The present invention relates to a leaf seal, and more particularly relates to a leaf seal for effecting a seal between first and second coaxial components arranged for relative rotation about a common axis.
Leaf seals may be used to form a seal between two relatively rotating components in order to maintain a relatively high pressure on one side of the seal and a relatively low pressure on the other. A leaf seal is arranged with a large number of typically rectangular leaves which are held at a defined angle (the “lay angle”) to the radial all the way round the seal circumference. The leaves give the seal a low stiffness, and the leaves are packed together such that the total leakage through the seal is reduced. Nonetheless, interleaf gaps do provide the seal with a porous aerodynamic working section. Such seals may be used, for example, in gas turbine engines.
The leaves 32 each have a root portion 40 and a working portion 41, and have a width w in the axial direction and a thickness t. The leaves alternate with spacer elements 33 at their root portions 40, and are secured thereat to a backing ring 34 of a housing, which typically also comprises front 35a (high pressure side) and rear (low pressure side) 35b rigid cover plates. The working portions 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 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 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. A leakage flow through these gaps 39 is induced by the pressure differential across the seal.
Previously proposed leaf seals of an air-riding configuration are designed such that the end edges 36 of the leaves adjacent the rotating component are presented with a small air gap therebetween such that the leaves ride on the air leakage through that gap to inhibit premature contact wear of the leaf seal elements against the surface of the rotating component. It will be understood that the air gap should be as narrow as possible such that air leakage is reduced to the minimal level possible whilst creating the air-riding effect so as to limit actual leakage across the seal. However, it has been found that it can be difficult to generate sufficient hydrodynamic lift between the leaf pack and the rotating component to provide a satisfactory air-riding cushion between the two, and so leaf seals in their conventional form provide limited potential for the generation of hydrodynamic lift. Conventional leaf seals are this prone to wear at the tips of the leaves and/or on the surface of the rotating component itself. This limits the useful life of the leaf seal.
Wear and frictional heat generation are known problems associated with conventional contacting leaf-type shaft seals in both gas and steam turbines, and can result in adverse consequences for part life and engine efficiency. The conventional leaf seal may also be exposed to such conditions in engine operation. Aero-elastic vibrations of the leaves under a pressure loading may also cause leaf damage. Further, excessive deflection and distortion of the leaf pack in operation can also lead to damage, or can give rise to high torques (due to “blow-down” of the leaves against the rotating shaft surface) and heavy wear
It is a preferred object of the present invention to provide an improved leaf seal.
According to the present invention, there is provided a leaf seal for effecting a seal between first and second coaxial components arranged for relative rotation about a common axis, the seal having: an annular pack of stacked leaves mountable to said first component at root portions of the leaves distal to the second component and extending towards the second component such that end edges of the leaves are proximal to the second component; and a plurality of seal elements which cooperate to define an annulus extending around a surface of the second component; wherein each seal element forms a sector of the annulus; is mounted to the leaf pack at the end edges of a respective group of said leaves; and defines a respective seal surface which is presented for interaction with said surface of the second component during relative rotation between the components, said seal surfaces cooperating such that, in use, a pressure drop is maintained axially across the leaf seal.
Preferably, said first component is provided around said second component and the annulus defined by the seal elements is provided between the two components so as to extend around the outside of said second component.
In preferred arrangements, said seal elements are of equal circumferential length.
Each said seal element is optionally removably connected to the leaf pack. Alternatively the seal elements may be permanently connected to the leaf pack.
The end edges of each said group of leaves may be embedded in a respective said seal element.
In preferred arrangements, each seal element has a maximum axial dimension which is greater than or equal to the axial width of the leaves.
Said seal surfaces of the seal elements are optionally radially stepped so as to have a first region and a second region separated by a step therebetween, the first region of the seal surface being radially spaced further from said surface of the second component than the second region of the seal surface.
Said pack has a high pressure side and a low pressure side across which said pressure drop is maintained in use, and said first regions of the respective seal surfaces are preferably proximal to the high pressure side, with said second regions of the respective seal surfaces being proximal to the low pressure side.
Each seal element can have hydrodynamic lift-generating features formed in its seal surface. In such arrangements, it is preferable for said hydrodynamic lift-generating features to be formed only in the second region of the seal surfaces.
Each seal element (52) may include at least one internal flow passage (57, 62) extending from an inlet port (61) in a surface of the seal element remote from the seal surface (53) to an outlet port (58) formed in a said hydrodynamic lift-generating feature (56).
Said seal elements may include respective removable plates defining at least the majority of the seal surface of each seal element.
In preferred arrangements, said seal elements cooperate to define said annulus such that gaps are defined between circumferentially adjacent seal elements.
Each said gap may be defined by opposing parallel end surfaces of a pair of circumferentially adjacent seal elements. Said opposing parallel end surfaces of the adjacent seal elements can extend radially relative to the common rotational axis of the two components. Alternatively, said opposing parallel end surfaces of the adjacent seal elements can make an acute angle to the radial direction relative to the common rotational axis of the two components.
Said gaps may be configured to present a tortuous leakage path to an axial flow. For example, the gaps may be axially stepped in some embodiments.
The seal surface of each seal element may have a profile comprising a planar region proximal and parallel to said surface of the second component, and at least one edge region extending from the planar region towards an edge of the seal element and away from said surface of the second component.
So that the invention may be more readily understood, and so that further features thereof may be appreciated, embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:
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 has one or more leaf seals installed, for example, between an interconnecting shaft and a casing for the shaft.
The leaf seal 131 is thus shown to have an annular pack 50 of approximately rectangular leaves 132 terminating at radially inward end edges 136 which are proximal to the rotor 51. Although the end edges 136 of the leaves are proximal to the rotor 51 The leaves are held at an inclined angle to the radial. Interleaf gaps are formed between the leaves 132, giving a porous aerodynamic working section and sufficient compliance to adjust to the rotor. Nonetheless, the leaves are packed sufficiently tightly together so that the total leakage through the seal is low.
As illustrated most clearly in
The leaf seal 131 further comprises a plurality of arcuate seal elements 52 which, as illustrated most clearly in
Each seal element 52 defines a smooth and radially inwardly facing arcuate seal surface 53 which is presented for sealing interaction with the surface 137 of rotor 51 in a manner which will be described in more detail below. It is envisaged that when the rotor 51 is stationary and thus not rotating relative to the pack 50, or when the rotor is moving only at low speed relative to the pack 50, the seal surfaces 137 of the seal elements 52 leaves may lightly touch the surface 137 of the rotor 51 for wiping contact therewith during low-speed rotation.
The seal elements 52 are mounted to the leaf pack 50 at the end edges 136 of respective groups of leaves 132, and are thus accommodated within the radial space defined between the end edges 132 and the surface 137 of the rotor 51. In the particular arrangement illustrated the seal elements 52 are fixedly connected to the end edges 136 of the leaves 132 by embedding the end edges 136 of the leaves 132 in the seal elements 52. The seal elements 52 are thus permanently connected to the leaf pack 50. However, it is to be appreciated that the seal elements 52 can be mounted to the end edges 136 of the leaves in other ways, and can also be removably connected to the end edges 136 in order to facilitate easy removal of the seal elements 52 from the leaf pack 50 for possible replacement during servicing.
The width w of each leaf 132 in the pack 50 is generally similar to the width of the leaves in conventional leaf seals, and is thus most likely to be in the range of 3 to 8 mm. Gaps g1 and g2, provided between the upstream and downstream side edges 60a, 60b of the leaves and the adjacent cover plates 135a, 135b, are used to control the lift-up/blow-down behaviour of the leaf pack under pressurization, as well as a means of leakage control. In the particular arrangement illustrated, as shown in
During relative rotation between the rotor 51 and the housing 134, and hence relative rotation between the rotor 51 and the seal elements 52, seal surfaces 137 of the seal elements run in close proximity to the surface 137 of the rotor 51, or even in wiping contact therewith during low-speed rotation. The seal elements 52 thus cooperate to provide a seal at the surface 137 of the rotor such that a pressure drop is maintained axially across the leaf seal 131.
Because the seal elements 52 are mounted to the leaf pack 50 at the end edges 136 of respective groups of leaves 132, a degree of rigidity is introduced to the pack 50 at the end edges 136 of the leaves 132, which provides significant reduction in leaf vibration and leaf distortion, thereby making the seal 131 more robust to a range of different operating conditions.
In order to improve the air-riding function of the seal elements 52 relative to the surface 137 of the rotor 51, the seal elements 52 can be provided with hydrodynamic lift-generating features in their seal surfaces 53, such as the lift-generating grooves 56 illustrated in
The grooves 56 are configured to generate lift from the leakage flow of air between the seal surfaces 53 and the surface 137 of the rotor 51, thereby providing an improved air-riding effect between the seal elements 52 and the rotor 51. The exact configuration of the grooves 56 and their relationship to one another can be configured to the particular operating parameters of any given installation For example, the pitch (denoted by m, n in
It is to be appreciated, however, that the lift-generating grooves 56 can be susceptible to blockage from the gradual deposition of particulates during extended use, which can reduce the lift-generating effect of the grooves 56. This can be reduced by providing a number of different configurations of flow passages in order to feed a flow of cleaning air, drawn from the working fluid within the engine, to the grooves 56. Such a cleaning flow of air can also serve to help cool the seal elements 52 in the event that they make contact with the rotor 51 during use which would generate heat through friction, and also to enhance the pressure increase between the sealing elements and the rotor thereby improving hydrostatic/hydrodynamic lift and thus the air-riding performance of the leaf seal.
The size and geometry of the outlet ports 58 in both of the arrangements illustrated in
It is also to be noted that in the case that the grooves 56 are fed with a flow of cleaning/cooling air drawn from the radially outermost side of the seal elements, as illustrated in
In the case that the grooves 66 are fed with a flow of cleaning/cooling air drawn from the high pressure side face of the seal element, in the manner illustrated in
The invention has been described above with reference to embodiments in which the seal surfaces 53 of the seal elements are continuous and uninterrupted except for the possible provision of the lift-generating grooves 56. However, other arrangements are also possible, such as the example illustrated in
In the arrangement of
The stepped configuration illustrated in
The arrangement of
Turning now to consider
The leaf seal of the present invention can also be embodied in an arrangement such as that shown in
The inlet space 67 serves to admit a flow of air between the seal surface 53 and the surface 137 of the rotor to assist in the generation of hydrostatic lift between the seal element 52 and the rotor 51.
In the particular arrangement illustrated in
Whilst the inlet space 6 and the downstream space 70 are shown in
It will be noted that despite the chamfers provided at the upstream and downstream edges of the seal element 52 illustrated in
As will be understood, the invention effectively introduces additional features into the conventional leaf seal design which enable closer running clearances, reduced leaf vibrations and distortions, and reduced rotor-contact during engine operation. This is to achieve a low leakage, long life seal. The invention provides a simple and cost-effective means of enhancing the current technology to ensure its durability in service, particularly in the more arduous engine locations where engine overhaul and repair is significantly more costly. The option for seal element (or plate) replacement in the case of some deterioration strengthens the business case for this technology. The presence of seal elements 52 in the tip-region of the leaf pack 50 improves the dynamic properties of the seal by reducing leaf flutter and leaf distortion. The use of segmented seal elements 52 maintains radial compliance with the rotor 51 around its circumference. Air-riding features on the rotor-side of the seal elements 52 ensure minimal rotor-contact at all operating conditions in the engine cycle. This invention can exploit both hydrostatic and hydrodynamic lift mechanisms to achieve this.
When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the e presence of other features, steps or integers.
The features disclosed in the foregoing description, or in the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.
While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the scope of the invention.
Number | Date | Country | Kind |
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1312766.7 | Jul 2013 | GB | national |