| Summary: | A gas turbine engine can over-speed due to various reasons, including shaft failure, variable
geometry mal-schedule or fuel system malfunction. In any case, engine manufacturers are required
to demonstrate that a shaft over-speed event will not result in an uncontained failure with high
energy debris being released from the engine.
Although the certification authority can be satisfied that the engine is shaft failure safe
by conducting large scale tests, a purely experimental approach would be very complex and
expensive. Moreover, today’s poor understanding of the event leads to conservative designs that
exert unavoidable penalties on the engine performance and weight. It is in this context that the
need for an analytical approach and small scale testing arises to model the progression of the event.
This work is part of a wider long term research collaboration between Cranfield University
and Rolls-Royce that attempts to enhance today’s modelling capability of gas turbine shaft overspeed/
failure events. The final aim of the project is the development of a generic advanced
performance prediction tool able to account for all the complex and heavily interrelated phenomena
to ascertain the terminal speed of the over-speeding turbine. This multidisciplinary “all in one” tool
will allow to include the shaft failure scenario early into the design process and eliminate current
conservative design approaches while maintaining the high standard of airworthiness required for
certification.
This thesis focuses on the aerothermal performance modelling of turbomachinery components
at the extreme off-design conditions experienced during a shaft over-speed event. In particular,
novel modelling techniques and methodologies at the forefront of knowledge have been developed
to simulate the performance of turbine vanes at high negative incidence angles, derive the extended
compressor characteristics in reverse flow and calculate the response of the air system during rapid
transient among others. These component models are ready to be integrated into a generic single
computational tool that, once validated against engine data available from the sponsor, can be
applied to different engines and scenarios.
The collaboration with Rolls-Royce provided the opportunity to conduct research on other
areas related to performance engineering apart from the shaft failure modelling. The present
study makes several noteworthy contributions on compressor variable geometry loss, deviation and
stall modelling, compressor variable geometry schedule optimisation and on the effect of using real
gas models instead of the perfect gas assumption in engine performance simulation codes.
|
|---|
