Numerical Study of Unsteady Rotating Structures in a Turbine Disk Cavity

The improvement of modern cooling systems in aero-engines has led to an increase in the turbine inlet temperature to improve cycle efficiency. In particular, the use of purge flow within the cavity between the stator rim and rotor platform has considerably reduced hot gas ingestion from the main flow and has ensured the maximum metal components operating life. The mechanisms governing axial turbine rim seal flow are complex and influenced by various factors, firstly by the geometry of the cavity. The resulting flow has a three-dimensional structure and a pattern that varies over time. For a wide axial cavity, hot gas ingestion zones can be mainly attributed to three phenomena. The first one is the disc pumping effect generated by the shaft rotation, which induces a radial outflow in the rotor disc boundary layer, creating a recirculation within the cavity that leads to ingestion in the stator disc area. The second one is the vane/blade relative position, which determines a circumferential pressure profile where the higher and lower pressure zones define the flow ejection or ingestion. The third phenomenon is the purge/mainstream flow interaction and its inherent instability that leads to large-scale unsteady flow features developing within the cavity. The formation of such rotating structures and their impact on the ingestion zones is a key research topic in this area. This study presents the results of unsteady numerical simulations (URANS) of an axial turbine's first-stage cavity with a focus on the analysis of 3D-unsteady flow structures. The simulations have been performed by imposing a circumferential pressure periodicity at the outlet, extracted from previous simulations of the stage. The static pressure values within the cavity have been validated by comparison with experimental data. Three purge flow rates have been tested, namely high purge, low purge, and ingestion flow. The paper provides a new perspective on the topic by performing a SPOD analysis on a characteristic plane to identify the main energy modes that dominate the flow and their influence on the ingestion phenomenon.