Abstract:
This application describes a leaf seal arrangement, including: a first component having a high pressure area and a low pressure area therein; a second component which passes from the high pressure area to the low pressure area; a leaf seal having an array of leaf elements between the high and low pressure areas, each leaf element having a fixed end and a free end, wherein the fixed end is attached to the first component and the free end defines a sealing surface through which the second component sealably passes, a sealing gap therebetween, and wherein the sealing gap generally converges from the high pressure area toward the lower pressure area. Also described is a method of providing a leaf seal.

Description:
TECHNICAL FIELD OF INVENTION 
     This invention relates to a leaf seal. In particular, a leaf seal which is configured to generate hydrostatic lift under normal operating conditions. The leaf seal may find particular use in a gas turbine engine where sealing is required to accommodate transitory deflections of relatively rotating components which are experienced in normal use. 
     BACKGROUND OF INVENTION 
     With reference to  FIG. 1 , a ducted fan gas turbine engine which may incorporate the invention is generally indicated at  10  and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , and intermediate pressure turbine  17 , a low-pressure turbine  18  and a core engine exhaust nozzle  19 . A nacelle  21  generally surrounds the engine  10  and defines the intake  11 , a bypass duct  22  and a bypass exhaust nozzle  23 . 
     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 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. In the case of a gas turbine engine this helps restrict the leakage of air or fluid from particular areas of the engine. The pressure barrier is provided with a large number of typically rectangular leaves which are held at a defined angle to the radial around 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. 
       FIG. 2  shows a schematic perspective cut-away view of a portion of a typical leaf seal  210  comprising a pack of leaves  212 .  FIG. 3   a  shows an end view of a segment of a leaf seal  210  viewed along the axis of rotation of a rotating shaft  222 .  FIG. 3   b  shows a face view of a single leaf  212 . 
     Each leaf  212  is in the form of a plate, each having a root end  214 , a free end  216 , axial width, w, and a thickness, t. The leaves  212  alternate with spacer elements  226  at the root end  214  and are secured to a backing ring  219  of a housing  218 , which typically also comprises front  220  (high pressure side) and rear  221  (low pressure side) rigid annular cover plates. The free ends  216  of the leaves  212  present end edges  216   a  towards a surface of a rotating component  222  (shaft) generally rotating in the direction depicted by arrowhead  224 . The leaves  212 , and in particular the free end edges  216  of the leaves  212 , act against the surface in order to create a seal across the assembly. Each leaf  212  is sufficiently compliant in order to radially adjust with rotation of the surface  222 , so that a good sealing effect is created and maintained during use. The spacers  226  ensure that flexibility is available to appropriately present the leaves  212  towards the surface which, as illustrated, is generally with an inclined angle between them. The spacers  226  also help to form interleaf gaps  228  between adjacent working portions of the leaves  212 . An axial leakage flow through these gaps  228  is induced by the pressure differential across the seal  210 . 
     Generally, leaf seals can be designed such that the expected axial leakage flow determines the extent of the contact between the leaf elements and the rotating component. Thus, the leakage flow can contribute to leaf blow-down where the free end edges are urged radially inwards so as to bear on the rotor surface. Or blow-up forces which act to lift the leaf elements, thereby reducing the contact pressure but increasing the leakage flow. A limited amount of blow-down can be the more desirable to create a good seal between the free end edges and the surface, but 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. 
     A further complication in the design of leaf seals is brought about by the variance in the operating conditions. This variance is determined by many factors including but not limited by thermal expansion (differential and single bodied), operating pressures, mechanical tolerances and vibration. One particular issue of concern to this invention are variables which lead to a misalignment between the two components which are bridged by the seal. 
     The present invention seeks to provide an improved leaf seal which addresses some of the issues presented by having a misalignment. 
     STATEMENTS OF INVENTION 
     In a first aspect, the invention provides a leaf seal arrangement, comprising: a first component having a high pressure area and a low pressure area therein; a second component which passes from the high pressure area to the low pressure area; a leaf seal having an array of leaf elements between the high and low pressure areas, each leaf element having a fixed end and a free end, wherein the fixed end is attached to the first component and the free end defines a sealing surface through which the second component sealably passes, a sealing gap therebetween, and wherein the sealing gap generally converges from the high pressure area toward the lower pressure area. The convergent portion extends across between 60% and 100% the width of the sealing surface. 
     Providing a convergent sealing gap which is larger towards the upstream or high pressure side of the seal ensures there is a relatively consistent hydrostatic uplift on the leaf elements can be achieved which can provide more reliable sealing and increase the longevity of the leaf seal. 
     The angle of the convergence may be between 1 degree and 20 degrees. The angle of convergence may be between 1 and 10 degrees. Preferably, the angle of convergence will be between 1 and 5 degrees. By angle of convergence it is meant the angle between two lines which extend from the narrowest portion of the sealing gap and the widest portion of the sealing gap. The convergence is provided between the leaf element tip and the second component. The convergence may be provided by a shaped leaf element or a shaped second component. For example, the leaf element edge which provides the sealing gap may have a tapered or curved profile. Alternatively or additionally, the second component may include one or more features which provide a curved or angled surface relative to the leaf element edge at the sealing gap. 
     The second component may be rotatably mounted relative to the first component and define a neutral rotational axis. The sealing gap may be tapered. The angle of taper may be between 1 degree and 20 degrees. The angle of taper may be between 1 and 10 degrees. Preferably, the angle of taper is between 1 and 5 degrees. The angle of taper may be relative to the neutral rotational axis. 
     The leaf elements may have an axial width between the high pressure area and the low pressure area. Alternatively, the convergent portion may extend across between 75% and 100%. 
     The sealing gap may converge at a constant rate. 
     The sealing gap may include more than one convergent portion. Each portion may have different rates of convergence. 
     The first portion may extend from a downstream portion of the leaf element and may be inclined to the neutral rotational axis by an angle α 1  which is in the range bounded by 1 degree and 30 degrees. The second portion may extend from the first portion and may be inclined to the neutral rotational axis by an angle α 2  which is greater than or equal to α 1  and in the range bounded by 1 degree and 30 degrees. Alternatively, the angles α 1  and α 2  may be between 1 degree and 20 degrees, between 1 and 10 degrees, or, preferably, between 1 and 5 degrees. 
     The profile of the leaf of the first or second portion may not be straight and the angles α 1  and α 2  may be defined by lines which extend from the respective upstream and downstream gaps of the upstream and downstream portions. 
     The leaf seal may further comprise a non-convergent portion. The non-convergent portion may be a downstream portion. The non-convergent portion may be provided by a portion in which the leaf element edge and second component are parallel to each other. 
     The non-convergent portion may extend across less than 20% of the axial width of the leaf element. 
     The leaf seal may comprise circumferential portions of convergent sealing gap and circumferential portions of non-convergent sealing gap. 
     The convergent sealing gap may be at least partially defined by tapered profile on the second component. 
     The leaf seal may be suitable for use in a gas turbine engine. 
     In a second related aspect, the invention provides a gas turbine engine including the leaf seal of the first aspect. 
     In a third aspect, there is a method of providing a leaf seal for defining a high pressure area and a low pressure area in a first component in which a second component passes from the high pressure area to the low pressure area; the leaf seal having an array of leaf elements, each leaf element having a fixed end and a free end, wherein the fixed end is attached to the first component and the free end defines a sealing surface through which the second component sealably passes, a sealing gap therebetween, and wherein the sealing gap generally converges from the high pressure area toward the lower pressure area, the method comprising: determining the expected amount of deflection which will be experienced between the first and second components at the sealing gap during normal operation; determining a required amount of convergence in the sealing gap to accommodate the expected amount of deflection; configuring either or both of the leaf seal and the second component to provide the required convergence. 
     The required convergence may be greater than the expected deflection. 
     The sealing gap may be configured to provide positive hydrostatic lift under all normal operating conditions. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       Embodiments of the invention will now be described with the aid of the following drawings of which: 
         FIG. 1  shows a conventional gas turbine engine. 
         FIG. 2  shows a conventional leaf seal. 
         FIGS. 3   a  and  3   b  show respective axial and circumferential sectional views of a typical leaf seal. 
         FIGS. 4   a - c  show a conventional leaf seal with a rotary component in various states of axial alignment. 
         FIG. 5  shows a leaf seal according to the present invention. 
         FIGS. 6   a  and  6   b  show a leaf seal of the invention in different operating conditions. 
         FIG. 7  shows a further embodiment of a leaf seal according to the present invention. 
         FIG. 8  shows a further embodiment of a leaf seal according to the present invention. 
         FIG. 9  shows a further embodiment of a leaf seal according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     In a conventional leaf seal  210 , the inner bore of the leaf pack is defined by the free ends  216  of the leaf elements  212  which act to provide a sealing surface against the rotating component  222 . Normally, the rotating component  222  will be a shaft which is generally cylindrical. When assembled, the cylindrical inner bore of the leaf pack is concentrically aligned with the rotational axis  222   a  of the shaft  222 . Hence, there is provided a circumferentially uniform and parallel sealing gap  230  between the surface of the rotating component  222  and the free ends  216  of the leaf elements  212 . This is shown schematically in  FIG. 4   a.    
     However, in use, there are a number of operating variables (as listed in the background section above) which may result in the shaft  222  being axially offset with regard to the axis  222   a  of the inner bore of the leaf seal  210 . Thus, in practice, the aerodynamic properties, performance and wear of the seal  210  can be operating outside the desired design envelope for portions of the operating cycle of the seal  210 . 
       FIGS. 4   b  and  4   c  show two scenarios which may occur. The first of these is shown in  FIG. 4   b  and involves a misalignment in which the upstream sealing gap  232  is increased and the leaf tip  216   a  pressure drop is presented predominantly at the downstream edges  234  of the leaf elements  212 . This misalignment results in upstream hydrostatic pressure on the leaf tips  216   a  which provide uplift and a reduction in wear. Though potentially not ideal, this is generally a satisfactory outcome in the case of an unavoidable misalignment. 
       FIG. 4   c  shows the second scenario in which the misalignment is such that the sealing gap is increased at the downstream edge of the leaf elements  212 . This results in the pressure drop being across the upstream side  236  of the leaf tips  216   a  and a reduction in the hydrostatic pressure experienced by the leaf elements  212 . The reduction in hydrostatic pressure acts to suck the leaf elements  212  in toward the shaft which is generally detrimental to the operation of the seal  210  and the wear inflicted on the shaft  222  surface and leaf tips  216   a.    
       FIG. 5  shows cross-sectional schematic view of a leaf seal  510  according to the present invention. The leaf seal  510  is described in more detail below but is largely similar to that described in relation to  FIGS. 2 ,  3   a  and  3   b . However, a significant difference of the seal  510  is that the free end edges  516   a  have been adapted to provide a leaf seal  510  which can accommodate some axial misalignment with a rotor  522  without the potentially deleterious effects caused by the reduction in hydrostatic pressure of known leaf seals  210 . 
     Thus, there is shown a leaf seal  510  which includes a housing  518  to which is attached a plurality of leaf elements  512  in the form of thin compliant plates, of which only one is shown. Each leaf element  512  extends radially inwards from a fixed end  514  at the housing  518  towards a free end  516  which is proximate to the rotating shaft  522 . The leaf elements  512  are presented in an annular array such that each leaf element  512  is inclined to the radial as previously described in connection with  FIGS. 2 and 3   a . The housing  518  is shown as comprising a backing plate  519  which can be attached to a stationary structure as required, an upstream high pressure cover plate  520 , and a downstream low pressure cover plate  521 . The primary purpose of the housing is to provide a fixture to which the leaves can be attached and other housing arrangements as known in the art may be used. 
     The free end edges  516   a  of the leaf elements  512  collectively define an inner bore which provides a sealing surface. The free end edges  516   a  of the leaf elements  512  in the described embodiment are generally straight and lie at an angle to the principal or neutral rotational axis  522   a  of the shaft  522  so as to provide tapered configuration. By neutral rotational axis  522   a , it is meant the longitudinal central axis of the shaft  522  when not rotating or rotating under relatively benign conditions which do not lead to any significant transitory deflections. 
     The taper of the free end  516  is such that the sealing gap  530  is greater at the upstream side  536  of the leaf  516  and converges towards the downstream end  534 . Thus, the sealing surface provided by the leaf elements  512  is conical rather than cylindrical as in known existing seals. As will be seen from the embodiments described below, the convergent sealing gap  530  can be provided by different configurations of leaf element  512  and shaft surface, some of which include continually curving free ends and some of which have discrete angled portions which are inclined by different amounts to the neutral rotational axis  522   a.    
     The extent of the angle in the straight tapered free end is sufficient that the separating gap  530  will always be convergent under normal transient operating conditions. Thus, the seal  510  is designed by determining or estimating the ordinary transitory deflection of the rotary component  522  to provide an angle of expected deflection. And the seal  510  is provided with a taper angle which is greater than the expected deflection angle by a given margin. The margin can be used to allow for manufacturing or design tolerances, or to help provide a predominant operating condition such as a given amount of hydrostatic pressure during normal and transitory deflective behaviour. 
     Thus, the leaf tip taper ensures that any angular mismatch between the leaf inner bore and the rotor surface does not cause a change in the position of the closest point between the free end  516  of the leaf elements  512  and the rotor  522 . This closest point is preferably at the downstream edge  534  of the leaf elements  512  such that the pressure drop is local to the downstream edge  534  in the tip region and that contact with the rotor  522 , if and when it occurs, will be local to the downstream edge  534 . This means that the upstream portion  536  of the leaf tips  516   a  are generally open to upstream fluid pressure which provides a hydrostatic lifting force to much of the free ends  516  of the leaf elements  512 . This arrangement provides a consistent flow field under the leaf elements  512  and a reliable hydrostatic lift force at different operating conditions, and a leaf seal  510  which is less susceptible to aeroelastic forcing at the leaf tips  516   a  because the pressure drop at the leaf tips  516   a  is concentrated at the downstream edge  534  of the leaf tips  516 . 
     Typical angular deflections in a gas turbine engine may be in the order of fractions of a degree. Thus, the taper angle α may be a minimum of 1 degree but may extend up to as much as 45 degrees. Preferably, the taper angle will be between 1 degree and 20 degrees to the neutral rotational axis. The angled portion of the leaf free end can extend from between 50% of the leaf axial width to 100%. Preferably the taper will extend over 80% of the axial width. More preferably, the taper will extend over 90% of the axial width. It will be appreciated that in some instances it may be preferable to provide a shorter taper such that the hydrostatic lifting force can be purposively reduced during steady state normal or transitory operating conditions. 
       FIGS. 6   a  and  6   b  show two hypothetical deflection scenarios for a leaf seal  510  arrangement of the invention. In  FIG. 6   a , the angle of deflection β of the shaft  522  is positive in relation to the neutral axis  522  so as to reduce the gap at the upstream edge  536  of the leaf element  512 . (The positive nomenclature is with reference to the neutral rotational and nominally chosen for the purpose of the description only).  FIG. 6   b  shows a negative angle of deflection, −β, which increases the sealing gap  530  at the upstream edge  536 . Thus, it can be seen that at the maximum positive and negative expected deflections there is a clear separation at the upstream edge  536  of the leaf element  512 , and the narrowest portion of the sealing gap  530 , and thus location of the majority of the pressure drop, is towards the downstream edge  534  of the leaf element  512 . 
     It will be appreciated that only some of the leaf elements  512  in the annular array may have the taper angle α required to provide the desirous hydrostatic lift with other leaf elements having less or no taper angle (or convergence), relying on the lift provided by the tapered leaf elements  512  to reduce the contact pressure during a transient deflection event. 
     It will also be appreciated that the convergent gap  530  between the leaf free end edge  516   a  and the shaft  522  may be provided by having a profile on the shaft  522 . Hence, the shaft  522  may be provided with a conical (and thus tapered in the section) section which is axially aligned with the leaf seal so as to radially oppose the free ends of the leaf elements  512 . 
     Having multiple angled portions at the free end may be preferential in the case where a taper is included for another reason. For example, in WO06016098 there is described a leaf seal in which an upstream front corner of the leaf is removed so as to provide a deflection adjusting gap between the upstream cover plate and the upstream edge of the leaf to control blow-down forces under different operating conditions. UK patent application no. 1309579 describes other tapered leaf seal elements which provide other benefits. However, each of these, and many other leaf element configurations, could benefit from having a convergent sealing gap according to the present invention. The tapered portion spread over the leaf tip as described by this invention dominates the hydrostatic lift created under the leaf pack. Further, the hydrostatic lift can be provided without introducing significant stress raising features in the leaf by virtue of the tapered arrangement. 
     Alternative configurations of leaf elements can provide the required convergent sealing gap.  FIGS. 7 to 9  give alternative examples. 
       FIG. 7  shows a leaf element  710  in which the free end  716  includes two discrete angled portions. Thus, there is a first downstream portion  716   a  having a taper angle α 1 , and a second portion  716   b  which extends from the first portion  716   a  to the upstream gap  736 . The second portion  716   b  has a taper angle α 2  which is greater than the downstream portion  716   a . The values of α 1  and α 2  will be application specific but it is envisaged that α 1  will typically be between 1 and 30 degrees with α 2  between 5 and 45 degrees. Preferably, α 2  between 1 and 30 degrees. Regardless, α 2  will be greater than α 1  to provide the necessary convergence. 
       FIG. 8  shows a leaf element  812  free end  816  which continuously curves from the downstream edge gap  834  to the upstream edge gap  836 . The curve can be any suitable as defined by the application and the operating performance required. In the shown embodiment, the curve radius decreases from the downstream edge gap  834  such that the sealing gap  830  becomes increasingly large in the upstream direction. The curve may be defined by one whose second derivative is continuous and does not change sign (i.e. a curve that does not include a turning point). 
     Another way to quantify the curvature of the leaf element  812  in  FIG. 8  would be to define two portions in which an imaginary line extends from the upstream to the downstream edge thereof, the lines having angles α 1  and α 2  with respect to the neutral rotational axis of the shaft  822 . 
     In  FIG. 9 , the free end of the leaf element includes three portions. The first is a downstream portion  916   a  which is generally parallel to the neutral axis of the rotor  922   a.  The first portion  916   a  is relatively short, extending for only around a tenth of the axial width w of the leaf element  912 . In other embodiments, the first portion may occupy a greater proportion of the axial width, however, too much will reduce the benefit of the invention as it will result a mid-leaf pressure barrier which is too far upstream of the downstream edge  934 . This will result in a negative hydrostatic pressure during a negative shaft transient which will draw the leaf element  912  in toward the shaft  922 . It is envisaged that the first portion will not extend beyond 20% of the axial width w of the leaf in the vast majority of cases and preferably will not extend beyond 10%. Nevertheless, there may be instances where the creation of some negative hydrostatic pressure on the leaf elements may be desirable. 
     The second portion  916   b  is axially upstream and in series with the first portion  916   a  and has a concave profile. The second portion  916   b  transitions into a third, convex, portion  916   c  through a broad point of inflection. The third portion  916   c  extends to the upstream edge of the leaf element  912 . 
     It is to be noted that with all of the additional embodiments, the free end is generally configured to prevent a mid-leaf pressure barrier which can lead to a negative hydrostatic pressure, and include portions which are separated from the sealing surface to provide some constant hydrostatic pressure which can be useful for providing a stable leaf design. 
     The tip taper may be made by creating each leaf with a tip taper through pressing, stamping, etching, etc. or may be created by grinding or final machining the inner bore of a completed leaf seal (or leaf seal segment). 
     It will be appreciated that the above describe embodiments are exemplary and not limiting to the scope of the invention defined by the appended claims. For example, the leaf seal of the invention may find use in an application in which the two components do not rotate relative to each other. Further, the convergent sealing gap may be implemented with a profiled component, rather than a profiled leaf element free end. In some embodiments, the convergent leaf elements may be interspersed with no tapering leaf elements such that the hydrostatic lift is provided by a percentage of leaf elements within the leaf pack.