This invention relates to a leaf seal. In particular, this invention relates to a leaf seal for use in a gas turbine engine.
With reference to
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
The engine may have one or more seals installed, for example, between an interconnecting shaft and a casing for the shaft. Such seals may be so-called leaf seals.
Generally, leaf seals are used to form a seal between two relatively rotating components in order to provide a pressure barrier which defines high and low pressure areas. The pressure barrier is provided with a large number of typically rectangular leaves which are held at a defined angle to the radial all the way round the seal circumference. The leaves are flexible and allow radial compliance which can accommodate changes in the radial position of the two relatively rotating components. The leaves are packed at a sufficient density to provide an effective pressure barrier but the use of leaves inevitably leads to interleaf gaps and porosity across the seal.
Each leaf 32 is in the form of a plate, each having a root end 40, a free end 41, axial width, w, and a thickness, t. The leaves alternate with spacer elements 33 at the root end 40 and are secured to a backing ring 34 of a housing, which typically also comprises front 35a (high pressure side) and rear (low pressure side) 35b rigid annular cover plates. The free ends 41 of the leaves 32 present end edges 36 towards a surface 37 of a rotating component (shaft) generally rotating in the direction depicted by arrowhead 38. The leaves 32, and in particular the free end edges 36 of the leaves 32, act against the surface 37 in order to create a seal across the assembly 31. Each leaf 32 is sufficiently compliant in order to radially adjust with rotation of the surface 37, so that a good sealing effect is created. The spacers 33 ensure that flexibility is available to appropriately present the leaves 32 towards the surface 37 which, as illustrated, is generally with an inclined angle between them. The spacers 33 also help to form interleaf gaps 39 between adjacent working portions 41 of the leaves 32. An axial leakage flow through these gaps 39 is induced by the pressure differential across the seal.
The leakage flow can contribute to leaf blow-down where the free end edges 36 are urged radially inwards so as to bear on the rotor surface 37. Although a limited amount of blow-down is desirable to create a good seal between the free end edges 36 and the surface 37, excessive blow-down causes excessive rotor loading and wear in the seal and rotor. The wear of the end edges and/or the rotor can limit the usable life of the seal.
Various configurations of leaf seal have been proposed to help control the amount of blow-down. One example of this is described in WO06016098 which implements a leaf with an edge chamfer and an associated feature on the opposing cover plate, the separation of the two defines a control gap which is dependent on the radial deflection of a leaf.
Another potential issue with leaf seals is that the creation of non-uniform circumferential pressure distribution which are a result of using solid continuous leaves. To help alleviate this, it can be advantageous to introduce pressure equalising features to help reduce the differential in forces across the solid leaf elements.
One method of providing pressure equalisation is to include one or more apertures in the leaf working section to provide a flow between two sides of the leaf. Although this can be effective for equalising the pressure in the locality of the aperture(s) and reducing the loading on the leaf, the apertures lead to increases in leaf stress. This largely negates any benefit achieved with the pressure equalisation.
If circumferential pressure equalization is to be achieved successfully then the apertures need to avoid the use of a stress raising feature and be small in comparison to the axial length of the leaf element. However, a standard rectangular section leaf is highly loaded at the root end by the pressure loads in use, so the inclusion of holes in this shape cross-section is generally problematic.
It is an object of the present invention to provide an improved leaf seal.
The present invention provides a leaf seal for sealing between a static component and a relatively rotating component having an axis of rotation, the leaf seal defining axially separated higher pressure upstream and lower pressure downstream areas when in use, the leaf seal comprising: a plurality of leaf elements having: a root end for attachment to the static component; a free end for wiping contact with the rotating component; an upstream edge extending from the root end to the free end; and, a downstream edge extending from the root end to the free end, wherein the leaf element width between the upstream edge and downstream edge at the root end is a, and width between the upstream edge and the downstream edge at the free end is b, where b<0.5α.
The taper of the leaf elements advantageously removes material from the free end of the leaf elements which is not necessary for carrying the axial pressure drop. Further, it allows the root end of the leaf elements to be wider for a given weight meaning that the leaf can include pressure equalisation features or be made more resilient against radial deflection.
Preferably, b is in the range 0.1α<b<0.5α.
The taper angle of either or both of the upstream edge and downstream edge of at least one leaf element relative to the normal of the rotational axis may be between 2 and 45 degrees. Preferably, the taper angle of the upstream edge relative to the normal of the rotational axis is between 5 and 20 degrees. Preferably, the taper angle of the downstream edge relative to the normal of the rotational axis is between 5 and 20 degrees.
The leaf seal may further comprise an upstream and a downstream cover plate. Each cover plate may have an inner wall which faces the leaf element in use. The angle of either or both of the upstream inner wall and downstream inner wall relative to the normal of the rotational axis may be between 2 and 45 degrees. Preferably, the angle of the upstream inner wall relative to the normal of the rotational axis is between 5 and 20 degrees. Preferably, the downstream inner wall relative to the normal of the rotational axis is between 5 and 20 degrees.
The difference in the angle of one or other of the upstream and downstream inner walls and the corresponding edge of the leaf element may be less than 10 degrees relative to the normal of the rotational axis. Preferably, the difference in the angle of one or other of the upstream and downstream inner walls and the corresponding edge of the leaf seal is less than 5 degrees relative to the normal of the rotational axis. The angle of inner wall of either or both of the upstream or downstream cover plate and the corresponding edge of the leaf seal may be substantially the same when the leaf seal is in a non-working cold state, or at a nominal or specific operating point as per a given application.
Either or both of the leaf element upstream and downstream edges may include either or both of a convex portion and a concave portion.
Either or both of the upstream or downstream cover plate inner walls may include either or both of a convex portion and a concave portion.
At least one of the upstream or downstream leaf element edges may be separated from a corresponding portion of the adjacent inner wall by a gap. The gap may be substantially constant along the radial length of the inner wall.
The gap may be axially larger at the root end compared to the separation at the free end. Alternatively, the gap may be axially smaller at the root end compared to the separation at the free end.
At least one leaf element may be asymmetric about a radially extending mid-line. The arrangement of the upstream and downstream cover plate inner walls may be asymmetric about the radially extending mid-line.
The cover plates may have axial thickness and either or both of the upstream and downstream cover plates may be tapered so as to have a thicker cross section at the root end.
At least one leaf element may include an aperture for pressure equalisation across the circumferential thickness of the leaf element. The aperture may be local to the root end. The aperture may pass from one circumferential face of the leaf element through the leaf element circumferential thickness to the other circumferential face. The aperture may extend from a backing plate onto which the leaf element is bonded. The aperture may be triangular so as to provide a substantially V or Y shaped leaf element in which the root end is bifurcated by the aperture.
The leaf element may include at least one labyrinth tooth. The labyrinth tooth may be in an aperture. The aperture may be the pressure equalisation aperture. The labyrinth tooth may be symmetrical in the radial cross section. The labyrinth tooth may be centrally located within the aperture. Alternatively, the labyrinth tooth may be axially offset within the aperture so as to provide an open passage on the upstream or downstream side of the labyrinth tooth. The labyrinth tooth may be a rib or flange which extends from the backing plate on to which the leaf element is mounted. There may be a plurality of labyrinth teeth within a common aperture. The plurality of labyrinth teeth may be axially separated and parted by a radial line. The radial line may be a radial mid line of the leaf element. The plurality of labyrinth teeth may be symmetrically arranged about the radial line. The plurality of labyrinth teeth may be axially offset in relation to the aperture or be asymmetrically arranged.
The leaf seal may further comprise a cavity in fluid connection with a pressure source which is external to the seal. The external pressure source may be taken from an upstream area relative to the seal. The external pressure source may be metered to provide an intermediate pressure in the cavity. By intermediate pressure it is meant that it will be in the range bounded by the areas immediately upstream and downstream of the seal. The cavity may be proximate to the aperture or labyrinth tooth. Where there are a plurality of labyrinth teeth, the cavity may be partially defined by one or more of the plurality of labyrinth teeth.
Any edge of the leaf seal any rounded in a cross section defined by a circumferential plane. The leaf seal may have bull nosed, race track or elliptical shape in the cross section. The leaf element may have constant axial thickness. The radial thickness may vary. The root end may be thicker in the plane defined by the normal of the radial axis.
In another aspect, the present invention provides a leaf seal for sealing between a static component and a relatively rotating component having an axis of rotation, the leaf seal defining axially separated higher pressure upstream and lower pressure downstream areas within the static component when in use, the leaf seal comprising: a plurality of leaf elements having: a root end for attachment to the static component; a free end for wiping contact with the rotating component; an upstream edge extending from the root end to the free end; and, a downstream edge extending from the root end to the free end, wherein the leaf element is tapered towards the free end, the taper angle of the upstream edge and downstream edge to the normal of the rotational axis are between 2 and 45 degrees in a non-working state.
The minimum taper angle may be 5 degrees. The taper angles of the upstream and downstream edges may be between 5 and 45 degrees. The taper angles may be between 2 and 20 degrees.
The seal may be used in a gas turbine engine.
Embodiments of the invention will now be described with the aid of the following drawings of which:
a shows an end view of a segment of a leaf seal viewed along the axis of rotation of the rotating shaft which is sealed around.
b shows a face view of a single leaf.
In use, the plurality of leaf elements 412 are arranged at a defined angle to the radial around the seal circumference and provide a barrier to airflow from an upstream side 429 of the seal to a downstream side 430. The free end 418 of the complaint leaf element 412 is provided in wiping contact with the rotatable shaft 432 so as to provide a sealing surface 434 therebetween. The spacing between individual leaf elements 412 is greater towards the root end 416 which leads to some porosity and leakage across the seal 410.
The radial length of the leaf elements 412 tapers from the root end 416 to the free end 418 so as to provide a trapezoidal shape, with the root end 416 having a greater axial width. The taper is symmetrical about a radially extending midline 436 and extends from the free end 418 to the junction with the backing plate 424 at the root end 416 such that the full lengths of the upstream 420 and downstream 422 edges are angled with respect to the radial midline 436. The upstream 420 and downstream 422 edges are straight so as to provide a uniform taper. The circumferential thickness of each leaf element 412 is uniform.
The cover plates 426, 428 of the embodiment shown in
As will be seen from the embodiments described below, the relationship between the angle and spacing of the cover plate inner walls to the corresponding upstream and downstream edges of the leaf seals can be altered to provide a required performance from the seal in question.
The taper of the leaf elements of the invention and configuration of the cover plates can be characterised using the dimensions shown in
The taper of the leaf elements 612 advantageously removes material from the free end 618 of the leaf elements 612 which is not necessary for carrying the axial pressure drop. Further, it allows the root end 616 of the leaf elements 612 to be wider. The axial pressure load carrying capacity on the leaf seal is determined by the bending moment M(z) along the radial length of the leaf element. However, this reduces from the root to the tip following the quadratic relationship M(z)=½(ρd−ρu)t(L−z)2 where ρu is the upstream pressure, ρd is the downstream pressure, t is the leaf thickness, L is the leaf length and z is the length of the leaf element from the root to the considered point. Hence, the bending moment decreases M(z) along the length of the leaf element from the root end. This means that, in a standard rectangular leaf element, the width at the free end is more than is necessary for carrying the bending moment due to axial pressure.
The provision of a tapered leaf element 612 accounts for the reduction in bending moment. However, the angle of the taper may be determined using other factors. For example, the taper angle φ may be optionally increased so as to have a root end 616 which is wider than in an equivalent non-tapered leaf element. This may provide additional strength to account for a narrower free end 618 and or additional girth for accommodating pressure equalising features as described below in connection with
It will be appreciated that the ratio of b to α and angle of the taper of the leaf element 612 and inner walls 638, 639 of the cover plates will be application specific. In one advantageous embodiment, the ratio of b to a can be defined as b<α, or, more specifically, b<0.5α. However, b is preferably between 0.5α and 0.1α. The taper angle φ of the leaf element may be in the range from 2 degrees to 45 degrees but is preferably between 5 degrees and 20 degrees. Where the cover plates have tapered inner walls 638, 639, the taper angle ψ may be comparable to the taper angle φ of the leaf edges and as such may be in the range from 2 degrees to 45 degrees but is preferably between 5 degrees and 20 degrees. However, it may be advantageous to provide a difference in the two angles φ, ψ. In this case, the angles φ, ψ may vary by as much as 10 degrees, but will preferably be within 5 degrees. The length Y over which the taper extends is also application specific but is generally greater than the radial extent of the cover plates 626, 628.
A typical length for the envisaged leaf elements of the invention are between approximately 20 mm and 40 mm. Here, α may be between approximately 4 mm and 15 mm, b may be between 1 mm and 4 mm. However, it will be appreciated that leaf elements having other dimensions may fall within the scope of the invention.
Generally, the inner walls of the cover plates in a leaf seal are provided in relation to the leaf elements to control airflow leakage through the seal and the resulting blow-down and lift-up forces on the leaf elements. Thus, in a conventional seal with rectangular leaf elements, the radial length of the cover plates and the separation from the leaf elements are determined to provide a desired level of blow-down or lift-up for a particular application.
There are two main contributors to blow-down: one is the pressure mismatch across the thickness of a leaf and the other is the flow momentum changes as the flow directions change on entering the leaf pack. These concepts are discussed in Franceschini, G., Jones, T. V., and Gillespie, D. R. H., Improved Understanding of Blow-Down in Filament Seals, Journal of Turbomachinery 132 (2010) 041004, TURBO-09-1028; and, Franceschini, G., Jones, T. V., and Gillespie, D. R. H., Improved Understanding of Blow-Down in Filament Seals, in ASME Turbo Expo 2008: Power for Land, Sea and Air, Berlin, Germany, 2008, ASME, GT2008-51197 which are incorporated by reference.
Having a tapered leaf in conjunction with tapering inner walls is particularly advantageous for controlling blow-down as it allows the separating gap between the inner walls of the cover plates and the respective edge of the leaf elements to be adjusted as the leaf element is radially deflected use. That is, as a leaf element is radially deflected it moves closer to the angled cover plate and the separating gap is reduced. The momentum change blow-down term will reduce as the fluid flow rate reduces and the pressure mismatch term reduces because more pressure is dropped in the tighter cover plate gaps and thus less pressure is dropped over the leaf. Thus the blow-down will reduce.
Straight and parallel leaf to cover plate gaps are necessary for rectangular leaf elements to allow them to deflect towards or away from the rotor when experiencing varying levels of blow-down and lift-up forces. However, having uniform, radially extending gaps means that the radial separation of the leaf elements and cover plates is always constant and it can be advantageous to adjust the gap to be able to vary the blow-down and lift-up behaviour of the leaf seal when deflections occur.
The narrowing of the gap G in the case of a tapered leaf element 610 and cover plate inner wall 638 is related to the local radial deflection of the leaf element 612 by the sine of the leaf taper angle φ as shown in
The taper of the leaf elements shown in the previously described Figures have uniform tapers and with straight edged leaves. However, it will be appreciated that the taper angles and general edge profile shape may be advantageously adapted to provide a required load capacity or tuned response to radial deflection.
The profiles shown in
As will be appreciated, as well as altering the profile of the leaf element to adjust the performance of the seal, it is also possible to modify the relationship between the leaf elements and the cover plates. This may include adjusting the relative axial spacing and the profile of the inner or outer wall of the cover plates.
Both cover plate inner walls 1338, 1339 are perpendicular in the embodiment shown in
It will be noted that the corners of the leaf element 1312 are softened or rounded to reduce any high stress points in the leaf. It will be appreciated that it may be advantageous to round the corners of the free end of the leaf elements in any of the described embodiments.
It will be appreciated that, in the case where each leaf has a pressure equalising aperture, the rib may be provided by an annular rib which extends from and optionally integrally formed with the backing plate.
The variants of the leaf element shape and corresponding cover plate arrangements described above may be used in conjunction with the pressure equalising apertures described in connection with
Variations to the described embodiments are envisaged within the scope of the invention. For example, the leaf elements may be circumferentially thicker at the root end than at the tip end so that there is a taper in thickness as well as a taper in width. The taper may also be such that the leaf tip is marginally thicker than further up the leaf element towards the leaf root. The thickness profile may be linear or engineered to a desired profile in order to achieve a suitable blow-down response. It will be appreciated that where the seal is fixed to the rotor or radially inner component, the free ends of the leaf elements are located at the larger diameter then the thickness should be larger at the free end than at the root end.
It will be appreciated, that the various features described above in the different embodiments may be implemented independently of other features where possible, and various features may be combined in some advantageous embodiments, even where not explicitly described.
Number | Date | Country | Kind |
---|---|---|---|
1209705.1 | May 2012 | GB | national |