Controlling Tip Leakage Flow Over a Shrouded Turbine Rotor Using an Air-Curtain

This paper describes an experimental and computational investigation into the performance of an air-curtain seal used to control the leakage flow over the tip shroud of a turbine rotor. The results show that a seal of this type has the potential to reduce or eliminate shroud leakage whilst having a practical level of clearance between the stationary and moving components. The experimental measurements were undertaken using a single-stage low-speed air turbine equipped with a continuous circumferential nozzle in the casing to deliver an axisymmetric jet into the cavity over the rotor shroud. The jet was angled at 45° to the axial direction so that its momentum opposed the shroud leakage flow. In this arrangement the air-curtain was able to sustain the pressure difference between the inlet and outlet of the rotor blade row without any leakage. The test facility had comprehensive instrumentation for obtaining accurate measurements of turbine efficiency that were corrected for the externally supplied additional flow required for the air-curtain. Measurements were obtained for a range of jet flows and show the change in the turbine efficiency as the jet flow is increased. The measurements have been compared with calculations.