Abstract:
A combustor transition adapted to guide combustion gases in a hot gas flow path extending between a can combustor and a first stage of turbine in a gas turbine is disclosed. The combustor transition includes a duct having an upstream end adapted for connection to the can combustor and a downstream end adapted for connection to a first stage of a turbine, wherein the downstream end includes an outer wall, an inner wall, a first and a second side wall. At least one side wall has a side wall extension which extends in a downstream direction beyond the outlet. The side wall extension at least partly encloses a first resonator volume and at least one side wall extension includes a resonator hole, which is configured as a neck of a Helmholtz-damper. A method for retrofitting a gas turbine and method for borescope inspection of a GT are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European application 12189722.7 filed Oct. 24, 2012, the contents of which are hereby incorporated in its entirety. 
     TECHNICAL FIELD 
     The invention relates to a combustor transition with a wall extension to provide space for a resonator volume arranged as a Helmholtz-damper for thermo acoustic decoupling of adjacent combustors, a turbine comprising such a combustor transition as well as a method for retrofitting a gas turbine with such a combustor transition. 
     BACKGROUND 
     Gas turbines with can combustors are known from various applications in power plants. The combustion process in such gas turbines can lead to dynamic can-to-can coupling. Such a dynamic or thermo acoustic coupling of gas turbine can combustors may lead to strong pulsations in particular to strong low frequency pulsations, which negatively affect the stability and lifetime of the combustor. This may lead to reduced lifetime or in extreme cases to a mechanical failure of the gas turbine. In order to mitigate thermo acoustic pulsations usually dampers or resonators are installed in the combustion chamber and/or staging of the fuel supply is done as described for example in the US2010/0313568. Since low frequency dampers require large volumes this solution is not favored. Fuel staging has a detrimental impact on the emission performance due to the creation of local hot spots (leading to NO x  emissions) and local cold spots (leading to CO emissions). 
     Coupling of the different can combustors takes place through:
         the turbine inlet in the area downstream of the combustors or the combustor transition the piece to the turbine and upstream of the leading edges of the turbine&#39;s first stage vanes,   the main air supply to the burners,   cooling and leakage air supply to combustor or   cross-ignition tubes arranged between cans.       

     SUMMARY 
     In order to avoid such pulsations effective decoupling of the can combustors is suggested. This invention is intended to decouple thermo acoustic interaction between cans via the turbine inlet, which is seen as the most dominant coupling path. This coupling path is dominant since it has the largest areas and involves the smallest pressure drop between two neighboring cans. In this case the can-to-can type thermo acoustic pulsations can be avoided in general without the need for staging. Hence lifetime is increased and emissions are reduced. 
     One aspect of the present disclosure is a combustor transition from a can combustor to the turbine inlet adapted to guide combustion gases in a hot gas flow path extending between a gas turbine can combustor and a first stage of turbine. The combustor transition comprises a duct having an inlet at an upstream end adapted for connection to the can combustor and an outlet at a downstream end adapted for connection to a first stage of a turbine. The downstream end comprises an outer wall, an inner wall, as well as a first and a second side wall. The outer and inner walls of adjacent combustor transitions form an annular flow path with an outlet, the outlet being connected to the turbine inlet. 
     The inlet of a combustor transition typically has the same cross section as the can combustor to which the transition piece is attached. These can for example be a circular, an oval or a rectangular cross section. The outlet typically has the form of a segment of an annulus. A plurality of combustor transitions installed in the gas turbine form an annulus for guiding the hot gas flow into the turbine. 
     According to a first embodiment at least one side wall has a side wall extension which is extending in a downstream direction beyond the outlet at the downstream end of the combustor transition. The side wall extension at least partly encloses a first resonator volume. In one embodiment, the side wall extensions of two combustor transitions are configured such that, when installed next to each other in a gas turbine, the side wall extensions at least partly enclose a first resonator volume. Further, at least one side wall extension comprises a resonator hole (also called damper hole), which is configured as a neck of a Helmholtz-damper, which fluidly connects the resonator volume with the hot gas flow path. 
     According to an embodiment the first resonator volume of the combustor transition is limited at the upstream end by a first separating member 
     In a further embodiment the first resonator volume of combustor transitions comprises a volume, which is at least partly enclosed by the side walls of two combustor transitions when installed next to each other in a gas turbine. This first resonator volume is limited at the upstream end by a second separating member. 
     In yet another embodiment the combustor transition comprises the first resonator volume limited the upstream end by the first separating member and in addition a second resonator volume, which is at least partly enclosed by the side walls of two combustor transitions when installed next to each other in a gas turbine. This second resonator volume is further limited at the upstream end by a second separating member. In addition, the first separating member comprises a second resonator hole, which connects the first resonator volume and the second resonator volume, and which is configured as neck of a Helmholtz-damper. Thus, at least two different pulsation frequencies can be suppressed by the arrangement of the two resonating volumes. 
     According to another embodiment the combustor transition comprises a hollow insert, which delimits the first resonator volume. A hollow insert can also be used to delimit the second resonator volume or a hollow insert can be used to delimit the first and second or another multitude of resonator volumes. 
     When installed in a gas turbine the combustor transitions including the side wall extensions are exposed to hot gases on the side walls facing the hot gas flow path. Cooling of the side walls and the side wall extensions is therefore advantageous. According to one embodiment the combustor transition comprises a cooling air supply to the first resonator volume and/or the second resonator volume for cooling of the side wall, respectively of the side wall extension. 
     In yet another embodiment the combustor transition has a first side wall which ends at the outlet of the combustor transition, and a second side wall which has a side wall extension, which is extending in a downstream direction beyond the outlet at the downstream end of the combustor transition. This side wall extension has a U-shaped cross-section, with a first leg of the U-shaped extension connected to the second side wall. The extension is separating a hot gas side from a cooling side and a second leg of the U-shaped extension is beginning directly downstream of the outlet on the cooling side of the first side wall extension, and can be arranged substantially parallel to the first leg. The second leg is connected to the first leg by a third leg at the downstream end. The U-shaped extension is thereby forming a resonator volume between the first leg, the second leg, and the third leg. The third leg is typically shorter than the first and second leg, for example less than half the length of the first leg. 
     The second leg of the U-shaped extension is configured such that the second leg of the extension begins directly downstream of the first sidewall of a neighboring combustor transition, which has no extension, to form one streamlined contour on the hot gas side of the first side wall/second leg, when two combustor transitions are installed next to each other in a gas turbine. 
     According to an embodiment the resonator volume formed by one or more side wall extensions is closed towards the outer wall, i.e. when installed in the gas turbine at the end of the resonator volume, which is facing the outer vane platform of the first turbine stage, and/or towards the inner wall, i.e. when installed in the gas turbine at the side of the cooling space, which is facing the inner vane platform of the first turbine stage. 
     The resonator volume can be closed towards the outer wall and/or towards the inner wall by an end plate. 
     According to a further embodiment the end plate towards the wall, and/or towards the inner wall is split into a first end plate and into a second end plate by the split line. Each of the first and second end plate can be connected to the first and second end wall extension (e.g. by brazing or welding) of form an integral part of the corresponding end wall extension (e.g. in a casted or machined part). 
     During operation the transition piece side walls and transition piece side wall extensions are exposed to the combustion chamber&#39;s hot gases. Therefore it can be advantageous for the live time of these parts to provide them film cooling and or effusion cooling. According to a further embodiment the film cooling and/or effusion cooling holes are provided in the walls of the resonator volume, i.e. in the side walls of the combustor transition side wall and/or the side wall extension. 
     According to another embodiment the end plate is at least partly separated from the first side wall extension by a gap and at least partly connected to the second side wall extension. This embodiment can be advantageous for cases in which the second side wall extension extends further downstream of the combustor transition outlet. When every second combustor is removed the respective side of the shorter first extension will then offer an unobstructed access for baroscopic inspection of the adjacent hot gas flow path. 
     Besides the transition piece a can combustor comprising such a combustor transition piece is an object of the disclosure. The transition piece can be a separate component, which is connected to the can combustor, or it can be an integral part of the can combustor. The can combustor and transition piece can for example be casted, extrusion formed, or manufactured by welding or brazing 
     Further, a gas turbine comprising such a combustor transition piece is an object of the disclosure. The gas turbine has at least one compressor, at least one turbine, and at least one can combustor. Further, the disclosed combustor transition is installed between the can combustor and the turbine. 
     When installed in a gas turbine the side wall extension of a combustor transition is extending downstream into a space between the inner and outer platform of a vane one of the turbine. When installed the side wall extension is ending directly upstream of an airfoil of the vane one. Adjacent first and second side wall extension and the subsequent airfoil can be arranged such that their surfaces are aligned to form one smooth surface facing the hot gas flow path. 
     To minimize losses during the operation of the gas turbine the at least one side wall extension is extending downstream to the leading edge of a vane one airfoil such that in only leave a gap which is sized to allow for thermal expansion between the can combustor and turbine. 
     The proposed combustor transition can be used for new gas turbines as well as for retrofitting existing gas turbines. A method for retrofitting a gas turbine comprises the steps of opening the gas turbine housing, removing at least one existing combustor transition, installing at least one of the disclosed combustor transitions with a side wall extension, and of closing the gas turbine housing. 
     To give access for baroscopic inspection of the hot gas flow path inspection the can combustor and/or combustor transition can be removed. To reduce the time required for removal of combustor transitions it is advantageous if only a part of the transition needs to be removed. However, with the side wall extension access from one combustor to the hot gas flow path of a neighboring hot gas transition is restricted. To reduce the number of combustor transitions, which have to be removed, a method for borescope inspection of a gas turbine with a combustor transition which has a no or only a short side wall transition on one side of the outlet is proposed: According to this method every second combustor transition is removed for inspection and the hot gas path downstream of the removed combustor transition and the inspection of the hot gas path of the neighboring combustor, which remains installed in the gas turbine. The neighboring combustor is inspected via the gap, which is opened by removing the side wall extension together with the removed combustor transition. 
     Inspection of the hot gas path can be done in combustor hot gas paths even further apart if the resonator holes are arranged in both side walls of a side wall extension, and these are sufficiently aligned and large enough to allow passing of a borescope. 
     The above described combustor transition, can combustor and gas turbine can be a single combustion gas turbine or a sequential combustion gas turbine as known for example from EP0620363 B1 or EP0718470 A2. It can also be a combustor transition of a gas turbine with one of the combustor arrangements described in the WO2012/136787. The disclosed retrofit method as well as baroscopic inspection method can be applied to a single combustion gas turbine or a sequential combustion gas turbine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, its nature as well as its advantages, shall be described in more detail below with the aid of the accompanying drawings. Referring to the drawings: 
         FIG. 1 a    shows an example of a gas turbine according to the present invention. 
         FIG. 1 b    shows a cross section of the turbine inlet with combustor transitions of the gas turbine from  FIG. 1   a.    
         FIG. 2  shows an example of a combustor transition arrangement with a vane one of a turbine according to the present invention. 
         FIG. 3  shows a cross section III-III of  FIG. 2  with the combustor transition arrangement and vane one. 
         FIG. 4   a, b, c, d  shows details of examples of different embodiments of combustor transition side wall extensions, 
     
    
    
     DETAILED DESCRIPTION 
     The same or functionally identical elements are provided with the same designations below. The examples do not constitute any restriction of the invention to such arrangements. 
     An exemplary arrangement is shown in  FIG. 1 a   . The gas turbine  9  is supplied with compressor inlet gas  7 . In the gas turbine  9  a compressor  1  is followed by a combustion chamber comprising a plurality of can combustors  2 . Hot combustion gases are fed into a turbine  3  via a plurality of combustor transitions  24 . The can combustors  2  and combustor transition  24  form a hot gas flow path  15  leading to the turbine  3 . The combustor transition  24  connects the can combustors  2  of the combustion chamber with the vane one  10  of the turbine  3 . 
     Cooling air  5 ,  6  is branched off from the compressor  1  to cool the turbine  3  and combustor. In this example the cooling systems for high pressure cooling air  5  and low pressure cooling air  6  are indicated. 
     Exhaust gas  8  leaves the turbine  3 . The exhaust gas  8  is typically used in a heat recovery steam generator to generate steam for cogeneration or for a water steam cycle in a combined cycle (not shown). 
     The combustor transitions  24  of the gas turbine  9  of the cross section B-B are shown in  FIG. 1 b   . The combustor transitions  24  guide the hot gases from the can combustors  2  to the turbine and are arranged to form an annular hot gas duct at the turbine inlet. 
     An example for the interface between combustor transition  24  and the vane one  10  is shown in more detail in  FIG. 2 . Inside the combustor transition  24  the combustor transition outer wall  11 , the combustor transition inner wall  12  and the side walls  21  confine the hot gas flow path  15 . At the outlet of the combustor transition  24  the cross section of each combustor transition has the geometrical shape of a sector of the annulus, which forms the hot gas flow path  15  at the turbine inlet. The flow path continues into the vanes one  10  of the turbine  3 . The inner platforms  14  and outer platforms  13  delimit the hot gas flow path in the turbine inlet. The airfoils  18  of the turbine vanes  10  extend in radial direction between the inner platform  14  and outer platform  13  of the vane one  10  and at least partly divide the hot gas flow in the circumferential direction. To separate the hot gas flow path  15  into decoupled sections the side wall extension  20  is reaching into the upstream end of the turbine  3 , extending into the space confined by the inner vane platform  14  and outer vane platform  13 . Decoupling is achieved by a resonator volume (only indicated by dotted walls in  FIG. 2 ). The resonator volume is fluidly connected the hot gas flow path  15  by at least one resonator hole  26  which is designed as a neck of a Helmholtz damper. In particular the cross sectional area of the at least one resonator hole  26  can be adjusted such that in combination with the resonator volume  28  at least one critical frequency can be dampened. 
     The side wall  20  of combustor transition  21  can be arranged upstream of the airfoil  18  and a side wall extension  20  is extending into the space confined by the inner vane platform  14  and outer vane platform  13 . In this case the side wall extension  20  ends upstream of the leading edge of the airfoil  18 . Thus decoupling is achieved by a combination of dampening with the Helmholtz damper and by at least partly blocking the fluid connection between two neighboring combustors. Since the flow velocity in the first vane typically can reach the speed of sound and coupling of two combustors via the downstream areas of the vane one  18  is not possible. As shown in  FIG. 2  typically a gap can remain between the airfoil  18  and the side wall extension  20  to allow for axial movements to thermal expansions in the turbine and in the combustor. Typically, the airfoil  18  and side wall extension  20  should not touch each other to avoid mechanical damage of the parts, in particular of a coating or thermal barrier coating which can be applied to the surface of the parts. 
     The cross section III-III from  FIG. 2  of the combustion transitions  24  and the vanes one  10  is shown in  FIG. 3 . In this example vane arrangements comprising two airfoils  18  arranged between one inner and one outer platform  13 ,  14 . In this example one such vane arrangement with two airfoils  18  is arranged downstream of each combustor transition  24 . 
     The number of airfoils per inner- and outer platform (vane arrangement) is not limited to two and can be any integer number. Also the number of airfoils allocated to each transition peace is not limited to two but can be any number. Because an arrangement with side wall extension only every other combustor transition or every second, third, fourth etc. combustor transition can be used, the number of airfoils allocated to each transition peace is not limited to integer numbers. Inside the combustor transition  24  the hot gas flow path  15  is divided into separate channels by the combustor transition side walls  21 . The vanes  10  are arranged downstream of the combustor transition  24 . Upstream of every second airfoil  18  a side wall extension  20  extends to the upstream end of the airfoil  18 . 
     Different ways to design a combustor transition side wall extension  20  are possible. The details of four examples of such side wall extensions are shown in  FIGS. 4   a, b, c  and  d.    
     In the example of  FIG. 4 a    the right combustor transition side wall  21   a  of a first combustor transition  24  and the left combustor transition side wall  21   a  of the neighboring combustor transition end next to each other at the outlet  22  of the combustor transition. The right combustor transition side wall  21   a  is extended downstream to form a right side wall extension  20   a  and the left combustor transition side wall  21   b  is extended downstream to form a left side wall extension  20   b . Both side wall extensions  20   a ,  20   b  are arranged next to each other (in this example parallel to each other) thereby forming a side wall extension  20  comprising a duct or first resonator volume  28  between the inner vane one platform  14  and the outer vane one platform. This first resonator volume  28  is closed towards the space between the right combustor transition side wall  21   a  of a first combustor transition  24  and the left combustor transition side wall  21   a  of the neighboring combustor transition by a first separating member  25  comprising a seal  27 . For cooling high pressure cooling air  6  can supplied to the first resonator volume  28  from the space between the right combustor transition side wall  21   a  of a first combustor transition  24  and the left combustor transition side wall  21   a  of the neighboring. In the example shown cooling air is supplied as leakage air via the seal  27 . To reduce the cooling air losses the left and right side wall extensions  20   a ,  20   b , can be bend towards each other at their downstream end as shown in  FIG. 4 a   . In addition, to reduce the cooling air losses the channel between the left and right side wall extensions  20   a ,  20   b  can be closed by an end plate  17  at the side radially outer and inner end of the side wall extensions  20   a ,  20   b , i.e. at the end facing the inner side vane platform  14  and/or at the end facing the outer vane platform  13 . In the example shown in  FIG. 4 a    the end plate  17  comprises a left end plate  17   a , which is attached to the left side wall extensions  20   a , and a right end plate  17   b , which is attached to the right side wall extensions  20   b.    
     Between the left and right end plates  17   a ,  17   b  at the inner and/or outer position a gap or split line  16  can remain open to allow for thermal extension and assembly tolerances. Also between the downstream ends of the left and right side wall extensions  20   a ,  20   b  a gap  23  can be foreseen to allow for thermal extension and assembly tolerances. To better defined, closed resonance volume  28 , and to reduce cooling air loses these gaps  16 ,  23  can be closed by seals  27 . 
     In the examples shown in  FIG. 4  the left and right side walls of the resonator volume  28  have resonator holes  26 . Embodiments with a resonator hole  26  in only the left or only the right side wall of the resonator volume  28  are also conceivable. 
       FIG. 4 b    shows an alternative resonator volume arrangement. In this example a second resonator volume  29  is at least partly enclosed by a downstream section of left combustor transition side wall  21   a , a downstream end of the right combustor transition side wall  21 , and a second separating member  34 . In this example the second separating member  34  comprises walls sections connecting two neighboring combustor transition side walls  21   a ,  21   b . To avoid direct contact of the two side walls a gap remains between the two side walls  21   a ,  21   b , which can be closed by a seal  27 . The first resonance volume  28  is fluidly connected to the second resonator volume  29  by the second resonator hole  29 . 
     The second separating member  34  comprises a cooling air supply hole  30  for the supply of high pressure cooling air  6  to the first resonator volume  28  and second resonator volume  29 . High pressure cooling air  6  first is introduced into the second resonator volume  29  via the cooling air supply hole  30 . Part of the cooling air can be used for cooling of the downstream side ends of the combustor transition side walls  21   a ,  21   b  for example by effusion and/or film cooling (not shown). The remaining cooling air is supplied to the first resonator volume  28  via the second resonator hole  35 . 
     For better cooling of the combustor transition side wall extension  20  film cooling and/or effusion cooling holes  19  are provided in the left and right combustor transition side wall extensions  20   a ,  20   b . Film cooling and/or effusion cooling holes can be provided for all of the examples in  FIGS. 4 a , 4 b , 4 c  and 4 d    as well as any other side wall extension arrangement. 
     The example of  FIG. 4 b    has the advantage that the two resonator volumes  28 ,  29  with two resonator holes  26 ,  35  allow for tuning of at least two frequencies. The increased volume of also allows dampening of low frequencies. 
     The third example shown in  FIG. 4 c    shows an alternative end wall extension. In this example the left combustor transition side wall  21   a  ends at the outlet  22  without an extension. Only the right combustor transition side wall  21   b  is extended to form the combustor transition side wall extensions  20 . Here the right combustor transition side wall extension  20   b  does not end at the downstream end but the side wall extension of the right side wall  21   b  has a U-form and the left combustor transition side wall extension  20   a  is connected to the right combustor transition side wall extension  20   b  at the downstream end. The U-form includes the first leg  20   b , the second leg  20   a , and a third leg  20   c . In this example the end plate  17  is provided as one piece connecting the left and right side wall extensions  20   a ,  20   b . In this example the first separating member  25  can be part of the right combustor transition side wall extension  20   b . Thus, the resonator volume  28  is enclosed can be enclosed by only one side wall extension  20   b  with end plates  17  closing it. This design does not require any seals and therefore a defined volume with defined openings, i.e. the resonator hole(s)  26  can be provided. This design also reduces or avoids cooling air losses along seal lines. Additionally, for inspections of the outlet  22  area downstream of two neighboring combustor transitions  24  only one combustor transition  24  has to be removed. 
     In the example of  FIG. 4 d    a hollow insert  32  is used to define the resonator volume. The hollow insert can be limited to the space between the side wall extensions  20   a ,  20   b . In this case the insert is arranged in the space between the side wall extensions  20   a ,  20   b  and extends into the space between the two neighboring the combustor transitions side walls  21 . The hollow insert  32  is designed to follow the contour of the side walls  20   a ,  20   b  and the side wall extensions  21   a ,  21   b  of two neighboring combustor transitions  24  on the side facing away from the hot gas flow path  15 . They are closed radially towards the outside and inside and thereby form a defined resonator volume. The side walls of the insert  32  has at least one hole aligned with the at least one resonator hole  26  in the side wall extension  21   a ,  21   b . On the upstream side the hollow insert  32  comprises a hole for high pressure cooling air  6  supply. 
     The hollow insert  32  can comprise a separating member (not shown) to divide the volume enclosed by the hollow insert  32  into two or more resonator volumes  28 ,  29 . 
     In the example of  FIG. 4 d    the insert is completely enclosing the resonator volume  28 ,  29 . However, a semi-closed insert, which is attached at least partly to a side wall  21  and/or side wall extension  20  can be used. In this case the resonator volume  28 ,  29  is delimited by a combination of the insert walls and the side wall  21  and/or side wall extension  20 . 
     For all embodiments the combustor transition side wall extension  20 ,  20   a    20   b  can be one integral part of the combustor transition side wall  21 ,  21   a    21   b , for example in a casted, bended, pressed or forged piece. They can also be attached or fixed to the combustor transition side wall  21 ,  21   a    21   b , for example by welding, brazing, screws or rivets. 
     The end plate  17 ,  17   a ,  17   b  can be one integral part of the side wall extension(s)  20 ,  20   a    20   b , for example in a casted, bended, pressed or forged piece. The can also be attached or fixed to the combustor transition side wall extension  20 ,  20   a    20   b , for example by welding, brazing, screws or rivets.