GB2117842A

GB2117842A - Means for equalising the temperatures within a gas turbine engine

Info

Publication number: GB2117842A
Application number: GB08208864A
Authority: GB
Inventors: George Pask
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1982-03-25
Filing date: 1982-03-25
Publication date: 1983-10-19

Abstract

In order to reduce thermal distortion ('hogging') due to differential temperatures caused by convection effects in a shut-down gas turbine engine, means are provided for equalising the internal temperatures. This means comprises circumferential ducting (16, 17) interconnecting the top and bottom of the engines gas flow annulus, and air moving devices (18, 19) which cause a flow of air which counteract the convectional flows. <IMAGE>

Description

SPECIFICATION Means for equalising the temperatures within a gas turbine engine This invention relates to means for equalising the temperatures within a gas turbine engine.

One-problem with modern gas turbine engines is the thermal distortion known as 'hogging'. This occurs when an engine is shut down after use, when the residual heat in the various components of the engine causes convention currents which caue the upper parts of the engine to retain their heat for longer than do the lower parts of the engine. This produces a temperature differential which in turn causes differential thermal expansion.

The effect of this expansion is to cause the various rotors and casings of the engine to bow upwardly or to 'hog'. In itself this distortion may not be harmful, but if it is desired to re-start the engine the distorted rotors may rub on parts of the distorted casings unless large clearances are left. These large clearances are undesirable for aerodynamic reasons.

Methods proposed for dealing with the hogging problem have included provision of a drive, or utilising the existing starter drive means, to keep the engine rotors turning for some time after the engine has shut down. This may, in itself, require the expenditure of a considerable amount of power and has other potentially undesirable effects.

The present invention provides a way of preventing these distortions by equalising the temperatures within the engine.

According to the present invention means for equalising the temperatures within a gas turbine engine comprises circumferential ducting interconnecting, the upper and the lower parts of its gas flow annulus and at least one air moving device adapted to cause air to flow in said circumferential ducting from the upper to the lower parts of the gas flow annulus or vice-versa.

It may be convenient to use the existing bleed air ducting as entrances and/or exits to the circumferential ducting.

Electric fans may be conveniently used as said air-mixing devices.

The invention will now be particularly described, merely by way of example, with reference to the accompanying drawings in which: Fig. 1 is a partly-sectioned side elevation of a gas turbine engine having means for equalising temperatures in accordance with the invention, and Fig. 2 is a section on the line 2-2 of Fig. 1.

In Figure 1 there is shown a gas turbine engine 1 0. In order to simplify the description, the engine 10 is shown as a very simple single-shaft design, but it will be appreciated that the invention is applicable to other more complex designs, and in fact its primary use may be in connection with large two-shaft fan engines with their long rotor assemblies.

In the illustrated engine a single axial flow compressor 11 compresses air which enters the combustion chamber 12 where it is mixed with fuel and burnt. The resulting hot gases drive the single stage axial flow turbine 13, which drives the compressor 11 through drive shaft 14, and exhaust through the nozzle 1 5 to provide propulsive thrust.

As so far described, the engine is quite conventional, and will be prone to the thermal distortion effect known as 'hogging'. Thus when the engine is shut down, the rotor consisting of the compressor 1 the shaft 14 and the turbine 13 comes to rest. There is still a large amount of heat retained in the various structures of the engine, indeed it is common for the complete cooling-down to take some hours to achieve.

During this time convection currents are set up in the air retained in the internal cavities of the engine, and these currents cause heat transfer from the bottom to the top of the engine. In this way, superimposed on the overall cooling of the engine, a temperature differential is set up between the top and the bottom of the engine, this differential affecting both static and rotary parts.

This temperature differential will cause the casings and rotors of the engine, which are substantially surfaces of revolution, to arch or bow upwardly as their upper parts expand in relation to their lower parts. This is the distortion known as 'hogging', and although not necessarily detrimental while the engine is static it can cause initial rubs between the rotor and static engine structure if the engine is operated in the 'hogged' condition.

In order to avoid, or at least to reduce this distortion, the engine 10 is provided with means in accordance with the invention for equalising its internal temperature while in the shut-down condition. This means comprises two circumferential ducts 1 6 and 1 7 which interconnect the upper and lower parts of the gas flow annulus of the engine and which are provided with electric fans 1 8 and 1 9 respectively to enable forced flow of air through the ducts.

In the case of the duct 1 6,use is made 6tan existing offiake manifold to provide the interface between the duct and the main gas flow annulus of the engine. The manifold 20 consists of an annular chamber formed in the casing of the compressor and communicating with the compressed airflow in the compressor via a ring of apertures 21. Air from the manifold 20 is passed via an offiake passage 22 at the top of the engine to its location of use. The circumferential duct 1 6 is teed into the passage 22 to provide an upper connection, while at the bottom of the engine it connects directly into the manifold.

In the duct 1 6 the fan 1 8 is arranged to blow air upwardly, although a downward flow would be acceptable, from the bottom inlet to the duct and into the passage 22. This air will be relatively cold compared with the rest of the air in this vicinity, and a current of air will be set up which will counter the convectional rise of hot air to the top of the engine. Operation of the fan 18, and of the fan 19, is effected by a control unit 23 which controls the flow of current from a battery 24.

The duct 17 and associated fan 19 acts in a similar fashion to the duct 1 6 and fan 1 8, but because there is not available manifild equivalent to 20 in the turbine region, the duct 1 7 simply breaks through the outer casing of the engine top and bottom and interconnects these portions of the gas flow annulus via the space outside the outer stator platforms and shrouds.

There may of course be additional ducts like 1 6 and 17, but the operation of all will be the same in that once the engine is shut down, the fans 18 and 19 and any additional equivalent will be operated. The circulation of air through the engine which they produce will counteract the conventional flow of hot air and will at least reduce the 'hogging' effect. The control unit 23 may be arranged to turn the fans on automatically once the engine is shut down and conditions are such that hogging might otherwise occur. The control unit can also be arranged to switch the fans off once a predetermined period of operation has elapsed.

It will be understood that various modifications could be made to the embodiment described above. Thus for instance the ducts 1 6 and 1 7 could be internal of the engine rather than external, and other forms of air moving devices such as ejectors could be used instead of the fans 18 and 19. It may also be desirable to cause the air moving devices to operate in the opposite sense from that shown, so that warmer air is carried through the ducts to the bottom of the engine instead of cold air being carried to the top.

Claims

1. Means for equalising the temperatures within a gas turbine engine comprising circumferential ducting interconnecting the upper and the lower parts of its gas flow annulus and at least one air-moving device adapted to cause air to flow in said circumferential ducting from the upper to the lower part of the gas flow annulus or vice versa.

2. Means as claimed in claim 1 and in which said circumferential ducting communicates with the gas flow annulus via a bleed air manifold.

3. Means as claimed in claim 1 or claim 2 and in which suid air-moving devices are electric fans.

4. Means as claimed in any one of the preceding claims and comprising control apparatus adapted to cause said air-moving means to operate when necessary.

5. Means for equalising the temperatures within a gas turbine engine substantially as hereinbefore particularly described with reference to the accompanying drawings.

6. A gas turbine engine having means for equalising the temperature within it as claimed in any one of the preceding claims.