Combustor rear facing step hot side contour method and apparatus

A combustor liner has a stepped combustor liner surface and an overhang portion forming an air cooling slot. A contoured rear facing edge of the overhang portion reduces turbulence of combustion gas flow and reduces a combustor liner surface area exposed to combustion gases. A thermal barrier coating is also applied to the contoured rear facing edge, reducing heat flow into the overhang portion and hence reducing the operating temperature of the combustor liner. Thus, the amount of cooling air is reduced, which can reduce exhaust emissions, increase engine performance and extend a working life of the combustor assembly.

BACKGROUND OF THE INVENTION
 This invention relates generally to turbine engines, and, more
 particularly, to slot cooled ring combustors for turbine engines.
 A turbine engine includes a compressor for compressing air which is
 suitably mixed with a fuel and channeled to a combustor wherein the
 mixture is ignited for generating hot combustion gases. The gases are
 channeled to a turbine, which extracts energy from the combustion gases
 for powering the compressor, as well as producing useful work for
 propelling an aircraft in flight and for powering a load, such as an
 electrical generator. Increased efficiency in gas turbine engines is
 accomplished at least in part by an increase in the operating temperature
 of the combustor. A principal limitation on the operating combustor
 temperature has been material limitations of a liner in the combustor.
 One effective technique for cooling the combustor liner is thin film
 convection cooling wherein a protective film boundary of cool air flows
 along an inner surface of the liner via air cooling slots to insulate the
 liner from hot combustion gases. Aside from forming a protective boundary
 between the liner and hot gases, the cooling air allows for convective
 cooling of the liner. See, for example, U.S. Pat. No. 4,259,842. However,
 the air slots tend to encourage turbulence of combustion gases separating
 off the ends of the slots, which increases a heat transfer coefficient on
 the ends of the slots and hence increases the heat load on the combustor
 liner.
 Another effective technique for cooling a combustor liner thermal barrier
 is the use of thermal barrier coatings that are applied to the inner
 surface of a combustor liner for providing thermal insulation against
 combustion gases. Thermal barrier coatings reduce the amount of cooling
 air required for a given combustion gas temperature, or allow an increase
 in a combustion gas temperature for increasing efficiency of the engine.
 See, for example, U.S. Pat. No. 5,960,632. However, process limitations
 for applying thermal barrier coating, namely undesirable buildup of
 thermal barrier coatings, prevent thermal barrier coating from being
 applied to rear facing edges of the combustor liner, thereby exposing the
 edges to hot combustion gases and allowing undesirable heat flow into the
 liner.
 Accordingly, it would be desirable to provide a combuster assembly with
 rear facing edges that may be thermal barrier coated without creating
 undesirable heat buildup in air cooling slots, that reduce combustion gas
 turbulence at each rear facing edge, and that reduce the combustor liner
 surface area of the overhang portions.
 BRIEF SUMMARY OF THE INVENTION
 In an exemplary embodiment of the invention, a combuster includes a
 combustor liner including a stepped combustor liner surface and at least
 one overhang portion forming an air cooling slot. A rear facing edge of
 the overhang portion is contoured to reduce turbulence of combustion gas
 flow and to reduce an exposed combustor liner surface of the overhang
 portion. A thermal barrier coating is applied to the contoured rear facing
 edge, further reducing heat flow into the overhang portion and lowering
 the operating temperature of the combustor. Thus, the amount of required
 air cooling is reduced, which can reduce exhaust emissions, increase
 engine performance, and extend a working life of the combustor assembly.
 In addition, because of the thermal barrier coating, higher combustion gas
 temperatures may be achieved to increase the efficiency of a turbine
 engine without having adverse effects on combustor assembly materials.

DETAILED DESCRIPTION OF THE INVENTION
 A known combustor assembly 10 that combines thin film convective cooling
 and thermal barrier coating techniques to lower the operating temperature
 of a combustor liner in a conventional turbine engine is illustrated in
 FIG. 1. A conventional fuel injector (not shown) injects atomized fuel
 into a combustion zone 12 of combustor assembly 10 forming a air-fuel
 mixture that is typically mixed with a swirler (not shown). An igniter or
 cross-fire tube (not shown) ignites the air-fuel mixture downstream of the
 fuel injector, and combustion gases exit combustor assembly through a
 turbine nozzle (not shown) that directs high energy combustion gases upon
 a row of turbine blades or buckets (not shown). The gases rotate a turbine
 wheel (not shown) that delivers rotational energy to the compressor,
 powers a load, and/or is converted into thrust.
 Combustion zone 12 is formed by annular, radially outer and inner
 supporting members or shells (not shown) and a respective outer liner 20
 and inner liner 22. Outer and inner liners 20, 22 each include a plurality
 of air cooling slots 24 formed by overhanging portions 26 of a combustor
 liner surface 28. Referring now to FIG. 2, combustor liner surface 28
 includes a series of steps 30, each of which form a distinct portion of
 combustor liner surface 28 that is separated from other portions of
 combustor liner surface 28 by air cooling slots 24. Air cooling slots 24
 include openings 32 to receive air from an air plenum (not shown) and form
 a thin protective boundary of air between high temperature combustion
 gases and combustor liner surface 28, as well as providing for convective
 cooling of combustor liner 22. Air flows from openings 32 through slots 24
 between combustor liner surface 28 and a bottom surface 36 of combustor
 liner overhang portions 26.
 A layer 38 of known thermal barrier coating is applied on combustor liner
 surface 28 and extends from overhang portion 26 to overhang portion 26 of
 each step 30 to further insulate combustor liner surface 28 from high
 temperature combustion gases. However, due to process limitations, a rear
 facing edge 40 of each overhang portion 26 is not coated with a thermal
 barrier coating 38 because of a resultant undesirable build up of thermal
 barrier coating 38 under each overhang portion 26. Thus, this type of
 combustor assembly 10 is disadvantaged in that rear facing edge 40 of each
 overhang portion 26 is exposed to hot combustion gases and consequently
 allows undesirable heat flow into each overhang portion 26.
 In addition, each rear facing edge 40 includes square corner geometry,
 i.e., each rear facing edge 40 is substantially perpendicular to combustor
 liner surface 28 and a bottom surface 36 of each overhang portion 26.
 Square corner geometry encourages combustion gas flow turbulence as the
 flow separates off each rear facing edge 40. Turbulence increases the heat
 transfer coefficient on each rear facing edge 40, which, in turn, leads to
 increased undesirable heat load on overhang portions 26. Moreover, square
 corner geometry exposes an undesirably large combustor liner surface area
 to combustion gases, thereby increasing the heat load of overhang portions
 26 and increasing the temperature of the metal therein.
 A first embodiment of a combustor liner that at overcomes these
 disadvantages is illustrated in FIG. 3, and includes a series of combustor
 liner surfaces 62 separated by air cooling slots 64. Combustor liner
 surfaces 64 are connected to one another and are arranged in steps
 relative to one another. Each combustor liner surface 62 includes an
 overhang portion 66 extending adjacent and forming air cooling slots 64.
 Each overhang portion 66 includes a contoured rear facing edge 70 to allow
 for thermal barrier coating, to reduce turbulence of exhaust gases and to
 reduce a combustor liner surface area that is exposed to high temperature
 combustion gases.
 Specifically, each rear facing edge 70 comprises a first, radius portion 72
 that is curved, and a second portion 74 that is substantially straight and
 perpendicular to a bottom edge 76 of overhang portions 66. Thus, the
 square corner geometry of known combustor liners is avoided, and a
 combustor liner surface area exposed to hot combustion gases, or the
 surface area of combustor liner "hot side," is reduced. Further,
 combustion gas turbulence as the gases separate from overhang portions 66
 near each rear facing edge 70 is reduced, thereby lessening heat input
 into overhang portions 66 and reducing an operating temperature of
 overhang portions 66. Furthermore, the geometry of rear facing edges 70
 allows for a layer 78 of thermal barrier coating to be applied to rear
 facing edges 70 by adjusting a spray angle (not shown) to coat rear facing
 edge surfaces while masking an opening of each cooling slot 64 with a
 rubber cord (not shown). Therefore, the operating temperature of rear
 facing edges 70 is further reduced by thermal barrier coating layer 78,
 thereby extending a working life of combustor liner 60 as well as reducing
 exhaust emissions and increasing engine performance.
 FIG. 4 illustrates a second embodiment of a combustor liner 90 including a
 series of combustor liner surfaces 92 separated by air cooling slots 94.
 Combustor liner surfaces 92 are connected to one another and are arranged
 in steps relative to one another. Each combustor liner surface 92 includes
 an overhang portion 96 extending adjacent and forming air cooling slots
 94. Each overhang portion 96 includes a contoured rear facing edge 100 to
 allow for thermal barrier coating, to reduce turbulence of exhaust gases
 and to reduce a combustor liner surface area that is exposed to high
 temperature combustion gases.
 Specifically, each overhang portion 96 is tapered and becomes thinner near
 each rear facing edge 100, thereby reducing a combustor liner surface area
 that is exposed to hot combustion gases. Each rear facing edge 100
 comprises a first, radius portion 102 that is curved, and a second portion
 104 that is substantially straight and perpendicular to a bottom edge 106
 of overhang portion 96. Thus, the square corner geometry of known
 combustor liners is avoided, and a combustor liner surface area exposed to
 hot combustion gases, or the surface area of combustor liner "hot side,"
 is reduced. Further, combustion gas turbulence as the gases separate from
 overhang portions 96 near each rear facing edge 100 is reduced, thereby
 lessening heat input into overhang portions 96 and reducing an operating
 temperature of overhang portions 96. Furthermore, the geometry of rear
 facing edges 100 allows for a layer 108 of thermal barrier coating to be
 applied to rear facing edges 100 by adjusting a spray angle (not shown) to
 coat rear facing edge surfaces while masking an opening of each cooling
 slot 98 with a rubber cord (not shown). Therefore, the operating
 temperature of rear facing edges 100 is further reduced by thermal barrier
 coating layer 108, thereby extending a working life of combustor liner 90
 as well as reducing exhaust emissions and increasing engine performance.
 FIG. 5 illustrates a third embodiment of a combustor liner 120 including a
 series of combustor liner surfaces 122 separated by air cooling slots 124.
 Combustor liner surfaces 122 are connected to one another and are arranged
 in steps relative to one another. Each combustor liner surface 122
 includes an overhang portion 126 extending adjacent and forming air
 cooling slots 124. Each overhang portion 126 includes a contoured rear
 facing edge 130 to allow for thermal barrier coating, to reduce turbulence
 of exhaust gases and to reduce a combustor liner surface area that is
 exposed to high temperature combustion gases.
 Specifically, each rear facing edge 130 comprises a first portion 132 that
 is chamfered, and a second portion 134 that is substantially straight and
 perpendicular to a bottom edge 136 of overhang portion 126. Thus, the
 square corner geometry of known combustor liners is avoided, and a
 combustor liner surface area exposed to hot combustion gases, or the
 surface area of combustor liner "hot side," is reduced. Further,
 combustion gas turbulence as the gases separate from overhang portions 126
 near each rear facing edge 130 is reduced, thereby lessening heat input
 into overhang portions 126 and reducing an operating temperature of
 overhang portions 126. Furthermore, the geometry of rear facing edges 130
 allows for a layer 138 of thermal barrier coating to be applied to rear
 facing edges 130 by adjusting a spray angle (not shown) to coat rear
 facing edge surfaces while masking an opening of each cooling slot 128
 with a rubber cord (not shown). Therefore, the operating temperature of
 rear facing edges 130 is further reduced by thermal barrier coating layer
 138, thereby extending a working life of combustor liner 1200 as well as
 reducing exhaust emissions and increasing engine performance.
 While the invention has been described in terms of various specific
 embodiments, those skilled in the art will recognize that the invention
 can be practiced with modification within the spirit and scope of the
 claims.