Patent Application
20060121265
The invention relates in general to turbine engines and, more specifically, to stationary airfoils in a turbine engine.
A variety of materials and construction methods have been used in connection with turbine airfoils. For example, laminated airfoil concepts are known that use monolithic ceramic materials. Reasons for using such constructions include the reduction of impact stresses, reduction of thermally induced stresses from differential cooldown rates (e.g., thin trailing edge sections versus thicker sections), and accommodation of attachment to metals. However, precise and costly machining of individual laminates preclude the viability of these concepts.
Another type of material used in connection with turbine airfoils is ceramic matrix composites (CMC). CMC includes a ceramic matrix reinforced with ceramic fibers. In one CMC airfoil construction, fabric layers are wrapped over each other so that the fibers are primarily aligned substantially parallel to the surface of the component. For a 0/90 degree fabric lay-up, the fibers in the vane would substantially be oriented parallel to the gas path around the vane and along the vane radially to the machine. Furthermore, the reinforcing fibers are continuous and form an integral shell.
CMC airfoil designs can provide advantages over the monolithic airfoils described above. For example, the higher strength and toughness of CMCs can resolve the impact and thermal stress issues associated with monolithic ceramics, and their superior strain tolerance makes them more amenable to attachment to metal structures.
While providing some advantages over monolithic ceramics, the use of CMC materials in airfoil design introduce a new set of challenges. For example, CMC materials suffer from their low interlaminar tensile and shear strengths, which present special challenges in situations where an internally cooled component, such as a turbine vane, experiences large through thickness thermal gradients and the resultant high thermal stresses. In the above-described CMC airfoil construction, high thermal gradients cause high interlaminar tension (i.e. high stresses) in the weakest direction of the CMC material, resulting in delamination of the CMC.
Prior attempts to mitigate these stresses include three dimensional fiber reinforcement and exotic cooling methods. However, these approaches carry numerous development and manufacturing disadvantages and performance penalties.
Further, prior CMC airfoil constructions pose various manufacturing challenges. For instance, current oxide CMCs exhibit anisotropic shrinkage during curing, resulting in interlaminar stress buildup for constrained geometry shapes. Further complicating matters is that non-destructive evaluation methods to discover interlaminar defects are difficult on large, complex shapes such as gas turbine vanes. In addition, dimensional control is unproven for complex shapes and may be difficult to achieve in close-toleranced parts such as airfoils. Further, achievement of target material properties in large and/or complex shapes has proved to be difficult. There are also scale-ability limitations as current processes are labor-intensive, requiring very skilled technicians to carefully hand lay-up each reinforcing layer. Conventional lay-up techniques provide low pressure containment capability for trailing edge regions. In one example, the reinforcing fabric wrapped around the pressure and suction sides of the vane meet at the trailing edge where they become tangent to each other and are bonded together in the same manner as each layer is bonded to the adjacent layer. Consequently, the trailing edge is only weakly held together and is vulnerable to the pressure of the cooling air in the trailing edge exit holes.
Thus, there is a need for a vane that can address the problems encountered in prior CMC airfoil design and construction. Specifically, there is a need for a stacked CMC laminate vane that aligns the reinforcing fibers in the anticipated direction of high thermal stresses, thereby pitting strength against stress. Ideally, the construction can allow for the inclusion of enhanced cooling and structural features.
In one respect, embodiments of the invention are directed to a ceramic matrix composite laminate. The laminate has an airfoil-shaped outer peripheral surface. In addition, the laminate has an in-plane direction and a through thickness direction; the through thickness direction being substantially normal to the in-plane direction. The laminate is made of a ceramic matrix composite (CMC) material having anisotropic properties. Specifically, the in-plane tensile strength of the laminate is substantially greater than the through thickness tensile strength of the laminate. For instance, the in-plane tensile strength can be at least three times greater than the through thickness tensile strength.
The CMC material can include a ceramic matrix hosting a plurality of reinforcing fibers therein. In one embodiment, substantially all of the fibers can be oriented substantially in the in-plane direction of the laminate. Further, a first portion of the fibers can extend in a first in-plane direction, and a second portion of the fibers can extend in a second in-plane direction, which can be oriented at about 90 degrees relative to the first in-plane direction.
In one embodiment, the laminate can include a series of through thickness holes extending about at least a portion of the laminate. The holes can be proximate to the outer peripheral surface. These holes can be used as cooling passages. The laminate can also include one or more through thickness cutouts so as to form ribs or spars in the laminate.
The laminate can have recesses, serrations and/or cutouts about at least a portion of the outer peripheral surface. Further, the outer peripheral surface can be tapered.
In other respects, embodiments of the invention relate to an assembly in which a plurality of airfoil-shaped laminates are radially stacked so as to define a turbine vane. The vane has an outer peripheral surface as well as an associated planar direction and radial direction. The radial direction is substantially normal to the planar direction. Each laminate is made of an anisotropic CMC material such that the planar tensile strength of the vane is substantially greater than the radial tensile strength of the vane. In one embodiment, the planar tensile strength can be at least three times greater than the radial tensile strength.
The CMC material can include a ceramic matrix and a plurality of fibers therein. In one embodiment, substantially all of the fibers are oriented substantially in the planar direction of the vane. The fibers can be arranged in any of a number of ways. For instance, the fibers can be arranged in two planar directions in the vane. For example, a first portion of the fibers can extend in a first planar direction, and a second portion of the fibers can extend in a second planar direction. The first and second planar directions can be oriented at about 90 degrees relative to each other.
In another embodiment, at least one pair of adjacent laminates in the stack can have a unidirectional fiber arrangement. The pair of laminates includes a first laminate and a second laminate. In the first laminate, substantially all of the fibers can extend in a first planar direction. In the second laminate, substantially all of the fibers can extend in a second planar direction. The first and second planar directions can be oriented at about 90 degrees relative to each other.
A vane assembly according to embodiments of the invention can include a number of features. For instance, the vane assembly can include series of radial holes extending about at least a portion of the vane. The holes can be proximate to the outer peripheral surface. Thus, a coolant can pass through the radial holes so as to cool the outer peripheral surface of the vane. Further, at least one of the plurality of laminates can include one or more radial cutouts so as to form ribs or spars in the laminate.
The plurality of laminates can be held together in several ways. For instance, at least one pair of adjacent laminates can be joined by co-processing, sintering and/or by applying bonding material between the laminate pair. In another embodiment, a fastening system can be provided for holding the plurality of laminates in radial compression. In one embodiment, the fastening system can include an elongated fastener and a retainer. The fastener can extend through a radial opening provided in the vane. At least one end of the fastener can be closed by the retainer. In some embodiments, a stiffened fastening system may be desired, for example, to minimize concerns of radial creep of the fasteners and laminates. The stiffened fastening system can include at least two tie rods extending radially through one or more openings provided in the vane. The tie rods can be joined so as to form a single rigid fastener. The ends of the tie rods can be closed by retainers so as to hold the plurality of laminates in radial compression.
The laminates can be shaped or stacked to form an irregular outer peripheral surface of the vane. For example, the plurality of laminates can include alternating large laminates and small laminates so as to form a vane having a stepped outer peripheral surface. One or more laminates can be staggered from the other laminates to form an irregular outer peripheral surface. In another embodiment, at least two of the laminates in the stack can have a tapered outer peripheral edge. In such case, the two laminates can be stacked such that the tapered edge of each laminate can extend in substantially the same direction or in substantially opposite directions. Yet another manner of forming a vane with an irregular outer peripheral surface is by providing at least one laminate with recesses, serrations, and/or cutouts about at least a portion of the outer peripheral surface of the laminate. A thermal insulating material can be applied over the outer peripheral surface of the vane. Any of the above irregular outer peripheral surfaces can, among other things, facilitate bonding of a thermal insulating material over the stepped outer peripheral surface of the vane.
If needed, the trailing edge of a vane assembly according to embodiments of the invention can be cooled. A radial coolant supply opening can be provided in the vane. Each of the laminates can have a leading edge and a trailing edge. At least one of the laminates can include a channel extending from the trailing edge and into fluid communication with the coolant supply opening.
Embodiments of the present invention address the shortcomings of earlier stacked laminate vane designs by providing a robust vane that makes use of the anisotropic strength orientations of ceramic matrix composite (CMC) materials such that the high stresses inherent in a cooled vane are aligned with the strongest material direction, while the stresses in the weakest material direction are minimized. Embodiments of the invention will be explained in the context of one possible turbine vane, but the detailed description is intended only as exemplary. Embodiments of the invention are shown in
The individual laminates 12 of the vane assembly 10 can be substantially identical to each other; however, one or more laminates 12 can be different from the other laminates 12 in the vane assembly 10. Each laminate 12 can be airfoil-shaped. The term airfoil-shaped is intended to refer to the general shape of an airfoil cross-section and embodiments of the invention are not limited to any specific airfoil shape. Design parameters and engineering considerations can dictate the needed cross-sectional shape for a given laminate 12.
Each laminate 12 can be substantially flat. Each laminate 12 can have a top surface 26 and a bottom surface 28 as well as an outer peripheral edge 30, as shown in
As will be described in greater detail below, the laminates 12 can be made of a ceramic matrix composite (CMC) material. A CMC material comprises a ceramic matrix 32 that hosts a plurality of reinforcing fibers 34. The CMC material can be anisotropic at least in the sense that it can have different strength characteristics in different directions. Various factors, including material selection and fiber orientation, can affect the strength characteristics of a CMC material.
A CMC laminate 12 having anisotropic strength characteristics according to embodiments of the invention can be made of a variety of materials, and embodiments of the invention are not limited to any specific materials so long as the target anisotropic properties are obtained. In one embodiment, the CMC can be from the oxide-oxide family. In one embodiment, the ceramic matrix 32 can be, for example, alumina. The fibers 34 can be any of a number of oxide fibers. In one embodiment, the fibers 34 can be made of Nextel™ 720, which is sold by 3M, or any similar material. The fibers 34 can be provided in various forms, such as a woven fabric, blankets, unidirectional tapes, and mats. A variety of techniques are known in the art for making a CMC material, and such techniques can be used in forming a CMC material having strength directionalities in accordance with embodiments of the invention.
As mentioned earlier, fiber material is not the sole determinant of the strength properties of a CMC laminate. Fiber direction can also affect the strength. In a CMC laminate 12 according to embodiments of the invention, the fibers 34 can be arranged to provide the vane assembly 10 with the desired anisotropic strength properties. More specifically, the fibers 34 can be oriented in the laminate 12 to provide strength or strain tolerance in the direction of high thermal stresses or strains. To that end, substantially all of the fibers 34 can be provided in the in-plane direction 14 of the laminate 12; however, a CMC material according to embodiments of the invention can have some fibers 34 in the through thickness direction as well. “Substantially all” is intended to mean all of the fibers 34 or a sufficient majority of the fibers 34 so that the desired strength properties are obtained. Preferably, the fibers 34 are substantially parallel with at least one of the top surface 26 and the bottom surface 28 of the laminate 12.
When discussing fiber orientation, a point of reference is needed. For purposes of discussion herein, the chord line 36 of the laminate 12 will be used as the point of reference; however, other reference points can be used as will be appreciated by one skilled in the art and aspects of the invention are not limited to a particular point of reference. The chord line 36 can be defined as a straight line extending from the leading edge 22 to the trailing edge 24 of the airfoil shaped laminate 12. In the planar direction 14, the fibers 34 of the CMC laminate 12 can be substantially unidirectional, substantially bidirectional or multi-directional.
In a bi-directional laminate, like the laminate 12 shown in
As noted earlier, the fibers 34 can be substantially unidirectional, that is, all of the fibers 34 or a substantial majority of the fibers 34 can be oriented in a single direction. For example, the fibers 34 in one laminate can all be substantially aligned at, for example, 45 degrees relative to the chord line 36, such as shown in the laminate 12a in
Aside from the particular materials and the fiber orientations, the CMC laminates 12 according to embodiments of the invention can be defined by their anisotropic properties. For example, the laminates 12 can have a tensile strength in the in-plane direction 14 that is substantially greater than the tensile strength in the through thickness direction 15. In one embodiment, the in-plane tensile strength can be at least three times greater than the through thickness tensile strength. In another embodiment, the ratio of the in-plane tensile strength to the through thickness tensile strength of the CMC laminate can be about 10 to 1. In yet another embodiment, the in-plane tensile strength can be from about 25 to about 30 times greater than the through thickness tensile strength. Such unequal directionality of strengths in the laminates 12 is desirable for reasons that will be explained later.
One particular CMC laminate 12 according to embodiments of the invention can have an in-plane tensile strength from about 150 megapascals (MPa) to about 200 MPa in the fiber direction and, more specifically, from about 160 MPa to about 184 MPa in the fiber direction. Further, such a laminate 12 can have an in-plane compressive strength from about 140 MPa to 160 MPa in the fiber direction and, more specifically, from about 147 MPa to about 152 MPa in the fiber direction.
This particular CMC laminate 12 can be relatively weak in tension in the through thickness direction. For example, the through thickness tensile strength can be from about 3 MPa to about 10 MPa and, more particularly, from about 5 MPa to about 6 MPa, which is substantially lower than the in-plane tensile strengths discussed above. However, the laminate 12 can be relatively strong in compression in the through thickness direction. For example, the through thickness compressive strength of a laminate 12 according to embodiments of the invention can be from about −251 MPa to about −314 MPa.
The above strengths can be affected by temperature. Again, the above quantities are provided merely as examples, and embodiments of the invention are not limited to any specific strengths in the in-plane or through thickness directions.
As noted earlier, a vane assembly 10 according to embodiments of the invention can be formed by a stack of CMC laminates 12. Up to this point, the terms “in-plane” and “through thickness” have been used herein to facilitate discussion of the anisotropic strength characteristics of a CMC laminate in accordance with embodiments of the invention. While convenient for describing an individual laminate 12, such terms may become awkward when used to describe strength directionalities of a turbine vane 10 formed by a plurality of stacked laminates according to embodiments of the invention. For instance, the “in-plane direction” associated with an individual laminate generally corresponds to the axial and circumferential directions of the vane assembly 10 in its operational position relative to the turbine. Similarly, the “through thickness direction” generally corresponds to the radial direction of the vane assembly 10 relative to the turbine. Therefore, in connection with a turbine vane 10, the terms “radial” or “radial direction” will be used in place of the terms “through thickness” or “through thickness direction.” Likewise, the terms “planar” or “planar direction” will be used in place of the terms “in-plane” and “in-plane direction.”
With this understanding, the plurality of laminates 12 can be substantially radially stacked to form the vane assembly 10 according to embodiments of the invention. The outer peripheral edges 30 of the stacked laminates 12 can form the exterior surface 20 of the vane assembly 10. As noted earlier, the individual laminates 12 of the vane assembly 10 can be substantially identical to each other. Alternatively, one or more laminates 12 can be different from the other laminates 12 in a variety of ways including, for example, thickness, size, and/or shape.
The plurality of laminates 12 can be held together in numerous manners. For instance, the stack of laminates 12 can be held together by one or more fasteners including tie rods 38 or bolts, as shown in
The fastener can be closed by one or more retainers to hold the laminate stack together in radial compression. The retainer can be a nut 42 or a cap, just to name a few possibilities. The fastener and retainer can be any fastener structure that can carry the expected radial tensile loads and gas path bending loads, while engaging the vane assembly to provide a nominal compressive load on the CMC laminates 12 for all service loads so as to avoid any appreciable buildup of interlaminar tensile stresses in the radial direction 15, which is the weakest direction of a CMC laminate 12 according to aspects of the invention. The fastener and retainer can further cooperate with a compliant fastener, such as a Bellville washer 44 or conical washer, to maintain the compressive pre-load, while permitting thermal expansion without causing significant thermal stress from developing in the radial direction 15. To more evenly distribute the compressive load on the laminates 12, the fastener and/or retainer can cooperate with a load spreading member 45, such as a washer. The load spreading member 45 can be used with or without a Bellville washer 44 or other compliant fastener.
The fastening system shown in
However, it should be noted that, in some turbines, the vane assembly 10 may only be supported at one of its radial ends 16,18. For example, the vane assembly 10 may only be supported at its radially outer end 16 by an outer shroud or platform. In such case, the vane 10 may act like a cantilevered beam, and the gas path loads can create a bending moment on the vane 10 in one or more directions, thereby subjecting the vane 10 and/or tie bolts 38 to bending stresses. Over time, such forces may cause creep in the CMC stack and/or in the tie bolts 38. As a result, there can be a reduction or loss of compressive force applied on the laminate stack 10, which, in turn, might lead to coolant losses as well as delamination. Alternative fastening systems according to embodiments of the invention can be provided to address such concerns.
One example of such a fastening system is shown in
When such a fastening system is used, the major bending loads can be carried by the stiffened structure 46, which can minimize creep-related concerns. To accommodate such a fastener 46, one or more lateral openings 50 can be provided in the laminates 12. To form the openings 50, material can be removed from a pair of adjacent laminates 12 or from a single laminate 12.
In addition or apart from using fasteners, at least some of the individual laminates 12 can also be bonded to each other. Such bonding can be accomplished by sintering the laminates or by the application of a bonding material between each laminate. For example, the laminates 12 can be stacked and pressed together when heated for sintering, causing adjacent laminates 12 to sinter together. Alternatively, a ceramic powder can be mixed with a liquid to form a slurry. The slurry can be applied between the laminates 12 in the stack. When exposed to high temperatures, the slurry itself can become a ceramic, thereby bonding the laminates 12 together.
In addition to sintering and bonding, the laminates 12 can be joined together through co-processing of partially processed individual laminates using such methods as chemical vapor infiltration (CVI), slurry or sol-gel impregnation, polymer precursor infiltration & pyrolysis (PIP), melt-infiltration, etc. In these cases, partially densified individual laminates are formed, stacked, and then fully densified and/or fired as an assembly, thus forming a continuous matrix material phase in and between the laminates.
It should be noted that use of the phrase “at least one of co-processing, sintering and bonding material,” as used herein, is intended to mean that only one of these methods may be used to join individual laminates together, or that more than one of these methods can be used to join individual laminates together. Providing an additional bond between the laminates (whether by co-processing, sintering or having bonding material between each laminate 12) is particularly ideal for highly pressurized cooled vanes where the cooling passages require a strong seal between laminates 12 to contain pressurized coolant, such as air, flowing through the interior of the vane assembly 10.
However, for designs in which little pressure is required in the vane interior, the mechanical clamping pressure of the fasteners may be sufficient by itself. For instance, during turbine operation, the outer peripheral edges 30 of the laminates 12 are typically the hottest region of a given vane cross section. Consequently, the thickness of each laminate 12 would expand at or near the outer peripheral edge 30 due to thermal expansion. Thus, the laminates 12 would primarily engage each other at or near their outer peripheral edges 30. In such case, the clamping load from the tie rods 38 would be focused greatest around the outer perimeter of the laminates 12, thereby providing sufficient mechanical sealing for low internal pressure loads.
The airfoil-shaped CMC laminates 12 according to embodiments of the invention can be made in a variety of ways. Preferably, the CMC material is initially provided in the form of a substantially flat plate. From the flat plate, one or more airfoil shaped laminates can be cut out, such as by water jet or laser cutting. Flat plate CMC can provide numerous advantages. At the present, flat plate CMC provides one of the strongest, most reliable and statistically consistent forms of the material. As a result, the design can avoid manufacturing difficulties that have arisen when fabricating tightly curved configurations. For example, flat plates are unconstrained during curing and thus do not suffer from anisotropic shrinkage strains. Ideally, the assembly of the laminates in a radial stack can occur after each laminate is fully cured so as to avoid shrinkage issues. Flat, thin CMC plates also facilitate conventional non-destructive inspection. Furthermore, the method of construction reduces the criticality of delamination-type flaws, which are difficult to find. Moreover, dimensional control is more easily achieved as flat plates can be accurately formed and machined to shape using cost-effective cutting methods. A flat plate construction also enables scaleable and automatable manufacture.
The operation of a turbine is well known in the art as is the operation of a turbine vane. During operation, a turbine vane can experience high stresses in three directions—in the radial direction 15 and in the planar direction 14 (which encompasses the axial and circumferential directions of a vane relative to the turbine). A vane according to aspects of the invention is well suited to manage such a stress field.
In the planar direction 14, high stresses can arise because of thermal gradients between the hot exterior vane surface and the cooled vane interior. The thermal expansion of the vane exterior and the thermal contraction of the vane interior places the vane in tension in the planar direction 14. However, a vane assembly 10 according to embodiments of the invention is well suited for such loads because, as noted above, the fibers 34 in the CMC are aligned in the planar direction 14, giving the vane 10 sufficient planar strength or strain tolerance. Such fiber alignment can also provide strength against pressure stresses that can occur in the turbine.
In the radial direction 15, thermal gradients and aerodynamic bending forces can subject the vane 10 to high radial tensile stresses. While relatively weak in radial tension, a vane 10 according to embodiments of the invention can take advantage of the though thickness compressive strength of the laminates 12 (that is, the radial compressive strength of the vane 10) to counter the radial forces acting on the vane 10. To that end, the vane 10 can be held in radial compression at all times by tie bolts 38 or other fastening system. As a result, radial tensile stresses on the vane 10 are minimized.
During operation, the vane assembly 10 can be exposed to high temperatures, so the vane assembly 10 may require cooling. One cooling scheme that can be used in connection with a vane assembly 10 according to aspects of the invention is shown in
The individual cooling passages 52 can be any of a number of cross-sectional shapes including, for example, circular, elliptical, elongated, polygonal and square. Preferably, the passages 52 can all be substantially identical, but one or more of the passages 52 can be different at least in terms of its geometry, size, position, and orientation through the vane 10. The passages 52 can be provided according to a pattern, regular or otherwise, or they may be provided according to no particular pattern. In one embodiment, the holes 52 can be spaced equidistantly about the vane, relative to each other and/or to the outer peripheral surface 20. The shapes and pattern of the holes 52 can be optimized for each application, if necessary, to minimize stress and to increase robustness of the design.
Coolant for the passages 52 can be routed from a high pressure air source near the outer shroud. The coolant can flow radially through the cooling passages 52 from the radial outer end 16 to the radial inner end 18. Once the coolant reaches the end of the passages 52 at the radial inner end 18 of the vane 10, the coolant can be routed to the trailing edge 24 for discharge into the gas path or it can be dumped at one or more points on the inner shroud or platform, as will be understood by one skilled in the art.
For cases where greater cooling is required at the trailing edge 24 of the vane 10, trailing edge exit passages 54 can be provided in one or more of the laminates 12, such as those shown in
The cooling passages 52,54 can be formed in a number ways including water jet cutting, laser cutting, stamping, die-cutting, drilling or any other machining operation. Alternatively, the passages 52,54 can be formed by inserting fugitive rods or pins through a semi-cured CMC plate. The fugitive rods can remain in the partially cured laminate; later, the laminate can be heated to fully cure the laminate. In such case, the fugitive material can be removed, such as by burning or melting prior to or during laminate curing, thereby leaving the passages 52,54 behind.
The stacked laminate vane design lends itself to the inclusion and implementation of various preferred features, some of which will be discussed below. For example, in some instances, it may be desirable to afford greater thermal protection for the vane assembly 10. In such case, one or more layers of a thermal insulating material or a thermal barrier coating can be applied around the outside surface of the vane 10. In one embodiment, the thermal barrier coating can be a friable graded insulation (FGI) 58, which is known in the art, such as in U.S. Pat. Nos. 6,670,046 and 6,235,370, which are incorporated herein by reference. When such the FGI 58 substantially covers at least the outer peripheral surface 20 of the vane assembly 10, the thermal gradient across the vane 10 in the planar direction 14 can be reduced.
Experience has revealed difficulty in bonding thermal insulating materials, such as FGI 58, to smooth surfaces. Therefore, one or more laminates 12 according to embodiments of the invention can include a number of features to facilitate bonding of the thermal insulating material to the outer peripheral surface 20 of the vane assembly 10. For example, the outer peripheral edge 30 of each laminate 12 can have a rough finish after it is cut from a flat plate. That is, the outer peripheral edges 30 of the laminates 12 are not substantially smooth. Further, the laminates can be stacked in a staggered or offset manner to create an uneven outer peripheral surface 20, as shown in
Alternatively or in addition to the above, the outer peripheral edges of the laminates 12 can be tapered 30T. Such tapered edges 30T can be formed when the airfoil shaped laminate 12 is cut from a flat plate. In one embodiment, the laminates 12 can be stacked such that the direction of the tapered outer peripheral edge 30 of each laminate 12 extends in substantially the same direction. For example, as shown in
Alternatively, the laminates 12 can be stacked such that, with respect to adjacent laminates, the tapered outer peripheral edges 30T extend in opposite directions. For example, as shown in
In other instances, particularly when greater bonding is required, the CMC laminates 12 can be cut at slightly different sizes so that the stacked vane 10 has a stepped outer surface 60, as shown in
A host of features can be provided in the outer peripheral surface 18 of one or more laminates 12 to facilitate bonding of a thermal insulating material to the outer peripheral surface 20 of the vane assembly 10, as shown in
The recesses 100 can be provided about a portion of the outer peripheral edge 30 of a laminate 12 or about the entire periphery 30 of the laminate 12. In addition, the recesses 100 can be provided at regular or irregular intervals. The recesses 100 can be substantially identical to each other, or one or more recesses 100 can be different from the other recesses 100 at least with respect to their width, depth and conformation.
Again, at least one of the laminates 12 in the stack can have the recesses 100. In one embodiment, each of the laminates 12 can include the recesses 100. When adjacent laminates 12 are provided with recesses 100, the recesses 100 can be substantially aligned with each other or they can be offset. When the recesses 100 are offset, the recesses 100 of one laminate 12 may or may not overlap with the recesses 100 in the adjacent laminate 12. Alternatively, a vane assembly 10 can be formed in which regular, or non-recessed, airfoil-shaped laminates 12 (like the laminate in
The recesses 100, serrations 102, and cutouts 104 can be used separately or in combination. The phrase “at least one of recesses, serrations and cutouts,” as used herein, means that a laminate can have one or more of these features. For purposes of forming a vane assembly 10 with an irregular outer peripheral surface 20, such features can also be used in combination with any of the features disclosed in
In addition, desirable features that are difficult to achieve in a vane can be readily formed in a CMC laminate according to aspects of the invention. For example, ribs or spars that connect the pressure-side and suction-side of the airfoil are difficult to form in typical two dimensional laminate lay-up (wrapping) construction. U.S. Pat. No. 5,306,554, which is incorporated herein by reference, discloses an airfoil having ribs. Such ribs can result in moderate thermal stresses due to temperature differences between the cool rib and the hot airfoil skin. The stresses resulting from thermal and internal pressure are sufficient to create problems at the triple points (reference no. 25 in U.S. Pat. No. 5,306,554) of the construction. However, as shown in
Further, it is known in the art that an airfoil having a parted spar arrangement can reduce thermal stresses. For example, U.S. Pat. No. 6,398,501, which is incorporated herein by reference, describes the intermittent use of spars in an airfoil to minimize radial thermal stresses. Such features, while desirable, are difficult to provide in an airfoil. However, as shown in
The foregoing description is provided in the context of one vane assembly according to embodiments of the invention. Of course, aspects of the invention can be employed with respect to myriad vane designs, including all of those described above, as one skilled in the art would appreciate. Embodiments of the invention may have application to other hot gas path components of a turbine engine. For example, the same stacked laminate construction can be applied to the inner and outer platforms or shrouds of the vane by changing the shape of the laminates so as to build up the required platform or shroud geometry. Thus, it will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention as defined in the following claims.
