The present invention relates to a combustion gas duct that includes a discrete insert having a different mechanical property than a remainder of the duct. The combustion gas duct may be formed by bi-casting duct material around the insert.
Conventional can-annular gas turbine engines include a plurality of individual combustor cans, where each can is secured to a respective transition duct that directs combustion gases from the combustor can, and through inlet guide vanes to a respective portion of a turbine inlet annulus. Each flow of combustion gas remains discrete from the combustor until exiting the respective transition duct. In contrast, in certain emerging gas turbine engines that use can combustors, the cans are repositioned such that combustion gas flows exiting the cans is already properly oriented for delivery onto the first row of turbine blades. An example of this may be seen in US Patent Application Publication Number 2011/0203282 to Charron et al., published Aug. 25, 2011, which is incorporated by reference in its entirety herein. The array of transition ducts are replaced with a duct arrangement that receives the discrete combustion gas flows, accelerates them to a speed appropriate for delivery onto the first row of turbine blades, and directs them into a common annular chamber where the combustion gas flows are no longer segregated from each other. The annular chamber exhausts directly into the turbine inlet. The proper orientation and speed created by the arrangement eliminates the need for a first row of inlet guide vanes present in the conventional arrangements
In conventional gas turbine engine combustor arrangements, since the compressed air flows are not accelerated in the transition ducts there is a relatively small static pressure difference between compressed air in the plenum surrounding the transition duct and a static pressure of the combustion gas flows within the transition. Consequently, there is a relatively small force pressing inward on the exterior surface of the transition ducts.
In contrast, in the emerging technology ducting arrangement the combustion gas flows are traveling at significantly greater speeds. This results in significantly greater pressure differences (up to six atmospheres) and resulting forces acting on the exterior surface of the ducting arrangement. The annular chamber experiences the greatest of these forces because the combustion gas flows are fully accelerated within the annular chamber. These forces act to deform the ducting arrangement and hence there is room in the art for improvements that resist this deformation.
The invention is explained in the following description in view of the drawings that show:
The present inventors have recognized at least one form of deformation that may occur when using one exemplary embodiment of a ducting arrangement and have identified a region of the ducting arrangement likely to experience substantial mechanical stresses as a result of this deformation. The inventors have further recognized that this region of relatively great mechanical stress is also a region of relatively great thermal stress which compounds the problem.
To accommodate this localized region of high mechanical and thermal stresses, and thereby extend the service life of the ducting arrangement, the inventors have proposed an innovative arrangement that lowers the mechanical stresses in the area. This, in turn, permits the use of an insert having a material that is better suited thermally for the local region, and this reduces an amount of cooling air that is required for the local region which, in turn, increases engine efficiency. Reducing the stresses at this location permits the use of an insert material that may possess a different mechanical property, for example a lower yield strength, when compared to a material of the ducting arrangement immediately surrounding the insert. The inventors have also proposed to thermally decouple the insert from the material of the ducting arrangement immediately surrounding the insert. This will reduce thermal growth mismatch and associated stresses, further extending the service life of the component.
These goals may be accomplished in one exemplary embodiment by bi-casting ducting material around the insert. Bi-casting securely locks the insert in place but does so without a metallurgical bond. This can partially or fully structurally and/or thermally decouple the insert from the remainder of the ducting arrangement. This, in turn, permits the selection of a material for the insert that is tailored to withstand the relatively harsh conditions present in the region, (greater high-temperature stability) while allowing the selection of different material sufficient to withstand the relatively less harsh conditions in the remainder of the ducting arrangement. This is particularly beneficial in terms of cost since it allows for the use of the more expensive material of the insert only where conditions require it, while less expensive material can be used elsewhere.
The opening for a respective insert 50 essentially matches a shape of the respective insert 50 that rests therein. The insert 50 and the remainder 52 of the common duct structure end 34 meet at an interface 54. The insert may include the upstream region 46 of the junction 44, and the upstream region 46 experiences relatively high temperatures when compared to other locations of the ducting arrangement 10. Consequently, even if a same material is used for the insert 50 and the material that defines the opening, the upstream region 46 may expand or contract differently than other locations of the ducting arrangement. In order to prevent excessive thermal growth mismatch, where relative growth causes internal stresses beyond a yield strength of either material, the ducting arrangement 10 may include a gap (not visible) between the insert 50 and a perimeter 56 of the opening at one temperature. The gap may shrink and/or disappear at another operating condition. This can be configured such that when the gap disappears there is no stress created in the insert 50, or only an amount of stress that is within the yield strength of the insert material. The gap may be uniform around the insert 50, or it may be locally tailored to accommodate thermal growth that may not be uniform. For example, if the geometry of the insert results in greater thermal growth in one direction over another, the gap may be sized to accommodate the local differences. Conversely, the arrangement can be configured such that a yield strength of the ducting material surrounding the insert 50 is not exceeded.
Accommodating the relative thermal growth may be particularly important when the insert material is relatively weaker than the ducting material that defines the opening. In this case it is important to ensure a yield strength of the insert 50 is not exceeded as a result of the insert 50 thermally growing into the perimeter 56 of the opening and then experiencing a compressive force imparted by the ducting material that may resist that thermal growth. While it is possible that the ducting material that defines the opening may have a lower yield strength, it is more likely that the material of the throat insert will have a lower yield strength that must not be exceeded. A lower yield strength often accompanies materials characterized by a better high-temperature stability. High temperature stability materials do not oxidize or disintegrate as quickly at higher temperatures. General examples of high temperature stability materials include oxide dispersion strengthened alloys and ceramic matrix composites. CMSX-4® (of the Cannon Muskegon Corporation, Muskegon Mich.), is an example high temperature stable material that may be used for the insert 50. An example of a material for the remainder 52 of the common duct structure end 34 is Inconel® 939 (of the Special Metals Corporation of Huntington W. Va.).
If the insert 50 and the ducting material that defines the perimeter 56 have different coefficients of thermal expansion, this relationship can also be exploited in order to control stresses in the insert. For example, the insert material may have a relatively small coefficient of thermal expansion when compared to the ducting material defining the opening. The ducting arrangement 10 can be configured such that the greater temperature increase experienced by the insert 50 and the lower coefficient of thermal expansion result in a thermal growth that equals (or is essentially equal to) a thermal growth of the ducting material that defines the opening, because it has a lesser temperature increase but a greater coefficient of thermal expansion. Controlling these factors can enable control of the amount of stress within the insert 50 so that a yield strength of the material of the insert 50 is not exceeded.
In an exemplary embodiment the common duct structure end 34 may be formed by bi-casting ducting material around the throat insert 50, where the throat insert 50 was previously cast. Such a bi-casting process is disclosed in U.S. Patent Application Publication Number 2011/0243724 to Campbell et al., published Oct. 6, 2011, which is incorporated by reference in its entirety herein. The bi-casting process necessarily occurs at elevated temperatures. However, a temperature of the insert 50 can be controlled in order to maintain a desired relationship between the thermal growth of the insert 50 and the remainder 52 of the common duct structure end 34. For example, in an instance when the insert material is characterized by a higher coefficient of thermal expansion, a temperature of the insert 50 may be elevated during the bi-casting such that the insert 50 has grown thermally to approximate a condition the insert 50 will experience during operation of the gas turbine engine. Both the insert 50 and the ducting material would cool to ambient temperatures, and the insert would contract more. A gap may be formed at lower temperatures, and this gap may be configured to fully disappear at operating conditions without the insert expanding to the point where it expands into the perimeter 56 of the opening. Expanding into the perimeter 56 would produce a compressive stress on the insert 50 that may not be desired. Alternately, a certain amount of thermal growth interference between the two may be desired, and could be controlled such that the resulting compressive stress did not exceed a yield strength of either material. This can be accomplished by heating the insert 50 to a lower temperature than in the last example. During subsequent operation the insert 50 would heat more and thereby grow more into the ducting material surrounding the insert 50. Should the insert 50 be characterized by a lower coefficient of thermal expansion, keeping the insert 50 cooler during the casting operation may permit it to expand more during subsequent operation, thereby minimizing any gap formed at the interface 54. Using these techniques and known variations can result in an insert that is thermally decoupled from, or less thermally coupled with the remainder 52 of the common duct structure end 34.
Bi-casting in this manner will trap the insert 50 in place but will do so without forming a metallurgical bond at the interface 54 of the insert and the ducting material defining the opening. As shown in
In a ducting arrangement 10 without the insert 50 a small angle 62 is formed by the corners 48 of the merging walls 40, 42 and this is a stress riser 64 that experiences relatively high mechanical stress when operating forces act on the discrete ducts, such as to increase the small angle 62. In the exemplary embodiment, the insert 50 includes the stress riser 64 of the upstream region 46 of the junction and a length 66 of the junction 44 from where the corners 48 begin to intersect until they have fully run together inside the common duct structure end 34. Such an insert 50 may be termed a throat insert. Since the insert 50 now includes this upstream region 46 of the junction 44, and since the insert 50 is not metallurgically bonded to the remainder 52 of the common duct structure end 34, the remainder 52 does not experience the stiffness of the upstream portion 46 of the junction 44 to the same extent as it would without the insert 50, and conversely, the remainder 52 does not transfer stresses to the upstream region 46 of the junction 44 to the same extent as without the insert 50.
The ducting arrangement 10 must then be configured to accommodate this interface 54 that is not characterized by a metallurgical bond and hence transfers load less effectively. To do so the remainder 52 of the common duct structure end 34 may be made flexible enough to flex yet withstand operating forces the ducting arrangement 10 experiences during operation including the cantilever prying action. Alternately, or in addition, the ducting material in the portion of the remainder 52 of the common duct structure end 34 that surrounds the insert 50 may be thickened to increase a structural strength of the remainder 52. In this way the upstream region 46 of the junction 44 is mechanically decoupled, or coupled to a lesser degree, with the remainder 52 of the common duct structure end 34 when compared to ducting arrangements 10 not having the insert 50. Consequently, the upstream region 46 of the junction experiences reduced stresses during operation. Lowering the stress experienced increases the number of materials can be used for the insert because many materials that satisfy the high temperature are characterized by lower yield strengths. Many of these lower yield strengths are satisfactory for use in the mechanically decoupled (or less-coupled) configuration but would not be satisfactory without the mechanical decoupling (or decreased coupling). Mechanical loads and thermal growth mismatch both contribute to the stresses in the upstream region 46 of the junction. Consequently, optimal designs can balance the amount of mechanical decoupling, thermal decoupling, high temperature tolerance of the material, and yield strength to reach an appropriate configuration. In another exemplary embodiment the ducting arrangement 10 may be configured such that the insert 50 experiences stresses associated only with its aerodynamic function.
In this exemplary embodiment the insert 50 also uniquely defines a portion 70 of a flow path 72 associated with the discrete flow path portion 38 as well as a portion 74 of a flow path 76 associated with the arc-segment 36, and hence with the annular chamber 14 and the common duct structure.
Separately forming the insert 50 provides yet another advantage. The upstream region 46 of the junction 44 experiences relatively high operating temperatures. While the remainder 52 of the common duct structure end 34 and the remainder of the ducting arrangement 10 may or may not require cooling, the upstream region 46 may benefit from cooling. However, when the upstream region 46 is cast with the remainder 52 of the of the common duct structure end 34 any cooling channels must be machined in subsequent to the casting operation. In contrast, when the insert 50 is cast separately, cooling channels can be cast using a fugitive casting insert. As a result, the cooling channels can be more complex and can incorporate features in a surface of the cooling channel such as pins, fins, and turbulators to increase a cooling effectiveness.
From the foregoing it can be seen that the inventors have recognized a new problem associated with an emerging technology, have created a solution, and have done so using existing technology but applying it in a novel manner. The solution increases service life and improves cooling efficiency and so it represents an improvement in the art.
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.
Development for this invention was supported in part by Contract No. DE-FC26-05NT42644, awarded by the United States Department of Energy. Accordingly, the United States Government may have certain rights in this invention.