BACKGROUND 1. Field

5 [0001] The present invention relates to gas turbine engines, and in particular to a pressure activated non-contact seal for sealing a clearance between a blade tip and a stationary component of a gas turbine engine.

2. Description of the Related Art

[0002] A gas turbine engine typically includes a compressor section for0 compressing ambient air, a combustor section for mixing the compressed air with fuel and igniting the mixture to form a hot working medium fluid and a turbine section for expanding the working medium fluid for extracting power from the working medium fluid. The compressor section and the turbine section may include multiple stages of alternating rows of stationary vanes and rotating blades. The clearance between the tip5 of the rotating blades and the surrounding stationary component (such as an outer casing or shroud), in either the compressor or turbine stages, has a significant impact on the efficiency and power output of the engine. The higher the clearances are, the more is the leakage of the working medium fluid, leading to a loss in efficiency due to secondary flow losses. For the next generation high efficiency gas turbine engine0 development, novel measures for better blade tip clearance control are required to achieve performance targets.

[0003] An adaptive self-adjusting seal, various configurations of which are disclosed in the patent publications US8919781B2, US8641045B2, US8172232B2, US8002285B2, US7896352B2, US7410173B2, US7182345B2 and US6428009B2,5 has been developed as an effective non-contact seal for traditional rotor-stator sealing systems. The use of such a seal has so far not been realized for blade tip clearance control. SUMMARY

[0004] Briefly, aspects of the present invention relate to a pressure activated non- contact seal for sealing a clearance between a rotating and a stationary component of a gas turbine engine.

[0005] According to a first aspect of the invention, a sealing apparatus is provided for sealing a radial clearance between a stationary component and a rotatable component of a gas turbine engine. The sealing apparatus comprises a seal shoe extending along the stationary component and having a first side facing the rotatable component to create a non-contact seal therewith and a second side facing oppositely to the first side. The seal shoe is movable with respect to the stationary component in a radial direction. The sealing apparatus further comprises at least one spring element connected to the stationary component and acting on the second side of the seal shoe to bias the seal shoe toward the rotatable component. The spring element is effective to deflect and move with the seal shoe in response to fluid pressure applied to the seal shoe by a fluid stream between the seal shoe and the rotatable component. A radially inner mechanical stop is defined by the stationary component to limit a radial movement of the seal shoe toward the rotatable component. The sealing apparatus includes a fluid inlet for introducing a pressurized fluid to act on the second side of the seal shoe. A fluid flow controller is connected to the fluid inlet for regulating a pressure acting on the seal shoe due to the pressurized fluid. The fluid flow controller is operable such that a first pressure acts on the second side of the seal shoe during a steady state operation of the gas turbine engine, the first pressure being effective to move the seal shoe toward the rotatable component and maintain contact with the radially inner mechanical stop.

[0006] According to a second aspect of the invention, a method is provided for operating a sealing apparatus to control a radial clearance between a stationary component and a rotating component of a gas turbine engine. The sealing apparatus comprises a seal shoe extending along the stationary component and having a first side facing the rotating component to create a non-contact seal therewith and a second side facing oppositely to the first side. The seal shoe is movable with respect to the stationary component in a radial direction. The sealing apparatus further comprises at least one spring element connected to the stationary component and acting on the second side of the seal shoe to bias the seal shoe toward the rotating component. The spring element is effective to deflect and move with the seal shoe in response to fluid pressure applied to the seal shoe by a fluid stream between the seal shoe and the rotating component. A radially inner mechanical stop is defined by the stationary component to limit a radial movement of the seal shoe toward the rotating component. The sealing apparatus includes a fluid inlet for introducing a pressurized fluid to act on the second side of the seal shoe. A fluid flow controller is connected to the fluid inlet for regulating a pressure acting on the second side of the seal shoe due to the pressurized fluid. The method comprises operating the fluid flow controller such that during a steady state operation of the gas turbine engine, a first pressure acts on the second side of the seal shoe, the first pressure being effective to move the seal shoe toward the rotating component and maintain contact with the radially inner mechanical stop. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention is shown in more detail by help of figures. The figures show preferred configurations and do not limit the scope of the invention.

[0008] FIG. 1 is a longitudinal sectional view of an example gas turbine engine, where aspects of the present invention may be implemented; [0009] FIG. 2 is a view, looking in an axial direction, of a segment of a sealing apparatus in accordance with an embodiment of the present invention;

[0010] FIG. 3 is a cross-sectional view of the sealing apparatus of FIG. 2, along a section III-III;

[0011] FIG. 4 is a view, looking in an axial direction, of a segment of a sealing apparatus in accordance with another embodiment of the present invention; and

[0012] FIG. 5 is a cross-sectional view of a portion of a sealing apparatus illustrating an alternate variant of a secondary seal. DETAILED DESCRIPTION

[0013] In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention.

[0014] In the context of the present specification, the terms“radial”,“axial” and “circumferential” are defined in relation to a rotational engine axis 11.

[0015] Referring to FIG. 1, a gas turbine engine 10 is illustrated, which is generally rotationally symmetrical in relation to the engine axis 11. The gas turbine engine broadly includes a compressor section 12 for compressing ambient air, a combustor section 14 for mixing the compressed air with fuel and igniting the mixture to form a hot working medium fluid, and a turbine section 16 for extracting power from the working medium fluid. The compressor section 12 typically comprises multiple compressor stages housed in a casing 18. Each compressor stage includes a row of stationary guide vanes 20 positioned upstream of a row of rotating compressor blades 22. In each compressor stage, a radial clearance is defined between the tip of the compressor blades 22 and the stationary casing component 18. The turbine section 16 is also typically comprised of multiple turbine stages, with each turbine stage comprising a row of stationary vanes 24 followed by a row of rotating turbine blades 26. In each turbine stage, a radial clearance is defined between the tip of the turbine blades 26 and a respective stationary shroud 28, which is also referred to as a ring segment.

[0016] For minimizing a loss in engine efficiency due to secondary flow losses, it is desirable to run a tight clearance between the tip of the rotating blades and the surrounding stationary components, in both the compressor and the turbine stages. During transient conditions, such as during engine start-up or shut-down, the rotating parts (blades, rotor, and discs) and stationary parts (outer casing, blade rings, and ring segments) thermally expand at different rates. As a result, blade tip clearances can actually decrease during transient conditions until steady state operation is achieved, at which point the clearances can increase due to rubbing or contact of the blade tip, thereby reducing the efficiency of the engine. Thus, a need exists to reduce the likelihood of blade tip rub and control this undesirably large blade tip clearance.

[0017] Embodiments of the present invention provide a hybrid pressure activated non-contact seal for blade tip clearance control that addresses at least some of the technical problems stated above.

[0018] An inventive sealing apparatus 30 will now be illustrated referring to FIG. 1-3. The sealing apparatus 30 may be provided at one or more of the compressor stages, as exemplarily depicted in FIG. 1, for controlling a radial clearance between the rotating compressor blades 22 and the stationary compressor casing 18. Additionally, or alternately, a sealing apparatus in accordance with embodiments of the present invention may be provided at one or more turbine stages, for controlling a radial clearance between the rotating turbine blades 26 and the stationary shroud or ring segment 28.

[0019] FIG. 2-3 illustrate an embodiment of a sealing apparatus 30, which creates a non-contact seal of the radial clearance G between a stationary component 32 and a rotatable component 34 of the gas turbine engine 10. The stationary component 32 may be, for example, part of a compressor casing 18, or a turbine shroud or ring segment 28, while the rotatable component 34 may accordingly be a compressor blade 22 or a turbine blade 26 respectively (see FIG. 1). The sealing apparatus 30 includes at least one, but preferably a number of arcuate segments, arranged circumferentially adjacent to each other to form a ring. One such segment is illustrated in FIG. 2. Each segment comprises a seal shoe 36 which is movable, at least in a radial direction, with respect to the stationary component 32. The seal shoe 36 extends circumferentially along the stationary component 32 and has a first side 38 and a second side 40. The first side 38 of the seal shoe 36 forms a sealing surface in non-contact relationship with an exterior surface of the rotatable component 34 (see FIG. 3). The second side 40 faces oppositely to the first side 38.

[0020] Each segment of the sealing apparatus 30 also comprises at least one spring element 42 connected to the stationary component 32. The spring element 42 is configured to act on the second side 40 of the seal shoe 36 to bias the seal shoe 36 toward the rotatable component 34. The spring element 42 is effective to deflect and move with the seal shoe 36 in response to fluid pressure applied to the seal shoe 36 by a fluid stream between the seal shoe 36 and the rotatable component 34. The radial movement of the seal shoe 36 is limited by a radially inner mechanical stop 48 and a radially outer mechanical stop 50, which are defined by the stationary component 32, as shown in FIG. 2. In the illustrated embodiment, the mechanical stops 48, 50 are each defined at both circumferential ends of the segment. In various embodiments (not shown), the mechanical stops may be located at other circumferential locations. As shown, the stationary component 32 comprises a first recess 52, and a second recess 54 spaced circumferentially from the first recess 52. The recesses 52, 54 extend into the stationary component 32 in a circumferential direction. The seal shoe 36 has a first leg 56 and a second leg 58, which extend radially outward from the seal shoe 36. The legs 56, 58 are circumferentially spaced from each other, being positioned proximate to the recesses 52 and 54 respectively. Each leg 56, 58 has a respective arm 60, 62 which is positioned to extend into the respective recess 52, 54. The recesses 52, 54 are configured to allow a defined radial movement of the respective arm 60, 62 therewithin, and act as mechanical stops to limit the radial movement of the arms 60, 62. In particular, a radially inner shoulder 52a of the recess 52 forms a radially inner mechanical stop 48 while a radially outer shoulder 52b of the recess 52 forms a radially outer mechanical stop 50, upon respective engagement with the arm 60. Likewise, a radially inner shoulder 54a of the recess 54 forms a radially inner mechanical stop 48 while a radially outer shoulder 54b of the recess 54 forms a radially outer mechanical stop 50, upon respective engagement with the arm 62. It should be appreciated that the above described configuration of the mechanical stops is exemplary, and that the geometry and position of the mechanical stops may vary based on design requirements.

[0021] In one embodiment, as shown in FIG. 2, the spring element 42 may be configured as a leaf spring, including a plurality of radially spaced arcuate bands, in this case, an outer band 44 and an inner band 46. Each band 44, 46 is attached to or integrally formed with the stationary component 32 at a first circumferential end, and is fixed to one of the legs 58 of the seal shoe 36 at a second circumferential end. In various embodiments, in addition to or alternate to a leaf spring, other equivalent structures may be employed that may be suitable for the same function as described herein for the spring element 42. For example, in accordance with an embodiment as shown in FIG. 4, multiple spring elements 42 may be provided, which may be arranged circumferentially spaced along the seal segment. The spring elements 42 in this case may include, for example, bellows, among other types of springs. Furthermore, in a preferred embodiment, as shown in FIG. 3, the sealing apparatus 30 may also be provided with at least one secondary seal 64, 66 which may be attached to the stationary component 32 and effective to act on the second side 40 of the seal shoe 36 in response to fluid pressure applied to the seal shoe 36 by the fluid stream between the seal shoe 36 and the rotatable component 34. The secondary seal may include at least one forward secondary seal 64 located axially upstream of the sealing element 42, and/or at least one aft secondary seal 66 located axially downstream of the sealing element 42. In one embodiment, the secondary seals 64, 66 may be formed of sheet metal or other suitable flexible heat resistant material. The secondary seals 64, 66 may be received in respective slots 68, 70 formed on the second side 40 of the seal shoe 36. In a further variant, the secondary seals 64, 66 may have a different cross-section, such as an I-beam or a dog-bone shape, with arms 92. 94 engaging with the respective slots, as shown in FIG. 5. Some non-limiting examples of the secondary seal configuration are also disclosed in the patent publications mentioned in the background section.

[0022] In operation of the gas turbine engine 10, aerodynamic forces are developed which apply a fluid pressure to the seal shoe 36 causing it to move radially with respect to the rotating component 34. The fluid velocity increases as the clearance or gap G between the seal shoe 36 and rotating component 34 increases, thus reducing pressure in the gap G and drawing the seal shoe 36 radially inwardly toward the rotating component 34. As the gap G closes, the velocity decreases and the pressure increases within the seal gap G thus forcing the seal shoe 36 radially outwardly from the rotating component 34. The spring element 42 deflects and moves with the seal shoe 36 to create a primary seal of the gap G between the rotating component 34 and the stationary component 32 within predetermined design tolerances. The secondary seals 64, 66 also deflect and move with the seal shoe 36 to create a secondary seal of the gap G between the rotating component 34 and the stationary component 32. The purpose of the mechanical stops 48, 50 is to limit the extent of radially inward and outward movement of the seal shoe 36 with respect to the rotating component for safety and operational limitation.

[0023] The present inventors have recognized that the use an adaptive self- adjusting a seal, such as that disclosed in the above-mentioned patent publications, for blade tip clearances may pose practical challenges due to the complicated aerodynamics in the blade tip area. Furthermore, the inventors recognize that the rotating blades tend to generate pressure impulses, which may make the seal unstable during steady state engine operation, hence shortening the seal life.

[0024] Embodiments of the present invention utilize the aforementioned pressure balancing principle and additionally introduce an adjustable external fluid pressure for improving blade tip clearance at various engine operating states, while addressing instabilities caused by pressure impulses generated by the blades. To this end, as shown in FIG. 3, the sealing apparatus 30 according to the present invention includes a fluid inlet 72 for introducing a pressurized fluid to act on the second side 40 of the seal shoe 36. The fluid inlet 72 may be defined, for example, by a manifold in the stationary component 32, such as the compressor casing or the turbine shroud. The pressurized fluid introduced through the fluid let 72 may be received in a pressurized fluid chamber 82 defined between the seal shoe 36 and the stationary component 32. The fluid inlet 72 is connected to a source 74 of pressurized fluid via a supply line 76. In one embodiment, the pressurized fluid may include compressed air diverted from a compressor stage of the gas turbine engine 10 via a bleed line (not shown in the drawings). In other embodiments (not shown), the fluid inlet 72 may be connected to an external source of pressurized fluid. A fluid flow controller 78, such as a valve, may be connected to the fluid inlet 72 for regulating a pressure acting on the seal shoe 36 due to the pressurized fluid. As shown in FIG. 1, the sealing apparatus 30 of a given stage may be provided with a dedicated fluid flow controller or valve 78 for the respective stage. In other embodiments, the sealing apparatus 30 of multiple stages may be jointly controlled by a single fluid flow controller or valve 78.

[0025] In accordance with aspects of the present invention, the valve 78 may be controlled as a function of the operating state of the engine 10. In particular, during a steady state engine operation, such as during baseload or part-load operation, the valve 78 may be operated such that the chamber 82 is at a first pressure Pl, which acts on the second side 40 of the seal shoe 36 and is effective to move the seal shoe 36 toward the rotating component 34 and maintain a steady contact with the radially inner mechanical stop 48. The first pressure Pl may be determined such that the total pressure acting on the second side 40 of the seal shoe 36 is always greater than the fluid pressure acting on the first side 38 of the seal shoe 36 by the fluid stream between the seal shoe 36 and the rotating component 34 during the steady state operation of the engine 10. In particular, the valve 78 may be controlled such that the first pressure Pl is stable, i.e., substantially constant, during a given steady state engine operation, such as during base-load or part-load operation. In one embodiment, this may be implemented via a closed loop control system, which may be configured to control pressure fluctuations in the supply line 76 / fluid chamber 82 to the smallest amplitude possible around a constant mean value. The seal shoe 36 is therefore pushed to the closest point to the rotating component 34 that it is designed to operate at, thereby minimizing secondary flow losses and maintaining optimum engine efficiency. Furthermore, the forced contact with the radially inner mechanical stop 48 suppresses any vibration of the seal during steady state engine operation. Thus, unstable behavior such as flutter of seal may be avoided, thereby enhancing the long term mechanical integrity and durability of the seal. In one embodiment, the first side 38 of the seal shoe 36 may be provided with an anti-rubbing abradable coating 80.

[0026] During a transient engine operation, when a pinch-point (closest distance between rotor and stator) is typically expected to occur, such as during engine start-up or shut-down, the valve 78 may be operated such that the chamber is at a second pressure P2, which acts on the second side 40 of the seal shoe 36. The second pressure P2 is lower than the first pressure Pl and is effective to allow the seal shoe 36 to deflect and move in response to the fluid pressure applied to the seal shoe 36 by the fluid stream between the seal shoe 36 and the rotatable component 34 in a manner as described above. Therefore, at the second pressure, the seal shoe 36 essentially follows the rotating component 34, thereby avoiding any rubbing of the rotating and stationary components. Thereby, a hybrid pressure activated blade tip clearance control may be realized, which is adaptive during transient operation and stable during steady state operation. [0027] While specific embodiments have been described in detail, those with ordinary skill in the art will appreciate that various modifications and alternative to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims, and any and all equivalents thereof.