Abstract:
A radial centrifugal turbomachine with an adjusting device operatively active at the axial inlet of the turbomachine fixed casing, including: an annular member delimiting at least one radially inner passage at the axial inlet, wherein the member front surface and abutment surface of the casing delimit a radial annular passage. An actuator is connected to the member and to move axially and/or rotate according to a predefined angle to vary the radial annular passage and/or passage sections delimited between stator blades of the turbomachine first radial stage. A transmission shaft coaxial with the rotation axis and integral with the member is operatively connected to the actuator. The adjusting device allows adjusting the mass flow rate at the inlet to better adapt the performances of the turbomachine to the loading of the process/cycle and to implement a safety system dedicated to stopping the turbomachine in case of an urgent stop request.

Description:
FIELD OF THE FINDING 
       [0001]    The object of the present invention is a radial centrifugal turbomachine. By radial centrifugal turbomachine (or “outflow” turbomachine) it is intended a turbomachine in which the flow of the fluid with which it exchanges energy is radially directed from the center towards the periphery of the turbomachine for at least one part of the travel completed in the turbomachine itself. The present invention is applied both to the drive turbomachines (turbines) and to the working turbomachines (compressors). Preferably but not exclusively, the present invention refers to the expansions turbines. Preferably but not exclusively the present invention relates to the expansion turbines for the production of electrical and/or mechanical energy. Preferably but not exclusively, the present invention refers to the expansion turbines used in the apparatuses for the production of energy which exploit geothermal sources, preferably by means of steam Rankine cycle or organic Rankine cycle (ORC). More in detail, the present invention is situated in the scope of methods and devices adapted to vary the geometry of such turbomachines. 
       BACKGROUND OF THE FINDING 
       [0002]    The public document WO 2012/143799, on behalf of the same Applicant, illustrates an expansion turbine which comprises a fixed casing having an axial inlet and a radially peripheral outlet, a single rotor disc mounted in the casing and rotatable around a respective rotation axis, multiple annular series of rotor blades mounted on a front face of the rotor disc and arranged around the rotation axis, multiple annular series of stator blades mounted on the casing, facing the rotor disc and radially alternated with the rotor blades. 
         [0003]    The public document WO 2013/108099 illustrates a turbine for the expansion of an organic fluid in a Rankine cycle provided with arrays of rotor and stator blades which are alternated in a radial direction. The supply of the vapor in the turbine is obtained in frontal direction. In a first section of the turbine, defined high pressure, a first expansion of the work fluid is provided in a substantially radial direction. In a second section, defined low pressure, second expansion of the work fluid is provided in a substantially axial direction. The stator blades are supported by an external casing of the turbine. 
         [0004]    Turbomachines are usually characterized by conditions of the entering fluid (pressure and temperature) that are different from the conditions of the same fluid upon exiting. In the expansion turbines (drive turbomachines) like those described above, the entering fluid has higher pressure and temperature than it does upon exiting. In the working turbomachines, inlet pressure and temperature are instead lower than those at the output. 
         [0005]    Turbomachines with variable geometry are known that are adapted to allow the operation thereof with different pressure and temperature conditions (such turbomachines allow for multiple different operation curves). 
         [0006]    The known document DE3844189 illustrates a centrifugal radial compressor provided at the outlet of a radial diffuser provided with a movable blading. In particular, the diffuser comprises main fixed blades and movable blades. The movable blades can translate parallel to the chord of the profile, guided by suitable guides. 
         [0007]    The known document U.S. Pat. No. 5,454,225 illustrates an exhaust turbine of a turbocompressor comprising an annular duct which surrounds the rotor and has inlet ports with guide vanes. The guide vanes comprise movable portions for adjusting the passage cross section and controlling the entering flow. 
         [0008]    The known document U.S. Pat. No. 4,403,914 illustrates a device with variable geometry usable in turbine motors. The turbine motor comprises a support housing, a rotor contained in the housing, and a pair of mutually spaced walls which form an annular passage, which is radially extended and is open at one end towards the rotor. An embedded annular channel is formed in one of the walls and a ring is mounted in such channel and is movable between a retracted position, in which it is retracted in the channel, and an extended position, in which it projects into and restricts the annular passage in a variable manner. A plurality of blades are fixed to the ring, are extended through the annular passage and are slidably housed in slits formed in the other wall. 
         [0009]    The document U.S. Pat. No. 3,378,229 illustrates a radial centrifugal turbine which is provided, at an axial inlet thereof, with a plurality of inlet nozzles that can be rotated around respective axes in order to operate outside of design conditions. 
         [0010]    The document U.S. Pat. No. 3,639,075 illustrates a radial flow compressor provided with inlet guide vanes which are adjustable by making them rotate around their own axes. 
       SUMMARY 
       [0011]    In such context, the Applicant has perceived the need to improve the above-described radial centrifugal turbomachines by making them more flexible and safer. 
         [0012]    In particular, the Applicant has set the following objectives:
       implement a device that allows regulating the mass flow rate in a radial centrifugal turbomachine independent of the pressure and temperature conditions at the machine inlet, in order to better adapt the performances of the machine to the loading of the process/cycle (optimization of the performances of the turbomachine and of the plant where it is situated);   implement a safety system dedicated to stopping the radial centrifugal turbomachine, in an effective and quick manner, in case of an urgent stop request, in this manner eliminating ON/OFF block valves present in the known turbomachines and usually placed on the inlet line (increase of safety of the turbomachine and of the entire plant);   ideate a device configured for obtaining the above-listed objectives, which is simultaneously structurally simple and reliable and whose maintenance is easy and facilitated;   ideate a device configured for obtaining the above-listed objectives which has the least possible effect on the entering fluid flow.       
 
         [0017]    The Applicant has found that the above-indicated objectives and still others can be achieved by integrating in the radial centrifugal turbomachine, at the axial inlet thereof, an adjusting device comprising an annular member with controlled axial and/or rotary movement, capable of varying a radial annular passage inside the turbomachine and adjacent to a first radially inner stage thereof and/or of varying passage sections delimited by the stator blades of the abovementioned first stage. In particular, the indicated objectives and still others are substantially reached by a radial centrifugal turbomachine according to one or more of the enclosed claims and/or in accordance with one or more of the following aspects. 
         [0018]    In the present description and in the enclosed claims, with the adjective “axial”, it is intended to define a direction directed parallel to a rotation axis “X-X” of the turbomachine. With the adjective “radial”, it is intended to define a direction directed like rays extended orthogonally from the rotation axis “X-X”. 
         [0019]    More specifically, the present invention relates to a radial centrifugal turbomachine, comprising: 
         [0000]    a fixed casing;
 
a rotor disc installed in the casing and rotatable in the casing around a respective rotation axis;
 
a ring of rotor blades mounted on a front face of the rotor disc and coaxial with the rotation axis, in which the rotor blades project axially from the front face of the rotor disc;
 
a ring of stator blades, radially inner and concentric with respect to the ring of rotor blades, in which the stator blades extend axially between the casing and the front face of the rotor disc, in which the stator blades delimit between them passage sections;
 
in which the casing has an axial inlet radially inner with respect to the ring of stator blades;
 
an adjusting device operatively active at the axial inlet;
 
in which the adjusting device comprises:
       an annular member delimiting at least one radially inner passage for the fluid situated at said axial inlet, in which a front surface of the annular member and an abutment surface of the casing delimit between them a radial annular passage;   at least one actuator connected to the annular member and configured to move axially and/or to rotate around the rotation axis (X-X) according to a predefined angle said annular member so as to vary the radial annular passage and/or the passage sections;   a transmission shaft coaxial with the rotation axis, integral with the annular member and operatively connected to the actuator.       
 
         [0023]    The ring of rotor blades together with the respective ring of stator blades form a radial stage of the radial centrifugal turbomachine. In some embodiments, the turbomachine has a single radial stage. In other embodiments, the turbomachine has a plurality of concentric radial stages placed radially in succession. In some embodiments, in addition to the radial stage or radial stages, the radial centrifugal turbomachine comprises at least one auxiliary axial stage situated at a radially peripheral zone of the rotor disc and situated downstream, with reference to the path of the work fluid, of the radial stages. Preferably, said at least one auxiliary axial stage comprises a plurality of auxiliary stator blades directed radially and mounted on the casing and a plurality of auxiliary rotor blades directed radially and placed downstream of the stator blades. Preferably, the auxiliary rotor blades of the auxiliary axial stage are mounted on the rotor disc and are extended radially away from the same. 
         [0024]    In some embodiments, the turbomachine is a turbine. 
         [0025]    In other embodiments, the turbomachine is a compressor. 
         [0026]    The present invention also regards a plant for the generation of electrical energy comprising the radial centrifugal turbomachine according to the invention. Preferably, the plant is of the type with ORC (Organic Rankine Cycle) cycle and exploits, as source, the energy of geothermal resources. 
         [0027]    In one embodiment, the annular member is movable both in rotation and axially upon command of an axial and rotation actuator. 
         [0028]    In one embodiment, the annular member is only movable axially upon command of an axial actuator. 
         [0029]    In one embodiment, the annular member is only movable in rotation upon command of an rotation actuator. 
         [0030]    In one aspect, the annular member is axially movable, like a piston, up to a closed configuration for blocking the flow of fluid through the turbomachine. Preferably, in said closed configuration, the front surface of the annular member/piston lies against the abutment surface of the casing and the radial annular passage is closed. Preferably, between the front surface of the annular member and the abutment surface of the casing, at least one seal is interposed, for example an annular gasket coaxial with the rotation axis and mounted on the casing and/or on the annular member. 
         [0031]    In one aspect, the annular member is movable between a configuration a) of complete opening of the radial annular passage (area of the annular passage equal to 100%), in which the front surface of the annular member lies at a maximum distance from the abutment surface of the casing, and the abovementioned closed configuration b) (area of the annular passage equal to 0%). In one aspect, the annular member can be stopped in any intermediate position c) between the two (complete opening and closure) described above. In this case, the area of the radial annular passage is a function of the axial position of the annular member (area of the annular passage comprised between 0% and 100%). In condition a), there is the maximum passage area and hence, given the same inlet pressure, there is the maximum mass flow rate, or vice versa, with defined mass flow rate, there is the minimum pressure. In condition b), there is the minimum passage area, with the annular member in abutment against the gaskets, in which the mass flow rate is zero for any inlet pressure. In condition c), there is an intermediate condition, in which, with the variation of the position, the passage area is adjusted and hence the flow rate is adjusted, independent of the pressure. 
         [0032]    The Applicant has verified that the adjusting device, object of the present invention, introduces an adjustment parameter, i.e. the area of the radial annular passage, which allows increasing or decreasing the mass flow rate, maintaining a constant pressure ratio between inlet and outlet, or maintaining the mass flow rate constant, while changing the conditions between the turbomachine inlet and outlet. Such possibility from the standpoint of the thermodynamic cycle of the plant, in which the turbomachine is situated, translates into greater efficiency in input conditions different from the nominal conditions. In the field of plants for energy generation that exploit geothermal sources, the present invention allows varying the pressure upstream of the turbomachine (turbine) in an efficient (reversible) manner, i.e. regulating the process/cycle in which the turbine works upon varying the temperature conditions of the resource and/or also the flow rate of the resource. 
         [0033]    In one aspect, the adjusting device incorporates a safety device configured to quickly bring the annular member into the closed configuration in case of emergency. Such possibility is of fundamental importance when it is necessary to block the mass flow rate as quickly as possible in order to prevent damaging the turbomachine. The most critical event is the case of lack of power or “load rejection” in a turbine, in which the “brake” is lacking while the turbine is in generation phase at maximum power, and all the torque of the turbine is converted into acceleration. In this phase, it is necessary to stop the work fluid as quickly as possible in order to limit the acceleration of the turbine. 
         [0034]    In one aspect, the safety device comprises elastic means configured to push the annular member/piston towards the abutment surface and into the closed configuration. Preferably, said elastic means are pre-loaded. Preferably said elastic means comprise at least one mechanical spring. Preferably, the actuator acts axially in opposition to said elastic means. Preferably, the actuator is single-acting and moves the annular member away from the abutment surface of the casing and towards the configuration of complete opening by opposing the elastic means. In the case of any one failure of the axial actuator, due to the elastic means, the annular member is closed, zeroing the area of the radial annular passage and blocking the flow that feeds the turbine, stopping the same. The turbomachine therefore has intrinsic safety. In addition, the mechanical structure of the safety device is simple and hence reliable and relatively inexpensive. The structure of the safety device according to the invention also allows closing in very quick times, nearly halved with respect to the conventional safety systems with ON/OFF block valves present in the known turbomachines and usually placed on the inlet line. In addition, since there is practically no volume comprised between the closure zone (at the radial annular passage) and the blades of the first stage, the problem of the further acceleration of the rotor disc thrust by the fluid placed upstream of the conventional block valve is completely eliminated. Preferably, the elastic means are configured to bring the annular member into the closed configuration in a closing time comprised between about 0.02 s and about 0.1 s. In one aspect, the annular member has a diverging surface which delimits the radially inner passage. Said surface diverges in the flow advancement direction. Preferably, said diverging surface is convex or at least partially convex. Preferably, the diverging surface comprises a radially inner substantially circular surface, a circular surface that is substantially perpendicular to the rotation axis and defining the front surface and a convex surface of connection between said radially inner surface and said front surface. 
         [0035]    In one aspect, the diverging surface delimits, with one surface of the casing facing the axial inlet, an annular duct converging towards the ring of stator blades. The axial inlet is continuously connected with the annular duct. The convergent annular duct serves to radially and outwardly deflect the flow of fluid that enters axially. In one aspect, the adjusting device comprises radial elements which connect the annular member to the transmission shaft. In other words, the annular member is borne by the transmission shaft by means of said radial elements. Preferably, the radial elements are struts. Preferably, the radial elements have aerodynamic shape in order to direct the flow and/or to minimize their affect on the entering flow. 
         [0036]    In one aspect, the adjusting device comprises a guide element integral with the casing, coaxial with the rotation axis and slidably engaged with the transmission shaft. In one aspect, the guide element is fixed to the surface of the casing facing the axial inlet. Preferably, the guide element is engaged with a distal end of the transmission shaft. In one embodiment, the guide element is a guide pin inserted in a hole of the transmission shaft. In a different embodiment, the guide element is a guide hole housing the distal end of the transmission shaft. In both cases, between said shaft and said guide element, a small chamber with variable volume is delimited. Such guide element ensures the precise positioning of the guide shaft and of the annular member and the fluidity of the movement thereof. Preferably, the guide element and/or the shaft have vent channels in fluid communication with the small chamber with variable volume. In this manner, the rapidity of the closure in case of emergency is further increased since the translation of the shaft does not encounter the resistance due to the compression of the air/gas contained in said small chamber with variable volume. 
         [0037]    In one embodiment, the annular member has a plurality of through slits, in which each of the stator blades is inserted in and slidably coupled with one of said through slits. In this manner, the axial movement of the annular member/piston involves a variation of the height of said stator blades and a variation of the passage sections delimited between said stator blades. Preferably, the through slits are obtained in the front surface of the annular member. In such embodiment, the stator blades are fixed with respect to the casing. 
         [0038]    In a different embodiment, each of the stator blades can be oriented around a respective adjustment axis parallel to the rotation axis and is operatively connected to the annular member. In one aspect, the annular member can be rotated around the rotation axis according to a predefined angle upon command of the rotation actuator. In one aspect, a rotation of the annular member involves a synchronized orientation of the stator blades around the respective adjustment axes and a variation of the passage sections delimited between said stator blades. In such embodiment, it is possible to vary the passage sections delimited between the stator blades of the first stage by means of the rotation of the annular member. Preferably, in such embodiment, the translational movement of the annular member, if present, is used only to quickly bring the annular member into the closed configuration in case of emergency while the rotary movement serves to throttle and adjust the mass flow rate. 
         [0039]    In one aspect, the adjusting device comprises a transmission ring coaxial with the rotation axis and radially outer with respect to the radially inner passage of the annular member. The transmission ring is axially fixed with respect to the casing and is movable in rotation with respect to said casing together with the annular member, upon command of the rotation actuator. The transmission ring is connected to each of the stator blades, preferably at points spaced from the respective adjustment axes, preferably by means of pins. 
         [0040]    In one aspect, the casing comprises an inlet duct mounted on the axial inlet, in which the transmission shaft is at least partly housed within the inlet duct. In one aspect, the inlet duct comprises a first portion directly connected to the axial inlet and coaxial with the rotation axis. In one aspect, the inlet duct comprises a second portion connected to the first portion and placed crosswise with respect to the rotation axis. 
         [0041]    In one aspect, the axial actuator and/or the rotation actuator are housed outside the inlet duct. Preferably, the axial actuator and/or the rotation actuator are connected to the transmission shaft at a proximal end, opposite the distal end of said transmission shaft, which exits outward from the inlet duct. This solution allows intervening on the actuator, in case of failure or for executing maintenance, in a quick and inexpensive manner, without having to dismantle parts of the turbomachine and without having to remove the fluid contained therein. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0042]    Such description will be set forth hereinbelow with reference to the enclosed drawings, provided only as a non-limiting example, in which: 
           [0043]      FIG. 1  illustrates a meridian section of a first embodiment of a radial turbomachine in accordance with the present invention; 
           [0044]      FIG. 1A  shows a front section of an enlarged portion of the turbomachine of  FIG. 1 ; 
           [0045]      FIG. 1B  shows an optional element of the turbomachine of  FIG. 1 ; 
           [0046]      FIG. 2  illustrates a meridian section of a second embodiment of a turbomachine in accordance with the present invention; 
           [0047]      FIG. 3  illustrates a meridian section of a third embodiment of a turbomachine in accordance with the present invention; and 
           [0048]      FIG. 4  illustrates a meridian section of a fourth embodiment of a turbomachine in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0049]    With reference to the abovementioned figures, reference number  1  overall indicates a radial centrifugal turbomachine or outflow turbomachine in accordance with the present invention. The turbomachine  1  illustrated in  FIG. 1  is an expansion turbine with a single rotor disc  2 . For example, such turbine  1  is employed in the field of plants for the generation of electrical energy of ORC (Organic Rankine Cycle) cycle type which exploit the geothermal resources as sources. 
         [0050]    With reference to  FIG. 1 , the rotor disc  2  is provided with a plurality of rotor blades  3 ,  3 ′ arranged in series of concentric rings on a respective front face  4 . Each series of rotor blades  3 ,  3 ′ is part of a rotor stage of the turbine  1 . The rotor disc  2  is rigidly connected to a shaft  5  which is extended along a rotation axis “X-X”. The shaft  5  is in turn connected to a generator (not illustrated). The rotor blades  3 ,  3 ′ are extended away from the front face  4  of the rotor disc  2  with leading edges thereof substantially parallel to the rotation axis “X-X”. The rotor disc  2  and the shaft  5  are housed in a fixed casing  6  and supported thereby such to be able to freely rotate around the rotation axis “X-X”. 
         [0051]    The fixed casing  6  comprises a front wall  7 , placed in front of the front face  4  of the rotor disc  2 , and a rear wall  8 , situated in front of a rear face  9  of the rotor disc  2  opposite the front face  4 . A sleeve  10  is integral with the rear wall  8  and rotatably houses the shaft  5  by means of the interposition of suitable bearings  11 . The front wall  7  has an opening defining an axial inlet  12  for a work fluid. Such axial inlet  12  is situated at the rotation axis “X-X” and is circular and concentric with respect to the same axis “X-X”. 
         [0052]    The fixed casing  6  also houses a plurality of stator blades  13 ,  13 ′ arranged in series of concentric rings and directed towards the front face  4  of the rotor disc  2 . The series of stator blades  13 ,  13 ′ are radially alternated with the series of rotor blades  3 ,  3 ′ to define a radial expansion path of the work fluid which enters through the axial inlet  12  and expands, moving radially away towards the periphery of the rotor disc  2 . In particular, the series of stator blades  13 ,  13 ′ and the series of rotor blades  3 ,  3 ′ are housed in an expansion volume  14 . The expansion volume  14  has a radially inner annular inlet opening  15  in fluid communication with the axial inlet  12  and a radially outer annular outlet opening  16 . Pairs of rings of stator blades  13 ,  13 ′ and rings of rotor blades  3 ,  3 ′ form radial stages of the turbine  1 . In the illustrated example, the turbine  1  has two radial stages. 
         [0053]    In the illustrated embodiment in  FIG. 1 , the turbine  1  also comprises an auxiliary axial stage comprising a plurality of auxiliary rotor blades  17  mounted on a peripheral edge  18  of the rotor disc  2  and extended radially away from said peripheral edge  18 . A plurality of auxiliary stator blades  19  is mounted fixed on an internal wall  20  of the fixed casing  6  and said auxiliary stator blades  19  radially project towards the interior of the casing  6 . Downstream of the radially outer annular outlet opening  16 , a passage  21  is situated that conveys the fluid towards the auxiliary axial stage  17 ,  19 . Downstream of the auxiliary axial stage  17 ,  19 , a diffuser  22  is placed in fluid communication with an exhaust of the turbine  1 . 
         [0054]    The turbine  1  comprises a support disc  23  integral with the casing  6  and mounted in front of a radially more internal portion of the front face  4  of the rotor disc  2  lacking rotor blades  3 . The support disc  23  lies facing the axial inlet  12  and bears, at a radially peripheral portion  24  thereof, the stator blades  13 ′ of a first radial stage  3 ′,  13 ′, that closest to the axial inlet  12 . Such stator blades  13 ′ of the first radial stage  3 ′,  13 ′ extend axially between the support disc  23  and the front wall  7  of the casing  6 . 
         [0055]    At the axial inlet  12 , an adjusting device  25  is operatively active, configured for controlling the mass flow rate of the entering fluid and for quickly blocking the entrance of fluid into the turbine  1  in case of emergency. 
         [0056]    The adjusting device  25  of  FIG. 1  comprises an annular member  26  coaxial with the rotation axis “X-X”. The annular member  26  is housed in the casing  6  immediately downstream of the opening defining the axial inlet  12  and lies axially interposed between the front wall  7  of the casing  6  and the support disc  23 . The annular member  26  has a radially inner surface  27  thereof which delimits a radially inner passage  28  and which diverges in the advancement direction of the flow, i.e. it opens radially towards the support disc  23 . Such diverging surface  27  comprises a substantially cylindrical radially inner surface  29 , a circular surface that is substantially perpendicular to the rotation axis “X-X” and defining a front surface  30  of the annular member  26  and a convex connector surface  31  between said radially inner surface  29  and said front surface  30 . The front surface  30  bears a sealing gasket  32 . The radially inner substantially cylindrical surface  29  has a diameter substantially equal to that of the axial inlet  12 . 
         [0057]    The diverging surface  27  of the annular member  26  delimits, together with a surface  33  of the support disc  23  directed towards the axial inlet  12 , an annular duct  34  converging towards the ring of stator blades  13 ′ of the first stage  3 ′,  13 ′. The passage section of such convergent annular duct  34  progressively decreases in moving from the axial inlet  12  towards the ring of stator blades  13 ′ of the first stage  3 ′,  13 ′. 
         [0058]    Between the front surface  30  of the annular member  26  and an abutment surface  35  belonging to the support disc  23  and placed in front of said front surface  30 , a radial annular passage  15  is delimited, adjacent to the stator blades  13 ′ of the first radial stage  3 ′,  13 ′, which defines the abovementioned radially inner annular inlet opening  15 . 
         [0059]    An inlet tube/duct  36  is mounted on the casing  6  at the axial inlet  12 . Such inlet duct  36  comprises a first portion  37  directly connected to the axial inlet  12  and coaxial with the rotation axis “X-X” and a second portion  38  connected to the first portion  37  and placed crosswise with respect to the rotation axis “X-X”. The illustrated first and second portion  37 ,  38  are placed at 90° with respect to each other. 
         [0060]    The annular member  26  is borne by a transmission shaft  39  which exits outward from the axial inlet  12  and is coaxial with the rotation axis “X-X”. The transmission shaft  39  is extended inside the inlet duct  36 . In particular, the transmission shaft  39  has a distal end  40  close to the annular member  26  and a proximal end  41 , opposite the distal end  40 , which exits outward from the inlet duct  36 . 
         [0061]    In the illustrated embodiment, the transmission shaft  39  is housed, in a manner such that it can axially slide and rotate, in a sleeve  42  connected and integral with the inlet duct  35  and with the casing  6 . The sleeve  42  is joined to the casing  6  by means of struts  43  which are radially extended between the sleeve itself  42  and a radially inner wall of the axial inlet  12 . The transmission shaft  39  crosses through the inlet duct  35  at a rear wall  44  of said inlet duct  36 . Also the rear wall  44  supports the transmission shaft  39 , in a manner such that said shaft  40  can axially slide and rotate. 
         [0062]    The distal end  40  of the transmission shaft  39  exits outward from the sleeve  42  and bears a plurality of radial elements  45  connected to the annular member  26 . The annular member  26  is borne by the transmission shaft  39  by means of said radial elements  45 . Such radial elements  45  preferably have an aerodynamic profile. 
         [0063]    The proximal end  41  of the transmission shaft  39  exits outward from the rear wall  44  of the inlet duct  36  and is operatively connected to an actuator  46 , schematically illustrated, which, when suitably controlled, allows axially moving the transmission shaft  39  in the sleeve  42  and in the rear wall  44  and making it rotate, within the sleeve  42  and the rear wall  44 , around a main axis thereof coinciding with the rotation axis “X-X” of the turbine  1 . The actuator  46  is also mounted on the rear wall  44 . 
         [0064]    Elastic means  47  are operatively active at the proximal end  41  of the transmission shaft  39  and are configured to push the annular member  26  towards the abutment surface  35 . The actuator  46  is single-acting and allows axially moving the annular member  26  away from the abutment surface  35 , opposing the elastic means  47 , and making it rotate according to a predefined angle around the rotation axis “X-X”. In the embodiment schematically illustrated in  FIG. 1 , the elastic means  47  comprise a preloaded mechanical spring integrated in the actuator  46 . 
         [0065]    A transmission ring  48  is arranged in the casing  6  around the annular member  26 . The transmission ring  48  is axially fixed with respect to the casing  6  but can rotate with respect to said casing  6  around the rotation axis “X-X”. In addition, the transmission ring  48  is constrained to the annular member  26 , for example by means of one or more keys  49 , such that it can rotate together with said annular member  26 . The key or keys  49  allow the free axial translation of the annular member  26  with respect to the transmission ring  48  and constrain said two elements in the rotation around the rotation axis “X-X”. The key or keys  49  are, for example, integrally mounted on the transmission ring  48  and inserted such that they can axially slide in slots  50  made in the annular member  26 . 
         [0066]    The stator blades  13 ′ of the first radial stage  3 ′,  13 ′, that closer to the axial inlet  12 , are mounted on the casing  6  and on the support disc  23  by means of small shafts that allow the orientation thereof around respective adjustment axes “Y-Y” parallel to the rotation axis “X-X” ( FIGS. 1 and 1A ). All the blades  13 ′ of the first radial stage  3 ′,  13 ′ are also connected, by means of pins  51  spaced from the respective adjustment axes “Y-Y” and parallel to the rotation axis “X-X”, to the transmission ring  47 . 
         [0067]    A control unit, not illustrated, is operatively connected and controls the actuator  46 . The control unit allows axially moving the annular member  26  by adjusting the axial size of the radial annular passage  15  ( FIG. 1 ) and also simultaneously orienting all the blades  13 ′ of the first radial stage  3 ′,  13 ′ around the adjustment axes “Y-Y” so as to vary passage sections  52  delimited between said stator blades  13 ′ ( FIG. 1A ). In the case of failure of the actuator  46  or of another drawback, the actuator  46  is deactivated and the elastic means  47  quickly push (in a few hundredths of a second, e.g. in about 0.05 s) the annular member  26  and the sealing gasket  32  against the abutment surface  35  of the support disc  23 , blocking the flow through the turbine  1 . 
         [0068]    In a second exemplifying embodiment of the turbine  1  according to the invention, illustrated in  FIG. 2 , the structure of the turbine  1  is substantially the same as that described above (for the same elements, the same reference numbers have been used) except for the following two aspects:
       the turbine is only radial, i.e. it does not have the auxiliary axial stage of  FIG. 1 , so that the annular opening of radially outer outlet  16  opens into the diffuser  22 ;   the annular member  26  can only rotate around the rotation axis “X-X” (as described for the first embodiment) but it cannot be axially moved, so that also the actuator  46  is only rotational and the elastic means  47  are not present.       
 
         [0071]    As is visible in  FIG. 2 , the adjusting device  25  lacks the transmission ring  48 . The annular member  26  is directly connected to the stator blades  13 ′ of the first stage  3 ′,  13 ′ by means of the pins  51  and can only rotate a predefined angle with respect to the casing  6  in which it is housed. The control unit allows simultaneously orienting all the blades  13 ′ of the first radial stage  3 ′,  13 ′ around the adjustment axes “Y-Y” so as to vary the passage sections  52  delimited between said stator blades  13 ′ (as in  FIG. 1A ). 
         [0072]    In a third exemplifying embodiment of the turbine  1  according to the invention, illustrated in  FIG. 3 , the turbine  1  is only radial, like that of the second embodiment. In addition, the annular member  26  can only be axially moved along the rotation axis “X-X” (as described for the first embodiment) but cannot rotate around said rotation axis “X-X”. The actuator  46  is only axial and the elastic means  47  are present. The blades  13 ′ of the first radial stage  3 ′,  13 ′ are mounted fixed on the casing  6 , i.e. they cannot be oriented by rotating them. The annular member  26  has a radial edge  53  in which the same number of through slits  54  are made as are on said blades  13 ′ of the first radial stage  3 ′,  13 ′. Each slit  54  has a shape with the form of the profile of the respective blade  13 ′. Each blade  13 ′ lies slidably inserted in the respective slit  54  and the radial edge  53  is free to translate around the blades  13 ′ while the annular member  26  is moved along the rotation axis “X-X”. The control unit allows axially moving the annular member  26  by adjusting a height “H” of the blades  13 ′ and the axial size of the radial annular passage  15 . In the case of failure of the actuator  46  or of another drawback, the actuator  46  is deactivated and the elastic means  47  quickly push (in a few hundredths of a second, e.g. in about 0.05 s) the annular member  26  and the sealing gasket  32  against the abutment surface  35  of the support disc  23 , blocking the flow through the turbine  1 . 
         [0073]    In a fourth exemplifying embodiment of the turbine  1  according to the invention, illustrated in  FIG. 4 , the turbine  1  is identical to that of the third embodiment but lacks the radial edge  53  and the slits  54 . 
         [0074]    In the first, third and fourth embodiments, the annular member  26  is axially mobile between a configuration of complete opening of the radial annular passage  15  (area of the annular passage equal to 100%), in which the gasket  32  of the annular member  26  lies at a maximum distance from the abutment surface  35  of the casing  6 , and the abovementioned closed configuration (area of the annular passage  15  equal to 0%). 
         [0075]    In the first embodiment, the annular member  26  can only be stopped in the configuration of complete opening (turbine in operation) or in that of complete closure (turbine blocked) while the adjustment of the mass flow rate is only carried out by means of the orientation of the blades  13 ′ of the first stage  3 ′,  13 ′. In other words, the axial movement of the annular member  26  is only exploited in order to place the turbine  1  in safety conditions in case of emergency. 
         [0076]    In the third and fourth embodiment, the annular member  26  can be stopped in any intermediate position between the two (complete opening and closure) described above, in order to carry out the throttling. In this case, the area of the radial annular passage is a function of the axial position of the annular member (area of the annular passage comprised between 0% and 100%). The axial movement of the annular member  26  (only movement) is exploited both to place the turbine  1  in safety conditions in case of emergency and to adjust the mass flow rate. 
         [0077]    In the above-illustrated first, third and fourth embodiments, the adjusting device  25  can comprise a guide element  55  integral with the casing  6 , coaxial with the rotation axis “X-X” and slidably engaged with the transmission shaft  39 . As is visible in  FIG. 1B , the guide element  55  is fixed to the support disc  23  and has a guide hole  56  housing the distal end  40  of the transmission shaft  39 . The guide hole  56  and the distal end  40  of the shaft  39  delimit a small chamber  57  with variable volume in fluid communication with the convergent annular duct  34  by means of vent channels  58  made in the shaft  39 . 
         [0078]    In other embodiments, not illustrated, the turbomachine  1  is a working machine (compressor). 
         [0079]    The present invention also relates to a plant which comprises the turbomachine  1  according to the invention. By way of example, the plant is of ORC (Organic Rankine Cycle) type for the generation of electrical energy and exploits, as source, the energy of geothermal resources. The plant includes a turbine  1  according to the invention. The turbine  1  allows improving the efficiency of the cycle (with respect to the known turbines with fixed geometry) in input conditions different from the nominal conditions as shown in the following examples. 
       EXAMPLES 
       [0080]    The plant is designed to produce a certain quantity of electrical power P (kW) with a consumption of source fluid m (kg/s) represented by water at 150° C. (temperature of the source) that can be cooled to not less than 80° C. (re-injection temperature). 
       Example 1 
       [0081]    Following the development of the geothermal field, a condition is encountered of a resource at lower temperature: 135° C. (instead of 150°). The objective is to maintain the produced power P substantially constant. 
         [0082]    If the temperature of the source is lowered with respect to the nominal temperature, it is necessary to decrease the evaporation temperature in order to maintain constant the “pinch point” (minimum temperature difference in the evaporator between the evaporation curve of the organic fluid and the cooling curve of the resource). 
         [0083]    In a known turbine with fixed geometry, the volumetric flow rate is constant and hence, by decreasing the evaporation pressure, the mass flow rate decreases and also the produced power decreases therewith. In order to overcome this drawback with a known turbine with fixed geometry, it is sufficient to increase the flow rate by increasing the re-injection temperature in order to maintain the “pinch point” constant. In order to increase the flow rate it is therefore necessary to make multiple geothermal wells with respect to those provided (increase of initial investment). 
         [0084]    With the turbine according to the invention, the “pinch point” is maintained by lowering the pressure at the turbine inlet and by increasing the flow rate (by varying the geometry at the inlet). As is visible in the following table, the increase of mass flow rate (139%) is in any case less than that required (156%) with a known turbine with fixed geometry with consequent lower resource consumption. 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                   
                   
                 VARIABLE 
               
               
                   
                 FIXED GEOMETRY 
                 GEOMETRY 
               
               
                   
               
             
             
               
                 Produced power 
                 100% 
                 100% 
               
               
                 Mass flow rate of source fluid 
                 156% 
                 139% 
               
               
                 Volumetric flow rate at the turbine 
                 100% 
                 134% 
               
               
                 inlet 
                   
                   
               
               
                 Turbine inlet pressure 
                 100% 
                  84% 
               
               
                 Thermal power input 
                 100% 
                 109% 
               
               
                 Source exhaust temperature 
                 90 
                 80 
               
               
                   
               
             
          
         
       
     
       Example 2 
       [0085]    Following the development of the geothermal field, a condition of the resource is encountered at higher temperature: 170° C. (instead of 150°). It is desired to maintain constant the mass flow rate of source fluid (number of geothermal wells). In this condition, in case of variable geometry one can proceed with an optimization of the thermodynamic cycle which allows more greatly exploiting the advantages of the higher temperature of the resource in terms of cycle efficiency. In case of variable geometry, the turbine inlet pressure is greater, with higher power P production, as is summarized in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                   
                 FIXED  
                 VARIABLE 
               
               
                   
                 GEOMETRY 
                 GEOMETRY 
               
               
                   
               
             
             
               
                 Produced power 
                 141% 
                 144% 
               
               
                 Mass flow rate of source fluid 
                 100% 
                 100% 
               
               
                 Volumetric flow rate at the turbine 
                 100% 
                  93% 
               
               
                 inlet 
                   
                   
               
               
                 Turbine inlet pressure 
                 124% 
                 131% 
               
               
                 Thermal power input 
                 129% 
                 129% 
               
               
                 Source exhaust temperature 
                 80 
                 80