Rotor bow management
A method of reducing rotor bow in a high pressure rotor of a gas turbine engine that has in axial flow a low pressure rotor and a high pressure rotor. The method involves storing bleed air from the gas turbine engine when the engine is running to provide stored pneumatic energy; and using that stored pneumatic energy after the engine has been shut-down to rotate the high pressure rotor at a speed and for a duration that reduces rotor bow. A gas turbine engine wherein rotor bow in the high pressure rotor after engine shut-down has been reduced by carrying out the aforesaid method is also disclosed.
1. A method of reducing rotor bow in a high pressure rotor of a gas turbine engine that has in axial flow a low pressure rotor and a high pressure rotor, the method comprising the steps of:
storing bleed air from the gas turbine engine when the engine is running to provide stored pneumatic energy, wherein the stored pneumatic energy is provided by a pneumatic energy storage system that comprises an air tank, a pneumatic motor, an accessory gear box and a microcontroller; and
using the stored pneumatic energy after the engine has been shut-down to rotate the high pressure rotor at a speed and for a duration that reduces rotor bow,
wherein the pneumatic energy storage system is powered by a rechargeable battery that is recharged by a permanent magnet alternator that is driven by the accessory gear box.
2. A method according to claim 1 , wherein the gas turbine engine comprises in axial flow a low pressure compressor, and a high pressure compressor, and wherein the bleed air is provided by the high pressure compressor.
3. A method according to claim 2 , wherein the gas turbine engine comprises in axial flow an intermediate pressure compressor.
4. A method according to claim 1 , wherein the stored pneumatic energy is in the form of compressed air.
5. A method according to claim 1 , wherein the high pressure rotor is rotated at a speed from 0.1 rpm to 20 rpm for 1 minute to 10 hours.
6. A method according to claim 5 , wherein the high pressure rotor is rotated continuously for that duration.
7. A method according to claim 5 , wherein the high pressure rotor is rotated discontinuously for that duration.
8. A method according to claim 7 , wherein the high pressure rotor is rotated by approximately 180° per rotation.
9. A method according to claim 1 , wherein the high pressure rotor is rotated at a speed from 0.5 rpm to 15 rpm for 5 minutes to 8 hours.
10. A method according to claim 9 , wherein the high pressure rotor is rotated continuously for that duration.
11. A method according to claim 9 , wherein the high pressure rotor is rotated discontinuously for that duration.
12. A method according to claim 11 , wherein the high pressure rotor is rotated by approximately 180° per rotation.
13. A pneumatic system for controlling rotor bow, the system comprising:
an air tank configured to receive compressor bleed air from a compressor of a gas turbine engine when the engine is running, the air tank further configured to provide stored pneumatic energy, wherein the gas turbine engine has, in axial flow, a low pressure rotor and a high pressure rotor;
a valve configured to control the flow of the compressor bleed air into the air tank;
an accessory gearbox configured to rotate the low pressure rotor and the high pressure rotor;
a pneumatic motor or an air turbine starter configured to rotate the high pressure rotor with the stored pneumatic energy after the engine is shut down for a duration that reduces rotor bow;
a permanent magnet alternator driven by the accessory gearbox;
a re-chargeable battery configured to be re-charged by the permanent magnet alternator, wherein the re-chargeable batter is configured to power the pneumatic system.