A TURBOMACHINE CONTAINING FIXED BLADES HAVING A SINGLE HOOK

Abstract

A stationary blade (17) component for an expander (1) of a turbomachine is disclosed, which comprises an outer platform (71), in turn including: a radially outer surface (71.1), a radially inner surface (71.2), a forward edge (71.3), an aft edge (71.4), and a mechanical coupling feature adapted to mechanically attach the outer platform to a supporting structure (18) of a turbomachine. The stationary blade component further includes at least one airfoil (75) extending from the radially inner surface of the outer platform and comprising a leading edge (75.1) and a trailing edge (75.2). The mechanical coupling feature comprises a single forward hook (77) projecting from the radially outer surface of the outer platform and oriented towards the aft edge (71.4) of the outer platform.

Description

TURBOMACHINE INCLUDING STATIONARY BLADES HAVING A SINGLE HOOKDESCRIPTIONTECHNICAL FIELD

[0001] Exemplary embodiments of the present disclosure pertain to turbomachines. Specifically, embodiments disclosed herein, pertain to power-generating turbomachines, i.e expanders or turbines. Embodiments disclosed herein are particularly adapted to supercritical CO2 expanders. As understood herein, a supercritical CO2 expander is an expander wherein the working fluid contains mainly or almost exclusively carbon dioxide and wherein in at least one portion of the flow path the carbon dioxide is in supercritical conditions.BACKGROUND ART

[0002] Turbines or expanders include combustors which ignite a pressurized gaseous mixture containing fuel and oxidant. The resulting pressurized flow of hot combustion gas is expanded in an expansion flow path, including one or more expansion stages. Each expansion stage includes at least one annular row or array of stationary vanes, aka stationary blades, and one annular row or array of rotor blades, which form part of a turbine rotor, arranged for rotation in a turbine casing.

[0003] In some expanders or turbines, the stationary vanes or stationary blades of at least one expansion stage are supported in a cantilever fashion from a supporting member, such as a ring mounted in the casing of the turbomachine. One or more stationary vanes, or vane components, usually include an arcuate outer platform, wherefrom one or more airfoils, forming the actual stationary blades or vanes, project radially inwardly, i.e. towards the rotation axis of the rotor. Usually, the innermost end of the airfoil(s) is (are) coupled to an arcuate inner platform, which forms part of the stationary vane component. The stationary vane components, including the outer platform, the airfoil(s) and the inner platform affixed thereto, are subject to high thermal and mechanical stresses as they face the expansion flow path and are therefore in contact with the expanding flow of hot and pressurized combustion gas.

[0004] A maj or concern regarding combustion of fossil fuels relates to the productionof carbon dioxide, a greenhouse gas which is considered one of the main contributors of global warming and climate changes.

[0005] In recent years, thermal cycles have been developed, aimed at reducing the environmental impact of the power generation cycles using fossil fuels. For this purpose, 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 is high, both in terms 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 the 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, have been developed, wherein fuel, such as natural gas, is blended into a mixture of an oxidant consisting mainly of oxygen (O2) and carbon dioxide (CO2) at high pressure. The blend of fuel, oxygen and carbon dioxide bums in a combustor of an expander, thus producing a pressurized combustion gas consisting exclusively or almost exclusively of carbon dioxide and water.

[0007] The combustion gas is expanded in the expander or turbine to generate mechanical power, which can eventually be converted into electric power by an electric generator driven into rotation by the expander or can be used for mechanical drive purposes. The exhausted combustion gas ,i.e., the flue gas discharged at the discharge side of the expander, is cooled in a regenerative heat exchanger and further chilled to condensate water, which is removed from the chilled flue gas. A main part of 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. A remaining part of the flue gas is removed and carbon dioxide contained therein is captured.

[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 notinclude nitrogen. The resulting combustion gas does not include nitrogen and the percentage carbon dioxide content thereof is substantially higher than in combustion gas from a standard gas turbine cycle. The higher carbon dioxide percentage in the combustion gas renders carbon capture more efficient and less expensive

[0009] Oxy-fuel cycles, such as those described above, are particularly promising in terms of efficiency, reduction of noxious emissions, and efficient carbon capture. However, they operate under CO2 supercritical conditions at the inlet of the expander and are characterized by elevated pressure and temperature values, as well as by high pressure drops in each stage of the expander and strong temperature gradients across machine components facing the expansion flow path.

[0010] These critical aspects affect the stationary vanes or stationary blades of the expander and the structure supporting them. Specifically, stationary vanes or blades experience high thermal gradients and require a degree of flexibility to accommodate mutual displacements of stationary components of the expander, caused by thermal expansion and mechanical load during operation. Conversely, mechanical coupling features, such as ribs or hooks, provided on the outer surface of the arcuate platforms wherefrom the stationary blades project radially towards the rotation axis of the expander, require enhanced stiffness thereby reducing flexibility of the platform. This increases thermally induced stresses, in particular in expanders comprising a combustor. These aspects become particularly challenging in supercritical carbon dioxide expanders, where pressure drops are particularly high, and more specifically in supercritical carbon dioxide expanders including a combustor, where thermal gradients are high.

[0011] An object of embodiments disclosed herein is to provide an improved design for the stationary vanes or blades of an expander, and relevant supporting structures thereof, which are particularly suited for use in expanders or turbines characterized by critical pressure and temperature conditions.SUMMARY

[0012] According to one aspect, disclosed herein is a stationary blade component for a turbomachine comprising an outer platform, which in turn includes: a radially outer surface, a radially inner surface, a forward edge, an aft edge, and a mechanicalcoupling feature adapted to mechanically attach the outer platform to a supporting structure of a turbomachine. The stationary blade component further includes at least one airfoil extending from the radially inner surface of the outer platform and comprising a leading edge and a trailing edge. The mechanical coupling feature comprises a forward hook projecting from the radially outer surface of the outer platform and oriented towards the aft edge of the outer platform, i.e towards the trailing edge of the airfoil forming the respective stationary blade of the stationary blade component.

[0013] In practical embodiments, each stationary blade component has a single hook, rather than two hooks, as commonly provided in the prior art. The single hook is positioned in an intermediate position between the forward edge and the aft edge of the outer platform, preferably nearer to the forward edge, and is oriented in the aft direction. In some embodiments the single hook can be discontinuous in a tangential direction. This can specifically be the case if the stationary blade component comprises more than one stationary blade or vane and has therefore a remarkable extension in the tangential direction, i.e. in a circumferential direction around the axis of the turbomachine.

[0014] The radial dimension of the aft portion of the stationary blade component is thus reduced, compared with the prior art designs, which reduces the thermally induced stresses in the stationary blade component, and improves the useful life thereof.

[0015] Moreover, mounting of the stationary blade components on the supporting structure is faster and simpler.

[0016] The forward hook can be positioned in an intermediate position between the forward edge and the aft edge of the outer platform. In some embodiments, the outer platform comprises a rib projecting radially outwardly from the radially outer surface of the outer platform and forming a resting surface adapted to rest against the supporting structure of the turbomachine. The rib can be positioned at the aft edge of the outer platform, or between the aft edge of the outer platform and the forward hook.

[0017] According to another aspect, disclosed herein is an expander comprising at least one annular array of stationary blade components housed in the casing, and a rotor, housed in the casing for rotation therein. The stationary blade components of the annular array of stationary blade components are mounted on at least one respectivesupporting structure housed in the casing. Each stationary blade component comprises an outer platform, which in turn comprises: a radially outer surface, a radially inner surface, a forward edge, and an aft edge. Each stationary blade component further comprises at least one airfoil extending radially from the radially inner surface of the outer platform towards the rotation axis of the rotor, and a forward hook, which projects from the radially outer surface of the outer platform and oriented towards the aft edge of the outer platform; wherein each forward hook engages into a circular groove formed in an annular surface of the respective supporting structure, the circular groove having a forward-facing opening for insertion of the forward hooks.

[0018] Differently from the stationary blade components of the prior art, the stationary blade components according to the present disclosure include each a single hook, oriented in an aft direction, adapted to engage an annular slot formed in a supporting structure of the turbomachine. The single hook is arranged in a position intermediate the forward-to-aft extension of the platform, near the forward edge thereof, for instance in an intermediate position between the forward edge and the aft edge. No aft hook is provided. Assembling the stationary vane components, or stationary blade components is thus much easier and faster than in the turbomachines of the prior art. Moreover, the stationary blade component can have an optimal shape in terms of heat-induced stresses.

[0019] Specifically, the design disclosed herein reduces the radial extension of the back side ribs radial height improving nozzle sectors life; 2) assembly the nozzle sectors along the longitudinal engine axisBRIEF DESCRIPTION OF THE DRAWINGS

[0020] Reference is now made briefly to the accompanying drawings, in which: Fig.l illustrates a sectional view of an expander in one embodiment;Fig.2 illustrates an enlarged detail of Fig.l;Fig.3 illustrates a sectional view of an expander in another embodiment; and Fig.4 illustrates an enlarged portion of Fig.3.DETAILED DESCRIPTION

[0021] A sectional view of one embodiment of an expander 1 according to the present disclosure is shown in Fig.l. The section is taken along a plane containing arotation axis A-A of the expander. The sectional view shows only half expander, which is axial-symmetrical.

[0022] The expander 1 includes an outer casing 3 and an inner casing 5. The outer casing 3 can include a main body 3 A and a closure 3B on the aft side of the expander. The main body 3A and the closure 3B are coupled, through respective flanges, along a plane orthogonal to the rotation axis of the expander. In this embodiment, therefore, the outer casing 3 is a so-called vertically split casing.

[0023] A combustor, such as a can combustor including a plurality of combustion chambers 7, is positioned at the forward side of the expander 1, upstream of the first annular row of stationary vanes or blades. An annular chamber 6 is positioned between the outer casing 3 and the inner casing 5.

[0024] As used herein “forward” and “aft” are referred to the direction of flow of the process gas through the expander 1. Therefore, “forward” indicates a position on the side of the combustor chambers 7 and “aft” indicates a position on the side opposite the combustor chambers 7, i.e., the discharge side of the expander 1.

[0025] The expander 1 further comprises a rotor 11 housed in the inner casing 5 and adapted to rotate around the rotation axis A-A. The rotor 11 comprises a rotor shaft 13 and a plurality of annular rows, arrays, or sets of rotor blades. In the exemplary embodiment of Fig.1, the rotor 11 comprises eight annular arrays of rotor blades. Each array of rotor blades comprises rotor blades circumferentially arranged around the rotation axis A-A of the rotor 11. The rotor blades are labeled 15.j, where j indicates the position of the array in the forward-to-aft direction. Specifically, the rotor blades of the first annular array are labeled 15.1, the rotor blades of the last annular array are labeled 15.8; the blades of the jtharray are labeled 15.j. As used herein, reference number 15 refers to rotor blades of a generic annular array of rotor blades.

[0026] An annular row, array, or set of stationary blades is positioned upstream of each annular array of rotor blades 15.j. The blades of the annular arrays of stationary blades are labeled 17.j and are circumferentially arranged around the rotation axis A- A. As will be explained below in more detail, each blade of the annular arrays of stationary blades forms part of a stationary blade component. Each annular array of stationary blades thus comprises a set, i.e. an array of annularly, i.e. circumferentially,arranged stationary blade components.

[0027] The stationary blades are also referred to as vanes. Pairs of adjacent stationary blades or vanes define respective nozzles which orient the expanding gas in the correct orientation with respect to the downstream annular row or rotor blades 15.

[0028] More specifically, the blades of the most upstream annular array of stationary blades are labeled 17.1, the blades of the most downstream array of stationary blades are labeled 17.8. In general, the blades of the jthannular array of stationary blades are labeled 17.j. As used herein reference number 17 refers to stationary blades of a generic array of stationary blades. As used herein “upstream” and “downstream” refer to the direction of flow of the flue gas flowing through the expander 1.

[0029] In embodiments, the stationary blades or vanes are formed by or include airfoils, which are coupled to respective inner and outer platforms, as will be described in more detail below.

[0030] In general, an outer platform, an inner platform, if present, and one or more airfoils extending radially inwardly from the outer platform, form a stationary blade component. Each annular array of stationary blades thus includes an annular, i.e. circular, array of stationary blade components arranged around the rotation axis of the expander 1.

[0031] As will be clarified in more detail below, a stationary blade component can include one or more airfoils, i.e. one or more blades, rigidly coupled with the outer platform. Stationary blade components comprising one, two or three airfoils integrally formed with the outer platform (and with the inner platform when present) are usually referred to as “singlets”, “doublets” or “triplets”, respectively.

[0032] Each annular array of stationary blades 17.j and respective annular array of rotor blades 15.j form together a stage of the expander 1.

[0033] The stationary blades 17 are housed in the inner casing 5. As will be described in more detail below, the stationary blades 17 of one, some or each annular array of stationary blades are not mounted directly on the inner casing 5, but rather mounted on at least one respective ring, or on a pair of adjacent rings. In some embodiments, the stationary blades of at least one expansion stage can be connected directly to thecasing 5, as shown for instance for the last expansion stage in Fig.1.

[0034] In the embodiment of Fig.1 the rings are formed as components which are mechanically separate from the casing and which can be retained against rotation by suitable locking pins, not shown, and against axial displacement by suitable axial abutments, as shown in Fig.l. Novel features pertaining to the manner in which the stationary blades or vanes, and more specifically the stationary blade components, are mechanically coupled to the supporting structure inside the casing can be used also in combination with other supporting structures, e.g. including supporting rings which are integral with the casing or part thereof.

[0035] In the embodiment disclosed herein, the expander comprises an inner casing 5 and an outer casing 3. Specifically, the inner casing 5 is a horizontally split casing and the outer casing 3 is a horizontally split casing. This is particularly beneficial in case of a supercritical carbon dioxide expander, or other expanders which processes gas under similar thermodynamic conditions involving high pressure drops across some at least of the expansion stages, and process gases having high heat transfer coefficients which determines high temperature gradients. Nevertheless, in other embodiments, the novel features relating to the way in which the stationary vane components or stationary blade components are coupled to the supporting structure can be used in a different expander structure, for instance including a single casing, or a vertically split inner casing and a horizontally split outer casing, for instance.

[0036] Turning now to Fig. l, in the embodiment shown therein all stationary blades 17 of each annular array or set of stationary blades, except those of the first stage and of the last stage, are supported by a respective single ring. The rings are labeled 18.

[0037] In the embodiment of Fig. l, each ring can be a one-piece ring, i.e. can be monolithic and develop around 360°.

[0038] In some embodiments, each ring 18 can be split into two ring components, each ring component extending around 360° and the ring components being coaxial to one another. The ring components can be coupled to one another and constrained to one another, for instance by interference-fitting or shrink-fitting.

[0039] The stationary blades of the first stage are supported by a radially outer ring and a radially inner ring. More specifically, each ring is labeled 18.j (wherein j=l -7).As used herein, reference number 18 refers to a generic ring. Each annular array of stationary blades 17.j (wherein j =1-7), and more precisely each stationary blade component, whereof the stationary blade is part, is mounted on a respective ring 18.j. The most downstream annular array of stationary blades 17.8 is mounted on a ring 18.8 which forms part of an aft portion 5B of the inner casing 5, i.e. is directly mounted on a portion of the casing itself. The casing therefore features itself a supporting structure, e.g. in the form of a groove, for the stationary blade components. An arrangement with a supporting structure not including rings inside the casing, simplifies the overall structure of the expander, and can be used, in particular in the most downstream expander stages, where the thermodynamic conditions (pressure and temperature) are less critical. The aft portion 5B of the inner casing is coupled to a main body 5A of the inner casing 5. The main body 5 A can in turn be formed by a plurality of casing portions. Each casing portion can be split along a plane containing the rotation axis A-A of the rotor 11, i.e., the inner casing 5 is a so-called horizontally split casing.

[0040] In the area of contact between abutting rings, or between ring and casing, stationary seals (not shown) can be positioned, to prevent, limit or control the amount of cooling or purging gas leaking through the mutually contacting surfaces.

[0041] Each annular array of rotor blades 15.j is surrounded by a respective shroud. The shrouds are labeled 19.1, ... 19.j, 19.8. In the embodiment of Fig.1, the shrouds 19.2 to 19.7 are supported each by the respective ring 18, on which the stationary blades positioned immediately upstream of the shroud are mounted. Thus, shroud 19.j is mounted on ring 18.j that supports the annular array of stationary blades 17.j, and more precisely the respective stationary blade components, whereof the stationary blades form part, wherein j=2 to 7.

[0042] In the embodiment of Fig.1, the first shroud 19.1 that surrounds the first, i.e., the most upstream annular array of rotor blades 15.1, is supported by an auxiliary ring 18.0, which does not support any stationary blade, and which is positioned between the first ring 18.1 and the second ring 18.2.

[0043] The rings 18 form a structure which surrounds the stationary blades 15 and the shrouds 19 and support them, and which separates the flow path of the hot expanding gas from the inner casing 5. As mentioned, the stationary blades of the first stage are supported also by an inner ring. As will be explained in greater detail below, therings 18 are structured such that they transfer to the inner casing 5 only axial reaction forces generated by the hot gas which expands along the flow path. The supporting structure thus designed, which prevents transfer of radial reaction forces between the rings and the casing allows a differential thermal expansion of the rings on the one side and the casing on the other, said differential thermal expansion being determined by a thermal gradient.

[0044] Moreover, the rings 18 decouple the hot gas flow path from the inner casing 5 and form with the latter a cooling fluid gap 61, described in greater detail below, between the rings 18 and the inner casing 5. Calibrated flow passages (see e.g. passage 62 in ring 18.5) are provided in the structure formed by the rings, such that a controlled amount of cooling gas can flow from the cooling fluid gap towards the hot gas flow path.

[0045] The first ring 18.1 supports the stationary blades 17.1 of the first annular array of stationary blades, i.e., the most upstream set of stationary blades, which are arranged at the outlet of the combustor chambers 7. The stationary blades 17.1 of the first row of stationary blades or vanes are constrained to the ring 18.1 and to an inner annular support 20, i.e. the stationary blades 17.1 have airfoils which are coupled are supported at both the radially outer end and the radially inner end thereof.

[0046] The remaining stationary blades or vanes 17.j are mounted in a cantilever fashion on the respective ring 18.j, as best shown in the enlargement of Fig.2. The latter illustrates an enlargement of the stationary blades or vanes 17.2 of the second expansion stage of the expander 1. As can be seen in Fig. l, however, in this embodiment also the stationary blades of the subsequent expansion stages, namely blades 17.3-17.8 are mounted in a cantilever fashion from the respective ring 18.3-18.7 and (with respect to blades 17.8) from the casing portion 5B. The following description of the mechanical structure which couples the stationary blades or vanes to the rings or to the inner casing referred to the second expansion state shown in Fig.2 applies also to the remaining expansion stages.

[0047] Referring now to Fig.2, but with general reference to any expansion stage and relevant stationary blades 17 starting from the second expansion stage, each stationary blade component or stationary vane component comprises at least one stationary vane or stationary blade 17 (the stationary blade 17.2 in Fig.2) and a respective outerplatform 71. Each stationary vane component, or stationary blade component, can further include an inner platform 73. The outer platform 71 features the coupling members which connect the stationary blade 17 to the respective supporting ring 18 (18.2 in Fig.2) or directly to the casing.

[0048] The outer platform 71 comprises a radially outer surface 71.1 facing the ring 18. The outer platform 71 further comprises a radially inner surface 71.2 facing the inner platform 73 and the rotation axis A- A of the rotor 11.

[0049] The inner platform comprises a radially outer surface 73.1 facing the outer platform 71, and a radially inner surface 73.2 facing the rotation axis A-A. The surfaces 73.1 and 73.2 represent the boundaries of the expansion flow path.

[0050] The outer platform 71 further comprises a forward edge 71.3 and an aft edge 71.4, as well as a mechanical coupling feature adapted to mechanically attach the outer platform 71 to a supporting structure, which in Fig. 2 is featured by the supporting ring 18.2. As noted, the remaining annular rows of stationary blades or vanes 17.3-17.7, and the stationary vane components, whereof they form part, are attached to a supporting structure featured by the respective ring 18.3-18.7, while in this embodiment the most downstream row of stationary blades 17.8 is directly attached, through the respective outer platforms, to the inner casing 5.

[0051] It shall be noted that each outer platform 71 can be integrally formed with a single stationary blade 17, i.e. each stationary blade 17 may have its own platform. In this case each stationary blade component will comprise a single airfoil. This, however, is not mandatory. In some embodiments, or for one or more expansion stages of the expander, the stationary blade components may have an arcuate extension and include two or more stationary blades 17, i.e. two or more airfoils, integrally formed with a single outer platform 71. The stationary blades integrally formed with a common outer platform 71 can also be coupled to a common inner platform 73.

[0052] In general, the inner platform 73 and the outer platform 71 are shaped such that, when the annular row of stationary blades 17 is assembled and connected to the respective ring 18, a plurality of platforms 71 and 73 are circumferentially arranged around the rotation axis A-A and form respective circular bands, framing therebetween a plurality of airfoils, described below.

[0053] Each stationary blade 17 comprises, or consists of, at least one airfoil 75, which extends from the radially inner surface 71.2 of the outer platform 71 to the radially outer surface 73.1 of the inner platform 73. Each airfoil 75 comprises a leading edge 75.1 oriented forwardly, and a trailing edge 75.2, oriented aftwardly.

[0054] The mechanical coupling feature comprises a forward hook 77, which projects from the radially outer surface 71.1 of the outer platform 71 and is oriented towards the aft edge 71.4 of the outer platform 71. The forward hook 77 comprises a foot 77.1 at the radially outer surface 71.1 of the outer platform 71 and a projection 77.2 at the distal end of the forward hook 77, the projection 77.2 being oriented in the aft direction, i.e. towards the aft edge 71.2 of the outer platform 71.

[0055] The forward hook 77 is positioned in an intermediate position between the forward edge 71.3 and the aft edge 71.4 of the outer platform 71. By ideally dividing the platform 71 into a forward portion 71F and an aft portion 71A, with a plane P-P orthogonal to the rotation axis A-A, and equidistant from the forward edge 71.3 and the aft edge 71.4, the foot 77.1 of the forward hook 77 is preferably positioned in the forward portion 7 IF of the outer platform 71. This means that the forward hook 77 is arranged on the side of the outer platform 71 nearer to the forward edge 71.1 thereof. In some embodiments, the distal projection 77.2 can project beyond the median plane P-P towards the aft edge 71.4 of the outer platform 71.

[0056] In some embodiments, the outer platform 71 comprises a rib 71.5 which projects radially outwardly from the radially outer surface 71.1 of the outer platform 71 and forms an abutment adapted to abut against the respective ring 18. The rib 71.5 extends adjacent the aft edge 71.4 of the outer platform 71 and, in the embodiment of Fig.2, the rib 71.5 engages a circumferential groove 18G formed in a radially inwardly facing surface of the ring 18. The bottom of the groove 18G forms a cylindrical resting surface on which the rib 71.5 abuts and whereto radially outwardly oriented reaction forces are transferred from the stationary blade 17 to the ring 18 or directly to the casing 5, or to any other intermediate supporting structure which connect the row of stationary blades 17 to the casing 5.

[0057] In some embodiments, the aft platform portion 71 A, which extends from the median plane P-P (and from the foot of the forward hook 77) to the aft edge 71.4, has a thickness in radial direction (arrow R in Fig.2), which is smaller thank the thicknessin radial direction of the forward platform portion 7 IF. Moreover, in some embodiments, the outer platform can comprise an indentation 71.6 on the radially outer surface, the indentation 71.6 can be positioned between the forward hook 77 and the aft edge 71.4 of the outer platform 71 and can extends approximately parallel to the forward edge 71.3 and the aft edge 71.4 of the outer platform 71.

[0058] Each forward hook 77 engages a circular groove 18A formed in an annular surface 18B of the respective ring 18. The annular surface 18B can be a planar, i.e. flat surface, orthogonal to the rotation axis A-A. The circular groove 18A has a forwardfacing opening for insertion of the forward hooks 77 and more specifically the distal projection 77.2 thereof, which is oriented in the aft direction.

[0059] A further embodiment of an expander according to the present disclosure is illustrated in Figs. 3 and 4. The sectional view of Fig.3 is taken along a plane containing a rotation axis A-A of the expander and illustrates only the first four expansion stages of the expander.

[0060] The expander 1 includes an outer casing 3 and an inner casing 5. The outer casing 3 can include a main body and a closure on the aft side of the expander 1, not shown, quite in the same way as main body 3 A and closure 3B of Fig.1. Similarly to Fig. l, also in the embodiment of Fig.3 the outer casing 3 can be a vertically split casing.

[0061] A combustor, such as a can combustor including a plurality of combustion chambers 7, is positioned at the forward side of the expander 1 upstream of the first annular row of stationary vanes or blades. An annular chamber 6 is formed between the outer casing 3 and the inner casing 5.

[0062] The expander 1 further comprises a rotor 11 housed in the inner casing 5 and adapted to rotate around the rotation axis A-A. The rotor 11 comprises a rotor shaft 13 and a plurality of annular rows, arrays, or sets of rotor blades circumferentially arranged around the rotation axis A-A. As mentioned, in Fig.3, just the first four expansion stages of the expander are shown, which include each a respective annular row or array of rotor blades 15 and a corresponding annular row or array of stationary vanes or stationary blades 17. Specifically, each annular row of stationary blades 17 and the corresponding annular row of rotor blades 15 form an expansion stage of the expander1.

[0063] The stationary blades 17 are housed in the inner casing 5. As will be described in more detail below, the stationary blades 17 of one, some or each annular array of stationary blades are not mounted directly on the inner casing 5, but rather mounted on at least one respective ring, labeled 18. In the embodiment of Fig.3, all stationary blades 17 of the first four expansion stages are supported by a respective single ring 18.

[0064] Each annular array of rotor blades 15 is surrounded by a respective shroud. The shrouds are labeled 19. In the embodiment of Fig.3, the shrouds 19 from the second expansion stage onwards are supported each by the respective ring 18, on which the stationary blades positioned immediately upstream thereof are mounted.

[0065] The shroud of the first expansion stage is supported by a ring 18 which is separate from the ring which supports the annular row of stationary blades of the first expansion stage, i.e. the row of stationary vanes or blades positioned immediately downstream of the combustor.

[0066] As described with respect to Figs 1 and 2, also in the embodiment of Fig. 3 the rings 18 form a structure which surrounds and supports the stationary blades 15 and the shrouds 19, and which separates the flow path of the hot expanding gas from the inner casing 5. As will be explained in greater detail below, the rings 18 are structured such that they transfer to the inner casing 5 only axial reaction forces generated by the hot gas which expands along the flow path. The supporting structure thus designed, which prevents transfer of radial reaction forces between the rings and the casing allows a differential thermal expansion of the rings on the one side and the casing on the other, said differential thermal expansion being determined by a thermal gradient.

[0067] Moreover, the rings 18 decouple the hot gas flow path from the inner casing 5 and form with the latter a cooling fluid gap 61, described in greater detail below, between the rings 18 and the inner casing 5. Calibrated flow passages (see e.g. passage 62 in Fig.l, not shown in Fig.3) are provided in the structure formed by the rings 18, such that a controlled amount of cooling gas can flow from the cooling fluid gap towards the hot gas flow path.

[0068] Referring now to the enlargement of Fig.4, but with general reference to any stationary blade 17 starting from the second expansion stage, each stationary blade or stationary vane 17 comprises an outer platform 71 and can further include an inner platform 73 (Fig.3). The outer platform 71 features the coupling members which connect the stationary blade 17 to the respective supporting ring 18, or to the casing directly.

[0069] The outer platform 71 comprises a radially outer surface 71.1 facing the ring 18 (or the casing). The outer platform 71 further comprises a radially inner surface 71.2 facing the inner platform 73 and the rotation axis A-A of the rotor 11.

[0070] The outer platform 71 further comprises a forward edge 71.3 and an aft edge 71.4, as well as a mechanical coupling feature adapted to mechanically attach the outer platform 71 to a supporting structure, which in Figs. 3 and 4 is featured by the supporting ring 18.

[0071] As mentioned in connection with Figs. 1 and 2, each outer platform 71 can be integrally formed with a single stationary blade 17, i.e. each stationary blade 17 may have its own outer platform 71. This, however, is not mandatory. In some embodiments, or for one or more expansion stages of the expander, the stationary blades 17 can be configured as arcuate segments, wherein each segment comprises two or more stationary blades 17, integrally formed with a single outer platform 71. The stationary blades integrally formed with a common outer platform 71 can also be coupled to a common inner platform 73.

[0072] In general, the inner platform 71 and the outer platform are shaped such that when the annular row of stationary blades 17 is assembled with the respective ring 18, a plurality of platforms 71 and 73 are circumferentially arranged around the rotation axis A-A and form respective circular bands, framing therebetween a plurality of airfoils, described below.

[0073] Each stationary blade 17 comprises, or consists of, an airfoil 75, which extends from the radially inner surface 71.2 of the outer platform 71 to the radially outer surface 73.1 of the inner platform 73 and comprising a leading edge and a trailing edge. Each airfoil 75 includes a leading edge 75.1 oriented forwardly, and a trailing edge 75.2, oriented aftwardly.

[0074] Similarly to Figs. 1 and 2, the mechanical coupling feature wherewith each stationary blade 17 is coupled to the ring 18 or other supporting structure housed in the casing, comprises a forward hook 77, which projects from the radially outer surface71.1 of the outer platform 71 and is oriented towards the aft edge 71.4 of the outer platform 71. More specifically, in the embodiment of Figs 3 and 4, similarly to Figs. 1 and 2, the forward hook 77 comprises a foot 77.1 at the radially outer surface 71.1 of the outer platform 71, and a projection or tooth 77.2 at the distal end of the forward hook 77, the projection 77.2 being oriented in the aft direction, i.e. towards the aft edge71.2 of the outer platform 71.

[0075] The forward hook 77 is positioned in an intermediate position between the forward edge 71.3 and the aft edge 71.4 of the outer platform 71. Considering a geometric plane P-P orthogonal to the rotation axis A-A, and equidistant from the forward edge 71.3 and the aft edge 71.4, which divides the outer platform 71 in a forward platform portion 71F and an aft platform portion 71 A, the forward hook 77 is preferably constrained to the outer platform 71 in the forward portion 7 IF thereof. Similarly to the embodiment of Figs. 1 and 2, therefore, the forward hook 77 is arranged on the side of the outer platform 71 nearer to the forward edge 71.1 thereof. In some embodiments, the distal projection 77.2 can project beyond the median plane P-P towards the aft edge 71.4 of the outer platform 71.

[0076] In some embodiments, the outer platform 71 comprises a rib 71.5 which projects radially outwardly from the radially outer surface 71.1 of the outer platform 71 and forms a resting, i.e. an abutting surface adapted to rest against the respective ring 18. The rib 71.5 extends adjacent the aft edge 71.4 of the outer platform 71 and, in the embodiment of Fig.4, the rib 71.5 is in contact with a cylindrical inner surface 18C of the ring 18. The diameter of the inner cylindrical surface 18C is such that each stationary blade 17 can be mounted on the ring 18 with a forward-to-aft insertion movement.

[0077] The cylindrical surface 18C forms a cylindrical resting surface on which the rib 71.5 abuts and whereto radially outwardly oriented reaction forces are transferred from the stationary blade 17 to the ring 18 or directly to the casing 5. If no rings 18 are provided, the cylindrical surface 18C can be formed on the inner surface of the casing 5.

[0078] In some embodiments, the aft platform portion 71 A, which extends from theforward hook 77 to the aft edge 71.4, has a thickness in radial direction, which is smaller thank the thickness in radial direction of the forward platform portion 7 IF. Moreover, in some embodiments, the outer platform can comprise an indentation 71.6 on the radially outer surface. The indentation 71.6 can be positioned between the forward hook 77 and the aft edge 71.4 of the outer platform 71, and more precisely between the foot of the forward hook 77 and the rib 71.5, and can extend approximately parallel to the forward edge 71.3 and the aft edge 71.4 of the outer platform 71. As in the embodiment of Figs. 1 and 2, the indentation 71.6 provides a reduction of the thickness of the aft portion 71 A of the outer platform 71.

[0079] Each forward hook 77 engages a circular front groove 18A formed in an annular surface 18B of the respective ring 18. The annular surface 18B can be a planar, i.e. flat surface, orthogonal to the rotation axis A-A. The circular groove 18A has a forward-facing opening for insertion of the forward hooks 77 of the outer platforms 71, and more specifically the distal projection 77.2 thereof.

[0080] Differently from the embodiment of Figs 1 and 2, in the embodiment of Figs. 3 and 4 the rib 71.5 does not engage a radial groove of the ring 18, but rests on the inner cylindrical surface 18C thereof, which extends in a forward direction till the front planar annular surface 18B where the circular groove 18A is formed. With this arrangement, each stationary blade 17, with its respective outer platform 71, can be mounted on the supporting ring 18 thereof with a simple movement in a forward-to- aft direction (arrow f71 in Fig.4). The stationary blades 17 can therefore be mounted on a monolithic ring, i.e. on a ring which has a continuous extension of 360° around the symmetry axis thereof, i.e. has a monolithic development around 360°. A monolithic ring having no discontinuities in the tangential or circumferential direction is particularly beneficial in terms of ability to resist mechanical stresses, specifically in case of high pressure drops across the respective expansion stage.

[0081] In supercritical carbon dioxide expanders, for instance, pressure drops of 20 barA or higher, for instance 30 barA or higher, such as 40 barA or above, are feasible. A monolithic annular structure would be particularly useful in such turbomachines.

[0082] In both embodiments each stationary blade or stationary vane 17 is constrained to the casing (directly or through another supporting structure, such as a ring 18, through a forward hook 77 which is positioned towards the forward edge 71.3 ofthe outer platform 71, but is oriented in the aft direction, i.e. has a distal projection 77.2 which is oriented towards the aft edge 71.4 of the outer platform 71. Reaction forces between the supporting structure (ring 18 or casing 5) and stationary blades 17 are transmitted through the single hook 77 located near the forward edge of the platform 71, and oriented in the aft direction, and through the resting surface formed by the ribs 71.5 abutting against the cylindrical surface formed by the supporting structure (bottom of groove 18G or cylindrical surface 18C.

[0083] This results in a reduced radial extension of the platform 71, i.e. the radial height of the annular row of stationary blades 17. The aft portion of the platform, i.e the portion 71 A positioned downstream of the median plane P-P does not feature a hook, as it is usually the case in the stationary blades of the prior art. Rather, at the aft side of the platform 71 the radial load is transferred to the supporting structure (the ring 18 or directly the casing 5) through pressure contact between the rib 71.5 and the inwardly facing surface of the supporting structure. In Figs. 1 and 2 said inwardly facing surface is the bottom of the groove 18G, while in the embodiment of Figs. 3 and 4 the inwardly facing surface is the cylindrical surface 18C of the ring 18.

[0084] In addition to a reduced radial dimension of each annular row of stationary blades 17, the reduced thickness in the radial direction enabled by the novel structure described above allows reduces thermally induced stresses in the outer platform 71, as the aft portion 71 A of the platform, being thinner, may thermally deform when subject to thermal gradients, thus reducing the stresses induced thermally inside the annular row of stationary platform.

[0085] 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 following claims.

Claims

1. A fixed blade element for a turbomachine comprising an outer platform comprising: a radially outer surface, a radially inner surface, a front end, a rear end, and a mechanical connection element configured to mechanically attach the outer platform to the supporting structure of the turbomachine; at least one feather extending from a radially inner surface of the outer platform and comprising a leading edge and a trailing edge; wherein the mechanical connection element comprises a front hook protruding from the radially outer surface of the outer platform and oriented toward the rear end of the outer platform.

2. The element according to claim 1, in which the front hook is a single hook of a mechanical connection element.

3. An element according to claim 1 or 2, in which the hook has breaks in the tangential direction.

4. An element according to any of the preceding claims, in which the front hook is located in an intermediate position between the front end and the rear end of the outer platform.

5. An element according to any of the preceding claims, in which the outer platform comprises a rib projecting radially outward from the radially outer surface of the outer platform and forming a support surface adapted to rest on a support structure of the turbomachine.

6. The element according to claim 5, in which the rib is located on the rear end of the outer platform or between the rear end of the outer platform and the front hook.

7. An element according to any of the preceding claims, wherein the outer platform comprises the front part of the platform, extending from the front end to the front hook; and a rear portion of the platform extending from the front hook to the rear end; wherein the front portion of the platform has a thickness in the radial direction that is greater than the thickness in the radial direction of the rear portion of the platform.

8. An element according to any of the preceding claims, in which the outer platform comprises: a recess on the radially outer surface, wherein the recess is located between the front hook and the rear end of the outer platform and extends approximately parallel to the front end and the rear end of the outer platform.

9. An element according to any one of the preceding claims, comprising an inner platform having a radially outer surface and a radially inner surface; wherein a feather extends from the radially inner surface of the outer platform to the radially outer surface of the inner platform.

10. An expander comprising at least one annular array of fixed blade elements housed in a housing; and a rotor housed in the housing for rotation therein; a rotor comprising an axis of rotation and at least one annular array of rotor blades around the axis of rotation; wherein the fixed blade elements of at least one annular array of fixed blade elements are mounted on at least one corresponding support structure located in the housing; and wherein each fixed blade element of at least one annular array of fixed blade elements comprises: an outer platform comprising: a radially outer surface, a radially inner surface, a front end and a rear end; at least one feather extending radially from the radially inner surface of the outer platform to the axis of rotation of the rotor; and a front hook protruding from the radially outer surface of the outer platform and oriented toward the rear end of the outer platform; wherein each front hook engages with an annular groove formed in the annular surface of the corresponding support structure, wherein the annular groove has a forward-facing opening for inserting the front hooks.

11. The expander according to claim 10, wherein each fixed blade element of at least one annular array of fixed blade elements comprises a single hook.

12. An expander according to claim 10 or 11, wherein the front hook has breaks in the tangential direction around the axis of rotation.

13. An expander according to any one of paragraphs 10-12, in which the front hook of each element of the fixed blade is located in an intermediate position of the outer platform between the front end and the rear end of the outer platform.

14. An expander according to claim 10 or 13, in which the outer platform of each fixed blade element comprises a rib projecting radially outward from the radially outer surface of the outer platform and forming a cylindrical support surface adapted to rest on a radially inner cylindrical surface of a corresponding support structure.

15. An expander according to claim 14, in which the cylindrical support surface extends in a straight direction to the annular surface of the support structure; and the diameter of the inner cylindrical surface is such that each element of the fixed blade can be mounted on the support structure with the possibility of an insertion movement back and forth, as a result of which the front hook engages with the annular groove.

16. An expander according to any one of paragraphs 10-15, in which the support structure comprises a ring installed in the housing.

17. An expander according to claim 16, wherein the supporting structure comprises a ring with a continuous sweep around the circumference of 360°.

18. An expander according to any one of paragraphs 10-17, in which the edge of each fixed blade element passes near the rear end of its outer platform.

19. An expander according to any one of paragraphs 10-18, in which the outer platform of each fixed blade element comprises the front part of the platform, extending from the front end to the front hook; and a rear portion of the platform extending from the front hook to the rear end; wherein the front portion of the platform has a thickness in the radial direction that is greater than the thickness in the radial direction of the rear portion of the platform.

20. An expander according to any one of paragraphs 10-19, in which the outer platform of each element of the fixed blade comprises a recess on the radially outer surface, and the recess is located between the front hook and the rear end of the outer platform and extends approximately parallel to the front end and the rear end of the outer platform.

21. An expander according to any one of claims 10 to 20, wherein each fixed blade element comprises an inner platform having a radially outer surface and a radially inner surface; and the blade extends from the radially inner surface of the outer platform to the radially outer surface of the inner platform.

22. An expander according to any one of paragraphs 10-21, in which the housing includes an outer housing and an inner housing; wherein the inner housing is placed in the outer housing, and the rotor is supported for rotation in the inner housing.

23. An expander according to any one of paragraphs 10-22, further comprising a combustion chamber located upstream of the expansion channel of the flow passing through at least one array of fixed blade elements and rotor blades.

24. An expander according to any one of paragraphs 10-23, in which the support structure comprises a groove in the inner surface of the housing.