A transition piece for a power-generating turbomachine, turbomachine including same, and method of manufacturing

Abstract

The transition piece (113) comprises an inner duct (201) extending from an upstream end (201F) to a downstream end (201A), and forming a hot gas path (207). The transition piece (113) further comprises an outer duct (205), extending from an upstream end (205F) to a downstream end (205A) and surrounding the inner duct (201). The inner duct (201) and the outer duct (205) form a cooling annulus (113A) therebetween. Cooling holes (209) extend from the cooling annulus (113A) to the hot gas path (207), through a thickness of the inner duct (201). The inner duct (201) is manufactured by additive manufacturing. The downstream end (201A) of the inner duct (201) and the downstream end (205A) of the outer duct (205) are coupled to one another by a flange (211) surrounding the hot gas path (207).

Description

A TRANSITION PIECE FOR A POWER-GENERATING TURBOMACHINE, TURBOMACHINE INCLUDING SAME, AND METHOD OF MANUFACTURINGDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure pertains to combustors for turbomachines and parts thereof. Embodiments disclosed herein specifically refer to combustors adapted for use in oxyfuel combustion expanders, such as supercritical carbon dioxide expanders (sCO2expanders).

[0002] As understood herein a sCO2expander is an expander wherein carbon dioxide in a supercritical state is present in at least a portion of the process gas flow path inside the expander.BACKGROUND ART

[0003] Fossil fuels are a major source of chemical energy used for the generation of mechanical power. Fossil fuels are mixed with air and combusted to generate a combustion gas at high pressure and temperature, which expands in an expander. The expander converts combustion gas enthalpy into mechanical power available on the output shaft of the expander and used to drive a load, such as a compressor or compressor train, or to rotate an electric generator and convert mechanical power into electric power.

[0004] One of the major concerns regarding combustion of fossil fuels relates to the production of carbon dioxide, a greenhouse gas which is considered one of the main contributors of global warming and climate changes.

[0005] To reduce the environmental impact of power generation through combustion of fossil fuels, the option of post combustion capture of carbon dioxide has been investigated. Carbon dioxide capture facilities have been developed, to process flue gas exhausted from gas turbines and remove carbon dioxide therefrom, prior to discharging the flue gas in the environment. The cost of a carbon dioxide capturing facility arehigh, both in term CAPEX, as well as in terms of energy required to run the facility, which reduces the overall thermodynamic efficiency of the system. The percentage of carbon dioxide in flue gas is low. This requires large volumes of flue gas to be processed through the carbon dioxide capturing facility and renders the capturing process particularly inefficient.

[0006] In recent years oxy-combustion cycles, also known as oxy-fuel cycles or oxyfuel combustion cycles, have been developed, wherein fuel, such as natural gas or another fossil fuel, is blended into a mixture of an oxidant consisting mainly of oxygen (O2) and carbon dioxide (CO2) at high pressure. The blend of fuel, oxidant and carbon dioxide bums in a combustor of an expander producing a pressurized flue gas consisting exclusively or almost exclusively of carbon dioxide and water.

[0007] The flue gas is expanded in the expander to generate mechanical power. The exhaust flue gas discharged at the discharge side of the expander is cooled in a regenerative heat exchanger and further chilled to condensate water which can thus be removed from the chilled flue gas. The low-temperature flue gas, consisting mainly or exclusively of carbon dioxide is pressurized and recycled through the regenerative heat exchanger towards the combustor of the expander.

[0008] Oxygen supplied to the combustor of the expander can be obtained by separation from ambient air, removing nitrogen therefrom, such that the working fluid supplied to the combustor mainly consists of oxygen and carbon dioxide and does not include nitrogen. The resulting flue gas mainly consists of water and carbon dioxide. Water is removed from the flue gas by condensation and the part of water-free flue gas, which is not recycled to the combustor, can be efficiently processed in a carbon dioxide capturing unit.

[0009] The oxy-fuel cycle summarized above is a semi-closed cycle, in that only a fraction of the flue gas exits the cycle after water has been removed therefrom.

[0010] Oxy-fuel combustion cycles, such as those described above, are particularly interesting in terms of efficiency, reduction of noxious emissions and ease of CO2 sequestration. However, they operate under CO2 supercritical conditions at the inlet of the expander and are characterized by high pressure and temperature values inside theexpander and specifically inside the combustor. These operating conditions pose difficult constraints in the casing design.

[0011] Improvements in the design of the combustors adapted for supercritical carbon dioxide expanders or other expanders operating in similar conditions are highly desirable.

[0012] The present disclosure relates to improvements to the transition piece for a turbomachine, which are particularly aimed at improving features of the transition piece adapted to be used in a supercritical carbon dioxide expander or a turbomachine operating in similarly challenging thermodynamic conditions. The present disclosure also pertains to a combustor for an expander, and to an expander, such as a supercritical carbon dioxide expander, comprising an improved transition piece.SUMMARY

[0013] According to one aspect, disclosed herein is a transition piece for a turbomachine combustor, including an inner duct, which extends from an upstream end to a downstream end, and forming a hot gas path, and an outer duct, which extends from an upstream end to a downstream end and surrounding the inner duct. The inner duct and the outer duct form a cooling annulus therebetween, the cooling annulus having an upstream end and a downstream end. The transition piece further includes a plurality of cooling holes extending from the cooling annulus to the hot gas path, through a thickness of the inner duct. According to the present disclosure, the inner duct is manufactured by additive manufacturing. Also the outer duct can be manufactured by additive manufacturing, for instance as a monolithic body together with the inner duct. The downstream end of the inner duct and the downstream end of the outer duct are coupled to one another by a flange surrounding the hot gas path and closing the downstream end of the cooling annulus. In embodiments disclosed herein, the inner duct and the outer duct terminate with respective upstream edges forming a fluid inlet of the cooling annulus therebetween.

[0014] According to a further aspect, a method for manufacturing a transition piece for a turbomachine. The method comprises the following steps: forming by additive manufacturing an inner duct extending from an upstreamend to a downstream end, the inner duct defining a hot gas path therein; forming an outer duct, extending from an upstream end to a down-stream end and surrounding the inner duct; wherein the inner duct and the outer duct define a cooling annulus therebetween, the cooling duct having an upstream end and a downstream end; providing a flange at the downstream end of the outer duct and inner duct, the flange coupling the inner duct and the outer duct to one another, surrounding the hot gas path and closing the downstream end of the cooling annulus; wherein a plurality of cooling holes extends from the cooling annulus to the hot gas path, through a thickness of the inner duct.

[0015] According to further aspects, disclosed herein are a combustor and an expander, i.e. a power-generating turbomachine, including one or more transition pieces as outlined above.

[0016] Further features and embodiments of the transition piece and of the method of manufacturing the same are described below and set forth in the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Reference is now made briefly to the accompanying drawings, in which:Fig. l illustrates a schematic of an expander system according to the present disclosure;Fig.2 is a sectional view of an expander in a simplified representation;Fig.3 is an enlarged and more detailed sectional view of the combustor of the expander of Fig.2;Fig.4 is a front view of the burner in one embodiment;Fig.4A is an enlargement of a central area of Fig.4;Figs. 5 is a cross-sectional view according to line V-V of Fig.4;Fig.5 is a sectional view of the burner of Fig.4;Fig.6 is a side view of the liner in an embodiment;Fig.7 is a sectional view of the liner of Fig.6 along a plane containing the longitudinal axis thereof;Fig.7A is an enlargement of Fig.7;Fig.8 is an axonometric view of the transition piece in one embodiment;Fig.9 is an axonometric sectional view of the transition piece of Fig.8;Fig.10 is an axonometric sectional view of a first component of the transition piece in another embodiment;Fig.11 is an axonometric view of the first component of FiglO;Fig.12 is an axonometric sectional view of a second component of the transition piece of Fig.10;Fig. 13 is an axonometric view of the second component of Fig.12; andFigs. 14, 15 and 16 are schematics of different combustion chamber layouts.DETAILED DESCRIPTION

[0018] The schematic of Fig. 1 illustrates a simplified power system including an oxy-fuel cycle operating with supercritical carbon dioxide at the expander inlet (shortly SCO2 cycle), such as an Allam cycle or NET Power oxy-fuel cycle.

[0019] The power generating system 1 shown in Fig.l comprises a turbine or expander 3 that includes an expansion section 5 and a combustor 7. The combustor 7 can be an annular combustor, a can-type combustor, a can-annular combustor (aka cannu- lar combustor), or the like, for instance. In the embodiments disclosed herein, the combustor is a can-type combustor. The combustor comprises a plurality of combustor chambers arranged around the rotation axis of the expander 3, as shown in more detail in the subsequent figures and described in detail later on. The combustor chambers are housed in an outer casing of the expander, as will described in more detail below.

[0020] Reference number 7.1 in Fig.2 designates one combustion chamber of a cantype or cannular combustor. In some embodiments, each combustion chamber 7.1 is housed in a respective seat formed in a casing, as will be described in greater detail below with reference to Fig.3. The combustion chambers 7.1 are arranged circumferentially around a rotation axis A-A of the expander 3.

[0021] The combustor 7 is supplied with an oxidant flow delivered by an oxidant source. The oxidant may be oxygen (O2). In some embodiments, the oxidant is a blend of oxygen and carbon dioxide (CO2). The oxidant flow, or the oxygen forming part of the oxidant blend can be produced by an air separation unit 9 (Fig.1) which represents an oxidant source. The air separation unit 9 may remove nitrogen or nitrogen andcarbon dioxide from ambient air to produce the required oxidant stream which is supplied through an oxidant supply line 11 to the combustor 7 of the expander 3.

[0022] Reference number 13 indicates a fuel supply line, for instance adapted to supply natural gas, such as methane, to the combustor 7, specifically to each combustor chamber 7.1. The oxidant and the fuel are supplied at a forward side of the expander 3 to the combustor 7 at high pressure, for instance at 50 barA or higher, preferably a pressure equal to or higher than 100 barA, more preferably equal to or higher than 150 barA, even more preferably equal to or higher than 200 barA. In some embodiments, the upper pressure of the cycle performed in the thermodynamic system depicted in Fig. 1 can be equal to or above 250 barA, or higher, for equal to or lower than 1000 barA, or equal to or lower than 800 barA, or equal to or lower than 600 barA. The oxidant-fuel blend is burned in the combustor 7. Pressurized, hot combustion gas resulting from the combustion expands in the expansion section 5 of the expander 3.

[0023] In some embodiments, the temperature at the inlet of the gas expansion flow path, i.e., at the inlet of the rotor of the expander can be at or above 800°C, and preferably at or below 1500°C.

[0024] After expansion, the exhausted combustion gas is discharged at a discharge side of the expander 3 in a discharge line 15. The combustion gas in the discharge line 15 can be at around 600°C, for instance, and at a pressure which may range between 10 barA and 100 barA, for instance between 20 barA and 60 barA.

[0025] The power rate of the expander 3 can be higher than 50MW, for instance equal to or higher than 100 MW, for instance 150 MW or higher, e.g. 200 MW or higher. In embodiments the rated power is equal to or higher than 300 MW. In some embodiments the rated power is equal to or lower than 2000 MW, for instance equal to or lower than 1500MW, or equal to or lower than 1000 MW. For instance the rated power can be comprised between 200 MW and 650 MW.

[0026] Intermediate values of the upper and lower limit of each range mentioned above are also expressly disclosed herein.

[0027] The power system 1 further comprises a regenerative heat exchanger 17, wherein hot exhausted combustion gas flowing through a hot side 17.1 of theregenerative heat exchanger 17 is cooled in heat exchange with a flow of chilled exhausted combustion gas, which flows through a cold side 17.2 of the regenerative heat exchanger 17. The combustion gas discharged from the hot side 17.1 of the regenerative heat exchanger 17 is further chilled in a chilling heat exchanger 19 to a temperature which causes condensation of water vapor contained in the exhausted combustion gas. Condensed water is removed from the exhausted combustion gas in a water / gas separator 21.

[0028] The de-hydrated exhausted and chilled combustion gas, consisting mainly (e.g. up to 90% by weight) or exclusively of carbon dioxide, is compressed in a combustion gas compressor 23 to the pressure at the inlet side of the expander 3. While in the schematic of Fig.1 the combustion gas compressor 23 is pictorially represented as a single compressor, in some embodiments a multiple compressor can be used. For instance, the combustion gas compressor 23 can be a multi-stage compressor, or a compressor train and can include one or more intercoolers.

[0029] The compressed combustion gas, consisting mainly of carbon dioxide and delivered by the combustion gas compressor 23, is partly removed from the cycle through a discharge line 24. The major part of the compressed combustion gas is divided into a first part of recycled combustion gas and a second part of recycled combustion gas. The first part of recycled combustion gas is delivered through the cold side 17.2 of the regenerative heat exchanger 17 and is heated by heat exchange with the hot combustion gas flowing through the hot side 17.1 of the regenerative heat exchanger 17 and recycled to the expander 3 through a recycle line 25. The combustion gas recycled through recycle line 25 is fed to the combustor 7 and mixed with combustion gas generated therein as will be described in more detail later.

[0030] A side stream of chilled and dehydrated combustion gas, consisting of the second part of recycled combustion gas, is delivered through a cooling line 27, which bypasses the regenerative heat exchanger 17, towards components of the expander 3 which require cooling. A further side stream of chilled, dehydrated combustion gas can be delivered through a line 28 to the air separator 9 and / or to the oxidant supply line 11 to add carbon dioxide to the oxygen from the air separation unit 9. The combustion gas from line 28 and the oxygen from the air separation unit are blended toform the oxidant flow which is delivered to the combustor 7. The oxidant flow delivered to the combustor 7 can contain for instance approximately 20% by volume of oxygen and 80% by volume of carbon dioxide. The addition of carbon dioxide to the oxidant stream prevents corrosive damages to the piping and expander components, which may be caused if pure oxygen were used as an oxidant. Moreover, carbon dioxide blended with oxygen in the oxidant stream mitigates safety issues related to the delivery of pure oxygen to the combustor, and helps to tune the reactivity of the mixture within the combustor. The percentages outlined above are by example only and shall not be understood as limiting the scope of the present disclosure.

[0031] To recover further heat from the regenerative heat exchanger 17, the oxidant supply line 11 can include a heating section 11.1 which extends through the regenerative heat exchanger 17, such that the oxidant is heated by heat exchange against the hot combustion gas flowing in the hot side 17.1 of the regenerative heat exchanger 17 prior to be fed to the combustor 7.

[0032] The expander 3 may include an output shaft end 31 which can be integral with the central portion of the rotor, or can be assembled with the central portion of the rotor by bolting, welding, Hirth or spline connections, or the like, or a combination thereof. The mechanical power generated by the expansion of the combustion gas in the expansion section 5 of the expander 3 is available on the output shaft end 31 for mechanical drive or power generation purposes. In the exemplary embodiment of Fig. 1 the output shaft end 31 is drivingly coupled to an electric generator 33 directly or through a gearbox, a joint or combinations thereof. The electric generator 33 is in turn electrically coupled to an electric power distribution grid 35. In Figs 1 and 2 the output shaft end 31 is shown at the aft side of the expander 3. In other embodiments, not shown, the output shaft end 31 can be arranged at the forward side of the expander. In yet further embodiments, not shown, two output shafts ends can be provided, one at the forward side and one at the aft side of the expander.

[0033] With continuing reference to Fig.1, Fig. 2 illustrates a simplified sectional view of the expander 3 in one embodiment. The expander 3 can comprise an outer casing 41, which houses the combustor 7. In embodiments, the outer casing 41 includes a high-pressure casing 41.1 and a low-pressure exhaust casing 41.2. The high-pressurecasing 41.1 can be in the form of a barrel including a monolithic body, for example manufactured by forging, casting, or combination thereof. I.e. the outer casing can be a vertically split casing. The monolithic body extends around the longitudinal axis of the expander, i.e., around the rotation axis.

[0034] The low-pressure exhaust casing 41.2 can be positioned on the discharge side, i.e. the aft side, of the expander 3, i.e., on the side opposite the combustor 7.

[0035] In some embodiments, the low-pressure exhaust casing 41.2 forms a discharge plenum 41.3, through which exhausted combustion gas is discharged from the expander 3.

[0036] Reference numbers 45, 47 indicate bearing arrangements, which rotatingly support a rotor 43 of the expander 3. For instance, the bearing arrangement 45 on the side opposite the combustor 7 may include an axial or thrust bearing in combination with a radial bearing, or a bearing having an axial-radial bearing capability. The bearing arrangement 47 on the combustor side may include a radial bearing. A reversed arrangement is also possible, with a bearing having axial load capability arranged on the combustor side.

[0037] The output shaft end 31 of the rotor 43 can be drivingly coupled to the driven machine (electric generator 33) through flanges 49. The bearing arrangements 45, 47 can be arranged in bearing housings, not shown in detail.

[0038] The rotor 43 is surrounded by an inner casing 51, which can be formed by plurality of sections arranged in sequence in a forward-to-aft direction. In Fig.2 the inner casing 1 comprises two casing sections sequentially arranged in the forward-to- aft direction, i.e., parallel to the rotation axis. The inner casing 51 can be horizontally split, i.e. can include two portions which are coupled to one another along a plane containing the rotation axis of the rotor 43. If the inner casing comprises two casing sections arranged in sequence in the axial direction, each section can in turn be split into two portions along a plan containing the rotation axis of the rotor 43.

[0039] The inner casing 51 is fully or partly housed in the high-pressure casing 41.1. In some embodiments, as shown in Fig.2, the inner casing 51 projects in the low-pressure exhaust casing 41.2.

[0040] One or more annular fluid chambers 42 are formed between the inner casing 51 and the outer casing 41. Specifically, in the exemplar embodiment shown in Fig.2 the annular chamber 42 includes two sequentially arranged annular fluid chambers 42.1 and 42.2 separated by a septum 44. The fluid pressure inside the two annular fluid chambers 42.1 and 42.2 can be different. For instance, the forward annular fluid chamber 42.1 can be at a higher pressure than the aft annular fluid chamber 42.2. In use, at steady state conditions, the aft fluid chamber 42.2 and the forward fluid chamber 42.1 can be fed with chilling or cooling fluid, e.g. with chilled, dehydrated combustion gas from cooling line 27.

[0041] In some embodiments, the inner casing 51 is provided with cooling ducts, one of which is schematically shown at 51.1 in Fig.2. The cooling ducts provide a fluid coupling between one or each of several annular fluid chambers 42.1, 42.2 with the interior of the inner casing 51. Compressed recycled combustion gas, consisting mainly of carbon dioxide, can flow from the annular fluid chambers into the interior of the inner casing 51 to cool or purge annular cavities inside the inner casing 51. External cooling ducts can be provided in combination or as an alternative to cooling ducts extending through the inner casing.

[0042] The expander can be adapted to expand the combustion gas through the gas expansion flow path with a pressure drop of at least 150 bar, preferably of at least 250 bar, more preferably between 250 and 400 bar. To expand the combustion gas generated in the combustor 7 a high number of expansion stages is preferred. In the exemplary embodiment of Fig.2, the expander 3 includes eight stages, each configured as an axial expansion stage. In other embodiments, a different number of expansion stages can be foreseen, preferably equal to or higher than four, more preferably equal to or higher than five. In some embodiments, the number of expansion stages can be higher than eight, for instance nine, ten, eleven or more.

[0043] Each expansion stage includes an annular row of stationary blades 53, aka vanes 53, which are stationarily arranged in the inner casing 51 and form nozzles at the inlet of the expansion flow path.

[0044] Each expansion stage further includes a respective annular row of rotor blades 55, arranged downstream the respective annular row of stationary blades 53along an expansion flow path which extends from the combustor 7 through the expansion section 5 to the discharge plenum 41.3 in a forward-to-aft direction.

[0045] The rotor blades 55 form part of the rotor 43, i.e., they are connected thereto for co-rotation with the rotor shaft. In some embodiments, each annular row of rotor blades 55 is connected to a respective rotor disk, not shown in detail. The structure of the rotor and of the rotor disks is not relevant and is not shown in detail.

[0046] In embodiments, the rotor 43 further comprises a forward shaft portion 65 and an aft shaft portion 67. In embodiments, the combustor 7 extends around the forward shaft portion 65. In some embodiments, the discharge plenum 41.3 extends around the aft shaft portion 67.

[0047] A balance drum 69 can be constrained to the rotor 43 for co-rotation therewith. In the embodiment of Fig.2 the balance drum 69 includes a first balance drum portion 69A and a second balance drum portion 69B connected to one another by tie rods 70.

[0048] With continuing reference to Figs. 1 and 2, a general layout of the combustor 7 in one embodiment is shown in Fig.3. The embodiment of Fig.3 is based on a cantype combustor architecture, wherein the expander comprises a plurality of can-shaped liners, wherein each liner contains one or more burners, and wherein each liner is surrounded by its own can-shaped casing. In other embodiments, as mentioned above, the combustor can be a cannular (aka tubo-annular or can-annular) combustor, or else an annular combustor.

[0049] Turning now to Fig.3, in the illustrated embodiment the expander comprises a can-type combustor, which includes a plurality of combustor units. The units are arranged around the rotation axis A-A of the expander. One combustor unit is shown in a longitudinal sectional view in Fig.3. The combustor unit will be referred herein simply as “combustor” and comprises a combustor casing 101 which houses a liner 103. Each liner 103 has a longitudinal axis B-B. Since the combustor units form part of a can-type combustor, the liners 103 are positioned with their longitudinal axes B- B on a conical surface, the axis whereof is coincident with the rotation axis A-A of the expander. In some embodiments, the angle between the axes A-A and B-B can bebetween 0° and 80°, or between 0° and 60°, in some embodiments between 15° and 40°. The angle between the axes A-A and B-B is selected as a compromise between the need to reduce radial dimensions of the expander and improve the combustor design (which would be improved using smaller angles), and the overall design constraints of the expander, such as the dimension and position of the rotor shaft and bearings (which require larger angles).

[0050] In some embodiments, the combustor units can be housed in a combustor casing, which can be manufactured separately from the main outer casing of the expander. The two casings can be connected to one another with a flanged coupling. This may improve casing manufacturability. In the enclosed drawings, a single casing is shown for the sake of simplicity.

[0051] In other embodiments, the combustor can be an annular combustor or a can- nular combustor. The general layouts of can-type combustors, cannular combustors and annular combustors are known per se and are summarized in the schematics of Figs. 14, 15 and 16, respectively. As shown in Fig.14, the can-type combustor includes individual liners 103, arranged around the axis A-A of the turbomachine and coupled to one another by crossfire tubes 104. A cannular type combustor, as schematically shown in Fig.15, includes an annular pressure shell 106, housing the liners 103 and crossfire tubes 104. A schematic representation of an annular combustor is shown in Fig.16 In such case, the combustor comprises a single liner 103 developing symmetrically around the rotation axis A-A of the expander and having an inner, generally conical liner wall and an outer, generally conical liner wall, coaxial to one another. Burners are arranged annularly around the machine axis A-A and are housed in the liner, as schematically shown in Fig.16.

[0052] The liner 103 will be described in detail later with reference to the next figures. As mentioned, the liner 103 extends along a longitudinal axis B-B and has a forward end 103F, and an aft end 103 A. A, generally cylindrical side wall 105 of the liner 103 extends between the forward end 103F and the aft end 103 A. The side wall 105 has an outer surface 105 A and an inner surface 105B and surrounds the combustion chamber 7.1 of the combustor or combustor unit 7.

[0053] The combustor 7 comprises at least one burner 107 at the forward end 103Fof the liner 103. In some embodiments, the burner 107 is coupled with a forward end closure or lid 106 of the liner 103. In some embodiments, the forward end closure 105 of the liner 103 comprises a plurality of apertures wherethrough recycled combustion gas, mainly consisting of carbon dioxide, can enter the combustion chamber 7.1.

[0054] As will be described in detail below, the burner 107 is fluidly coupled with a fuel inlet 109 and with an oxidant inlet 111. The fuel inlet 109 is in turn fluidly coupled with the fuel supply line 13 (Fig.1) and receives fuel, for instance gaseous fuel, such as natural gas, therefrom. The oxidant inlet I l l is fluidly coupled with the oxidant supply line 11 and receives oxidant therefrom, the oxidant mainly consisting of oxygen and carbon dioxide, as mentioned above.

[0055] The combustor 7 further comprises a transition piece 113 positioned at the aft end 103A of the liner 103. The transition piece 113 forms an extension of the duct formed by the liner 103 towards the array of stationary nozzles 53 and guides the combustion gas generated in the combustion chamber 7.1 toward the expansion flow path formed by the stationary and rotary blades of the expander.

[0056] In the embodiment of Fig.3, since the combustor is a can-type combustor, each combustion chamber 7.1 extending along the longitudinal axis B-B of the respective liner 103 merges in a respective one of a plurality of transition pieces 113 positioned circumferentially around the rotation axis A-A. In other embodiments, the combustor can be an annular combustor including a conically shaped liner. In such embodiment, not shown, the conical liner merges with a single, generally conical transition piece. In both cases, the transition piece(s) fluidly couple(s) the liner(s) with the row of stationary vanes 53 of the first expansion stage of the expander, positioned at the inlet of the expansion flow path.

[0057] The combustor 7 further comprises a sleeve 115, which is arranged around the liner 103 and substantially coaxial therewith. Specifically, the sleeve 115 is positioned between the liner 103, i.e. between the side wall 105 thereof, and the combustor casing 101. The sleeve 115 comprises a forward end 115F and an aft end 115A. The forward end 115F of the sleeve 115 is coupled to an inner surface of the combustor casing 101. The sleeve 115 divides a space between the combustor casing 101 and the liner 103 into an inner annular space 117 and an outer annular space 119. The outerannular space 119 surrounds the inner annular space 117. The inner annular space 117 and the outer annular space 119 are substantially co-axial.

[0058] As in the embodiment of Fig.3 the combustor is a can-type combustor, each generally cylindrical liner is surrounded by a generally cylindrical sleeve 115. The inner annular space and the outer annular space surrounding each liner 103 extend coaxial to the liner 103, i.e. in the direction of the longitudinal axis B-B and have a cylindrical cross-section. In other embodiments, not shown, where the combustor is an annular combustor, the sleeve is generally conical and coaxial to the rotation axis A- A of the expander. The inner and outer annular spaces are conical and co-axial to the rotation axis A-A.

[0059] As described above, recycled combustion gas is returned towards the combustor 7 of the expander 3. As described regarding Fig.l, combustion gas is added to oxygen separated from air by the air separation unit 9. The oxidant consisting of the blend of oxygen and recycled combustion gas is delivered to the combustor 7 through the oxidant inlet 111.

[0060] A further flow of recycled combustion gas, wherefrom condensed water has been removed is delivered to the expander through recycle line 25 and is fed to the combustor 7 through a first process gas inlet 120, which is fluidly coupled with the inner annular space 117 as described below. A yet further flow of recycled combustion gas is delivered to the expander 3 through the cooling line 27, which is fluidly coupled to a second process gas inlet 121. The second process gas inlet 121 is fluidly coupled with the outer annular space 119 in a position downstream of the forward end 115F of the sleeve 115, i.e. in a position between the expansion flow path of the expander and the forward end 115F of the sleeve 115. The first process gas inlet 120 is therefore connected to the combustor 7 in a position upstream of the position of the second process gas inlet 121. A thermal insulation chamber is formed between the second process gas inlet 121 and the forward end 115F of the sleeve 115. The insulation chamber is formed by the outer annular space 119, or part thereof, which extends around the sleeve 115 and the liner 103. The thermal insulation chamber can be filled with stagnant process gas representing an inert, thermally insulating gas, consisting mainly of carbon dioxide at high pressure, which represents an efficient insulation material.

[0061] In the embodiment of Fig.3 the forward end 115F of the sleeve 115 is positioned in a position intermediate the forward end 103F of the liner 103 and the aft end 103A of the liner, i.e. between the two ends 103F and 103A of the liner 103.

[0062] Thus, the liner 103 projects in a forward direction beyond the sleeve 115 which surrounds the liner 103. In this embodiment the combustor further comprises a forward plenum 127, which is positioned at the forward end 103F of the liner 103. Specifically, in Fig.3, the forward plenum 127 surrounds the forward end 103 of the liner 103, which projects inside the forward plenum 127.

[0063] In other embodiments, the forward end 103F of the liner and the forward end 115F of the sleeve 115 can be substantially aligned along the axis B-B. Therefore, the sleeve 115 will be flush with the liner 103 at the aft ends thereof.

[0064] In Fig.3, the forward plenum 127 extends parallel to an axis of the liner 103 from the forward end 115F of the sleeve 115 in a forward direction towards a lid or closure 129, which closes the forward side of the combustor casing 101. The fuel inlet 119 and the oxidant inlet 111 extend through the closure 129. The closure 129 closes the forward plenum 127 at the forward side thereof.

[0065] The forward plenum 127 is fluidly coupled with the inner annular space 117. The first process gas inlet 120 is fluidly coupled to the forward plenum 127, such that process gas, i.e. combustion gas recycled from the exhaust of the expander 3 through the recycle line 25 and the first process gas inlet 120, flows through the forward plenum 127 and therefrom into the inner annular space 117 in a forward-to-aft direction. The inner annular space 117 and / or the forward plenum 127 can be fluidly coupled with the combustion chamber 7.1 through holes, apertures, or ports extending through the side wall 105 of the liner. A description of the holes, apertures, or ports which extend through the side wall 105 of the liner will be given later herein. The ports, holes or apertures provided in the side wall 105 of the liner allow process gas, consisting mainly of carbon dioxide, which flow through the inner annular space 117, to flow into the combustion chamber 7.1 as a cooling fluid or dilution fluid.

[0066] In some embodiments, the combustor 7 further comprises an aft plenum 131 positioned at the aft end 103A of the liner 103. For instance, the aft plenum 131 cansurround the aft end 103 A of the liner 103. In some embodiments, the aft plenum 131 also surrounds the transition piece 113, i.e. the transition piece is at least partly housed in the aft plenum 131.

[0067] In Fig.3 the aft plenum 131 is fluidly coupled with the outer annular space 119, which surrounds the liner 103 and the sleeve 115. The outer annular space 119 can extend from the aft plenum 131 toward the forward end 103F of the liner 103.

[0068] In the embodiment of Fig.3, the second process gas inlet 121 is positioned at the aft plenum, i.e. directly coupled therewith. In other embodiments, not shown, the second process gas inlet 121 can be positioned in an intermediate position along the development of the outer annular spaec 119 between the aft end thereof and the aft plenum 131.

[0069] The aft plenum 131 can be fluidly coupled through cooling ducts (not shown) to components which face the expansion flow path, such as the stationary vanes and / or the rotary blades of the rotor.

[0070] In the embodiment of Fig.3, the inner annular space 117 is fluidly coupled at the aft end thereof with a cooling annulus 113 A of the transition piece 113. The cooling annulus 113A has an upstream end, i.e. a forward end, and a downstream end, i.e. an aft end. The forward end of the cooling annulus 113 A is fluidly coupled with the aft end of the inner annular space 117. As will be explained in more detail below, the cooling annulus can be formed between an inner duct and an outer duct of the transition piece 113, wherein the inner duct forms a hot gas path adapted to fluidly connect the combustion chamber 7.1 with the expansion flow path of an expander 3. The aft plenum 131 extends around the outer duct of the transition piece 113.

[0071] The process fluid delivered through the first process gas inlet 120 flows through the inner annular space 117 and partly enters the combustion chamber 7.1 through the apertures extending through the wall of the liner 103. The remaining process gas flows from the first inner annular space 117 into the cooling annulus 113 A in a direction of flow concordant with the direction of flow of the combustion gas generated in the combustion chamber 7.1 and which flows towards the expansion flow path. The process fluid flows from the cooling annulus 113 A through cooling aperturesformed in the transition piece, which will be described in more detail below and cools the inner surface of the transition piece 113 by effusion or film cooling.

[0072] An embodiment of the at least one burner 107 positioned at the forward end of the combustion chamber 7.1 is illustrated in Figs. 4, 4A and 5.

[0073] The burner 107 comprises a concentrical arrangement of ducts, wherethrough oxidant from the oxidant inlet 111 and fuel from the fuel inlet 109 are delivered into the combustion chamber 7.1, where they are intimately mixed and burned to generate the flow of compressed and hot combustion gas which then expands through the expansion flow path of the expander.

[0074] In the embodiment of Figs 4, 4A and 5 the burner 107 comprises a centerbody 135, which extends in a longitudinal direction parallel to the longitudinal axis B- B of the liner 103. If a single burner faces the combustion chamber 7.1, the centerbody 135, and therefore the burner 107 can be coaxial with the liner 103 and the combustion chamber 7.1, as shown in Fig.3.

[0075] The center-body 135 is surrounded by an intermediate annular wall 137, which extends coaxially around the center-body. The center-body 135 and the intermediate annular wall 137 form a first annular oxidant flow path 139 therebetween. More specifically, the first annular oxidant flow path 139 is formed between an outer wall of the center-body 135 and an inner wall of the intermediate annular wall 137. In some embodiments, the outer surface of the center-body 135 and the inner wall of the intermediate annular wall 137 are cylindrical, at least in the most downstream portion thereof, such that the first annular oxidant flow path 139 has a constant cross-section in the shape of a circular annulus.

[0076] The intermediate annular wall 137 forms a fuel passage 141 therein. The fuel passage 141 is fluidly coupled with at least one fuel port 143. In the embodiment of Figs. 4, 4A, and 5, the fuel passage 141 comprises a plurality of fuel ports 143. In some embodiments, the fuel port 143 or the plurality of fuel ports 143 are positioned on a front surface 137A of the intermediate annular wall 137. The fuel ports 143 are positioned at or near the distal end, i.e. the aft end of the intermediate annular wall 137. In the embodiment of Figs. 4, 4A, and 5, the fuel ports 143 are formed in a front endsurface of the intermediate annular wall 137. The front end surface can be planar.

[0077] If a plurality of fuel ports 143 are foreseen, they can be arranged with a constant pitch around the axis B-B of the burner, as shown in Fig.4.

[0078] In the embodiment of Figs. 4, 4A and 5, the burner 107 further comprises an outer annular wall 145, which surrounds the intermediate annular wall 137. The intermediate annular wall 137 and the outer annular wall 145 form a second annular oxidant flow path 149 therebetween. In some embodiments, the cross-sectional area of the first annular oxidant flow path 139 is smaller than the cross-sectional area of the second annular oxidant flow path 149. For instance, the cross-sectional area of the first annular oxidant flow path 139 can be comprised between 50% and 70%, preferably between 60% and 65% of the cross-sectional area of the second oxidant flow path 149.

[0079] More specifically, the second oxidant flow path 149 is formed between an outer surface of the intermediate annular wall 137 and an inner surface of the outer annular wall 145. In some embodiments, the outer surface of the intermediate annular wall 137 and the inner surface of the outer annular wall 147 are cylindrical, at least in the most downstream portion thereof, such that the second annular oxidant flow path 149 has a constant cross-section in the shape of a circular annulus.

[0080] To impart a tangential or swirling motion to the oxidant which flows in the first annular oxidant flow path 139, a first swirling flow generator 151 can be positioned in the first annular oxidant flow path 139. This swirling motion is beneficial for enhancing mixing, improving combustion efficiency, and controlling flow pattern within the combustion chamber 7.1. In the embodiment of Figs 3 and 4, the first swirling flow generator 151 comprises a plurality of inclined first blades, which span between the outer surface of the center-body 135 and the inner surface of the intermediate the intermediate annular wall 137, and configured to impart a tangential motion to an oxidant flow in the first annular oxidant flow path.

[0081] In some embodiments, the burner 107 further comprises a second swirling flow generator positioned in the second annular oxidant flow path. In the embodiment of Figs 4, 4A and 5, the second swirling flow generator 153 comprises a plurality of inclined first blades 153.1, which span between the outer surface of the intermediateannular wall 137 and the inner surface of the outer annular wall 145, and configured to impart a tangential motion to an oxidant flow in the second annular oxidant flow path.

[0082] In other embodiments, not shown, the first swirling flow generator, or the second swirling generator, or both the first swirling generator and second swirling generator can comprise inclined oxidant inlet ports, which are configured to feed oxidant flow having a tangential speed component in the respective annular oxidant flow paths.

[0083] In the embodiment of Figs. 4, 4A and 5, the first swirling flow generator and the second swirling flow generator are adapted to induce a tangential motion in the same direction in both the first annular oxidant flow path and the second annular oxidant flow path, respectively.

[0084] In the embodiment of Figs. 4, 4A and 5, at least one fuel feed duct 155 extends through at least one of the inclined second blades 153.1 from the outer annular wall 145 to the intermediate annular wall 137. Each fuel feed duct 155 comprises an inlet end 155.1 which is fluidly coupled with a fuel inlet plenum 157, and an outlet end 155.2 fluidly coupled with the fuel passage 141 extending along the intermediate annular wall 137.

[0085] In some embodiments, in addition to the first annular oxidant flow path 139 and the second annular oxidant flow path 149, the burner 107 can feature a third oxidant flow path inside the center-body 135. The third oxidant flow path can comprise an oxidant duct 159 in the center-body 135, which is fluidly coupled with one or more oxidant outlet ports 161, which are positioned at or adjacent the front distal end of the center-body 135. In Figs. 4 and 5 the oxidant outlet ports 161 are positioned at a front distal end surface of the center-body. The third oxidant flow path as well as the first annular oxidant flow path 139 second annular oxidant flow path 149 are fluidly coupled with the oxidant inlet 111 (Fig.3). An embodiment of the liner 103 is shown in detail in Figs. 6, 7 and 7A. The side wall 105 of the liner 103 comprises a plurality of cooling holes 171. Each cooling hole 171 extends through the side wall 105 from the outer surface 105 A to the inner surface 105B of the side wall 105.

[0086] The liner 103, and in particular the side wall 105 thereof, can be manufacturedas a single monolithic component by additive manufacturing, for instance starting from the forward end 103F to the aft end 103 A of the liner 103. Using additive manufacturing for the production of the liner 103 the thickness of the side wall 105 can be made larger than with standard manufacturing technologies.

[0087] In some embodiments, the inner diameter of the liner, i.e the cross-sectional diameter of the inner surface 105B of the wall 105, can be equal to or less than 300 mm, or equal to or less than 250 mm, for instance equal to or less than 200 mm. The ratio between the thickness of the side wall 105 and the inner cross-sectional diameter can be equal to or higher than 0.015, preferably equal to or higher than 0.02. For instance, the ratio between the wall thickness and the inner diameter can be comprised between 0.02 and 0.05, or 0.2 and 0.04, or 0.02 and 0.03.

[0088] If the liner has a non-circular inner cross section, the above-mentioned ratio can be referred to the equivalent diameter defined as follows: s Decl= jv where S is the area of the cross section of the hot gas path in the liner.

[0089] In some embodiments, specifically for liners of large dimensions, which may be difficult to produce in a single additive manufacturing step due to the dimension of available additive manufacturing machines, the liner 103, and specifically the side wall 105 thereof, can be manufactured in individual ring-shaped portions or sections which are then welded to one another. Specifically, the liner can be formed by producing individual ring-shaped portions by additive manufacturing and subsequently welding the portions together. For instance, the ring-shaped portions can be bonded or coupled to one another by electron-beam welding.

[0090] Each ring-shaped portion represents a part of the axial development of the liner and extends circularly around the B-B axis of the liner. Each part can be delimited by planes orthogonal to the axis B-B of the liner. Once the individual ring-shaped portions of the liner have been manufactured by additive manufacturing, they are aligned with each other in the axial direction and welded peripherally along the annular edges of two consecutive portions abutting each other.

[0091] In some embodiments, the perforated intermediate section of the side wall 105 can be manufactured as a single piece by additive manufacturing, and only nonperforated end sections of the wall 105, which form the forward end 103F and the aft end 103 A, can be manufactured in separate additive manufacturing steps and welded to the perforated central or intermediate portion of the wall 105.

[0092] In some embodiments, the cooling holes are generated by additive manufacturing. This renders the manufacturing process faster. However, in other embodiments, some or all the cooling holes can be formed by drilling once the liner 103 has been manufactured by additive manufacturing. If the liner is formed by a plurality of ringshaped portions welded to one another, cooling holes can be machined in each portion prior to welding, or after welding the portions to one another. The cooling holes can be machined by electrical discharge machining (EDM).

[0093] In the embodiment of Figs 6, 7, 7A the cooling holes 171 are distributed according to annular arrays of cooling holes 171, sequentially spaced from one another from the forward end 103F to the aft end 103 A of the liner 103. Each annular array of cooling holes is positioned at a bridge 173, which connects two consecutively arranged ring panels 175 of the side wall 105 of the liner 103.

[0094] In some embodiments, each array of cooling holes 171 is associated with one or more lips or projections 177, which project from the inner surface of the side wall 105 and extend in a forward-to-aft direction inside the wall 105 of the liner 103. In the embodiment shown in Figs. 7, 7A, each lip 177 is ring-shaped and extends around the entire circumference of the inner surface of the side wall 105. Each lip 177 forms a respective annular slot which forms a guide for a cooling fluid which enters the combustion chamber 7.1 through the cooling holes 171. The lips 177 are oriented such as to impart to the cooling fluid a velocity component which is tangent to the inner surface of the side wall, i.e. oriented in a forward-to-aft direction inside the combustion chamber 7.1. Each lip 177 forms a ring-shaped gap 179 in front of the respective annular array of cooling holes 171.

[0095] In preferred embodiments, the lips 177 are generated by additive manufacturing while forming the side wall 105 and form, therefore, a monolithic body therewith. If the side wall 105 is manufactured by producing individual ring-shaped portionswhich are subsequently welded to one another, each portion is manufactured by additive manufacturing with the respective lips 177 forming a monolithic integral body with the ring-shaped portion of the wall 105.

[0096] The liner can further comprise dilution holes 181, which extend from the outer surface to the inner surface of the side wall 105. The dilution holes 181 can be distributed in various ways along the length of the wall 105 of the liner 103, to optimize the fuel combustion and the flow of combustion gas along the liner 103. The distribution of dilution holes illustrated in the drawing is merely illustrative.

[0097] In some embodiments the dilution holes 181 can be generated in the wall 105 of the liner by additive manufacturing, i.e. during the additive manufacturing process which generates the liner 103 or part thereof. In other embodiments, the dilution holes 181 can be added once the liner 103, or a ring-shaped portion thereof has been generated by additive manufacturing. For instance, the dilution holes 181 can be machined by electrical discharge machining (EDM).

[0098] The liner shown in Figs. 6, 7, 7A can be manufactured as a single piece by additive manufacturing starting from the forward end 103F and ending with the aft end 103 A. The cooling holes 171 and the dilution 181 are formed during the additive manufacturing process, along with the lips 177.

[0099] The liner 103 can also comprise one or more additional holes in the side wall 105 thereof, for connection of cross fire tubes and / or sensors, transducers, and other instruments useful for the control of the combustion process. These additional holes are not shown.

[0100] At the end of the additive manufacturing process, the liner 103 is completely formed.

[0101] In other embodiments, the cooling holes 171 and / or the dilution holes 181 can be machined in the wall 105 after the latter has been formed.

[0102] If the dimension of the liner so requires, the wall 105 can be manufactured by separately forming by additive manufacturing two or more ring-shaped portions of the wall 105 in separate processes. The ring-shaped portions can then be welded togetherto form the liner 103. Each ring-shaped portion is manufactured monolithically with the respective lips 177. The cooling holes 171 and / or the diffusion ports 181 can be formed by additive manufacturing in each ring-shaped portion of the side wall 105, or can be machined therein after manufacturing of the ring-shaped portion, before or after welding the ring-shaped portions together.

[0103] Figs. 8 and 9 illustrate an embodiment of the transition piece 113. In this embodiment, the transition piece 113 comprises an inner duct 201 which extends in a forward-to-aft direction from an upstream end, or forward end, labeled 20 IF to a downstream end, or aft end 201 A. The inner duct 201 forms a hot gas path 207 which extends from the liner 103 to the stationary nozzles 53, where the expansion flow path of the expander 3 starts. The inner duct 201 has an inner surface 20 IB facing the hot gas path, and an outer surface 201C.

[0104] The outer surface 201C of the inner duct 201 faces an outer duct 205, which extends from an upstream end, or aft end 205F, to a downstream end, or aft end 205 A. The outer duct 205 surrounds the inner duct 201 and has an inner surface 205B which faces the inner duct 201, and an outer surface 205 C which faces the aft plenum 131.

[0105] The cooling annulus 113 A is formed between the inner surface 205B of the outer duct 205 and the outer surface 201C of the inner duct 201. The inner duct 201 and the outer duct 205 terminate with respective upstream edges which form a fluid inlet of the cooling annulus therebetween. As shown in Fig.9, in advantageous embodiments, the cooling annulus 113A is empty, i.e. free of material forming the outer duct 205 and the inner duct 201, i.e. represents a free duct through which a coolant can freely flow.

[0106] The cooling annulus 113 A can be fluidly coupled with the hot gas path 207 through one or more effusion cooling holes 209. Each effusion cooling hole 209 extends through the thickness of the inner duct 201 from the outer surface to the inner surface thereof. In contrast, the outer duct 205 is solid. By “solid”, it is understood that the duct 205 is devoid of apertures, such that no coolant flows from the cooling annulus 113A outside the transition piece 113.

[0107] In some embodiments, the inner duct 201 and the outer duct 205 aremanufactured by additive manufacturing as a single monolithic block, in the same 3D- printing process. The effusion cooling holes can be manufactured by additive manufacturing during the process of generating the inner duct 201 and outer duct 205.

[0108] The downstream, i.e. aft end 201 A of the inner duct 201 and the downstream, or aft end 205 A of the outer duct 205 can be coupled to one another by a flange 211 surrounding the hot gas path and closing a downstream end of the cooling annulus 113A. If the outer duct 205 and the inner duct 201 are manufactured by additive manufacturing in the same process, with the inner and outer ducts growing simultaneously from one end toward the other end thereof, the additive manufacturing process can start from the flange 2011 and proceed further from the aft end 201 A and 205 A of the inner duct 201 and of the outer duct 205 towards the forward end 20 IF and 205F of the inner duct 201 and of the outer duct 205 respectively. During the additive manufacturing process, the effusion cooling holes 209 are also generated in the inner duct 201.

[0109] Thus, the inner duct 201, the outer duct 205 and the flange 211 form in this embodiment a monolithic structure manufactured by additive manufacturing in the same additive manufacturing sequence.

[0110] According to a different embodiment, the outer duct is 205 formed by a first shell and a second shell, which are manufactured separately from one another and separately from the inner duct 201, and connected to one another by welding, and further connected by welding to the inner duct 201.[OHl] In this embodiment, the inner duct 201 can be manufactured in one additive manufacturing sequence. The flange 211 can be manufactured during this first additive manufacturing sequence monolithically with the inner duct. The resulting transition piece component generated in this additive manufacturing sequence is shown in a sectional axonometric view in Fig.10, while Fig.11 shows a full axonometric view of the entire transition piece component.

[0112] In a modified embodiment, the flange 211 can be manufactured separately from the inner duct 201 and welded thereto afterwards, for instance by electron-beam welding.

[0113] The effusion cooling holes 209 can be produced by additive manufacturing during this manufacturing step.

[0114] The outer duct 205 is produced by additive manufacturing in two separate steps, each step producing a portion of the outer duct 205 in form of an outer shell. Fig.12 illustrates one of said shells, the other being substantially symmetrical. In Fig.13 the two shells, labeled 205X and 205 Y are welded to one another along a welding line 206. While in Fig. 13 the outer duct 205 produced by welding together the two shells 205X and 205Y is shown in isolation, it shall be understood that the two shells 205X and 205 Y are actually welded to one another after placing them around the inner duct 201. The aft end of each shell 205X, 205Y is welded to the flange 211.

[0115] If the transition piece 113 is produced by assembling three separate pieces 201, 205X, 205 Y which are manufactured separately, the effusion cooling holes 209 can be machined in the inner duct 201 before welding the two shells 205X, 205 Y therearound.

[0116] Producing the transition piece 113 by additive manufacturing as described above, the inner duct 201 and / or the outer duct 205 can have a thickness which is higher than usually achievable the manufacturing technique of the current art. Specifically, a higher ratio between the thickness of the duct 201, 205, and equivalent diameter of the hot gas path can be achieved. In some embodiments the ratio between the thickness of the inner duct 201 and / or of the outer duct 205 and the equivalent diameter of the hot gas path can be equal to or higher than 0.020, preferably equal to or higher than 0.025 along the entire extension of the transition piece. The equivalent diameter of the transition piece can be defined aswhere S is the area of the cross section of the hot gas path.

[0117] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the followingclaims.

Claims

CLAIMS1. A transition piece for a turbomachine combustor, the transition piece comprising: an inner duct extending from an upstream end to a downstream end, and forming a hot gas path; an outer duct, extending from an upstream end to a downstream end and surrounding the inner duct; wherein the inner duct and the outer duct form a cooling annulus therebetween, the cooling annulus having an upstream end and a downstream end; a plurality of cooling holes extending from the cooling annulus to the hot gas path, through a thickness of the inner duct; wherein the inner duct is manufactured by additive manufacturing; and wherein the downstream end of the inner duct and the downstream end of the outer duct are coupled to one another by a flange surrounding the hot gas path.

2. The transition piece of claim 1, wherein the cooling holes are formed by additive manufacturing.

3. The transition piece of claim 1 or 2, wherein the outer duct is manufactured by additive manufacturing.

4. The transition piece of any one of the preceding claims, wherein the inner duct and the outer duct terminate with respective upstream edges forming a fluid inlet at the upstream end of the cooling annulus, the downstream end of the cooling annulus being closed by said flange..

5. The transition piece of any one of the preceding claims, wherein the cooling annulus has a fluid inlet at the upstream end of the transition piece, between an edge of the inner duct and an edge of the outer duct.

6. The transition piece of any one of the preceding claims, wherein the flange is manufactured by additive manufacturing.

7. The transition piece of claim 6, wherein the inner duct, the outer duct and the flange form a monolithic structure manufactured by additive manufacturing.

8. The transition piece of any one of claims 1 to 6, wherein the outer duct is formed by a first shell and a second shell which are manufactured separately from one another and connected to one another by welding.

9. The transition piece of claim 8, when dependent on claim 4 or 5, wherein the first shell and the second shell are connected to the flange by welding.

10. The transition piece of any one of the preceding claims, wherein the ratio between a thickness of the inner duct and an equivalent diameter of the inner duct is at least 0.02, preferably at least 0.025 along the entire length of the transition piece.

11. The transition piece of any one of the preceding claims, wherein the cooling holes are inclined in a forward to aft direction from an outer surface to an inner surface of the inner duct, such as to generate a cooling gas flow in the hot gas path, which has a speed component in a forward-to-aft direction.

12. The transition piece of any one of the preceding claims, wherein the cooling annulus is empty and the outer duct is solid.

13. A combustor for a turbomachine, comprising a liner and a transition piece according to any one of the preceding claims.

14. The combustor of claim 13, configured as a can-type combustor.

15. A power-generating turbomachine comprising a combustor according to claim 13 or 14.

16. The turbomachine of claim 15, wherein the turbomachine is a supercritical carbon dioxide expander.

17. A method for manufacturing a transition piece for a turbomachine, comprising the following steps: forming by additive manufacturing an inner duct extending from an upstream end to a downstream end, the inner duct defining a hot gas path therein; forming an outer duct, extending from an upstream end to a downstream end and surrounding the inner duct; wherein the inner duct and the outer duct define a cooling annulus therebetween, the cooling duct having an upstreamend and a downstream end; providing a flange at the downstream end of the outer duct and inner duct, the flange coupling the inner duct and the outer duct to one another, surrounding the hot gas path and closing the downstream end of the cooling annulus; wherein a plurality of cooling holes extends from the cooling annulus to the hot gas path, through a thickness of the inner duct.

18. The method of claim 17, wherein the step of forming an outer duct comprises the step of forming said outer duct at least partly by additive manufacturing.

19. The method of claim 17 or 18, wherein the flange is formed by additive manufacturing.

20. The method of any one of claims 17 to 19, wherein the inner duct and the outer duct terminate with respective upstream edges forming a fluid inlet of the cooling annulus therebetween at the upstream end of the cooling annulus.

21. The method of any one of the preceding claims, wherein the outer duct is formed by additive manufacturing starting from a flange at the downstream end of the inner duct and outer duct.

22. The method of claim 21, wherein the inner duct and the outer duct are formed by additive manufacturing simultaneously, starting from the downstream ends thereof at said flange, and ending with the upstream ends thereof.

23. The method of any one of claims 17 to 20, further comprising the following steps: producing by additive manufacturing a first shell, the first shell forming a first portion of the outer duct and having an upstream end and a downstream end, the first shell comprising first longitudinal edges extending from the upstream end to the downstream end; producing by additive manufacturing a second shell, the second shell forming a second portion of the outer duct and having an upstream end and a downstream end, the second shell comprising second longitudinal edges extending from the upstream end to the downstream end;placing said first shell and said second shell around the inner duct; connecting said first shell and second shell to one another by welding along the first edges of the first shell and the second edges of the second shell, thus forming the outer duct therewith; and further connecting the downstream end of the first shell and of the second shell to the downstream end of the inner duct by welding.

24. The method of any one of claims 17 to 23, further comprising the step of forming said cooling holes by additive manufacturing while forming the inner duct.

25. The method of claim 23, further comprising the step of forming said cooling holes by boring the inner duct prior to placing said first shell and second shell around the inner duct.

26. The method of any one of claims 17 to 25, wherein the ratio between a thickness of the inner duct and an equivalent diameter of the inner duct is at least 0.02, preferably at least 0.025 along the entire length of the transition piece.

27. The method of any one of claims 17 to 26, wherein the cooling annulus is empty and the outer duct is solid.

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