Influence of Cavity Flows on High-Pressure Turbine Stage Efficiency

The continuous improvement of aero-engine efficiency has been the primary objective in the development strategies of manufacturers since the beginning of the aircraft propulsion era. Efficiency gains imply a reduction in burnt fuel for the same power output, a reduction in the overall weight of the aircraft, and a decrease in maintenance costs. Improvements in modern cooling systems, including the ejection of purge flow from the cavity between the stator rim and the rotor platform, have allowed for increasing the turbine inlet temperature and, consequently, the turbine module efficiency. However, the potential efficiency gain associated with an increase in turbine inlet temperature is not fully exploited. Indeed, mixing cold air from the cavity with hot flow from the vanes reduces the average enthalpy of the main flow, generates mixing losses, and enhances the intensity of secondary flows. Hence, the effect of cooling systems with special interest in the inherently unsteady cavity flow ejection into the stage should be accurately quantified in terms of efficiency impact. This article focuses on the definition of a reliable approach to be used to estimate stage efficiency, accounting for the unsteady nature of component interaction, the ejection of coolant, and the influence of secondary flows development. The results obtained from verified and validated unsteady 3D RANS simulations of a high-pressure turbine stage experimentally analyzed at the von Karman Institute for Fluid Dynamics are used to calculate efficiency through instantaneous and averaged definitions. Three Cases are considered, varying rotational speed and cavity mass-flow rate. In addition, the paper focuses on the generation of entropy within the simulated domain. The fluid domain is divided into subregions to identify the main sources of entropy production. This approach considers the dominant, irreversible losses occurring within the flow field, providing a local and physically-consistent evaluation of energy degradation.