ON THE MODELLING OF THE OVERSPEED PERFORMANCE OF A SINGLE STAGE TURBINE

This manuscript is on the aerodynamic characterization of a turbine particularly when the turbine loses its load permanently such as after a shaft or transmission failure. These conditions cause excessive acceleration of the turbine stage or stages. If not mitigated, a disk burst may lead to hazardous conditions threatening the passenger and the flight safety. Modelling brings the possibility of controlling the event and provides design flexibility for a lightweight and fail-safe system design.

Additional to the control system action through fuel shut-off, there are variety of conditions which affect the acceleration. Friction and blade damage in unlocated failures are examples of these. The aerodynamic performance is another important aspect as the turbine operates at far off-design conditions.

NASA E3 single stage high pressure turbine has fewer airfoils than the base two stage version and operate at a higher loading. Single stage turbine RANS CFD model has been prepared and the turbine is characterized at overspeed conditions up to 200% speed. Later, the tip clearance of the stage was increased up to 15% of the blade height to assess the performance deterioration. Finally, two more damage scenarios were introduced through truncating the blade tips from leading edge only and trailing edge only. Study shows that significant efficiency decrease occurs, which can be considered as a parameter in safety calculations and flight certification.