JP2008128240A - Turbine seal guard - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an improved system for blocking solid particulate impurities, moving through a flow passage, from seals or other components. <P>SOLUTION: A seal guard in a turbine comprises: a plurality of nozzles 12 mutually spaced in a circumferential direction; a plurality of turbine blades 14 mutually spaced in a circumferential direction including a blade cover 20; and an opening 38 defined by a trailing edge of the nozzle 12 and a leading edge of the blade cover 20. The seal guard includes: an upstream axial fin 52 disposed to the trailing edge of the nozzle 12 and extended downstream to cross at least a part of the opening 38; or a downstream axial fin 58 disposed to the leading edge of the blade cover 20 and extended upstream to cross at least a part of the opening 38. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention generally relates to a system for protecting turbine components from erosion and deposition of solid particles. In particular, the present invention relates to a system that provides a seal guard for removing solid particles from turbine seal components, but is not limited to such a system.

  Steam turbines and / or gas turbines typically utilize an annular seal between stationary and rotating parts to minimize secondary leakage. By preventing or reducing such leakage, the efficiency of the turbine can be maximized because the exhaust flow is forced to flow to the turbine blades at a greater rate. However, impurities or solid particles are often present in the working fluid. In general, solid particles are accelerated at high speed while passing through the diaphragm stage of the turbine (ie, while moving along a stationary nozzle blade). Such high-speed moving solid particles cause erosion and degradation, resulting in increased surface roughness of downstream turbine components with which the solid particles contact, including the annular seal. In addition to erosion, solid particles can stick to rotating or stationary components, thereby changing the flow area and creating frictional damage.

  Turbine components present in the main steam flow path are generally protected by special coatings or mechanical curing processes. However, the components associated with the annular seal are typically manufactured from a softer material than the components in the main steam flow path, and no special coating film treatment or mechanical curing process is performed. This is because a flexible material is desirable when friction occurs between stationary and rotating components. Thus, solid particles in contact with the annular seal component often cause erosion and degradation. Such erosion can flatten overlapping teeth that form an annular seal or scrape off the corners of the teeth, and over time, if the integrity of any tooth structure is compromised, It can lead to performance problems and even more serious reliability problems. Solid particles can accumulate inside the annular seal, and such solid particle deposition can cause serious damage to the seal, considering the rotational speed of the rotating parts and the seal tolerance between stationary and rotating parts. give. In such situations, the solid particles rub against the rotating and stationary surfaces, impairing the integrity of the structure of the annular seal and / or damaging the overlapping teeth that form the seal. As a result, leakage increases and turbine efficiency may decrease.

  Accordingly, steam turbine and gas turbine annular seals and other sensitive components must be more adequately protected from their solid particulate impurities. Prior systems have not fully addressed this open issue. Accordingly, there is a need for an improved system for shielding seals and other components from solid particle impurities that move through a flow path.

  Thus, this application may describe a system for preventing solid particle erosion in a turbine. The turbine may include a turbine blade that includes a blade cover and an opening defined by a trailing edge of the nozzle and a leading edge of the blade cover. The system may include at least one of an upstream axial fin disposed at the rear edge of the nozzle and a downstream axial fin disposed at the front edge of the blade cover. The trailing edge of the nozzle may include an outer sidewall.

  In some embodiments, the upstream axial fin may include a fin that protrudes downstream from the outer sidewall. The upstream axial fins may extend substantially continuously in the circumferential direction. In other embodiments, the upstream axial fins may include fins that extend axially across at least a portion of the opening.

  In some embodiments, the downstream axial fins may include fins that protrude upstream from the blade cover. The downstream axial fin may further include a fin extending axially across at least a portion of the opening. The downstream axial fins may extend substantially continuously in the circumferential direction.

  In some embodiments, the system may include both upstream axial fins and downstream axial fins. The upstream axial fin and the downstream axial fin may have substantially the same radial position and extend substantially over the axial distance of the opening. In other embodiments, the downstream axial fins may be positioned slightly offset radially outward from the upstream axial fins. The downstream axial fin and the upstream axial fin may overlap each other in the axial direction.

  The upstream axial fin may be an integral part of the outer sidewall. In some embodiments, the upstream axial fin may be attached to the outer sidewall by welding or peening.

  The system may further include a downstream groove disposed at the leading edge of the blade cover. The upstream axial fin may extend so as to cover the opening and terminate inside the downstream groove. In other embodiments, the system may further include an upstream groove located at the trailing edge of the nozzle. The downstream axial fin may extend so as to cover the opening, and may terminate inside the upstream groove.

  The present application relates to a plurality of nozzles spaced apart from one another in the circumferential direction, a plurality of turbine blades spaced apart from one another in the circumferential direction, each including a blade cover, a trailing edge of the nozzle and a blade cover A seal guard in a turbine including an opening defined by a leading edge may be further described. The seal guard is disposed at the rear edge of the nozzle and extends in the downstream direction so as to cross at least a part of the opening, or the opening guard is disposed at the front edge of the blade cover. A downstream axial fin extending in the upstream direction so as to traverse at least a portion thereof. In some embodiments, the system may include both an upstream axial fin and a downstream axial fin, the upstream axial fin and the downstream axial fin having substantially the same radial position. , And may extend substantially over the axial distance of the opening. In other embodiments, the downstream axial fins may be positioned slightly offset radially outward from the upstream axial fins, and the upstream axial fins and the downstream axial fins are axially positioned. They may overlap each other.

  In some embodiments, the system may include a downstream groove disposed at the leading edge of the blade cover. The upstream axial fin may extend so as to cover the opening and terminate inside the downstream groove. In other embodiments, the system may include an upstream groove disposed at the trailing edge of the nozzle. The downstream axial fin may extend so as to cover the opening, and may terminate inside the upstream groove.

  These and other features of the present application will become apparent upon review of the following detailed description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.

  Referring now to the drawings, wherein like numerals represent like parts throughout the several views. FIG. 1 illustrates an example of an ambient environment in which an embodiment of the present application may operate, ie, a cross-sectional view of a turbine stage 10. The turbine stage 10 may be a stage within a steam turbine or gas turbine, or other type of turbomachine. The turbine stage 10 may include a plurality of nozzles 12 that are spaced apart from each other in the circumferential direction. The nozzle 12 may be a stationary component that conveys the flow of working fluid to a plurality of turbine blades 14 that are spaced apart from each other in the circumferential direction. The turbine blade 14 may include an airfoil 16 that extends from the base 18. In operation, the configuration of the airfoil 16 and the working fluid flow cause the turbine blade 14 to rotate with respect to the rotor blades (not shown), thereby converting the working fluid stream energy into mechanical energy. The base 18 of the turbine blade 14 may be coupled to the rotor blades.

  Each turbine blade 14 may be coupled to a blade cover 20 at the radially outer end of the airfoil 16. The blade cover 20 may be integral with the turbine blade 14 or may be attached to the turbine blade 14 according to conventional methods. In general, the blade cover 20 may be a surface or structure that is at the radially outer end of the airfoil 16 and is substantially perpendicular to the surface of the airfoil 16. In particular, the blade cover may prevent working fluid from leaking beyond the end of the airfoil 16. This leakage prevention is beneficial because the amount of work performed by the working fluid that leaks beyond the end of the airfoil 16 is small, resulting in reduced turbine efficiency. The blade cover 20 is also known as a tip shroud or bucket cover. Each blade cover 20 may extend circumferentially toward the blade cover 20 of an adjacent turbine blade 14. Each blade cover 20 may abut against two adjacent blade covers 20. Thus, the blade cover 20 may form a substantially continuous peripheral ring within the turbine (although it is divided). In another type of configuration, the blade cover may span multiple turbine blades. In that case, blade covers (not shown) larger than the blade cover described above may contact each other to form a peripheral ring inside the turbine. One or more cover teeth 24 may be disposed on the radially outer surface 22 of the blade cover 20. The cover teeth 24, also known as vernier teeth, may include tapered or stepped protrusions that are pointed from the radially outer surface 22 radially outward.

  Outside the blade cover 20, the turbine stage 10 may further include a spill strip 26. The spill strip 26 may be a stationary component that includes a series of abutting arcuate segments that form a generally continuous peripheral ring within the turbine when assembled. One or more spill strip teeth 30 may be disposed on the radially inner surface 28 of the spill strip 26. The spill strip teeth 30 may be vernier type, high-low type, interlocking type or any other commonly used type of sealing teeth, with a pointed shape radially inward from the radially inner surface 28. A certain taper or step protrusion may be included. The spill strip teeth 30 and the cover teeth 24 may be arranged such that they form a seal 34 between the blade cover 20 and the spill strip 26. The seal 34 may be called an annular seal, a high / low seal, an interlocking seal, or a vernier seal. In particular, the spill strip teeth 30 and the cover teeth 24 may overlap teeth along the axial length of the blade cover 20 / spill strip 26 so that a seal is formed that limits the axial movement of the working fluid in this region. Interlocking teeth may be formed. The spill strip 26 may be coupled to the turbine casing 34 or may be integral with the turbine casing 34. The turbine casing 34 may form an outer casing that surrounds the turbine. Furthermore, in other constructions, the spill strip 26 may be mounted on a casing, diaphragm blade / ring support or downstream diaphragm / blade ring.

  A working fluid that is air in a gas turbine and steam in a steam turbine may flow through the turbine stage 10. Typically, the working fluid flows through the main flow path of the turbine (ie, through the nozzle 12 and then through the airfoil 16). The flow of the main flow path is indicated by arrows 36 in FIG. In the conventional turbine structure, the opening 38 is disposed between the outer rear edge of the nozzle 12 and the inner front edge of the blade cover 20. The working fluid may escape from the main flow path and flow into the opening 38. Such flow direction is indicated by arrow 40 in FIG. The working fluid that flows into the opening 38 may then contact components associated with the seal 32, including the spill strip teeth 30 and the cover teeth 24.

  The working fluid may contain some impurities. The term “impurity” as used herein may include solid particles or other impurities. Those solid particles flow into the main flow path (indicated by arrow 36) and contact turbine components such as nozzle 12 and airfoil 16 inside the main flow path. As described above, the components of the main flow path are generally protected by a special coating or mechanical curing process so that little or no erosion or degradation of the components occurs when in contact with solid particles. The solid particles may flow into the opening 38. In the conventional turbine structure, the axial distance of the opening 38 (that is, the axial distance defined by the outer rear edge of the nozzle 12 and the inner front edge of the blade cover 20) is relatively long. Frequent inflows occur frequently.

  Furthermore, since the degree of curvature of the front edge portion of the opening 38 is relatively small, the function of preventing the solid particles flowing into the opening 38 cannot be performed. Without such an obstruction, most of the solid particles will be swept into the opening 38 due to the rotational speed of the turbine blade 14. In particular, in the case of a conventional turbine structure, the rotation of the turbine blade 14 causes the solid particles to move radially outward, and as a result, the proportion of solid particles that enter the opening 38 increases. Note that the leading edge of the opening 38 (ie, the trailing edge of the nozzle 12) is often defined by the outer sidewall 46 component. In general, a turbine nozzle assembly can be constructed using several methods. The first method involves using a nozzle 12 having an integrally formed outer side wall 46 and inner side wall 47. The outer side wall 46 and the inner side wall 47 are used to weld the nozzle 12 directly between the outer ring 48 and the inner ring 49. A nozzle with this configuration is often called a singlet nozzle. The second nozzle assembly method (not shown) uses a band / ring configuration. In this type of structure, the nozzle is first welded between an inner band and an outer band that extend at an angle of about 180 °. Those arcuate bands to which the airfoils have been welded are then assembled and welded between the inner and outer support rings of the turbine stator. According to this second nozzle assembly method, the leading edge of the opening 38 will be defined by an outer band not shown. A third assembly method (not shown) uses one or more large pieces of material from which the nozzle is machined. This method is sometimes referred to as a “bring” configuration.

  For simplicity, the front edge of the opening 38 is generally referred to herein as the rear edge of the nozzle 12, and in particular, the rear edge of the outer sidewall 46. If other nozzle assembly configurations are used, the leading edge of the opening 38 is defined by the outer band components described above and other components such as the outer ring 48, nozzle 12 and / or other stationary components. It will be appreciated by those skilled in the art that this may be possible. Accordingly, when referred to herein as the trailing edge of the nozzle 12 or outer sidewall 46, it is intended to include those other components that define the leading edge of the opening 38 in other types of nozzle assembly configurations.

  In the opening 38, the solid particles come into contact with components, such as the cover teeth 24 or spill strip teeth 30, which are usually made of a soft material and have not been treated by a special coating film or mechanical hardening process. In many cases, solid particles in contact with those components of the seal 32 cause erosion. Over time, erosion can seriously damage the cover teeth 24 and / or spill strip teeth 30 so that the performance of the seal 32 is adversely affected and leakage increases. That is, as the teeth wear and the overlap between cover teeth 24 or spill strip teeth 30 is reduced or broken, a greater amount of working fluid can escape through seal 32 and leak. Naturally, no energy is extracted from the working fluid that passes through the seal, thus reducing the efficiency of the turbine.

  Referring now to FIG. 2, one embodiment of a seal guard 50 according to the present application is shown in connection with the turbine stage 10. It will be appreciated by those skilled in the art that the use of turbine stage 10 is merely an example and seal guard 50 may be used in combination with other turbine stage configurations. The seal guard 50 may include an upstream axial fin 52 that protrudes axially downstream from the outer side wall 46 of the nozzle 12. In other embodiments, more than one axial fin 52 may be provided. In particular, the upstream axial fin 52 may protrude axially from the outer sidewall 46 across the opening 38. In some embodiments, the upstream axial fin 52 may protrude from the outer ring 48 (ie, the upstream axial fin 52 protrudes axially from the outer ring 48 across the opening 38). Also good). The rear edge of the upstream axial fin 52 that is the terminal end may be disposed very close to the front edge of the blade cover 20. The upstream axial fin 52 may extend over some or all of the axial distance of the opening 38. In some embodiments, the upstream axial fin 52 may taper toward the tip 54 as shown in FIG.

  The upstream axial fins 52 may extend circumferentially substantially continuously within the turbine. That is, in some embodiments of the nozzle configuration, the outer sidewalls 46 of adjacent nozzles 12 may abut to form a substantially continuous peripheral ring. The upstream axial fins 52 may be formed in the outer sidewall 46 so as to extend continuously (although divided) along the perimeter of the turbine (ie, the upstream axial fins 52 are outside). Formed over the entire length of the side wall 46 in the circumferential direction). In other embodiments of the nozzle configuration, the upstream axial fins 52 may extend along the periphery of the turbine due to the shape of the outer sidewall 46 or the configuration of the outer ring 48.

  In some embodiments, the seal guard 50 may include a downstream axial fin 58 that protrudes upstream from the blade cover 20. In other embodiments, two or more downstream axial fins 58 may be included. In particular, the downstream axial fin 58 may protrude axially from the blade cover 20 across the opening 38. The downstream axial fin 58 may extend over some or substantially all of the axial distance of the opening 38. In some embodiments, the end of the downstream axial fin 58 may taper toward the tip 60 as shown in FIG.

  The downstream axial fins 58 may extend substantially continuously in the circumferential direction. That is, the blade covers 20 of adjacent turbine blades 14 may abut to form a substantially continuous peripheral ring. The downstream axial fins 58 may be formed in the blade cover 20 so as to extend continuously (although divided) along the periphery of the turbine (ie, the downstream axial fins 58 are blades). Formed over the entire length in the circumferential direction of the cover 20).

  In some embodiments, the upstream axial fin 52 and the downstream axial fin 58 are arranged such that the upstream axial fin 52 and the downstream axial fin 58 substantially cover the axial distance of the opening 38. Combinations may be used. In other embodiments, as shown in FIG. 2, the downstream axial fins 58 may be positioned slightly offset radially outward from the upstream axial fins 52. Although not shown, in some embodiments, this arrangement causes the upstream axial fins 52 and the downstream axial fins 58 to overlap each other in the axial direction.

  Several methods can be used to form the upstream axial fins 52. In some embodiments, the upstream axial fin 52 may be machined as an integral part of the outer sidewall 46 or the outer ring 48 in a conventional manner. As mentioned above, in some turbines, the upstream edge of opening 38 may be defined by an outer band. In such a case, the upstream axial fin 52 may be machined as an integral part of the outer band. The upstream axial fin 52 may be manufactured from alloy steel (such as 12-chrome, stainless steel or low alloy steel) and may be mechanically hardened by flame quenching or other similar mechanical processing, Mechanical curing is not essential. In another embodiment, a coating film, such as a coating film typically used for components in the flow path, may be used for the upstream axial fins 52 rather than a mechanical curing process. The purpose of this mechanical curing process or the formation of a protective coating is to improve the resistance of the upstream axial fin 52 to erosion by solid particles.

  As shown in FIG. 3, the upstream axial fin 52 may be welded to the outer sidewall 46. In other cases (not shown), the upstream axial fin 52 may be welded to the outer ring 48. As shown, the upstream axial fins 52 may be welded to the outer sidewall 46 that does not have such components. In such a case, a long tapered member 70 may be welded to the rear edge of the outer side wall 46 so that the upstream axial fin 52 is formed. A weld 72 disposed on the upward radial surface of the long tapered member 70 and a weld 74 disposed on the lower leading edge of the long tapered member 70 may be used, although other weld configurations may be used. One skilled in the art will recognize that it may be used. In such embodiments, the welded upstream axial fin 52 may be made from a different material than the outer sidewall 46. In general, Stellite®, Inconel®, or other similar alloys or materials may be used that are more resistant to erosion by solid particles than the material of outer sidewall 46 or outer ring 48. In other embodiments, the upstream axial fin 52 to be welded may be manufactured from the same material as the outer sidewall 46 or the outer ring 48.

  As shown in FIG. 4, the upstream axial fins 52 may be attached to the outer sidewall 46 by mechanical coupling or by peening the components in place, otherwise (not shown). May be attached to the outer ring 48. In the present embodiment, a tapered or rectangular member 80 may be used to form the upstream axial fin 52. Rectangular member 80 may include a dovetail or hook 82 at one end, but may include other configurations that allow for mechanical coupling or peening. During mounting, the hook 82 may be inserted into the dovetail or groove 84 of the outer side wall 46. As is well known in the art, the outer sidewall 46 is then peened at a location adjacent to the groove 86 so that the outer sidewall 46 is deformed and the hook 82 is mechanically locked into the groove 84. Also good. The outer sidewall 46 or the outer ring 48 may further include small secondary grooves (not shown) that aid in mechanical deformation of the peening process.

  When employing mechanical coupling or peening of the upstream axial fins 52 in place, the upstream axial fins 52 may be manufactured from a material different from the material of the outer sidewall 46 or outer ring 48. Generally, stellite, inconel, or other similar alloys or materials may be used that are more resistant to erosion by solid particles than the material of outer sidewall 46 or outer ring 48. In other embodiments, the mechanically coupled or peened upstream axial fins 52 may be manufactured from the same material as the outer sidewall 46 or the outer ring 48. Further, by attaching the upstream axial fin 52 by mechanical coupling or peening, any existing outer sidewall 46 or outer ring 48 (or any of the other components defining the upstream edge of the opening 38) may be used. The upstream axial fin 52 can be efficiently retrofitted to the component). Thus, upstream axial fins 52 may be added to a turbine that is undergoing significant erosion by solid particles to prevent the occurrence of more severe erosion. Therefore, the upstream axial fin 52 may be applied to either an existing channel or a new channel.

  The downstream axial fins 58 may be formed in a manner similar to that described above with respect to the upstream axial fins 52. That is, the downstream axial fins 58 may be machined as an integral part of the blade cover 20, or may be welded, mechanically coupled to the existing blade cover 20, or in place. May be peened. The downstream axial fins 58 may be manufactured from the same materials described above with respect to the upstream axial fins 52.

  During operation, the seal guard 50 may form a shield for deflecting solid particle impurities from entering the opening 38. Also, the seal guard 50 may change the flow characteristics around the opening 38 so that solid particle impurities are carried away from the opening 38. Solid particle impurities so deflected or redirected within the main flow path cannot contact the seal 32 components (and other turbine components in this region of the turbine) or erode the seal 32 components. And turbine performance is not adversely affected by increased leakage. In particular, the upstream axial fin 52 and the downstream axial fin 58 significantly reduce the axial extent of the opening 38, thereby reducing the number of solid particles that enter the opening 38. In addition, the upstream axial fin 52 and its tapered tip 54 are used in place of the existing slight curvature at the front edge of the opening 38 that has infused solid particles to enter the opening 38. .

  In some embodiments, the upstream axial fin 52 may be used without the downstream axial fin 58. In such an embodiment, the upstream axial fin 52 may be configured to cover most of the opening 38. Further, as shown in FIG. 5, the upstream axial fin 52 may be used in combination with the downstream groove 90. The downstream groove 90 may be a groove or a recess formed in the blade cover 20. In such an embodiment, the upstream axial fin 52 may cover the opening 38 and terminate within the downstream groove 90.

  Similarly, in another embodiment, the downstream axial fin 58 may be used without the upstream axial fin 52. In such an embodiment, the downstream axial fin 58 may be configured to cover most of the opening 38. Further, as shown in FIG. 6, the downstream axial fin 58 may be used in combination with the upstream groove 94. The upstream groove 94 may be a groove or a recess formed in the outer side wall 46 or the outer ring 48. In such an embodiment, the downstream axial fin 58 may cover the opening 38 and terminate within the upstream groove 94.

  In other embodiments, as shown in FIG. 2, the upstream axial fins 52 may overlap or substantially overlap the downstream axial fins 58. In such an embodiment, the downstream axial fin 58 may be positioned slightly offset radially outward from the upstream axial fin 52. In such an embodiment, the solid particles inevitably progress to sew between overlapping or closely spaced axial fins before reaching the seal 32 and associated components. become. However, depending on the relative radial position of the axial fins and the flow direction imparted to the working fluid by the upstream axial fins 52, solid particles are unlikely to make such movements. Because the number of solid particles that flow to the seal 32 is significantly limited, erosion to the seal 32 and adjacent components is greatly reduced.

  From the above description of preferred embodiments of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims. Furthermore, the foregoing description relates only to the described embodiments of the present application, and numerous changes and modifications may be made without departing from the scope of the present application as defined by the appended claims and their equivalents. It will be clear that can be implemented.

1 is a diagram schematically illustrating a cross-sectional view of a conventional turbine stage that is an example of an ambient environment in which an embodiment of the present application may operate. FIG. 1 is a diagram schematically illustrating a cross-sectional view of a turbine stage having a seal guard according to an embodiment of the present application. FIG. FIG. 3 is a diagram schematically illustrating a cross-sectional view of a turbine stage having a seal guard according to another embodiment of the present application. And FIG. 6 is a diagram schematically illustrating a cross-sectional view of a turbine stage having a seal guard according to yet another embodiment of the present application. And FIG. 6 is a diagram schematically illustrating a cross-sectional view of a turbine stage having a seal guard according to yet another embodiment of the present application. And FIG. 6 is a diagram schematically illustrating a cross-sectional view of a turbine stage having a seal guard according to yet another embodiment of the present application.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Turbine stage, 12 ... Nozzle, 14 ... Turbine blade, 20 ... Blade cover, 32 ... Seal, 38 ... Opening, 46 ... Outer side wall, 48 ... Outer ring, 52 ... Upstream axial fin, 58 ... Downstream side Axial fin, 90 ... downstream groove, 94 ... upstream groove

Claims (10)

  A solid in a turbine comprising a blade cover (20) and a plurality of turbine blades (14) including a rear edge of the nozzle (12) and an opening (38) defined by the front edge of the blade cover (20). In a system for preventing particle erosion, an upstream axial fin (52) disposed on a rear edge of the nozzle (12) and a downstream axial fin (52) disposed on a front edge of the blade cover (20). 58).   The system of claim 1, wherein the rear edge of the nozzle (12) comprises an outer sidewall (46).   The system of claim 2, wherein the upstream axial fin (52) comprises a fin projecting downstream from the outer sidewall (46).   The system of any preceding claim, wherein the upstream axial fin (52) comprises a fin extending laterally across at least a portion of the opening (38).   The system of claim 1, wherein the downstream axial fin (58) comprises a fin projecting upstream from the blade cover (20).   The system of claim 1, wherein the system includes both an upstream axial fin (52) and a downstream axial fin (58).   The upstream axial fin (52) and the downstream axial fin (58) have substantially the same radial position and extend substantially over the axial distance of the opening (38). 6. The system according to 6. The downstream axial fin (58) is positioned slightly offset radially outward from the upstream axial fin (52);
The system of claim 6, wherein the downstream axial fin (58) and the upstream axial fin (52) overlap each other in the axial direction. Further comprising a downstream groove (90) disposed at a front edge of the blade cover (20);
The system of claim 1, wherein the upstream axial fin (52) extends to cover the opening (38) and terminates within the downstream groove (90). Further comprising an upstream groove (94) disposed at the rear edge of the nozzle (12);
The system of claim 1, wherein the downstream axial fin (58) extends to cover the opening (38) and terminates within the upstream groove (94).

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