Crack resistant combustor

This invention relates to combustion chamber liners for turbine engines and specifically to a crack resistant combustion chamber louver assembly.

Turbine engine combustion chamber liners may be made of multiple, axially successive louvers circumscribing a combustion chamber centerline. A typical louver has a forward panel, an aft panel and a short bulkhead that projects radially to connect the forward and aft panels to each other. A louver assembly comprises a forward louver and an aft louver arranged so that the aft panel of the forward louver nests radially inside the forward panel of the aft louver. The aft panel of the forward louver also extends axially past the connecting bulkhead of the aft louver to define a lip. A weld joint extends circumferentially to join the forward panel of the aft louver to the aft panel of the forward louver. The lips of certain louvers, particularly louvers that are not near the axially forward end of the liner, may include a series of circumferentially distributed slots. These lip slots help relieve thermal stresses that could cause cracks in the lips of those louvers. Experience shows that such lip slots are unnecessary in the louvers residing closer to the forward end of the liner.

Turbine engine manufacturers strive to minimize undesirable exhaust emissions arising from combustion of a fuel and air mixture in the combustion chamber. U.S. Pat. Nos. 6,101,814 and 6,715,292 (the contents of both of which are incorporated herein by reference) describe a combustor liner and associated fuel injector that produce considerably reduced emissions in comparison to early generation combustion liners. Throughout this specification the low emissions liner described in the aforementioned patents will be referred to as an intermediate generation liner; the predecessor to the intermediate generation liner will be referred to as an early generation liner. Experience reveals that a louver near the forward end of the early generation liner, specifically the second louver L 2 , does not require lip slots in order to resist cracking of the lip. Similarly, no lip slots are required in the second louver L 2 of the intermediate generation liner to resist cracking of the lip. However in the intermediate generation liner, the forward panel of the axially adjacent aft louver (louver L 3 ) exhibits susceptibility to cracking in the immediate vicinity of the weld joint that secures the louvers to each other. The cracking is believed to arise because a portion of forward louver L 2 that is relatively hot during engine operation nests radially inside of a portion of aft louver L 3 that is relatively cool during engine operation. The relatively cool portion of aft louver L 3 is unable to withstand the cyclic, thermally induced radial expansion (and contraction) of the relatively hot portion of forward louver L 2 . The cracking is undesirable because it requires more frequent inspections than would otherwise be necessary and may also require replacement or reconditioning of an otherwise serviceable liner or its louvers.

What is needed is a combustor liner louver assembly whose louvers exhibit improved forward panel crack resistance.

One embodiment of the louver assembly described herein includes an aft louver having a forward panel that extends axially from a louver leading edge to a corner, and a forward louver joined to the forward panel of the aft louver. A lip defined by a portion of the forward louver that extends axially past the corner to a louver trailing edge includes circumferentially distributed trailing edge slots extending forwardly from the trailing edge a nominal distance equal to about 88% to 95% of the length of the lip.

The foregoing and other features of the various embodiments of the louver assembly will become more apparent from the following description of the preferred embodiment and the accompanying drawings.

FIG. 1 is a side elevation view of a combustor can comprising multiple, axially successive louvers L 1 through L 11 .

FIG. 2 is a perspective view of a portion of the combustor can of FIG. 1 .

FIG. 2A is an enlarged, view of portion 2 A of FIG. 2 .

FIG. 3 is a developed view taken in the direction 3 - 3 of FIG. 1 .

FIG. 4 is a graph showing the improved service life of a louver assembly as disclosed herein.

FIG. 5 is a schematic illustration of an annular combustor

This invention is predicated in part on the recognition that crack susceptibility in the forward panel of a louver is related to differences in thermal expansion of that louver relative to an adjacent louver. Moreover, the remedy for mitigating the crack susceptibility involves modifying the adjacent louver at a location offset from the crack initiation site of the crack susceptible louver.

FIGS. 1, 2 and 2A show a combustion chamber liner 12 ′ for a gas turbine engine. The illustrated liner is a combustor can and is one of nine such cans circumferentially distributed about an engine axis 14 to form a can-annular combustor. Each liner 12 includes eleven axially successive louvers 16 (individually labeled L 1 through L 11 ) each in the form of an integral ring. Each liner circumscribes a combustion chamber centerline 18 to define a combustion chamber 20 . A fuel injector, not shown, projects through opening 22 at the forward end of the can. A typical louver 16 has a forward panel 24 extending axially from a louver leading edge 26 to a rounded, radially inner corner 28 , and an aft panel 30 extending from a rounded, radially outer corner 32 to a louver trailing edge 36 . A bulkhead 38 projects radially outwardly from the forward louver to connect the forward and aft panels to each other. Circumferentially distributed coolant admission holes 40 penetrate the bulkhead. The holes in louver L 2 are all of equal size. The holes 40 in louvers L 3 through L 11 are grouped in clusters of holes. Each cluster occupies a sector of the bulkhead circumference. All the holes in a given cluster are of the same size (i.e. flow area) however the holes in a given cluster are not necessarily the same size as the holes in other clusters. The described clustering arrangement accommodates the three dimensional distribution of gas temperature in the combustion chamber.

Referring additionally to FIG. 3 , twenty four circumferentially distributed flexure slots, such as slot 42 , define a series of tabs 44 in the forward panel of louvers L 3 through L 11 . Each slot extends from the louver leading edge 26 to an associated coolant hole 40 , however only one of every three coolant holes is associated with a flexure slot. The flexure slots impart flexibility to the forward panels to facilitate mating of axially adjacent louvers.

As seen best in FIGS. 2 and 2A , a louver assembly comprises a forward, radially inner louver such as L 2 , and an aft, radially outer louver such as L 3 arranged so that part of the aft panel 30 of the forward louver nests radially inside the forward panel 24 of the aft louver. The aft panel of the forward louver also extends axially past the bulkhead 38 of the aft louver to define a lip 48 . Specifically, the aft panel of the forward louver extends axially past the radially inner corner 28 of the aft louver to define the lip 48 and an associated annulus 50 . The lip has a length L measured from the trailing edge to the radially inner corner 28 . The annulus receives a coolant fluid by way of the coolant holes 40 . The coolant forms a coolant film on the louvers to help protect them from the intense heat of combustion occurring inside the combustion chamber 20 .

A weld joint 52 joins the forward louver to the forward panel of the aft louver. As seen best in FIG. 3 , the weld joint is circumferentially quasi-continuous since it is locally interrupted by the flexure slots 42 . The weld joint includes a weld nugget 54 axially bordered by regions of weld runout 56 ( FIG. 2A ).

Referring to FIG. 2 , louvers L 3 through L 10 include twenty four circumferentially distributed lip slots 58 . Each lip slot has a keyhole configuration comprising a linear portion and a circular or otherwise rounded terminus. Each lip slot 58 is circumferentially aligned with one of the twenty four flexure slots 42 in the same louver. However the lip slots of a given louver are circumferentially offset from the flexure slots of the axially neighboring louver by 7.5 degrees. Each lip slot, as measured from the louver trailing edge to the center of the circular terminus, has a length L 80 of about 80% of the length L of the louver lip. Since the lips on louvers L 3 through L 10 are all about 0.350 inches (0.89 centimeters) long, each of the slots is about 0.280 inches (0.71 centimeters) long. These slots 58 help relieve thermal stresses that could cause cracks in the lips of the louvers L 3 through L 10 .

In early generation liners, the dilution hole pattern differs from that of the intermediate generation liners, and the lip of louver L 2 is devoid of slots analogous to slots 58 . Experience has shown that the lip of louver L 2 in these early generation liners is not susceptible to cracking related to thermal stress. The intermediate generation liners employ the dilution hole pattern described in the patents incorporated herein by reference, but, like the early generation liners, also do not employ slots analogous to slots 58 in louver L 2 . These intermediate generation liners also are not known to be susceptible to cracking in the lip of louver L 2 . However the intermediate generation liners exhibit a susceptibility to cracking in the relatively cool forward panel 24 of aft louver L 3 . The crack initiation site is aft of the weld runout 56 immediately aft of the weld nugget 54 . This cracking of the forward panel of aft louver L 3 is believed to arise from thermally induced radial expansion of the relatively hot portion of louver L 2 (which is the forward louver from the perspective of louver L 3 ) in the vicinity of the forward panel 24 of louver L 3 . The cracking of the forward panel is believed to occur in the intermediate generation liner, but not in the early generation liner, because of a modified gas temperature distribution arising from interactions attributable to the dilution hole pattern and the innovative fuel injector described in the patents incorporated by reference.

Louver L 2 includes circumferentially distributed trailing edge slots 60 axially offset from the forward panel of louver L 3 . The slots are keyhole slots comprising a linear portion 62 and a circular or otherwise rounded terminus 64 . Each trailing edge slot 60 is circumferentially aligned with one of the twenty four flexure slots 42 in louver L 2 . However the trailing edge slots of louver L 2 are circumferentially offset from the flexure slots of louver L 3 by 7.5 degrees. The slots 60 extend forwardly from the trailing edge 36 of louver L 2 a nominal distance L 95 equal to about 88% to 95% of the length L of the lip and preferably about 95% of the length L of the lip. The nominal distance is the distance from the trailing edge 36 to the center of the circular terminus 64 . The length L of the lip on louver L 2 is about 0.425 inches (1.08 centimeters). Accordingly, the preferred length of the slot is about 0.405 inches (1.03 centimeters).

FIG. 4 shows a graph of the predicted life of louver L 3 expressed as the life of a slotted louver divided by the life of an unslotted louver (i.e. a louver with a trailing edge slot length of zero) where louver life is measured in engine cycles. The life expectancy is shown as a function of slot length expressed as a percentage of lip length. The prediction is based on a coarse grid finite element model of a sector of the disclosed liner. As is evident from the graph, the slot in louver L 3 yields an impressive gain in louver life.

Intermediate generation combustion liners can be upgraded by cutting through louver L 1 at the approximate location 68 ( FIG. 1 ) thereby allowing the aft end of louver L 1 and louvers L 2 through L 11 to be separated as a unit from the forward end of L 1 . The separated louver assembly can then be scrapped or recycled. An upgraded louver assembly comprising the aft end of a louver L 1 and louvers L 2 through L 11 and also including the trailing edge slots 60 in L 2 is then attached, for example by welding, to the residual forward end of the original louver L 1 . As already described, the trailing edge slots 60 measure about 88% to 95% and preferably 95% of the length L of the louver lip. This upgrade method is not only time efficient but is also cost effective because the forward end of L 1 is the most costly part of the liner.

Combustor liners can also be upgraded, albeit less cost effectively and less time efficiently, by installing the trailing edge slots 60 in an unslotted louver (e.g. louver L 2 ) of those liners. The upgrade involves removing the unslotted louver from the liner, and securing a slotted louver having trailing edge slots measuring about 88% to 95% and preferably 95% of the length L of the louver lip to the liner in place of the unslotted louver. The slotted louver may be the same louver as the previously unslotted louver upgraded to include the slots 60 , or may be a newly manufactured replacement louver or may be a used, previously unslotted louver taken from a pool of louvers that have been upgraded by installing the slots 60 therein.

The foregoing discussion describes the liner and associated method of upgrade in the context of a combustor can for a can-annular combustor. However as seen in FIG. 5 , the concept can be applied to the louvers of an annular combustor 70 having radially inner and outer liners 72 , 74 constructed of louvers substantially as already described.

Although this disclosure refers to specific embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the subject matter set forth in the accompanying claims.