turbine (LPT) stage typical of geared turbofan engines. Fast response pressure measurements are employed to in both test sections, each operated under similar annulus conditions (Ma 0.8) and at different purge flow rates. The characteristics of the coherent structures developed within the rim seal are carefully analyzed by utilizing spectral analysis techniques, including recently developed methodologies that exploit the properties of the cross-power spectral density and the analytic function of the Hilbert transform. In the HPT stage, shallow cavity modes appear to trigger instabilities, whereas the behavior of the LPT stage suggests Kelvin-Helmholtz-type shear-layer mechanisms. Further analysis at the outlet of the stage reveals rim-seal-induced fluctuations, which are isolated and quantified for both test articles. Minimal effects are detected in the HPT stage, whereas the LPT stage displays a higher sensitivity to the rim-seal instability.