Cavity Flow Instabilities in a Purged High-Pressure Turbine Stage

As designers push engine efficiency closer to thermodynamic limits, the analysis of flow instabilities developed in High-Pressure Turbine (HPT) is crucial to minimizing aerodynamic losses and optimizing secondary air systems. Purge flow, while essential for protecting turbine components from thermal stress, significantly impacts the overall efficiency of the engine and is strictly connected to cavity modes and rim-seal instabilities. This paper presents an experimental investigation of these instabilities in an HPT stage, tested at engine-representative flow conditions in the short-duration turbine rig of the von Karman Institute. As operating conditions significantly influence instability behavior, this study provides valuable insight for future turbine design. Fast-response pressure measurements reveal asynchronous flow instabilities linked to ingress-egress mechanisms, with intensities modulated by the Purge Rate (PR). The maximum strength is reached at PR=1.0%, with comparable intensities persisting for higher rates. For lower PRs, the instability diminishes as the cavity becomes unsealed. An analysis based on the cross-power spectral density is applied to quantify the characteristics of the rotating instabilities. The speed of the asynchronous structures exhibits minimal sensitivity to the PR, approximately 65% of the rotor speed. In contrast, the structures length scale shows considerable variation, ranging from 11-12 lobes at PR=1.0%, to 14 lobes for PR=1.74%. The frequency domain analysis reveals a complex modulation of these instabilities and suggests a potential correlation with low engine-order fluctuations.