GB2221724A - Bladed rotor assembly and sealing wire therefor - Google Patents

Abstract

A sealing wire 27', suitable for sealing the gap G between the platform edges (23, 25) of adjacent turbomachine rotor blades (5), is lodged in the gap G under centrifugal force when the turbine is rotating. The wire 27' is substantially straight in the longitudinal sense but has a flattened oval or "race-track" shaped cross-section to prevent it rotating and thereby wearing out rapidly while lodged in the inter-platform gap G. <IMAGE>

Description

BLADED ROTOR ASSEMBLY AND SEALING WI Pr THEREFOR The present invention relates to means for sealing gaps between adjacent platforms of rotor blades in a turbomachine, and specifically to wires used for sealing such gaps.

In one known type of axial flow gas turbine rotor assembly comprising turbine blades mounted onto a hub member, such as a disc or drum, for incorporation into, e.g., an aircraft engine, the edges of the radially inner platforms of adjacent rotor blades do not abut each other because manufacturing and assembly tolerances require that there is a small inter-platform gap to ensure avoidance of harmful interference between platform edges at all points in the turbine's operational cycle. Turbine rotor discs and blades in modern gas turbine aeroengines are cooled by means of compressed cooling air which is fed respectively to the disc rims and to blade cooling passages having their entries under the blade platforms in the root portions of the blades. However, the cooling air can escape from under the platforms through inter-platform gaps unless the gaps are sealed in some way.Such cooling air leakage detracts from the efficiency of the engine.

One known way of sealing inter-platform gaps is to provide a groove in one of each pair of mutually confronting platform edge faces which define the gaps, the opposing platform edge face of each pair being left plane.

The groove is coextensive with the major part of the width of the platform edge face in which it is formed. Its radially outer wall is relieved in the radial direction to allow a length of wire housed in the groove to centrifuge outwards when the rotor assembly rotates, the wire thus becoming lodged between the relieved wall of the groove and the opposing platform edge face. In this way the wires substantially seal the inter-platform gaps.

A problem has been found in this arrangement in that if, as is most convenient, straight round-section wires are used, relative vibration and other movement between adjacent platforms, plus the effect of slight leakages of pressurised air past the wires, tends to cause the wires to rotate in their grooves. The wires may then wear quite rapidly until they are of small enough diameter to either jam in or be released through the gaps between the platforms. In the former case, the vibration characteristics of the blades may be deleteriously changed, leading to much increased stresses in the blades. In the latter case, not only is the efficiency of the engine reduced because of increased cooling air leakage, but also subsequent stages of the turbine may be damaged by impact, or by interference of the wires with turbine blade tip seals, etc.

To combat the tendency of these wires to spin, it is known to bend them longitudinally so that they are slightly curved or kinked rather than straight. Of course, in this case the radially outer (relieved) wall of the slot in which each wire is housed must have a profile to match the longitudinal curvature of the wire, otherwise the wire will not seat correctly in the interplatform gap under centrifugal force.It has been found that, although this is a satisfactory solution to the problem in the case of larger engines, such as the Rolls-Royce RB211 (Registered Trade Mark), it is not such a satisfactory solution in the case of medium size engines such as the Rolls-Royce Tay (Trade Mark), due to the fact that the turbine blade platform dimensions, and therefore the slots housing the sealing wires, are smaller than in the RB211, sometimes causing the bent wires to become wedged in the slots in incorrect positions, giving poorer sealing of the inter-platform gaps.

The invention conveniently overcomes the above problems by utilising a sealing wire having a particular cross-sectional shape.

According to the present invention, a sealing wire, for sealing gaps between the platform edges of adjacent turbomachine rotor blades, is substantially straight in the longitudinal sense but has a non-circular cross-section including at least one flat: preferably, the non-circular cross-section is racetrack-shaped, comprising two curves and two flats.

When the wires are lodged in the inter-platform gaps under centrifugal forces, such a non-circular wire section substantially prevents rotation of the wires in the previously mentioned grooves which house them.

Preferably, the turbomachine rotor blades are air-cooled gas turbine blades as hereinbefore mentioned.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying illustrations, in which: Figure 1 is a part-sectional side elevation of part of an axial flow gas turbine rotor assembly and immediately adjacent structure in a gas turbine aeroengine; Figure 2 is a partial view on section II -II in Figure 1, showing only detail in the plane of the section; Figure 3 is an enlarged partial view on section III III in Figure 2; Figure 4 is a partial view on section IV - IV in Figure 3; Figure 5 is a further enlarged view similar to Figure 4, but showing an embodiment of the invention; and Figure 6 is a photograph of the results of a comparative test involving an embodiment of the invention.

Referring first to Figures 1 and 2, there is shown a turbine rotor assembly 1 comprising a rotor disc 3 and a plurality of rotor blades 5 which are attached to the periphery of the disc 3 for rotation with it. Attachment of the blades 5 to the disc 3 is by means of specially shaped root portions 7 which are received in correspondingly shaped axial slots or apertures 9 in the disc rim. The slots 9, and hence the blades 5, are equiangularly spaced around the disc 3.

In addition to its root portion '., each blade 5 comprises a radially inner platform portion 11, an aerofoil portion 13 and a radially outer shroud portion 15. The aerofoil portions 13 experience the direct heating effects of the turbine gases in the turbine annulus passage and are actively cooled by means of compressed air which is supplied via pre-swirl nozzles 14 in adjacent static structure 16 and which flows through various passages 18 internally of the root, platform and aerofoil portions of the blades 5. Cooling air is exhausted from each blade, as known, through small holes (not shown) on various parts of the aerofoil surface and through exhaust holes in the shroud 15.Due to the reactions experienced by the blades 5 as the rotor 1 is driven round by the flow of hot gases which the blades intercept, it is necessary to ensure that the blades are prevented from sliding axially rearwards in their slots 9, and this is accomplished by lockplates 20, which engage an undercut hooked portion or slot 22 on the rear edge of each platform 11, and a similar circumferential slot 17. The slot 17 is packed with a ring member 24.

Bounded by adjacent blade root portions 7, the undersides of the platforms 11, the rim of rotor disc 3 and the forward faces of the lock plates 20, are under-platform chambers 21. The compressed air for cooling the blades is fed to these chambers 21 from the pre-swirl nozzles 14 in the static structure 16 just forward of the rotor assembly 1 and enters the blades through air entry holes 26 under the platforms 11 near their junctions with the root portions 7.

As will be seen from Figure 2, the platforms 11 of the blades are not quite circumferentially contiguous because it is required that there is a small inter-platform gap G between circumferentially adjacent blades 5. However, this leads to excessive escape of compressed air from chambers 21 through the gaps G, unless they are satisfactorily sealed. Such leakage of cooling air has an adverse effect on the specific fuel consumption of the engine.

As previously described, it is already known to reduce leakage of air through the gaps G between pairs of confronting platform edge faces 23,25, by means of straight round-section sealing wires 27 housed in grooves 29 provided in one face 23 of each pair of platform edge faces. The grooves 29 and the wires 27 are coextensive with the major portions of the widths of the edge faces 23, which occupy radial planes transverse of both the circumferential and the axial directions. The details of a platform edge face 23, its groove 29 and the sealing wire 27 are shown in Figures 3 and 4.

As shown in detail in Figure 4, the radially outer wall 31 of each groove 29 is relieved in the radial direction to allow the wires 27 to centrifuge outwards to the position shown in dashed outline when the rotor assembly 1 rotates. The wires 27 should then wedge between the relieved walls 31 and the plane platform edge faces 25, so substantially filling the gaps G where they are coextensive with the wires. Nevertheless, the problem of the sealing wires 27 rotating and fretting in the grooves 29 has been encountered as previously described.

Figure 5 shows that in accordance with the present invention the lengths of sealing wire 27' have been given (e.g. by cold or hot rolling or by extrusion through an appropriately shaped die) a flattened oval or so-called "racetrack" shaped cross-section comprising two end semi-circles 33 and two flats 35. As shown, when these flattened sealing wires 27' are centrifuged outwards upon rotation of the rotor, they seal the gaps G in the same manner as the round-section wires, but are restrained from rotating by virtue of their shape. Hence, fretting and wear rates are much reduced.

Note that the sealing wires 27' may be retained in place in the grooves 29 during assembly of the blades 5 onto the disc 3 by means of a light mineral grease or jelly, which will disappear during the first run of the engine.

Figure 6 is a photograph showing three blades A,B and C which were subjected to an endurance test. The blades are of the type illustrated in Figures 1 to 4, and the photograph shows the witness marks left on the plane platform edge faces 25 by the three types of sealing wires mentioned in this specification.

Blade A shows the witness mark left by a longitudinally straight circular section wire. The mark although continuous (showing good sealing), is relatively thin, showing that the sealing wire was constantly turning in its groove instead of wearing a flat contact face on itself by virtue of its contact with the platform. If the test had continued long enough, the wire might have worn down to a sufficiently small diameter to pass into the inter-platform gap, jam between the platforms, and perhaps ultimately be released into the turbine annulus passage.

Blade B shows the witness mark left by a longitudinally straight "race-track" section wire according to the invention. The mark is continuous and relatively thick, showing that one of the "flats" on the wire had consistently rested against the platform edge face.

Blade C shows the witness mark left by a circular section wire given a slight longitudinal curvature to prevent it rotating in its groove. The mark is discontinuous but relatively thick, showing that although the wire had consistently stayed in one position in the inter-platform gap without rotating, and has therefore worn "flats" on itself by virtue of its contact with the platform, the contact had nevertheless been discontinuous due to the wire not quite being in the correct relationship to the gap, thereby illustrating the previously mentioned problem of ensuring adequate sealing in small to medium size engines with this form of wire seal.

Although in the above specific embodiment of the invention, the cross-sectional shape of the sealing wire has been described as a "flattened oval" or "race-track" shaped, because this is easily and conveniently produced, there is no reason why other cross-sectional shapes within the scope of the claims should not be utilised, so long as they exhibit at least one flattened portion sufficient to substantially prevent rotation of the wire in the interplatform gap.

Although the sealing wires according to the present invention have been described above only in relation to sealing between the platforms of air cooled turbine blades, they are also applicable to sealing between the platforms of uncooled blades, in order to reduce leakage of rotor disc rim cooling and sealing air through the interplatform gaps. Such leakage would otherwise cause aerodynamic losses in the blade annulus passage and would reduce fuel efficiency due to the need to supply more disc cooling air from the compressor.

Furthermore, such sealing wires are effective to prevent unwanted flow of gases from the blade annulus passage into the blade root area under the platforms and back out again, which would otherwise cause overheating of the blade roots and disc rim, plus aerodynamic losses.

Claims (5)

Claims:

1. In a turbomachine, a bladed rotor assembly comprising a rotor hub member and a plurality of blade members attached thereto and arranged circumferentially thereof in substantially equiangularly spaced relationship to each other, said blade members having radially inner platforms, circumferentially adjacent platforms being provided with sealing means for reducing gaseous flow through gaps defined between confronting platform edge faces, said sealing means comprising, for each pair of confronting platform edge faces, a sealing wire housed in a groove provided in a first one of said pair of edge faces, said groove and said sealing wire being coextensive with the major part of the width of said one platform edge, the radially outer wall of said groove being relieved in the radial direction to allow said wire to centrifuge outwards when said rotor assembly rotates and lodge between said relieved wall and the second one of said pair of edge faces, whereby said wires substantially fill coextensive portions of said gaps, said wires having a substantially oval cross-section including at least one flat, thereby to substantially prevent rotation of the wires when lodged as aforesaid.

2. A bladed rotor assembly according to claim 1 in which the cross-section of the sealing wires is race-track shaped, comprising two curves and two flats.

3. A sealing wire, suitable for sealing gaps between the platform edges of adjacent turbomachine rotor blades, the wire being substantial straight in the longitudinal sense and having a substantially oval cross-section including at least one flat.

4. A sealing wire according to claim 3, the crosssection of the sealing wire being race-track shaped, comprising two curves and two flats.

5. A bladed rotor assembly substantially as herein described with reference to, and as illustrated by, the accompanying drawings.