GB2024336A - Gas turbine rotor tip clearance control apparatus - Google Patents

Abstract

The apparatus comprises an annular stator shroud member (18) having an inner frusto-conical face (19) cooperating with the tips of the rotor blades (16) to form a seal. Actuation means (29) are provided to move the shroud member (18) axially so as to vary the clearance (44) between shroud member (18) and the blade tips in a predetermined manner. <IMAGE>

Description

SPECIFICATION Tip clearance control apparatus for a gas turbine engine This invention relates to a rotor tip clearance control apparatus for a gas turbine engine.

In gas turbine engines there are a number of situations where a high speed rotor co-operates with static structure to form a seal. One particular instance of this construction lies in the turbine where the turbine disc carries a plurality of rotor blades whose tips seal against a static shroud structure.

Such structures require that the clearance between the rotating and the static structure is very carefully controlled in order to provide the necessary efficient sealing. This is very difficult to achieve because the rotating and static components tend to differentially expand under the influence of heating and centrifugal loads. Various proposals have been made in the past to provide cooling or heating to the shroud structure or outer static structure but this problem has still not been completely solved.

The present invention provides a control apparatus which may be used to vary the clearance of such a device in a predetermined fashion.

According to the present invention a rotor tip clearance control apparatus for a gas turbine engine comprises an annular shroud member forming part of the static structure of the apparatus, said member having an inner frusto-conical face adapted to cooperate with the outer extremities of said rotor to form a seal, and actuation means adapted to move said shroud member axially so as to affect the clearance between said member and the rotor extremities in a predetermined manner.

Normally the clearance would be arranged to be a minimum value at all times.

In one embodiment the actuation means comprises a mechanism which causes radial expansion or contraction of the shroud to be reflected in axial movement of the shroud.

Thus the radial expansion or contraction may be transmitted back to the shroud by levers to cause the axial movement.

In an alternative embodiment the tip clearance between the rotor extremities and the shroud is measured and used to control a system of rams which move the shroud member axially.

The invention is of most benefit where the shroud member comprises the complete annulus.

The invention also comprises gas turbine engine having rotor tip clearance control apparatus as set out above.

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 gas turbine engine incorporating rotor tip clearance control apparatus in accordance with the invention, Figure 2 is an enlarged section through the rotor tip clearance control apparatus of the engine of Figure 1, Figure 3 is a view similar to Figure 2 but of an alternative embodiment and, Figure 4 is a view similar to Figure 3 but of a third embodiment.

In Figure 1 there is shown a gas turbine engine comprising a casing 10 within which are mounted in flow series a compressor 11, a combustion system 12 and a turbine 13, and which forms at its downstream extremity a final nozzle 14. Operation of the engine overall is conventional and notelabo- rated on in this specification. It will be seen that the turbine includes nozzle guide vanes 15 which direct hot gases onto the turbine rotor blades 16 which are carried from a rotor disc 17. Attheirtips the blades 16 are closely spaced from a shroud ring 18.

Figure 2 shows in greater detail the structure in the area of the shroud ring 18. It will be seen that the ring 18 comprises a hollow annular member whose front and rear faces and outer surface are at right angles to each other and in this embodiment lie approximately parallel and at right angles to the axis of the engine. The inner face 19 of the ring is frusto-conical and diverges in the direction of airflow of the engine, that is from left to right in the drawings.

The shroud ring 18 is provided at one end of its outer facing with a projecting flange 20 from which are carried the guides 21 of a sliding trunnion 22 which has a central aperture 23. The aperture 23 engages with a pin 24 set in a bell crank lever 25. The lever 25 is pivoted about a pin 26 carried from the casing 10 by a flange 27.

The arm 28 of the lever 25 between the pins 24 and 26 is relatively short while the other arm 29 of the lever is relatively long and carries at its extremity remote from the pivot 26 an actuating pin 30. The pin 30 engages in a depression 31 formed in an actuating ring 32. The ring 32 is retained to the shroud ring 18 by a projecting flange 33 which extends into a channel 34 in a projection 35 from the shroud ring 18.

It should be noted that the ring 18 and actuating ring 32 are engaged by a plurality of levers 25 and associated mechanisms; there may for instance be three of these lever systems spaced around the periphery of the rings 18 and 32.

To prevent leakage of gases from the main flow duct of the gas turbine engine the ring 18 is provided with an annular channel 36 in a projection 37 from its outer surface. The channel 36 engages a piston ring 38 which in turn engages with the cylindrical inner surface of a sealing member 39 carried by a flange 40 from the casing 10.

In a similar manner the ring 32 has a channel 41 which carries a piston ring 42. The ring 42 seals against the outer surface of the flange 43 whose primary purpose is to carry the nozzle guide vanes 15. Between them the piston rings 38 and 42 prevent gases escaping round the front or the back of the shroud ring 18.

It will be understood that the operation of the bell crank lever is that should the ring 18 expand because of an increase in temperature the trunnion block 22 will be pushed outwardly relative to the pivot 26.

This will rotate the bell crank about the pivot and will cause the actuating pin 30 to move in a direction substantially axially of the engine. Because the arm 28 is considerably shorter than the arm 29 a considerable geometrical advantage is involved and a relatively small radial expansion of the ring 18 will cause a relatively large axial displacement of the ring 30.

By virtue of its engagement with the ring 32 the pin 30 causes axial movement of this ring and therefore of the ring 18. Clearly because the inner surface 19 of the ring 18 isfrusto-conical an axial movement of the ring 18 will have an effect on the clearance 44 between the tips of the blade 16 and the surface 19.

Thus if the ring 18 moves to the right it will tend to decrease the clearance whilst if it moves to the left it will tend to increase the clearance.

It will therefore be seen that in the present embodiment if the ring 18 expands radially it will tend to increase the clearance 44. However, the action of the lever 25 will displace the ring to the right and will therefore tend to reduce the clearance. An opposite effect occurs when the ring contracts. In this embod iment the apparatus therefore tends to reduce the effect of thermal expansion on the clearance 44, however, it is clear that by carefully choosing the relative dimensions of the arms 28 and 29 various different characteristics could be achieved for the variation in the clearance 44.

Obviously the casing described in which the ring 18 is a complete ring leads to the worst effects with regard to radial expansion and contraction. However, even in the less difficult situation where the ring 18 is made of a plurality of smaller segments the invention may still be used.

One difficulty with the embodiment of Figure 2 lies in the fact that the axial movement of the ring is substantially a linear function of its radial expansion and it may not always be desirable to have the position of the inner surface 19 determined in this linear fashion. Figure 3 is a modified embodiment in which the axial position of the ring 18 may be made to respond to the actual value of the clearance 44. In Figure 3 the ring 18 is basically similar to that in Figure 2 as is the inner surface 18. Once again piston ring seals are provided at 38 and 42. However, in this instance a series of hydraulic, electric or pneumatic rams 45 are provided which operate through push rods 46 and ball joints 47 on the ring 18. In the embodiment illustrated the pistons 45 are mounted in the radially extending part of the flange 43.A small orifice 48 formed in the face 19 provides a signal indicative of the value of the clearance 44 and the signal is fed through ducting 49 to a control unit 50.

The control unit 50 which may be pneumatic, hydraulic or electronic translates this signal into a supply of pressurised fluid through the pipes 51 and 52 to the rams 45.

In this way by correctly programming the control device 50 the clearance 44 may be maintained at a pre-determined value by axially moving the ring 18 so as to counteract the effect of relative expansions between the ring 18 and the tips of the blades 16.

The lever system described in relation to Figure 3 is not the only method of providing a mechanical system which translates radial expansion of the ring 18 into axial movement and Figure 4 shows an alternative. Once again the basic constructional features of the Figure 4 embodiment are similar to those of Figure 2 and are not described but in this embodiment the ring 18 is provided with a plurality of angled cam faces 54 supported from its outer surface. The cam faces 54 each engage between pairs of rollers 55 and 56 carried on projections 57 from the casing 10.

Operation of this embodiment is quite simple in that radial expansion of the ring will cause radial movement of the cams 54. This radial movement will cause the cams to translate axially between the rollers 55 and 56 and will thus cause axial motion of the ring 18. The degree of axial movement per radial movement will be determined by the angle of the cams 54 and it would be possible to arrange that the cams were not straight lines but had curved shapes to achieve a pre-determined variation in the motion of the ring.

The most useful application of the present invention is to the shroud of high temperature areas of the engine such as the turbine described. However, the matching of radial expansions is a problem in other parts of the engine such as in compressors or in labyrinth seals in general and the present invention could clearly be applied to such compressors and seals in just the same way as described above in relation to turbines. It is also possible to conceive of a number of different ways in which mechanical or pneumatic or hydraulic actuation of the ring could be achieved.

Claims (7)

1. A rotor tip clearance control apparatus for a gas turbine engine comprising an annular shroud member forming part of the static structure of the apparatus, said member having an inner frustoconical face adapted to co-operate with the outer extremities of said rotor to form a seal, and actuation means adapted to move said shroud member axially so as to affect the clearance between said member and the rotor extremities in a predetermined manner.

2. A rotor tip clearance control apparatus as claimed in claim 1 and in which said actuation means comprises a mechanism adapted to cause radial expansion or contraction of the shroud member to be reflected in axial movement of the shroud.

3. A rotor tip clearance control apparatus as claimed in claim 2 and in which said actuation means comprises a lever mechanism.

4. A rotor tip clearance control apparatus as claimed in claim 3 and in which said lever mechanism comprises a plurality of bell-crank levers each engaging at one extremity with a radially facing part of said shroud ring so as to be rotated by radial expansion or contraction of said shroud, and engaging at the other extremity with an axially facing part of said shroud so that said rotation causes axial movement of the shroud.

5. A rotor tip clearance control apparatus as claimed in claim 1 and comprising measuring means adapted to measure said clearance and to provide an output indicative of the clearance and control means adapted to use said signal in controlling said actuation means to maintain a predetermined value of said clearance.

6. A rotor tip clearance control apparatus as claimed in claim 5 and in which said actuation means includes fluid pressure operated rams.

7. A rotor tip clearance control apparatus substantially as herein particularly described with reference to Figures 1 and 2, or Figure 3, or Figure 4 of the accompanying drawings.