EP0623189A1 - Coolable outer air seal assembly for a turbine. - Google Patents

Abstract

An external air seal assembly (44) cooled by combined shock and film cooling. Several construction features are shown including the spraying of coolant against a radially outer side of an outer air seal assembly (44) and the ejection of a film of coolant onto a radially inner surface ( 52) of the outer air seal assembly (44). In a particular embodiment, an external air seal assembly (44) consists of a substrate (54), an apertured layer (56) placed outside the substrate (54), and a layer of abradable material (58) extending radially inward from the substrate (54) and defining a flow surface (52). The cooling fluid is routed into a cavity (66) through openings (72), and strikes the outer surface of the substrate (54). A plurality of film cooling ports (70), which extend through the substrate (54) and the abradable layer (58), provide fluid communication between the cavity (66) and the flow path (14). Said cooling orifices (76) are oriented so as to eject the cooling liquid in film form onto the flow surface (52). In another embodiment, the cavity (66) includes a longitudinally extending chamber (112) located in a flared portion (108) of the air seal (46) to improve cooling and reduce the stresses inside it.

Description

Description

Coolable Outer Air Seal Assembly For a Turbine

This is a File Wrapper Continuation-In-Part

Application of Serial No. 07/980,815 filed November 24, 1992, now abandoned.

Technical Field This invention relates to turbomachines, and more particularly to outer air seal assemblies used in turbines.

Background of the Invention A typical turbomachine such as an axial flow gas turbine engine has an annular flowpath for conducting working fluid sequentially through a compressor section, a combustion section, and a turbine section. The compressor section includes a plurality of rotating blades which add energy to the working fluid. The working fluid exits the compressor section and enters the combustion section. Fuel is mixed with the compressed working fluid and the mixture is ignited to add more energy to the working fluid. The resulting products of combustion are then expanded through the turbine section. The turbine section includes another plurality of rotating blades which extract energy from the expanding fluid. A portion of this extracted energy is transferred back to the compressor section via a rotor shaft interconnecting the compressor section and turbine section. The remainder of the energy extracted may be used for other functions. The work output of the gas turbine engine is dependant upon many factors. Among these factors is the heat generated during the combustion process. The amount of heat generation is controlled by the fuels used and the fuel/air ratio but is limited by the allowable temperature within the turbine section. In modern gas turbine engines, working fluid temperatures beyond the melting temperature of the turbine materials are achieved by directing cooling fluid to the turbine section. Typically this cooling fluid is comprised of a portion of working fluid that exits the compressor section and bypasses the combustion process.

Another factor related to the work output is the efficiency of the energy transfer between the products of combustion and the turbine rotor blades. The turbine section includes arrays of aerodynamically shaped vanes upstream of each array of rotor blades to optimize the orientation of the working fluid prior to engagement with the rotor blades. The turbine rotor blades have airfoil portions aerodynamically shaped to efficiently engage the working fluid. The rotor blade includes a platform to provide a radially inner flow surface and the turbine section includes an outer air seal assembly to provide a radially outer flow surface. The combination of these two flow surfaces confine the flow of working fluid to the airfoil portion of the rotor blade.

The outer air seal assembly typically includes a plurality of arcuate segments arranged to form an annular structure extending about the longitudinal axis of the gas turbine engine. An array of rotor blades rotates within the confines of one of the outer air seal assemblies. Each rotor blade includes a radially outer tip which, during rotation of the rotor assembly, passes within close radial proximity to or in contact with the outer air seal assembly.

Contact between the tips of the blades and the flow surface of the outer air seal assembly may occur due to the proximity required to confine the flow of working fluid to the airfoil portion. To prevent damage to the rotor blades during contact, the tips of the rotor blades are coated with an abrasive material and the outer air seal assembly has a layer of abradable material over its flow surface. Therefore, as the tip passes over the flow surface any contact will result in particles of the abradable material being dislodged rather than the blade being worn or damaged.

The segments of the outer air seal assembly are exposed to the hot working fluid. Typically, segments are cooled to prevent overheating of the substrate material. Cooling fluid is flowed radially inward through the stator assembly and over the radially outer surfaces of the substrate. This cooling fluid then flows radially inward between adjacent segments and exits out into the flowpath.

One type of abradable coating used in outer air seal assemblies is formed from ceramic materials. Ceramic materials are useful because of their ability to withstand high temperatures such as those found in turbines. Unfortunately, there have been difficulties associated with bonding the ceramic coating to the metal substrates because of thermal stresses caused by having two materials with different rates of thermal expansion exposed to a very hot environment. This is especially true for the first stages of the turbine, which are exposed to the highest temperature working fluid, and has lead to cracking and debonding of the ceramic coating from the substrate.

The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop an outer air seal assembly with minimal cracking and/or debonding of an abradable layer from a seal segment.

Disclosure of the Invention

According to the present invention, an outer air seal assembly includes a plurality of segments having a substrate, an abradable layer, impingement cooling means for cooling the substrate, and film cooling means for flowing a buffer of cooling fluid over the abradable layer. According further to the present invention, the impingement cooling means includes a cover plate positioned radially outward of the substrate defining a cavity therebetween and having a plurality of apertures. Cooling fluid flows through the apertures and impinges upon a surface of the substrate facing into the cavity. According further still, the film cooling means includes a plurality of cooling holes extending through the substrate and abradable layer and being angled to eject a film of fluid over the surface of the abradable layer. The cooling holes are aligned with the blade passing direction to eject cooling fluid in the same direction as a passing blade tip.

According to a particular embodiment, an outer air seal segment includes a pair of axially spaced cavities extending between a substrate and a cover- and means to generate a pressure differential between the cavities, with higher pressure in the first cavity than in the second cavity, the cover has a first and second plurality of apertures. The first cavity is in fluid communication with a source of cooling fluid through the first plurality of apertures. The second cavity is downstream of the first cavity and is in fluid communication with the same source of cooling fluid through the second plurality of apertures. In this particular embodiment, the pressure differential means is defined by the first plurality of apertures having a diameter D, greater than a diameter D₂ of the second plurality of apertures. Cooling fluid exits the cavities through cooling holes extending through the substrate and an abradable ceramic layer and through intersegment holes. The cooling holes are angled relative to a radial axis and have a flared portion to diffuse a film of cooling fluid over the abradable layer. The cooling holes are laterally aligned with the blade passing direction to eject fluid into the direction of a passing blade tip. The intersegment holes eject cooling fluid into the circumferential space between adjacent segments to provide convective cooling of the segment, impingement cooling of an adjacent segment, and to purge the intersegment space of working fluid.

Where, for structural reasons, the air seal segment includes an enlarged end portion, such as the upstream end portion, the cavity adjacent thereto may include a longitudinally extending chamber disposed in the enlarged end portion. The chamber passes cooling fluid to the end portion for improved cooling thereof and has the additional benefit of providing stress relief in that end portion.

A principle feature of the present invention is the combination of impingement cooling means and film cooling means in an outer air seal segment. Another feature is the alignment of the cooling holes with the blade passing direction. A feature of a particular embodiment is the pair of cavities defined by the impingement cover and the substrate. Another feature of the particular embodiment is the means to generate a pressure differential between the cavities. A further feature is the angle and shape of the cooling holes. A still further feature is the intersegment holes disposed along the lateral edge of the segment. An advantage of the present invention is the level of thermal stress within the segment as a result of the impingement cooling and film cooling. The impingement cooling maintains the substrate within an acceptable temperature range for the substrate material. The film cooling generates a buffer of cooling fluid between the abradable layer and the working fluid to cool the abradable layer. The two cooling means in conjunction minimize the temperature gradients between the layer of abradable material and the substrate to minimize thermal stresses between the two materials. Another advantage is the expected useful life of the segment as a result of angling the holes relative to the radial axis and aligning the cooling holes with the blade passing direction. Angling the cooling holes enlarges the opening in the abradable layer to reduce the likelihood of plugging the cooling hole with particles disclosed as a result of abrasive contact between the blade tip and segment. Aligning the cooling holes with the blade passing direction further minimizes the likelihood of plugging the cooling hole with dislodged particles. An advantage of the particular embodiment is the availability of high pressure film cooling at the upstream, high pressure end of the segment as a result of the multiple cavities and pressure differential means. Another advantage of the particular embodiment is the level of thermal stress near the lateral edges of the segment as a result of the intersegment cooling. The intersegment cooling holes provide convective cooling and impingement cooling to the lateral edges of the segments. In addition, the cooling fluid flowing into the intersegment region purges this region to block the ingestion of hot working fluid between adjacent segments. The foregoing and other objects, features and advantages of the present invention become more apparent in light of the following detailed description of the exemplary embodiments thereof, as illustrated in the accompanying drawings.

Brief Description of the Drawings

FIG. 1 is a cross sectional side view of a gas turbine engine.

FIG. 2 is a sectional side view of an outer air seal assembly, a turbine rotor assembly, and an upstream and downstream vane assembly.

FIG. 3 is a radially outward view of a single outer air seal segment.

FIG. 4 is a radially inward view of a single outer air seal segment.

FIG. 5 is a partially sectioned side view taken along line 5-5 of FIG. 3.

FIG. 6 is an axially upstream view of the outer air seal segment with arrows showing the direction of flow of the film cooling.

FIG. 7a is a sectional view of a film cooling hole taken along line 7-7 of FIG. 5. FIG. 7b is an illustration of the film cooling hole after a build-up of dislodged abradable particles.

FIG. 8 is a partially sectioned side view similar to Fig. 5, but illustrating an alternate embodiment of the invention.

Best Mode for Carrying Out the Invention

Illustrated in Fig. 1 is an axial flow gas turbine engine 12 shown as an example of a typical turbomachine. The gas turbine engine includes an axially directed flowpath 14, a compressor 16, a combustor 18, and a turbine 22. The compressor includes a plurality of compressor blades 24 which extend through the flowpath and engage working fluid in the flowpath. The engagement between the working fluid and the compressor rotor blades transfers energy to the working fluid. Working fluid exits the compressor and enters the combustor where it is mixed with a supply of fuel. The mixture is ignited in the combustor to add more energy to the working fluid. The products of the combustion are then expanded through the turbine. The turbine includes a plurality of axially alternating stages of turbine vanes 26 and turbine rotor blades 28. The turbine rotor blades extend through the flowpath and engage the working fluid to transfer energy from the working fluid to the turbine rotor blades. A portion of this energy transferred to the turbine rotor blades is transferred to the compressor section via a pair of rotor shafts 32 interconnecting the turbine and compressor.

As shown in Fig. 2, each stage of turbine rotor blades 28 is axially downstream of a stage of turbine vanes 26. Each turbine rotor blade includes an airfoil portion 34 having a radial tip 36 and a platform 38 disposed radially inward of the airfoil portion. The platform includes a flow surface 42 which faces radially outward towards the flowpath. The platform flow surface discourages working fluid within the flowpath from flowing radially inward. Radially outward of the airfoil tip is an outer air seal assembly 44. The outer air seal assembly includes a plurality of segments 46 spaced circumferentially about the turbine rotor blades. The plurality of segments define an annulus having a flow surface 52 which faces radially inward towards the flowpath. The outer air seal flow surface is in radial proximity to the airfoil tip and discourages working fluid from flowing radially outward.

The segment is illustrated in more detail in Figs. 3-6. Each segment includes a substrate 54, an impingement cover 56, and a layer of abradable ceramic material 58. The substrate includes a plurality of hooks 62 which engage stator structure 64 within the turbine to retain each of the segments. Ceramic material is suggested for the abradable layer because of its insulating characteristics, although non-ceramic materials may also be applicable.

The impingement cover is disposed on the outward side of the substrate. The impingement cover and substrate define a first cavity 66 and a second cavity 68 disposed axially downstream of the first cavity. The impingement cover, as shown more clearly in Figs. 4 and 5, includes a first plurality of impingement holes 72 and a second plurality of impingement holes 74. The first plurality of impingement holes provide fluid communication between the first cavity and the source of cooling fluid. The second plurality of impingement holes provide fluid communication between the second cavity and the source of cooling fluid.

The segment includes means to generate a pressure differential between the two cavities, with higher pressure in the first cavity than in the second cavity. As shown in FIG. 4, the means is defined by having different diameter impingement holes. Each of the first plurality of cooling holes has a diameter D_r Each of the second plurality of cooling holes has a diameter D₂, wherein D₂ is less than D The larger diameter cooling holes permit a greater flow of cooling fluid into the first cavity. Since the cavities have approximately the same number of film cooling holes, and they are approximately the same size, the difference in impingement hole diameter generates a higher pressure in the first cavity. Although described and shown at thus, other means to generate a pressure differential may be used, such as a greater quality of impingement holes in the first cavity.

A plurality of film cooling holes 76 extend through the substrate and abradable layer as shown in Figs. 3 and

5. A first plurality of film cooling holes 78 extends between the first cavity and the flowpath. A second plurality of film cooling holes 82 extends between the second cavity and flowpath. The film cooling holes are closely spaced over the entire surface of the abradable layer to provide optimal coverage taking into account the engine efficiency costs of the cooling fluid. The broad extent of the coverage of film cooling holes results in a uniform film of cooling fluid over most of the abradable layer flow surface.

Each of the film cooling holes is shaped and oriented as shown in Fig. 7a. Each film cooling hole includes a constant diameter portion 84 and a flared portion 86. The flared portion opens into the flow surface and provides diffusion of the cooling fluid flowing through the film cooling hole. By diffusing the cooling fluid before ejecting it over the flow surface, the area of the cooling fluid is increased and the velocity exiting the film cooling hole is reduced. Increasing the area of the ejected fluid correspondingly increases the coverage each film cooling hole provides over the flow surface. Reducing the velocity of the cooling fluid ejected from the film cooling hole encourages the ejected fluid to remain attached to the flow surface.

Each of the film cooling holes is canted at an angle a relative to a radial axis 88 of the gas turbine engine. Angling the holes results in an elliptical opening in the flow surface of the abradable layer. The elliptical opening is less likely to become closed due to particles deposited in the opening by flow over the flow surface. In addition, angling the cooling holes relative to the radial axis ejects cooling fluid tangentially over the flow surface as shown by arrows 90 to further encourage the development of a film of cooling fluid.

The majority of the film cooling holes are oriented such that the direction of cooling fluid ejection is laterally aligned with the blade passing direction is shown by arrow 92. Aligning the cooling holes as such results in film cooling holes which are less likely to become blocked by dislodged particles of the abradable layer. Each segment includes a plurality of intersegment cooling holes 94. The intersegment cooling holes provide fluid communication between the cavities and the lateral space 96 between adjacent segments. The cooling fluid flows through the intersegment holes to provide convective cooling of the substrate in the region of the intersegment cooling holes. Cooling fluid exiting the intersegment cooling holes (shown by arrows 96) is impinged upon the lateral edge 102 of the adjacent segment to provide cooling of the adjacent segments lateral edges. After the impingement occurs, the cooling fluid then flows radially inward between the adjacent segments and out into the flowpath. Flowing cooling fluid into the intersegment space provides means to purge the space and prevents the ingestion of working fluid into the intersegment space.

During operation, hot working fluid passes over the flow surface of the outer air seal and heats the outer air seal assembly.. Cooling fluid flows radially inward into the space radially outward of the impingement cover. This cooling fluid flows through the impingement cooling holes and into the cavities. The internal pressure of the first cavity is greater than the internal pressure of the second cavity. As a result of the larger impingement cooling holes. Cooling fluid within the cavities then flows through the film cooling holes and exits the film cooling holes to form a film or buffer of cooling fluid over the flow surface of the segment. The pressure on the abradable layer caused by the working fluid is greatest at the upstream end of the abradable layer and decreases towards the downstream end of the abradable layer. Having separated cavities that are axially spaced within the segment provides means to have high pressure cooling fluid flowing through the upstream film cooling holes where it is needed and lower pressure cooling fluid flowing through the downstream holes. This ensures that an adequate supply of film cooling fluid is provided over the axial extent of the flow surface. A portion of the cooling fluid within the cavities flows to the intersegment cooling holes to provide convective cooling to the segment, impingement cooling to an adjacent segment, and to purge the intersegment space of hot working fluid.

Abrasive contact between the airfoil tip and the abradable layer may result in (dislodged particles of the abradable layer) . These particles 104 may be deposited within the film cooling holes and result in a reduction in the flow area of the film cooling hole. As shown in Figs. 7a and 7b, however, the angle, orientation, and shape of the film cooling holes make this event less likely than a radially oriented, constant diameter cooling hole. Since the cooling holes are aligned with the blade passing direction, and since the cooling holes are angled relative to a radial axis, the effective diameter of the cooling is maximized. This effect minimizes the likelihood of dislodged particles closing the film cooling holes. In addition, the larger opening resulting from the flared portion and the angle of the film cooling hole relative to the radial axis also reduces the likelihood of the film cooling hole becoming completely blocked or plugged. Reducing the likelihood of blocked film cooling holes increases the life expectancy of the segment by ensuring that, even after some degradation of the abradable layer, cooling fluid will continue to flow through the film cooling holes to provide cooling of the segment. Referring to Fig. 8, an alternate embodiment of seal segment 46 is shown. For various structural reasons which need not be set forth for purposes hereof, this seal segment includes an enlarged upstream end portion 108 provided with hook 110 similar to hook 62 disposed at the downstream end of the segment and described hereinabove. Like hook 62, hook 110 is captured within a slot in stator structure 64, for mechanical retention of the seal segment. For purposes of cooling enlarged end portion 108, first cavity 66 is provided with a chamber 112 extending longitudinally into end portion 108. Chamber 112 functions as a passage for channeling cooling air from cavity 66 to end portion 108. It will be appreciated that such provision of cooling air within the interior of end portion 108 will reduce thermally induced stresses therewithin and enhance the thermal isolation of hook 110 (and the contiguous stator structure) from the extremely hot working fluid flowing past abradable ceramic material 58. Such a reduction in thermal stress in end portion 108 enhances the reliability and performance of the seal and is accompanied by increased flexibility of end portion 108 due to the void therein defined by chamber 112. This increased flexibility and thermal stress reduction results in a minimization of seal distortion (maximization of seal concentricity) for improved performance of the gas turbine engine. The cooling scheme illustrated and described with respect to the seal segments illustrated in Figs. 2-7 is equally well suited for the seal segment illustrated in Fig. 8.

Claims

ClaimsWhat is claimed is:

1. An outer air seal assembly (44) for a turbomachine (12), the turbomachine (12) including a flowpath (14), a rotor blade assembly being rotatable about a longitudinal axis of the turbomachine (12) , and a source of cooling fluid, the rotor blade assembly including a plurality of blades (28) , each of the blades (28) including a radially outward tip (36) , the outer air seal assembly (44) disposed radially outward of the rotor blade assembly, wherein the outer air seal assembly (44) is characterized by including: a plurality of circumferentially spaced arcuate segments (46) forming an annulus axially aligned with the rotor blade assembly and defining a radially outer flow surface (52) for the flowpath (14) , the flow surface (52) in radial proximity to the radial tips (36) , wherein each segment (46) includes a substrate (54) including means to retain the segment, an impingement cover (56) disposed radially outward of the substrate (54) and including a plurality of apertures (72) extending radially through the impingement cover (56) , a first cavity (66) defined by a separation between the impingement cover (56) and the substrate (54) , a layer of abradable material (58) radially inward of the substrate (54) to form the radially outer flow surface (52) , and wherein the apertures (72) provide fluid communication between the cavity (66) and the source of cooling fluid such that cooling fluid passing through the apertures (72) impinges upon the substrate (54) , wherein the substrate (54) and abradable layer (58) include a plurality of cooling holes (76) permitting fluid communication between the first cavity (66) and the flow path (14) , the cooling holes (76) having an exit in the abradable layer (58) , wherein the cooling holes (76) are angled relative to a radial axis of the turbomachine (12) such that the cooling fluid exiting the cooling hole (76) is urged to form a film of cooling fluid over the flow surface (52) .

2. The outer air seal assembly (44) according to Claim 1, 8, 9 or 10, wherein the cooling holes (76) are aligned with the direction of movement of the rotor blade (28) relative to the segment (46) .

3. The outer air seal assembly (44) according to Claim 1, 2, 7, 8, 9 or 10, wherein each cooling hole (76) has a downstream end and includes a flared portion (86) at the downstream end, the flared portion (86) diffusing the cooling fluid passing through the cooling hole (76) .

4. The outer air seal assembly according to Claim 1, 2, 3, 7, 8, 9 or 10, further including a plurality of intersegment cooling holes (94) disposed along a lateral edge of the segment (46) , the intersegment cooling holes (94) providing fluid communication between the cavity

(66) and the lateral space (96) between adjacent segments (46) , wherein the intersegment cooling holes (94) provide convective cooling to the lateral edge of the segment (46) , wherein the intersegment cooling holes (94) direct cooling fluid towards the adjacent segment (101) provide impingement cooling of a lateral edge (102) of the adjacent segment (101) , and wherein the cooling fluid exiting the intersegment cooling holes (94) flows between the segments (46,101) such that the lateral space (96) between the adjacent segments (46,101) is purged.

5. The outer air seal assembly (44) according to Claim 1, 2, 3, 4, 7, 8, 9 or 10, further including a second cavity (68) extending between the cover (56) and the substrate (54) , the second cavity (68) being downstream of the first cavity (66) , a second plurality of apertures (74) extending through the cover (56) and providing fluid communication between the second cavity (68) and the source of cooling fluid, and means to generate a pressure differential between the first cavity (66) and the second cavity (68) .

6. The outer air seal assembly (44) according to Claim 5, wherein the pressure differential means is defined by each of the first plurality of apertures (72) having a diameter D.,, each of the second plurality of apertures (74) having a diameter D₂, and wherein D₁ > D₂ such that the internal pressure of the first cavity (66) is greater than the internal pressure of the second cavity (68) .

7. An outer air seal assembly (44) for a turbomachine (12) , the turbomachine (12) including a flowpath (14) , a rotor blade assembly being rotatable about a longitudinal axis of the turbomachine (12) , and a source of cooling fluid, the rotor blade assembly including a plurality of blades (28) , each of the blades (28) including a radially outward tip (36) , the outer air seal assembly (44) disposed radially outward of the rotor blade assembly, wherein the outer air seal assembly (44) is characterized by including: a plurality of circumferentially spaced arcuate segments (46) forming an annulus axially aligned with the rotor blade assembly and defining a radially outer flow surface (52) for the flowpath (14) , the flow surface (52) in radial proximity to the radial tips (36) , wherein each segment (46) includes a substrate having at least one enlarged end portion (108) and including means to retain the segment an impingement cover (56) disposed radially outward of the substrate (54) and including a plurality of apertures (72) extending radially through the impingement cover (56) , a first cavity (66) defined by a separation between the impingement cover (56) and the substrate (54) and having a longitudinally extending chamber (112) disposed in said enlarged end portion, a layer of abradable material (58) radially inward of the substrate (54) to form the radially outer flow surface (52) , and wherein the apertures (72) provide fluid communication between the cavity (66) and the source of cooling fluid such that cooling fluid passing through the apertures (72) impinges upon the substrate (54) , wherein the substrate (54) and abradable layer (58) include a plurality of cooling holes (76) permitting fluid communication between the first cavity (66) and the flow path (14) , the cooling holes (76) having an exit in the abradable layer (58) , said chamber (112) providing a passage for channeling cooling air to said enlarged end portion (108) as well as reducing internal stresses therewithin.

8. The outer air seal assembly according to Claim 7, wherein the cooling holes (76) are angled relative to a radial axis of the turbomachine (12) such that the cooling fluid exiting the cooling hole (76) is urged to form a film of cooling fluid over the flow surface (52) .

9. A segment (46) for an outer air seal assembly (44) of a turbomachine (12) , the turbomachine (12) including a flowpath (14) oriented about a longitudinal axis, the segment (46) having an installed condition within the turbomachine (12) wherein the segment (46) forms a portion of a radially outer flow surface of the flowpath (14) , the segment (46) being characterized by including: a substrate (54) including means to retain the segment (46) , an impingement cover (56) disposed outward of the substrate (54) and including a plurality of apertures (72) extending through the impingement cover (56) , a first cavity (66) defined by a separation between the impingement cover (56) and the substrate (54) , a layer of abradable material (58) inward of the substrate (54) to form a flow surface (52) , and wherein the apertures (72) provide fluid communication between the cavity (66) and a source of cooling fluid such that cooling fluid passing through the apertures (72) impinges upon the substrate (54), wherein the substrate (54) and abradable layer (58) include a plurality of cooling holes (76) extending therethrough, the cooling holes (76) having an exit in the abradable layer (58) , wherein in the installed condition the cooling holes (76) are angled relative to a radial axis of the turbomachine (12) such that the cooling fluid exiting the cooling hole (76) is urged to form a film of cooling fluid over the flow surface (52) .

10. A segment (46) for an outer air seal assembly (44) of a turbomachine (12), the turbomachine (12) including a flowpath (14) oriented about a longitudinal axis, the segment (46) having an installed condition within the turbomachine (12) wherein the segment (46) forms a portion of a radially outer flow surface of the flowpath (14) , the segment (46) being characterized by including: a substrate having at least one enlarged end portion (108) and including means to retain the segment (46) , an impingement cover (56) disposed outward of the substrate (54) and including a plurality of apertures (72) extending through the impingement cover (56) , a first cavity (66) defined by a separation between the impingement cover (56) and the substrate (54) and having a longitudinally extending chamber (112) disposed in said enlarged end portion (108) , a layer of abradable material (58) inward of the substrate (54) to form a flow surface (52) , and wherein the apertures (72) provide fluid communication between the cavity (66) and a source of cooling fluid such that cooling fluid passing through the apertures (72) impinges upon the substrate (54) , wherein the substrate (54) and abradable layer (58) include a plurality of cooling holes (76) extending therethrough, the cooling holes (76) having an exit in the abradable layer (58) , said chamber (112) providing a passage for channeling cooling air to said enlarged end portion (108) as well as reducing internal stresses therewithin.