Rotating Instabilities in Shrouded Low Pressure Turbine at Design and Off-Design Conditions

Abstract

The present study aims to analyze rotating instabilities that may occur inside shroud cavities above rotors of low pressure turbine configurations. To do so, unsteady simulations on two configurations, one of which being a multistage configuration, at design and off-design conditions were carried out. Unsteady flow structures, uncorrelated from blade passing frequencies and depending on operating points, are identified in every rotor shroud exit cavity under investigation. Similarities regarding flow patterns and interactions with the main flow are observed: hot spots of gas, with different azimuthal periodicity and rotational speed, coming from the shroud, rotating at the interface with the main flow path, and modifying local aerodynamics. The influence on main flow aerodynamics and the origin of these instabilities are then discussed. Last but not least, the study at off-design operating points deepens the analysis and allows us to identify physical parameters driving the instabilities and propose a mechanism for instabilities’ onset. Those phenomena, which are physically sound and in agreement with scaling laws, still need to be experimentally observed. Advanced experiments are currently set up in the community for that purpose.