User:EvanOscarSmith/sandbox

Gas Turbine Thermal Rotor Bow

After a gas turbine engine is shut down it will develop a circumferential thermal gradient vertically across the compressor due to hot air rising from the cooling metal components and pooling at the top, both inside the compressor rotor drum and within the gas path annulus. As the hot compressor rotor drum cools and contracts in the presence of this thermal gradient, it does so non-uniformly, and therefore will bend slightly, in a phenomenon known as rotor bow.

Starting an engine under bowed conditions can result in damage, representing a risk to both airworthiness and operational capability. If an engine is started while the rotor is sufficiently bowed, rubbing between the rotor and stator (e.g. seals) can introduce frictional heat to the rotor, causing the rotor bow to increase in magnitude. This runaway effect is known as the Newkirk Effect. Ultimately, rubbing can occur between compressor blades and the compressor casing, leading to crack initiation at the root of the blades, resulting in eventual blade failure. This damage sequence can be catastrophic.

Operational Impact

Engines which suffer from thermal rotor bow must be managed carefully during their start sequence. An engine will begin to bow immediately after shutdown, reaching a critical level after a period of time, eventually reaching a peak bow before starting to equilibrate and re-straighten. After a sufficiently long time (hours or days depending on the engine's application) the engine can be restarted without issue. If, however, an engine is to be restarted during its bowed phase, the engine must be cranked slowly to remove or reduce the thermal imbalance so as to avoid damage on startup. Alternatively, some engines may be idled for extended periods prior to shutdown to reduce their temperature, such that their subsequent bow is less severe. In situations where there is an operational imperative to start an engine quickly (be it military or commercial), these delays can be costly. This issue has been featured in the media recently, with several modern engines suffering from the problem to such an extent that orders for aircraft have been delayed or cancelled.

Current Research

A body of active research is currently under way at the University of New South Wales in Canberra, Australia. Using simple analogue models, 3D transient conjugate heat transfer computational fluid dynamics simulations, coupled through one-way fluid thermal structural interaction with transient structural models allow a range of geometric, operational, and environmental parameters to be studied with respect to their contribution to the thermal bow behaviour of an engine undergoing natural cooling. Validation of this approach has been performed experimentally, using a small representative rig which is heated up and measured as it cools naturally. It is generally understood that longer engines (those with longer compressor rotors) will be less stiff, and subsequently suffer from bow more severely, however, other parameters which define the geometry and operation of gas turbines have not been studied in depth for their contribution to the rotor bow problem until now.

References