Patent Application
20140366544
The present invention relates generally to gas turbine engines, and more particularly to combustors for such engines.
Combustor performance directly impacts the overall pollutant emission and performance of a gas turbine engine. While cooling air is typically required to cool hot surfaces of combustor liners, the introduction of such cooling air into the main gas path dilutes the hot combustion gas flowing to the turbine and thus reduces the combustor performance. Combustor to turbine transition ducts, particularly in reverse flow combustor configurations, allow the combustion products to have a longer distance to mix with the cool dilution air before striking the turbine blades. This extra mixing length reduces the combustion maximum gas exit temperature. With the help of such longer combustor to turbine transition ducts, lower peak combustor gas exit temperature can be achieved with less dilution flow, so that more can be used for cooling, carbon formation and emission control. The fuel nozzle count may also be able to be reduced, which reduces weight and costs of the combustion system. Also, such reverse flow transitions ducts which feed the combustion gases into the turbine shorten the overall length of the combustor and thus the engine, which greatly reduces weight, cost and simplifies shaft/bearing design. However, these transition ducts are extra areas that need to be cooled and therefore an effective combustor wall construction and/or cooling system thereof is required.
There is provided a gas turbine engine combustor comprising outer and inner annular liners and an exit duct at a downstream end, the exit duct circumscribing an annular combustor exit opening defining a combustion gas path therethrough, the exit duct including annular first and second exit duct walls radially spaced apart to define therebetween the combustor exit opening, at least one of the first and second exit duct walls having a double-skin wall section at a most downstream end thereof, the double-skin wall section including an inner hot wall facing said combustor exit opening and an outer cold wall radially spaced away from the hot wall to define a radial gap therebetween, the outer cold wall being disposed outside of said combustion gas path, the inner hot wall and the outer cold wall being fastened together by at least one joint therebetween, the outer cold wall having a coefficient of thermal expansion greater than that of the inner hot wall to reducing thermal growth mismatch between the outer cold wall and the inner hot wall during operation of the combustor and reduce thermal stress at the joint.
There is also provided a combustor exit duct for a gas turbine engine, the combustor exit duct comprising annular first and second exit duct walls radially spaced apart to define therebetween a combustor exit opening, at least one of the first and second exit duct walls having a double-skin wall section at a most downstream end thereof, the double-skin wall section including an inner hot wall facing said combustor exit opening and an outer cold wall radially spaced away from the hot wall to define a radial gap therebetween, the outer cold wall being disposed outside of said combustion gas path, the inner hot wall and the outer cold wall being fastened together by at least one joint therebetween, the outer cold wall having a coefficient of thermal expansion greater than that of the inner hot wall to reducing thermal growth mismatch between the outer cold wall and the inner hot wall during operation of the combustor and reduce thermal stress at the joint.
There is further provided a method of forming a gas turbine engine combustor, the combustor having outer and inner annular liners and an exit duct at a downstream end, the method comprising: providing a first and a second annular wall of the exit duct which circumscribe an annular combustor exit opening defining a combustion gas path therethrough; forming a double-skin wall section on at least one of the first and second annular walls of the exit duct, by welding an annular outer cold wall flange to an inner hot wall portion facing said combustor exit opening to form an annular welded joint therebetween, the annular outer cold wall flange being spaced apart from the inner hot wall downstream of said welded joint to define a radial gap therebetween at a downstream end of the double-skin wall section; and reducing thermal stress at the welded joint between the outer cold wall flange and the inner hot wall of the double-skin wall section by forming the outer cold wall flange from a material having a coefficient of thermal expansion that is greater than that of the inner hot wall to thereby reducing thermal growth mismatch between the outer cold wall flange and the inner hot wall.
Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below.
Reference is now made to the accompanying figures depicting aspects of the present invention, in which:
Referring to the drawings and more particularly to
The combustor 16, and more particularly the combustor exit duct 16′ thereof, will now be described in further detail. As described above, it is desirable to keep cool the combustor-to-turbine transition duct portions but preferably without introducing undue amounts of diluting cooling air flow into the main combustion gas stream.
Accordingly, the combustor exit duct 16′ of the presently described combustor 16 includes a double-skin exit duct wall configuration, which uses the coolant air flow twice, firstly by impingement cooling and secondly by film cooling of the downstream turbine section. As best seen in
This impingement/film cooling combination using a double-skin duct configuration, however, requires the cooler outer skin to be attached, such as by welding, to the hotter inner skin adjacent the exit end of the combustor exit duct. If left unchecked, this can lead to a thermal fight between the two skins due to the different temperatures to which they are exposed, thereby producing high thermal stresses at the joint (weld, braze, fastening point, etc) interconnecting the two skins. This can cause a reduced fatigue life. Accordingly, the present combustor duct configuration, as will be described in further detail below, provides a design which reduces the high thermal stress at the weld junction between the two skins of the double-skin configuration. By reducing the thermal fight between the double-skins, the fatigue life of the weld junction can be greatly increased.
The combustor exit duct 16′ as described herein includes at least one double-skin wall section which is formed of two formable metal sheets having different coefficients of expansion, thereby greatly reduce the thermal fight between the two skins of the double-skin wall section and thus reducing the thermal stress at the weld joint between these two skins. This combination of welded skins having respective low and high coefficients of expansion allows the designer to have the options of (a) higher fatigue life due to lower stress resulting from the lower thermal fight or for the same fatigue life, (b) higher metal temperature gradient between the two skins for the same fatigue life which results in lower cooling flow and higher combustion performance, (c) thinner skin for less weight and cost or (d) less expensive material.
Referring now to
At least one of the first and second exit duct walls, in this embodiment the LED portion 20 and the SED portion 23, have a double-skin wall section at a most downstream end thereof. As will be seen, the double-skin wall section includes inner hot walls 19, 23 facing the combustor exit opening 33 and outer cold walls 37, 24 radially spaced away from the hot wall to define a radial gap 36, 32 therebetween. The outer cold walls 37, 24 are disposed outside of the combustion gas path flowing through the combustor exit opening 33, and are thus exposed to lower temperatures during operation of the engine. The inner hot walls and the outer cold walls are fastened together by at least one joint therebetween, which may include an annular welded joint for example.
As can be see in
Similarly, as best seen in
Referring back to
As the inner hot wall 23 is directed exposed to the hot combustion gases within the combustor 16, it is subjected to higher temperatures than the outer cold wall 24 which is both spaced apart from the hotter inner hot wall 23 of the SED 29 and also exposed to more cooling air disposed around the combustor. Accordingly, in order to compensate for this temperature difference, the outer cold wall 24 has a coefficient of thermal expansion that is higher than the coefficient of thermal expansion of the inner hot wall 23 of the SED 29. This acts to reduce the thermal imbalance in the double skin wall which forms the SED 29 at the exit end 33 of the combustor exit duct 16′. In other words, as the inner hot wall 23 heats up during operation of the combustor 16, the difference in the coefficients of thermal expansion between the inner hot wall 23 and the outer cold wall 24 will result in both walls, or skins, expanding at approximately the same rate and approximately the same amount. Thus, the thermal growth of the two “skins” of the double-skin SED 29 are more closely matched, as a result of this mismatch in the coefficients of thermal expansion of the two walls 23 and 24.
This difference in thermal expansion may be achieved, for example, by forming the inner and outer walls, or skins, 23 and 24 of the SED 29 out of different materials, such as two different sheet metals for example, each having a different coefficient of thermal expansion (i.e. the outer wall 24 expanding more at a given temperature than the inner wall 23). Alternately, this difference in thermal expansion between the two walls 23, 24 may be achieved by other means, rather than by having different coefficients of thermal expansion, for example by making the two walls of different thickness or different material properties such as to achieve a similar thermal growth match during operation of the engine, when the inner wall 23 of the SED 29 is exposed to higher temperatures than the outer wall 24 thereof.
In at least the depicted embodiment, the outer cold wall 24 is formed as an annular flange having a ring shape skin with an outer flat portion 30 positioned against a planar section of the combustor inner annular wall, where the weld joint 27 is formed. The outer wall 24 also has a curved portion 31 which has a radius of curvature different from the radius of curvature of the inner wall 23 to form the annular gap or plenum 32 therebetween.
It at least one embodiment, the LED 20 is similarly constructed with a double-skin wall section, as per the SED 29 described above. Referring now to
The outer cold wall 37 is also formed from a formable metal sheet of a type compatible for fastening, such as by a weld or brace, to the inner hot wall 19. The outer cold wall 37 has an outer portion 26 for connection to the outer liner 21 and an outwardly offset wall section 37′. The radially extending annular gap 36 is formed between an end section of said outer liner 21 and the outwardly offset wall section 37′.
In one particular embodiment, the outer cold walls 24 and 37 may be formed from a Hastalloy X (registered trademark) sheet metal, and the hot inner walls 19 and 23 are formed from one of 1N625, Haynes 188 and Haynes 230 sheet metals.
In an exemplary operating environment of the combustor 16, the hot annular walls 19 and 23, of the LED 20 and the SED 29 respectively, which are directly exposed to the hot combustion gases, are subjected to temperatures of at least 1650° F. The outer cold walls 37 and 24, of the LED 20 and the SED 29 respectively, are disposed in relatively cooler areas surrounding the combustor, outside the main gas path, and are thus subjected to lower temperatures of at least 800-1500° F. This difference in temperature would typically cause, in a double-wall construction wherein the two skins are the same material, the hotter inner walls to expand more than the cooler outer walls. In the combustor exit duct 16′ of the present combustor 16, the aforementioned differential in the coefficients of thermal expansion between the hot and the cold walls of the double skin liner construction results in the two walls expanding approximately a similar amount, thereby substantially compensating for the differential in temperature during operation of the combustor. Accordingly, this reduction of the thermal growth between the two walls results in less stress being placed on the welded joints 27 and 38 between the hot and cold walls of the double-wall construction.
The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without department from the scope of the invention disclosed. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.
