GB2068470A

GB2068470A - Casing for gas turbine engine

Info

Publication number: GB2068470A
Application number: GB8003577A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1980-02-02
Filing date: 1980-02-02
Publication date: 1981-08-12

Abstract

The casing includes arcuate segments (27) which together make up a shroud ring defining the outer boundary of the gas flow path through a rotor of the engine. Each segment (27) is mounted on supporting structure (21, 22) by means of at least one cam track (28, 29) which is angled so that upon circumferential movement of the segment its radial position is also altered. Actuation means (40) can move the segments circumferentially and thus adjust their radial position to correct the clearance between the segments and the tips of their associated rotor blades (19). <IMAGE>

Description

SPECIFICATION Casing for a gas turbine engine This invention relates to a casing for a gas turbine engine.

Casings of gas turbine engines often comprise or carry shrouds which cooperate with the tips of rotor blades to define the outer boundaries of the gas flow through the particular part of the engine concerned. Thus in particular the compressor and turbine casings carry such shrouds.

The clearance between the tips of the rotor blades of the turbines and compressors of a gas turbine engine has a significant effect on the efficiency of the engine. With recent increases in fuel costs, the additional efficiency which may be achieved by close control of these clearances has become very desirable.

In the past, the so-called 'active' tip control arrangements in which some form of variable geometry is used with servo control to maintain relatively constant clearances has been largely aimed at the turbines of the gas turbine engine. In particular, our earlier patent application 7905999 relates to a device using the 'hade' of the turbine blade tips to provide such an active clearance control. However, it is clear that if a simple and relatively light weight construction could be designed which did not rely on the 'hade' present in some turbines but unusual in compressors this 'active' control could provide benefits when applied to a compressor.

The present invention relates to a casing for a gas turbine which is particularly useful in the environment of a compressor but which could be applied to a turbine.

According to the present invention a casing for a gas turbine engine comprises a plurality of arcuate segments together forming a shroud ring which defines the outer boundary of the flow of gas through a rotor of the engine, each segment being mounted on supporting structure by means of at least one cam track which is angled so that upon circumferential movement of the segment, the radial position of the segment is also altered, and actuation means for causing circumferential movement of the segments, and thus via the cam tracks radial movement of the segments.

The segments may have projections which engage with the cam tracks which are formed in fixed structure of the casing.

In a preferred embodiment a servo-control system is incorporated which maintains the clearance between the inner face of the shroud ring and the tips of the associated rotor blades within a predetermined range of values.

The invention will now be particularly described, merely by way of example with reference to the accompanying drawings in which: Figure 1 is a partly broken-away view of a fantype gas turbine engine having a casing in accordance with the present invention; Figure 2 is an enlarged axial section through the casing of Figure 1, Figure 3 is a section on the line 3-3 of Figure 2, Figure 4 is a view similar to Figure 2 but of a further embodiment, and Figure 5 is a section on the line 5-5 of Figure 6.

In Figure 1 there is shown a gas turbine engine comprising a core engine 10 driving a fan rotor 11 which compresses air within a fan casing 12. Part of this air enters the core engine 10 while the remainder passes through the annular nozzle formed between the downstream extremity of the casing 12 and the core engine casing 14 to provide propulsive thrust.

Within the core engine 10 its proportion of the fan air is further compressed within a compressor section 14, fuel is added to the compressed air and burnt in a combustion section 15, the hot gases drive turbines in the turbine section 1 6 and exhaust through a nozzle 1 7 to provide additional propulsive thrust. The turbines are connected to drive the compressors of the compressor section 14 and the fan rotor 11.

As will be appreciated, the core engine 10 may comprise a single or a multi-spool system; usually the multi-spool arrangement will involve high pressure and intermediate pressure assemblies each consisting of a turbine, a compressor and an interconnecting shaft. A separate low pressure or fan turbine then drives rhe fan through a further shaft. It does not significantly affect the present invention whether it is applied to the compressor casing of a single or multi-spool core or to a separate engine or even to a turbine of any of these engines. However, in the illustrated embodiment the core is taken to be a multi-shaft device having high and intermediate pressure rotating systems, and where the casing of the core 10 is broken away at 1 8 the blades 19 of the high pressure compressor are exposed to view.

In order to achieve the best efficiency from the high pressure compressor it is necessary to maintain the clearance between the tip of the blades 1 9 and the associated static casing at a minimum value, and Figure 2 shows in enlarged section the construction used to achieve this.

Although only part of the casing is described, controlling the clearance of one rotor stage, it will be understood that other stages may have similar casings as necessary.

It will be seen that the basic casing structure comprises a substantially cylindrical member 20 from which extend two radial flanges 21 and 22, each carrying at its outer extremity a slugging ring 23 and 24 respectively which serve to control thermal expansion of the flanges and thus the member 20. Tha flanges 21 and 22 also extend within the member 20 to form relatively shallow supporting flanges 25 and 26 which carry between them a shroud ring made up of a plurality of segments 27. To carry the segments 27, the supporting flanges 25 and 26 are each provided with sets of facing cam tracks 28 and 29 respectively in which projections 30 and 31 from the segments 27 engage.

As can be seen from Figure 3, each segment 27 is carried by the engagement of its projections 30 and 31 with cam tracks 28 and 29 adjacent each of its ends. Figure 3 actually shows the region of abutment between adjacent segments 27. The cam tracks 28 and 29 extend at a shallow angle to the circumferential direction, so that if the segments are moved circumferentially the engagement between the projections and the cam tracks will cause the segments to move radially inwards or outwards along the slope of the tracks.

Therefore, by adjusting the circumferential position of the segments their radial position can be altered, and hence the clearance between the inner face 32 of the segment, which forms the outer boundary of the gas flow passage through the engine, and the tips of the rotor blades 1 9 may be adjusted.

In order to provide the necessary circumferential motion of the segments 27, each is provided with a pair of racks 33 at each end, aligned circumferentially with the projections 30 and 31. Pinions 34 engage with the racks 33, these being carried on a shaft 35 which extends between bearings 36 and 37 in the flanges 21 and 22 respectively. Beyond the flange 22 the shaft 35 extends to carry a drive gear wheel 38 which engages with the drive pinion 39 of an electric motor 40. It will be understood that rotation of the motor drive pinion 39 will rotate the wheel 38, the shaft 35 and the pinions 34 and will thus translate the racks 33 circumferentially.Because the motion of the segments 27 is a compound of radial and circumferential movement, the racks 33 do not extend in the circumferential direction but instead run parallel with the cam tracks 28 and 29 so that engagement between the pinions 34 and rack 33 is maintained over the range of movement of the segments.

By operating the motor 40 it is therefore possible to vary the clearance between the face 32 and the blades 19. In order to control this clearance a sensor 41 is mounted in the face 32, the sensor providing an output dependent upon the value of the clearance. For convenience it is assumed that the sensor operates on an electrical parameter such as magnetic or electrical field and produces an electrical output to the output wires 42, but of course many forms of transducer could be used to act as a sensor.

The output from the sensor 41 is conducted along output wires 42 to a control unit 43 which conveniently comprises a micro-computer using a microprocessor. The output from the unit is in the form of a motor drive signal which operates the motor 40. It will be appreciated that this is a fairly straightforward form of closed loop servo control system, and that it is well within the capability of one skilled in the art to design a system which will maintain the clearance close to a desired value, or at least within a desired range.

It is clearly possible to use alternative methods of effecting circumferential movement of the segments 27 and Figures 4 and 5 illustrate one such alternative. The basic structure of casing member, segments, and cam tracks and projections is almost identical to that of Figures 2 and 3, and will not be described again. However, in this case each segment 23 is provided with a pair of upstanding ears 50 adjacent to each of its ends the ears 50 retaining a pin 51 which passes through the bush 52 which forms one end of a push rod 53. The othr end of each push rod 53 comprises a bush 54 which engages with a pin 55 held in an H-section unison ring 56 which extends circumferentially outside of the shroud ring formed by the segments 27.

It will be seen that circumferential movement of the ring 56 will cause, via the push rods 53, corresponding circumferential movement of the segments 27; and that the articulation of the push rods will allow the segments to perform their compound circumferential and radial motion. In order to cause the necessary motion of the unison ring 56, it is provided with a rack 57 on one of its outside surfaces, the output pinion 58 of an electric motor 59 engaging with the rack. As in the case of the previous embodiment, the motor may be controlled by a servo loop comprising a sensor 60 and controller 61 to maintain a steady clearance between the segment and the rotor blades.

It will be appreciated that considerable modifications could be made to the embodiments described above. In particular the prime mover for the control of the segments could be electrical, hydraulic or pneumatic and could drive the segments via a wide range of linkages, gearings or cam arrangements. It should also be noted that the layout of the cam tracks could be altered; thus the tracks could be on the segments rather than on the fixed structure, and it may be desirable to vary the shape of the tracks to achieve a nonlinear response for the system.

Claims

1. A casing for a gas turbine engine comprising a plurality of arcuate segments together forming à shroud ring which defines the outer boundary of the flow of gas through a rotor of the engine, each segment being mounted on supporting structure by means of at least one cam track which is angled so that upon circumferential movement of the segment, the radial position of the segment is also altered, and actuation means for causing circumferential movement of the segments, and thus via the cam tracks radial movement of the segments.

2. A casing as claimed in claim 1 and in which the segments have projections which engage with the cam tracks which are formed in fixed structure of the casing.

3. As casing as claimed in claim 1 and comprising a servo control system which measures the clearance between an internal surface of the shroud ring and the tips of rotor blades which cooperate with the ring, and controls the circumferential movement of the segments to maintain the clearance within predetermined limits.