Air turbine starter with nozzle holding mechanism

Abstract

An air turbine starter has a housing defining an interior having a main inlet and a main outlet to define a main airflow path from the main inlet to the main outlet. A nozzle is located in the interior and has circumferentially spaced blades. A shroud is also located in the interior and circumscribes at least a portion of the blades. A retaining mechanism constrains axial movement of the nozzle.

Description

Technical Field

[0001] The present disclosure relates generally to an air turbine starter, and more particularly to a retention mechanism for a turbine in an air turbine starter. Background Art

[0002] Turbine engines, such as gas turbine engines, are periodically started using an air turbine starter that is mechanically coupled to the rotor of the turbine engine. The air turbine starter is typically mounted to the engine through a gearbox or other transmission assembly. The transmission transfers power from the air turbine starter to the engine to assist in starting the engine. The air start turbine includes an air driven turbine that rotates a drive gearbox. A high pressure air source flows air through the turbine to affect the rotation of the turbine, which drives the gearbox, which in turn rotates the rotor of the turbine engine during the starting sequence. During operation, the high pressure air applies an axial load on the turbine and affects the rotation of the turbine. Summary of the invention

[0003] Aspects and advantages of the disclosure will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the disclosure.

[0004] In one aspect, the present disclosure is directed to an air starter comprising a housing defining an interior having a primary inlet and a primary outlet to define a primary airflow path from the primary inlet to the primary outlet. A nozzle is located in the interior and has circumferentially spaced blades. A turbine is located in the interior, is rotatable about an axis of rotation, and has circumferentially spaced blades. A physical interference is at least one between the nozzle and the shroud or between the shroud and the housing, wherein the nozzle is axially constrained in downstream movement along the primary airflow path by the physical interference.

[0005] In another aspect, the invention is directed to an assembly including a housing defining an interior having a primary inlet and a primary outlet to define a primary airflow path from the primary inlet to the primary outlet. A nozzle is located in the interior and has circumferentially spaced vanes. A shroud is located in the interior and circumscribes at least a portion of the vanes. A physical interference is at least one between the nozzle and the shroud or between the shroud and the housing, wherein the nozzle is axially constrained in downstream movement along the primary airflow path by the physical interference.

[0006] These and other features, aspects and advantages of the present disclosure will become better understood with reference to the following description and appended claims.The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various aspects of the present disclosure and, together with the description, serve to explain the principles of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In the attached figure:

[0008] Figure 1 is a schematic diagram of a turbine engine with an air turbine starter.

[0009] Figure 2 is a cross-sectional view of an air turbine starter incorporating several aspects of the present disclosure.

[0010] Figure 3 is an exploded view showing the nozzle and shroud of an air turbine starter.

[0011] Figure 4A yes Figure 3 An enlarged perspective view of a portion of the nozzle and shroud in an assembled state.

[0012] Figure 4B is along Figure 2 An enlarged cross-sectional view of the nozzle and shroud within the housing taken along line IV.

[0013] Figure 4C Shown along Figure 2 An enlarged cross-sectional view of another variation of the nozzle and shroud within the housing, taken along line IV.

[0014] Figure 5 yes Figure 1 Schematic diagram of the turbine engine and starter inside the vehicle. DETAILED DESCRIPTION

[0015] Aspects of the disclosure described relate to an air turbine starter that can be used to start a turbine engine. These aspects can be an air turbine starter alone or in combination with a turbine engine. The air turbine starter includes a nozzle upstream of a rotatable turbine that directs an incoming air flow to the rotatable turbine. One aspect of the disclosure relates to constraining the movement of a nozzle upstream of a rotatable turbine of the air turbine starter. The relatively high pressure of the pressurized air supplied to rotate the turbine can apply a load to the nozzle as it passes through the nozzle to affect the rotation of the turbine. The load on the nozzle can be sufficient to cause a portion of the nozzle to abut a portion of the rotating turbine, which can prematurely wear the parts. An example of such a load is an axial load on the nozzle.

[0016] An existing solution to prevent axial movement of the nozzle when subjected to pressure loads is to cast the nozzle and the surrounding casing (such as a surrounding shroud) as a single component. Castings limit the type of material and require a minimum thickness depending on the nature of the material used to form the casting. Castings can also limit the geometry of the nozzle, especially the shape and arrangement of the vanes that form the nozzle.

[0017] The present disclosure describes a solution for making an air turbine starter such that the nozzle is properly constrained in its movement, especially axial movement, without the need to cast the nozzle and the surrounding casing, especially the shroud, as a single component.

[0018] For purposes of illustration, the present disclosure will be described with respect to a starter for a turbine engine. For example, the present disclosure may have applicability in other vehicles or engines and may be used to provide benefits in industrial, commercial, and residential applications.

[0019] As used herein, the term "upstream" refers to the direction opposite to the direction of fluid flow, and the term "downstream" refers to the direction in the same direction as the direction of fluid flow. "Front" or "forward" refers to the front of something, and "rear" or "backward" refers to the back of something. For example, when used with respect to fluid flow, front / forward means upstream, and rear / backward means downstream.

[0020] Additionally, as used herein, the term "radial" or "radially" refers to a direction away from a common center. For example, in the general context of a turbine engine, radial refers to a direction along a ray extending between a central longitudinal axis of the engine and an outer periphery of the engine. Additionally, as used herein, the term "group" of elements or a "group" of elements may be any number of elements, including only one.

[0021] All directional references (e.g., radial, axial, proximal, distal, upper, lower, upward, downward, left, right, side, front, rear, top, bottom, above, below, vertical, horizontal, clockwise, counterclockwise, upstream, downstream, forward, backward, etc.) are only used for identification purposes to help the reader understand the present disclosure, and do not create limitations, especially with respect to the position, orientation or use of the aspects of the present disclosure described herein. Unless otherwise specified, connection references (e.g., attachment, connection, fixing, fastening, connection and engagement) will be understood broadly and may include intermediate members between element sets and relative movement between elements. Therefore, connection references do not necessarily infer that two elements are directly connected and connected to each other in a fixed relationship. The exemplary drawings are only for illustrative purposes, and the sizes, positions, orders and relative sizes reflected in the drawings may vary.

[0022] Reference Figure 1, an air turbine starter (ATS) 10 is coupled to an auxiliary gearbox (AGB) 12, both of which are schematically shown as being mounted to a turbine engine 14 (such as a gas turbine engine). The turbine engine 14 defines an axial direction 15 as shown. The turbine engine 14 includes an air inlet having a fan 16 that supplies air to a high-pressure compression region 18. The air inlet with the fan 16 and the high-pressure compression region 18 are collectively referred to as the "cold section" or turbine section of the turbine engine 14 upstream of combustion. The high-pressure compression region 18 provides high-pressure air to the combustion chamber 20. In the combustion chamber 20, the high-pressure air is mixed with fuel and burned. Before being discharged from the turbine engine 14, the hot and pressurized combustion gases pass through the high-pressure turbine region 22 and the low-pressure turbine region 24. As the combustion gases flow through the high-pressure turbine region 22 and the low-pressure turbine region 24, rotational energy is extracted from the gas flow through the turbine engine 14. A shaft can connect the high-pressure turbine region 22 to the high-pressure compression region 18 to provide power for the compression mechanism. The low pressure turbine may be coupled to the fan 16 at the air inlet via a shaft to provide power to the fan 16 .

[0023] The AGB 12 is coupled to the turbine engine 14 at the high pressure turbine region 22 or the low pressure turbine region 24 via a mechanical power take-off 26. The mechanical power take-off 26 includes a plurality of gears and a plurality of components for mechanically coupling the AGB 12 to the turbine engine 14. The ATS 10 may be mounted outside of the intake region containing the fan 16 or outside of the core near the high pressure compression region 18.

[0024] Reference now Figure 2 , a cross-section of an air turbine starter 10 is shown. For clarity, the axial direction 15 is shown as passing through the center of the ATS 10. The ATS 10 includes a starter housing 30 that defines an interior 28 having a main inlet 32, a main outlet 34. A flow path 36 extends between the inlet 32 ​​and the outlet 34 for communicating a gas flow therethrough. In a non-limiting example, the gas is air and is supplied from a ground-operated air cart, an auxiliary power unit, or from a cross-drain start of an already running engine.

[0025] The turbine section 38 is located within the starter housing 30. The turbine section 38 includes a nozzle 42, a shroud 44, and a turbine 46 having circumferentially spaced blades 40. The circumferentially spaced blades 40 of the turbine 46 may be disposed within the flow path 36 to rotatably extract mechanical power from the airflow along the flow path 36. The transfer of the extracted mechanical power may be managed using a turbine shaft 58.

[0026] The nozzle 42 includes a central hub 48 and a set of circumferentially spaced blades 50. The blades 50 may be in the form of an airfoil, and the blades 40 may also be in the form of an airfoil. The nozzle 42 and the shroud 44 may be made into two separate parts assembled together using heat fitting, which may eliminate the traditional method of casting the nozzle and the shroud as one part. The advantage of the nozzle 42 and the shroud 44 being two separate parts is that it may allow the nozzle 42, the shroud 44, and the housing 30 to be made of the same or different materials. Examples of such materials may include light metals (such as aluminum), stainless steel, iron, or titanium. The material may also include, but is not limited to, additive manufacturing or die casting of high-strength materials with a thinner trailing edge thickness, which provides structural and aerodynamic performance advantages. In addition, although a single turbine 46 is shown, it is conceivable that multiple turbines 46 may be provided to form multiple stages within the ATS 10.

[0027] The gearbox section 51 is downstream of the turbine section 38 and includes a gearbox 52 mounted within the starter housing 30. A gear train 54 may be disposed within the gearbox 52 and drivingly coupled to the turbine 46. In the illustrated example, the gear train 54 is a planetary gear system including a ring gear 56, planetary gears, and a sun gear. It should be understood that the gear train 54 may include any suitable gear assembly, including but not limited to a pinion assembly, etc. A turbine shaft 58 is coupled to the gear train 54 and is rotatably supported by a pair of turbine bearings 60.

[0028] Reference Figure 3 , the nozzle 42 and shroud 44 of the ATS 10 are shown in an exploded view to better illustrate the two separate components. For clarity, the axial direction 15 is shown as passing through the center of the nozzle 42 and the shroud 44. The shroud 44 is generally in the shape of a ring having a nozzle edge 64 and a casing edge 66. The shroud 44 may include a plurality of slots 68 leading to the nozzle edge 64. A plurality of blades 50 are circumferentially spaced about the hub 48. The blade 50 may include at least one radial protrusion 62 at the distal end or tip of the blade 50. The protrusion 62 mates with a corresponding slot 68. When the air starter is assembled, the protrusion 62 is received in a corresponding one of the plurality of slots 68.

[0029] The benefit of the protrusion 62 being accommodated in a plurality of grooves 68 is that physical interference is generated between the protrusion 62 and the groove 68. This physical interference limits the relative axial movement between the nozzle and the shroud. The physical interference also limits the relative rotational movement of the nozzle 42 and the shroud 44. Therefore, assuming that the shroud 44 is substantially fixed to resist axial movement and / or rotational movement, the nozzle is fixed accordingly. Therefore, during operation, when high-pressure air flows through the nozzle 42, the load from the high-pressure air acting on the nozzle will not cause the nozzle to move axially or rotate due to the physical interference provided by the protrusion 62 and the groove 68. Utilizing this physical interference structure, the nozzle 42 and the shroud 44 can be manufactured separately, unlike in the past. By manufacturing these parts separately and then connecting these parts together, the cost of manufacturing these parts is much lower, especially because these parts used to be single castings, which is complex and relatively expensive. Another benefit of the two-piece structure is that the nozzle 42 and the shroud 44 can be made of different materials. Another benefit is that the assembly of these parts is now a simple drop in or build up assembly.

[0030] Reference Figure 4A , a portion of the nozzle 42 and shroud 44 of the ATS 10 are shown assembled to better illustrate the operational state of the protrusion 62 and the slot 68. In the assembled and / or operational state, the protrusion 62 is received in the slot 68, and the nozzle 42 is physically coupled to the shroud 44, creating a first physical interference 70. In other variations, the shroud 44 may have an element similar to the protrusion 62, and the bucket 50 may have a complementary element similar to the slot 68. In both variations, when the protrusion 62 is trapped in the slot 68, the nozzle 42 is able to effectively "grab" the shroud 44, thereby creating the first physical interference 70. In another variation, the bucket 50 may lack the protrusion 62, and the shroud 44 may lack the slot 68. Therefore, the first physical interference 70 may include contact between the nozzle 42 and the shroud 44 and / or contact between the housing 30, the nozzle 42, and / or the shroud 44.

[0031] The reception of the protrusion 62 in the groove 68 need not be perfect. There may be some slope or tolerance between the protrusion 62 and the groove 68, which may manifest itself as a space between the protrusion 62 and at least a portion of the groove 68. One example of such a space is a gap 72. The gap 72 may compensate for thermal expansion or contraction of the protrusion 62 and / or the groove 68 relative to each other, particularly when the nozzle 42 and the shroud 44 are made of different materials having different thermal expansion rates. The thermal expansion or contraction of the protrusion 62 may include an interference fit, also known as a press fit, a friction fit, a shrink fit, etc., between the protrusion 62 and the groove 68 during high inlet air pressures.

[0032] Figure 4B Shown along Figure 2 FIG. 2 is an enlarged cross-sectional view of the nozzle 42 and the shroud 44 within the housing 30 taken along line IV of FIG. The housing 30 may include a seat 74 that accommodates a portion of the nozzle 42. The seat 74 accommodates a portion of the assembled nozzle 42 and shroud 44. The seat 74 limits axial upstream movement of the nozzle 42 and the shroud 44. The seat 74 also limits radial movement of the nozzle 42 and the shroud 44. The housing may include a radial gap 75 between the nozzle 42 and the seat 74 and / or between the shroud 44 and the seat 74. The radial gap 75 provides room for tolerance variations and / or thermal expansion between the nozzle 42, the shroud 44, and the housing.

[0033] The radial clearance 75 may also allow the nozzle 42 and / or shroud 44 to be coupled to the seat 74 of the housing 30 by a shrink fit. The shrink fit may include high pressure air entering the compression region 18 and increasing the load on the nozzle 42 and shroud 44. The increased load may cause friction between the nozzle 42, shroud 44, and the housing 30, which may increase the temperature within the compression region 18. The change in temperature may allow the nozzle 42 and shroud 44 to expand or contract, limiting the radial clearance 75 within the housing 30.

[0034] The housing 30 may also include a turbine bushing 76. The turbine bushing 76 may be integral with a portion 78 of the housing 30 or, circumscribed with a portion 78 of the housing 30, thereby forming a housing bushing 80. The housing 30 may also include an axial gap or housing gap 82. The housing gap 82 is between the housing edge 66 of the shroud 44 and the housing bushing 80. At high pressures, the nozzle 42 and the shroud 44 may become unable to maintain the load from the air inlet within the high pressure compression region 18. In response to the nozzle 42 and the shroud 44 becoming unable to maintain the load, the nozzle 42 and the shroud 44 may move axially into the housing gap 82. The axial movement of the nozzle 42 and the shroud 44 may be radially aligned or contacted with the housing bushing 80, thereby eliminating the housing gap 82 and creating a second physical interference 84 within the ATS (10). The second physical interference 84 may limit the shroud 44 and the nozzle 42 from contacting the blades 40 of the turbine 46. Structures other than the turbine bushing 76 may be provided in the housing 30 to create the second physical interference 84.

[0035] Figure 4C Shown along Figure 2FIG. 1 is an enlarged cross-sectional view of another variation of the nozzle 42 and the shroud 44 within the casing 30, taken along line IV of FIG. The shroud 44 may also be integrated with the bushing 76, or circumscribed with the bushing 76, thereby forming a shroud bushing 86. The shroud bushing 86 may include a first end 88 and a second end 90. The first end 88 may be coupled to the nozzle 42 and the casing 30. The first end 88 may include a first physical interface 70. The casing gap 82 may be located between the second end 90 of the shroud bushing 86 and the casing 30. In response to the nozzle 42 and the shroud 44 becoming unable to hold a load, the nozzle 42 and the shroud 44 may move axially into the casing gap 82. The axial movement of the nozzle 42 and the shroud 44 may contact the casing 30, thereby eliminating the casing gap 82 and generating a second physical interference 84 within the ATS (10). The second physical interference 84 may limit the nozzle 42 and the shroud bushing 86 from contacting the blades 40 of the turbine 46.

[0036] Figure 5 is based on Figure 1 Schematic diagram of a turbine engine and ATS 10, wherein the turbine engine 14 may be in a vehicle or structure 100. As a non-limiting example, the vehicle or structure 100 may be any vehicle on land, at sea, or in the air. In addition, the vehicle or structure 100 may be, but is not limited to, a ship power plant, a wind turbine, or a small power plant. Further contemplated, the turbine engine may be any engine used with a turbine with the ATS 10, or a generator required by the vehicle or structure 100.

[0037] Some advantages associated with the present disclosure discussed herein may include, but are not limited to, eliminating causes of high stress and deformation of the nozzle by not requiring a high degree of fit between the vanes and the main airflow inlet of the nozzle. Another advantage is that a wide selection of materials can be used to form the nozzle, shroud, and main airflow inlet since the parts do not need to be cast as one component. Advantages of eliminating the need for casting may include reduced thickness and high material strength. In addition, the present disclosure reduces component failures by allowing the nozzle to maintain high pressure to achieve better performance in a reliable, low-cost solution.

[0038] In addition to those shown in the above-mentioned figures, the present disclosure also contemplates many other possible configurations. To the extent not yet described, the different features and structures of each aspect may be used in combination with other features and structures as desired. The inability to describe a feature in all aspects does not mean that it cannot be explained, but is explained for the sake of brevity of description. Therefore, whether or not the new aspect is explicitly described, the various features of the different aspects may be combined and matched as needed to form the new aspect. The combination or arrangement of the features described herein is covered by the present disclosure.

[0039] Further aspects of the invention are provided by the subject matter of the following clauses:

[0040] An air starter comprises: a housing defining an interior having a main inlet and a main outlet to define a main airflow path from the main inlet to the main outlet; a nozzle located in the interior and having circumferentially spaced blades; a shroud located in the interior and circumscribing at least a portion of the blades; a turbine located in the interior, rotatable about an axis of rotation, and having circumferentially spaced blades; a physical interference, at least one of between the nozzle and the shroud or between the shroud and the housing; and wherein, by the physical interference, the nozzle is axially constrained in downstream movement along the main airflow path.

[0041] The air starter of any preceding clause, wherein the physical interference comprises a first physical interference between the nozzle and the shroud and a second physical interference between the shroud and the housing.

[0042] The air starter of any preceding clause, wherein the first physical interference comprises a protrusion on at least one of the bucket or the shroud and a slot on the other of the bucket or the shroud to receive the protrusion.

[0043] An air starter as in any preceding clause wherein the protrusion is on the blade and the slot is in the shroud.

[0044] The air starter of any preceding clause, further comprising a turbine liner circumscribing at least a portion of the casing and having a portion radially aligned with a portion of the shroud to form a second physical interference.

[0045] An air starter as claimed in any preceding clause, wherein the housing comprises an inlet housing defining a main inlet and a turbine housing defining a main outlet, and the turbine liner is part of the turbine housing.

[0046] An air starter as in any preceding clause, wherein the inlet housing includes a seat that receives at least a portion of the nozzle.

[0047] An air starter as claimed in any preceding clause, wherein the nozzle is radially constrained by the seat but not axially constrained by the seat.

[0048] An air starter as claimed in any preceding clause wherein a radial gap is provided at least one of between the nozzle and the seat or between the shroud and the seat.

[0049] An air starter as claimed in any preceding clause wherein an axial clearance is provided between the shroud and the turbine liner.

[0050] An assembly includes: a housing defining an interior having a main inlet and a main outlet to define a main airflow path from the main inlet to the main outlet; a nozzle located in the interior and having circumferentially spaced vanes; a shroud located in the interior and circumscribing at least a portion of the vanes; a physical interference, at least one of between the nozzle and the shroud or between the shroud and the housing; and, wherein, by the physical interference, the nozzle is axially constrained in downstream movement along the main airflow path.

[0051] The assembly of any preceding clause, wherein the physical interference comprises a first physical interference between the nozzle and the shroud and a second physical interference between the shroud and the housing.

[0052] The assembly of any preceding clause, wherein the first physical interference comprises a protrusion on at least one of the bucket or the shroud and a slot on the other of the bucket or the shroud that receives the protrusion.

[0053] The assembly of any preceding clause, wherein the protrusion is on the bucket and the slot is in the shroud.

[0054] The assembly of any preceding clause, further comprising a bushing having a portion radially aligned with a portion of the shroud to form a second physical interference.

[0055] The assembly of any preceding clause, wherein the housing comprises an inlet housing defining a primary inlet and an outlet housing defining a primary outlet, and the liner is part of the outlet housing.

[0056] The assembly of any preceding clause, wherein the inlet housing includes a seat that receives at least a portion of the nozzle.

[0057] An assembly of any preceding clause wherein the nozzle is radially constrained by the seat but not axially constrained by the seat.

[0058] The assembly of any preceding clause, wherein a radial gap is provided at least one between the nozzle and the seat or between the shroud and the seat.

[0059] An assembly of any preceding clause, wherein an axial clearance is provided between the shroud and the bushing.

[0060] This written description uses examples to describe aspects of the disclosure described herein, including the best mode, and also enables those skilled in the art to practice aspects of the disclosure, including making and using any device or system and performing any combined methods. The patentable scope of aspects of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements that do not differ significantly from the literal language of the claims.

Claims

1. An air starter, characterized in that: include: a housing defining an interior having a primary inlet and a primary outlet to define a primary airflow path from the primary inlet to the primary outlet; a blade located in the interior and rotatable about an axis of rotation; A shield is located in the interior, the shield having: a radially inner surface at least partially defining a corresponding portion of the primary airflow path; a nozzle edge extending from the radially inner surface; and a plurality of grooves, the plurality of grooves being arranged along an edge of the nozzle; and a plurality of vanes at least partially surrounded by the shroud, the plurality of vanes being disposed within the primary airflow path, and each vane of the plurality of vanes having: a corresponding outer wall extending between the root and the tip and between the leading edge and the trailing edge, wherein the plurality of buckets have: a projection defining a radially raised portion of the tip relative to the axis of rotation, the projection extending through a corresponding slot in the radially inner surface and into the shroud such that the blade moves axially downstream along the primary airflow path through the shroud with the remainder of the tip facing the radially inner surface of the shroud; Wherein a physical interference is formed between the protrusion and the shroud such that the plurality of vanes are axially constrained in downstream movement along the primary airflow path by the physical interference.

2. The air starter according to claim 1, characterized in that: Also included is a turbine liner surrounding at least a portion of the blades, the turbine liner having a portion radially aligned with a portion of the shroud.

3. The air starter according to claim 2, characterized in that: The housing includes an inlet housing defining the main inlet and a turbine housing defining the main outlet, and the turbine liner is a part of the turbine housing.

4. The air starter according to claim 3, characterized in that: The inlet casing includes seats that receive at least respective portions of the plurality of buckets.

5. The air starter according to claim 3, characterized in that: An axial gap is provided between the shroud and the turbine bushing.

6. The air starter according to claim 1, characterized in that: The shroud terminates upstream of the blades.

7. A component, characterized in that include: a housing defining an interior having a primary inlet and a primary outlet to define a primary airflow path from the primary inlet to the primary outlet; a nozzle located in the interior and having circumferentially spaced vanes, wherein the circumferentially spaced vanes include: a respective outer wall extending between the leading edge and the trailing edge and between the root and the tip; and a respective projection extending from an upstream end axially aligned with the leading edge at the tip and a downstream end axially upstream of the trailing edge at the tip; and a shroud positioned within the interior and surrounding at least a corresponding portion of the bucket; Wherein a physical interference is formed between the protrusion and the shroud such that the nozzle is axially constrained in downstream movement along the primary airflow path by the physical interference.

8. The assembly according to claim 7, characterized in that The corresponding protrusions are received in slots provided on the shield.

9. The assembly according to claim 8, characterized in that Also included is a bushing having a portion radially aligned with a portion of the shroud.

10. The assembly according to claim 9, characterized in that The housing includes an inlet housing defining the main inlet and an outlet housing defining the main outlet, and the liner is a part of the outlet housing.

11. The assembly according to claim 10, characterized in that The inlet housing includes a seat that receives at least a portion of the nozzle.

12. The assembly according to claim 11, characterized in that At least one of a radial gap is provided between the nozzle and the seat or a radial gap is provided between the shroud and the seat.

13. The assembly according to claim 11, characterized in that An axial gap is provided between the shield and the bushing, and the bushing restricts axial movement of the shield within the axial gap.

14. The assembly according to claim 7, characterized in that The shroud includes a plurality of slots spaced apart about the shroud, each slot of the plurality of slots corresponding to a single respective protrusion of the circumferentially spaced buckets.

15. The assembly according to claim 7, characterized in that The respective protrusion is disposed along a portion of the tip where the tip intersects the leading edge of the bucket.

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