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

The present study aims to analyze the rotating instabilities that may occur inside shroud cavities above rotors of low-pressure turbine configurations. Unsteady simulations have been carried out at design and off-design conditions on two configurations, one being a multi-stage configuration. Unsteady flow structures, uncorrelated from blade passing frequencies and depending on operating points, are identified in every rotor tip shroud cavity. Similarities regarding flow patterns and interactions with the main flow are observed: hot spots of gas having different azimuthal extend come from the tip shroud cavity and rotate at the interface with the main flow path. The influence on the main flow and the origin of these instabilities are discussed. The study at off-design operating points deepens the analysis and allows the identification of physical parameters which drive the instability.