This invention relates generally to the field of combustion turbine engines, and more particularly to the use of ceramics and ceramic matrix composite materials in a combustion turbine engine.
A combustion turbine engine has a rotating shaft with several circular arrays of radially oriented aerodynamic blades mounted around the circumferences of disks on the shaft. Closely surrounding these blades is a refractory shroud that contains the flow of hot combustion gasses passing through the engine. This shroud must withstand temperatures of over 1400° C. reliably over a long life span. Close spatial tolerances must be maintained in the gap between the blade tips and the shroud for engine efficiency. However, the shroud, blades, disks, and their connections are subject to wide thermal changes during variations in engine operation, including engine shutdowns and restarts. The shroud must insulate the engine case from combustion heat, and it must be durable and abrasion tolerant to withstand occasional rubbing contact with the blade tips.
Ceramics are known to be useful in the inner lining of shrouds to meet these requirements. A shroud is assembled from a series of adjacent rings, each ring having an inner surface typically of one or more refractory materials such as ceramics. Each ring is formed of a series of arcuate segments. Each segment is attached to a surrounding framework such as a metal ring that is attached to the interior of the engine case. However, ceramic components are difficult to attach to other components. Ceramic material cannot be welded, and it is relatively brittle and weak in tension and shear, so it cannot withstand high stress concentrations. It differs from metal in thermal conductivity and growth, making it challenging to attach ceramic parts to metal parts in a hot and varying environment. Thus, efforts are being made to advance technologies for use of ceramic components in combustion turbine engines, including technologies for reliable ceramic-to-metal connections.
An example of this advancement is disclosed in U.S. Pat. No. 6,758,653, which shows the use of a ceramic matrix composite (CMC) member connected to a metal support member. A CMC member using this type of connection can serve as the inner liner of a combustion turbine engine shroud. Ceramic matrix composite materials typically include layers of refractory fibers in a matrix of ceramic. Fibers provide directional tensile strength that is otherwise lacking in ceramic. CMC material has durability and longevity in hot environments, and it has lower mass density than competing metals, making it useful for combustion turbine engine components. However, it is not ideal for components with stress in areas of sharp curvature, because the fiber layers tend to separate from each other during formation and sintering, leaving voids that weaken the material at curves.
The invention is explained in the following description in view of the drawings that show:
A refractory shroud ring segment 32 of a combustion turbine engine is an exemplary application of the technology of the present invention. This technology can also be applied to other components of combustion turbine engines.
A wide range of ceramic matrix composites (CMCs) have been developed that combine a matrix material with a reinforcing phase of a different composition (such as mullite/silica) or of the same composition (alumina/alumina or silicon carbide/silicon carbide). The fibers may be continuous or long discontinuous fibers. The matrix may further contain whiskers, platelets or particulates. Reinforcing fibers may be disposed in the matrix material in layers, with the plies of adjacent layers being directionally oriented to achieve a desired mechanical strength. The CMC skin 40 may be a continuously wrapped structure as known in the art of fabrication of composite structures. This means that the fibers are wrapped continuously around the core 42 to avoid discontinuities that cause weak points and unevenness in the skin.
One or more pin bores 48 are formed through the core 42, such as by drilling or by the removal of a fugitive material used during the casting of the ceramic core. The pin bores 48 may be oriented in the circumferential direction as shown, or in an axial direction. An axial pin orientation may facilitate insertion/assembly. Pin access wells 50 are formed into the cold side 52 of the ring segment 32, such as by machining or through the use of a fugitive material, intersecting the pin bores 48. Two such wells 50 intersect each pin bore in the illustrated embodiment.
Additional details of the pin-loaded attachment scheme are shown in
The pin bore 48 may have a compliant layer such as bushing 62 to help distribute the pin loading and to protect the metal pin 60 and ceramic core 42 from fretting and sliding wear. An example of a suitable type of bushing is a “slotted spring pin” available from Spirol International, Inc. as shown in
Such a slotted spring pin may be effective in other high temperature applications where a ceramic structure is attached to a metal structure, such as when a ceramic matrix composite material is supported by a metal support bar inserted through a bore in the CMC material. Should the metal support bar be sized for a tight fit at room temperature, the CMC material defining the bore adjacent to the bar would be crushed at high temperatures by the differential thermal expansion between the metal and the ceramic. This would cause an increase in the size of the bore, resulting in an increasingly loose fit, with subsequent high cycle wear of the CMC material against the metal bar. An intervening spring member allows the metal-to-ceramic fit to remain tight in spite of differential thermal expansion, thereby eliminating dynamic vibration between the CMC and the metal material. Such a design may still experience some localized sliding between the CMC and the metal material as the temperature cycles between room temperature and a high operating temperature, but such wear is low cycle (e.g. 102 cycles) when compared to the high cycle wear (e.g. 106 cycles) experienced by a design not including such a slotted spring pin. This concept may be applied with a pin/bore having a circular cross section, such as illustrated herein, or with a pin/bore having other shapes, such as elliptical, slotted, etc. The concept may further be applied to applications of oxide or non-oxide ceramics, and to monolithic or composite ceramics. Applications may include gas turbine engine components as well as other types of equipment experiencing operation at an elevated temperature.
The back surface 52 of the integrated refractory component 32 includes a central generally flat section 52C and two radially-inwardly sloping side sections (surfaces 52S in
To optimize the design, the bearing stress can be reduced by increasing the diameter of the pins 60, thus increasing the contact area; the shear tear-out stress can be reduced by increasing the diameter of the pins, thus increasing the shear area; and/or the shear tear-out stress can be reduced by locating the bin bores 48 farther from the cold side 52 of the segment 32, thus increasing the shear area. Bending stress on the segment 32 is reduced for two reasons. First, the loading pins 60 may be located at locations that minimize bending stress. Second, the structure is thick, and the CMC/core/CMC cross-section is quite strong in bending since the CMC 40 effectively carries the bending load as a primary membrane stress (either tensile or compressive) in the fiber direction.
Another advantage of the pin-loaded, CMC wrapped core structure is that it minimizes stress in areas that are particularly difficult to fabricate with CMC. CMC manufacturing development efforts have repeatedly shown that it is difficult to achieve good microstructure around a radius of curvature. The problems are related to the difficulties in compacting the fabric around a corner, and to sintering shrinkage anisotropy between the fiber and the matrix. The net result is that the as-manufactured CMC tends to have a level of delamination and void formation around the radius of curvature. This results in low interlaminar tensile and shear strength around sharp curves in a CMC structure. Prior attachment devices based on hooks, pins, or T-joints carry the pressure load as a shear load and a moment at the radius of curvature, which generate an interlaminar shear stress and an interlaminar tensile stress, respectively. In order for these attachment types to be viable, the CMC must possess sufficient interlaminar shear and interlaminar tensile strength to carry such pressure loads with sufficient margin. Even if the manufacturing difficulties were resolved, and the CMC microstructure were perfect, this is not a favorable load path for a 2D laminated CMC material
In comparison, the present pin-loaded core concept does not rely on CMC strength around a radius of curvature as a primary load path. First, there is minimal shear stress due to the small bending load. Second, since the core prevents an opening moment at the radius of curvature, there is essentially zero interiaminar tensile stress. Therefore, there is little driving force for delamination cracks to propagate, even if they exist in the as-manufactured CMC. An additional benefit of the pin-loaded core structure is that a continuously wrapped CMC structure may be used to minimize CMC free-edges, which reduces the likelihood of catastrophic delamination cracking, because delaminations are trapped.
The present segment structure is self-constrained against thermal deformations because of its large thickness and due to the complexity of the thermal gradients. There are both positive and negative aspects to the structure being self-constrained. On the negative side, it is unlikely that the structure can deform to relieve the thermal stress. Therefore, all thermal gradients manifest as a corresponding thermal stress, and sometimes these stresses can be quite high. The magnitude of the thermal stress state may be reduced to acceptable levels by the use of a lower stiffness and a highly strain tolerant core material as described in U.S. patent application publication 2004/0043889. On the positive side, it should be easier to control the gas path surface and tip clearances for a self-constrained structure. For structures that deform under a thermal gradient, the blade tip clearances must be set such that blade incursion does not occur at any temperature condition (hot or cold). Therefore, the blade tip clearance must be set according to the closest incursion point of the cycle. At other operating conditions the tip clearance would be greater than necessary. If the ring segment is self-constrained and does not deform, it is not necessary to account for deformations of the ring segment surface, and the blade-tip clearances can be decreased. It is well known that a decrease in blade tip clearance results in an increase in engine performance.
Another advantage of the embodiment described above is its resistance to pressure fluctuations (e.g., caused by a passing blade) and resistance to a blade strike. The resilience of this ring segment concept is related to two features. First, the large mass of the ring segment due to the solid core design will help the structure resist pressure fluctuations and/or impact events by acting as a highly damping material. Second, the ability to apply a significant preload to the structure may help the structure to resist pressure fluctuations and/or impact events.
The following summarizes some of the advantages of the ring segment described above.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
Number | Name | Date | Kind |
---|---|---|---|
4066384 | DeFerdinando | Jan 1978 | A |
4245954 | Glenn | Jan 1981 | A |
5035573 | Tseng et al. | Jul 1991 | A |
5116199 | Ciokajlo | May 1992 | A |
5201847 | Whidden | Apr 1993 | A |
5616001 | Boyd | Apr 1997 | A |
5720597 | Wang et al. | Feb 1998 | A |
5921749 | McLaurin et al. | Jul 1999 | A |
6164903 | Kouris | Dec 2000 | A |
6200092 | Koschier | Mar 2001 | B1 |
6241469 | Beeck et al. | Jun 2001 | B1 |
6325593 | Darkins et al. | Dec 2001 | B1 |
6478537 | Junkin | Nov 2002 | B2 |
6550777 | Turnquist et al. | Apr 2003 | B2 |
6648597 | Widrig et al. | Nov 2003 | B1 |
6709230 | Morrison et al. | Mar 2004 | B2 |
6758653 | Morrison | Jul 2004 | B2 |
6896484 | Diakunchak | May 2005 | B2 |
6932566 | Suzumura et al. | Aug 2005 | B2 |
20040043889 | Campbell | Mar 2004 | A1 |
20040047726 | Morrison | Mar 2004 | A1 |
20040062639 | Glynn et al. | Apr 2004 | A1 |
20060292001 | Keller et al. | Dec 2006 | A1 |
Number | Date | Country | |
---|---|---|---|
20070031258 A1 | Feb 2007 | US |