Patent Publication Number: US-9896951-B2

Title: Turbine vane with cooled fillet

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European application 14160874.5 filed Mar. 20, 2014, the contents of which are hereby incorporated in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a turbine vane, and more particularly to a cooled vane with a fillet interposed between a platform and an airfoil of the vane. Further, it relates to a method for cooling such a vane. 
     BACKGROUND 
     The thermodynamic efficiency of power generating cycles depends on the maximum temperature of its working fluid which, in the case for example of a gas turbine, is the temperature of the hot gas exiting the combustor. The maximum feasible temperature of the hot gas is limited by combustion emissions as well as by the operating temperature limit of the parts in contact with this hot gas, and on the ability to cool these parts below the hot gas temperature. In particular blades, i.e. rotating blades and vanes (stationary blades), are exposed to high temperature combustion gases, and consequently are subject to high thermal stresses. Methods are known in the art for cooling the vanes and reducing the thermal stresses. Typically high pressure air, discharged from a compressor, is introduced into an interior of an air-cooled vane from a vane root portion. After cooling the vane the cooling gas is discharged from the vane into a hot gas flow path of the gas turbine. 
     The region of a vane where the airfoil is connected to the platform is highly loaded and often subject to additional stresses due to thermal mismatches and different thermal expansions of the airfoil and the platform. For a smooth transition and to reduce peaks in the stress distribution a rounded transition from platform to airfoil has been suggested. Such rounded transitions or connections are typically called fillet. 
     However cooling of fillets is difficult and requires additional cooling gas flow, which can lead to a reduction in power and efficiency. 
     SUMMARY 
     The object of the present disclosure is to propose a vane, which avoids high stresses in the fillet region and assures safe efficient cooling of the fillet as well as efficient use of the cooling gas, i.e. the disclosed vane provides adequate cooling for the platform-to-airfoil transition region in a vane. 
     According to a first embodiment the vane comprises a platform, and airfoil extending in longitudinal direction away from the platform. A fillet is connecting the platform to the airfoil. The airfoil can extend from the platform to an airfoil tip or to an opposite platform. The airfoil has a pressure side delimited by a pressure side wall, and a suction side delimited by a suction side wall. Pressure side wall and suction side wall join at a leading edge and at a trailing edge. An impingement tube can be inserted into the airfoil delimiting a cooling channel between the impingement tube and the side walls. The vane further comprises a baffle structure positioned adjacent the fillet which follows the inside contour of the fillet and is delimiting a first cooling passage between the fillet and the baffle structure. The inside of the vane, e.g. of the fillet, is the side facing away from the hot gas side during operation of a turbine with such a vane. A first obstruction is arranged on the inside of the airfoil at the connection of the fillet to the side walls for separating the first cooling passage from the cooling channel. This obstruction can further guide the cooling gas away from the airfoil side walls. 
     Due to this separation cooling gas which has been used in the cooling channel can be reused for further cooling purposes. To reduce stresses the fillet can have a large curvature in the order of up to the thickness of the airfoil at the root (i.e. connection region to the platform). To minimize stresses due to different thermal expansions during transients in the gas turbine operation the fillet ideally has a constant wall thickness. In case the wall thickness of the airfoil side walls is different from the wall thickness of the platform a continuous change of fillet wall thickness can be advantageous. As a result the inner contour of the fillet can have a bell mouth like shape. Due to the curvature and resulting large surface area of this bellmounth shaped fillet a large amount of cooling gas might be needed for cooling of the fillet. The reuse of the fillet cooling for further cooling of the vane can therefore significantly contribute to a good overall efficiency of the turbine. 
     It can be advantageous if the fillet cooling is supplied independently from the airfoil cooling. Preferably the fillet cooling gas is reused for cooling the airfoil. With an independent cooling scheme and reuse of the cooing air it is possible to increase the coolant consumption in this region without affecting the airfoil cooling design and without increasing the overall cooling consumption of the vane. In this way the airfoil cooling performance can be independently optimized. 
     The cooling gas can be air which has been compressed by a compressor of a gas turbine if the vane is installed in an air breathing gas turbine. It can be any other gas or mixture of gases. For example it can be a mixture of air and flue gases for a gas turbine with flue gas recirculation into the compressor inlet. The vane can have a platform at one end of the airfoil and ending with a tip at the other end of the airfoil. In this case the cooling gas is supplied from the side of the platform. The vane can also have a platform on both sides of the platform. In a vane with platforms on both sides the cooling gas can be supplied from both sides or from either side. If the cooling gas is supplied only to one side of a vane with two platforms the vane typically includes a channel or duct in the hollow airfoil for feeding cooling gas from the side with cooling gas supply to the opposite side. 
     According to another embodiment the vane comprises a second impingement structure adjacent the platform which follows the contour the platform. This second impingement structure delimits a second cooling passage between the platform and the second impingement structure. The impingement structure can partly or completely cover the platform, i.e. the platform is partly completely impingement cooled through the impingement structure. 
     In one embodiment of the vane cooling gas used to impingement cool the platform in the region of the second cooling passage can flow to the first cooling passage to convectively cool the fillet while passing through the first cooling passage. 
     In one embodiment of the vane the baffle structure comprises impingement holes for impingement cooling of the fillet. 
     In a further embodiment of the vane a second obstruction is arranged on the inside of the platform at the connection between the second cooling passage and the first cooling passage for separating the first cooling passage from the second cooling passage. The obstruction avoids a cross flow of cooling gas from the second cooling passage through the first cooling passage which could have a detrimental effect on the impingement cooling in the first passage. The second obstruction can partly or completely separate the first cooling passage from the second cooling passage. 
     The cooling gas used for impingement cooling the platform can for example be fed from the second cooling passage to impingement tube of the airfoil for further use. 
     In one embodiment of the vane the second obstruction spans around the circumference of the fillet. In an alternative embodiment the second obstruction extends around the leading edge and or the trailing edge for shielding the impingement cooling of the filet from a cross flow of cooling gas coming from second cooling passage towards the first cooling passage in the leading edge region and/or trailing edge region of the fillet. 
     In another embodiment of the vane the second cooling passage has an opening to the first cooling passage such that cooling gas flows from the second cooling passage to first cooling passage. The opening can be a seamless connection of the baffle structure with the second impingement structure. These can even be combined into one structure or in one piece or one plate. The cooling gas leaving the second cooling passage can thus be reused for subsequent convective cooling of the fillet during operation. 
     In another embodiment of the vane the second cooling passage has an opening and connection such as a flow channel or connecting plenum to the impingement tube such that cooling gas flows from second cooling passage to the impingement tube for subsequent impingement cooling of the airfoil during operation. 
     In yet another embodiment of the vane the first cooling passage has an opening or flow channel to the impingement tube such that cooling gas flows from first cooling passage into impingement tube for subsequent impingement cooling of the airfoil during operation. 
     It can further be advantageous if the fillet or fillet region comprises a row of film cooling holes arranged in the fillet wall such that during operation cooling gas from the first cooling passage is used for film cooling of the fillet after impingement cooling. Further or alternatively, the platform can comprise at least one convective cooling hole arranged in the platform such that during operation cooling gas from the second cooling passage is used for convective cooling of the platform after impingement cooling. This convective cooling hole can discharge the cooling gas into the hot gas flow path. 
     Film cooling of the fillet and convective cooling of the platform can be used to discharge all of the cooling gas flowing into the first cooling passage and into the second cooling passage thereby completely decoupling the airfoil cooling from the platform and fillet cooling. The film cooling holes in the fillet and convective cooling holes in the platform can also be arranged in combination with an opening or flow channel connecting the first cooling passage to the impingement tube of the airfoil such than part of the cooling gas is reused for impingement cooling of the airfoil and part of the cooling gas is used for film cooling and/or convective cooling. 
     In a further embodiment of the vane the fillet has a curved shape with an outer surface facing the hot gases during operation wherein the curvature is tangentially to the outer surface of the platform at the connection of the filet to the platform and tangentially to the outer surface of the airfoil at the connection the filet to the airfoil. 
     In yet another embodiment the fillet has wall thickness which is equal to wall thickness of the platform at the connection to platform and which is equal to the wall thickness of the airfoil side walls at the connection to the airfoil side walls to minimize stresses. The wall thickness of the fillet can for example continuously decreases or continuously increases along the extension of the fillet from the platform to the side walls. The wall thickness can for example also change with continuous first order derivative, i.e. the thickness changes continuously without any steps along the extension of the fillet from a connection to the platform to the connection to the side walls. 
     In another embodiment of the vane the impingement tube is arranged inside a leading edge section of the airfoil, and a convective cooling section is arranged inside a trailing edge section of the airfoil. A wall is dividing the convective cooling section into a first convective cooling section adjacent to the platform and into a second convective cooling section extending towards the vane tip, respectively extending towards a platform at the opposite end of the airfoil. 
     The rib can further serve to guide the cooling gas in the first passage along the root of the airfoil. 
     Convective cooling in the first and/or second convective cooling section can be enhanced by turbulator such as for example a pin field and/or cooling ribs. 
     In a further embodiment a cooling gas feed is connecting the first cooling passage to the first convective cooling section for directly feeding cooling gas from the first cooling passage to first convective cooling section. Thus the cooling gas leaving the first passage is not flowing via the impingement tube into the convective cooling section but directly from the first cooling passage. The pressure of the cooling gas therefore remains higher in the first cooling passage to effectively cool the root section of the airfoil. 
     Besides the vane a method for cooling a vane is an object of the disclosure. 
     The disclosed vane allows good cooling of a fillet and reduces stresses in the fillet. Further, it allows the reuse of the cooling gas spent for cooling the fillet. 
     The vane which is to be cooled by that method has a platform, an airfoil extending in longitudinal direction away from the platform extending form the platform and connected to the platform by a fillet. The airfoil has a pressure side and a suction side with a pressure side wall and a suction side wall, which join at a leading edge and at a trailing edge. An impingement tube is inserted into said airfoil delimiting a cooling channel between the impingement tube and the side walls. The method for cooling such a vane comprises the following steps:
         supplying cooling gas to a baffle structure positioned adjacent the fillet which follows the inside contour of the fillet and delimits a first cooling passage between the fillet and the baffle structure,   impinging the cooling gas onto the fillet for impingement cooling,   after impingement guiding the cooling gas leaving the first cooling passage with the help of an obstruction arranged on the inside of the airfoil at the connection of the fillet to the side walls into the impingement tube, and   impinging the cooling gas on the side walls of the airfoil.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure, its nature as well as its advantages, shall be described in more detail below with the aid of the accompanying schematic drawings. Referring to the drawings: 
         FIG. 1  shows a perspective view of an exemplary turbine vane; 
         FIG. 2 a , 2 b    shows bottom view of the foot of the vane from  FIG. 1 ; 
         FIG. 3  shows an example the cross-section the platform and a cut out of the airfoil at the connection to the platform; 
         FIG. 4  shows a modified detailed of the platform of  FIG. 3 ; 
         FIG. 5  shows another example the cross-section the platform and a cut out of the airfoil at the connection to the platform; 
         FIG. 6  shows another example the cross-section the platform and a cut out of the airfoil at the connection to the platform; 
         FIG. 7  shows another example the cross-section the platform and a cut out of the airfoil at the connection to the platform; 
         FIG. 8  shows exemplary cross-section of the airfoil. 
     
    
    
     DETAILED DESCRIPTION 
     A vane  10  of a turbine according to an exemplary embodiment of the disclosure is shown in  FIG. 1 . The vane  10  has an airfoil  11  which extends in the longitudinal direction from a platform  18  to a vane tip  17 . The longitudinal direction of the airfoil  11  in this context is the direction from platform to tip, respectively from platform to opposite platform of the vane. This direction is typically practically perpendicular to the flow direction of hot gases in the flow path of a turbine. The airfoil  11  has a pressure side  14  and a suction side  15  and also a leading edge  12  and a trailing edge  13 . The platform  18  is provided with hook-like fastening elements  19   a  and  19   b  on the top. The airfoil  11  merges into the platform  18  with a fillet  16  at a root. At the trailing edge  13 , discharge openings  21  for cooling gas are arranged in a distributed manner along said trailing edge  13  and are separated from each other by means of ribs  32  disposed in between. The airfoil  11  is outwardly delimited by a pressure-side wall  14   a  and a suction-side wall  15   a . Film cooling gas holes can be arranged on the surface of the suction-side wall  15   a  and pressure-side wall  16   a  (not shown). These can be advantageous in leading edge region of the side walls  14   a ,  15   a    
     The vane shown in  FIG. 1  has an airfoil  11  extending from one platform  18  and ending at a tip  17 . Depending on the design and application a vane can comprise two platforms  18  with an airfoil  11  extending from one platform to another platform. 
       FIG. 2 a    shows the platform  18  in a top view of the vane in  FIG. 1  In the top view of  FIG. 2 a    impingement plates, and baffles for guides the cooling gas are omitted to allow a view into the vane. The  FIG. 2 a    shows platform  18 . The airfoil itself is not visible as it is pointing away from the platform  18  but an opening with the aerodynamic profile of the platform is visible. A curved fillet  16  connecting the platform  18  to the airfoil encircles the profiled vane opening. During operation cooling gas  33  flows from the platform  18  across the fillet following the curvature of the fillet  16 . To further guide the cooling gas flow  33  a first obstruction  25  is arranged on the inside of the vane at the connection of the fillet  16  to the airfoil. Second obstructions  28  are arranged on the platform  18  at the connection of the fillet  16  to the platform  18  in the leading edge as well as in a trailing edge region. The second obstructions  28  shield the leading edge and the trailing edge regions of the fillet  16  from a cross flow of cooling gas from the platform  18  during operation. 
       FIG. 2 b    is based on  FIG. 2 a   . Here examples for the location of impingement cooling holes  36  are indicated. In this example cooling holes  36  are distributed above the platform and in a leading edge as well as in a trailing edge region of the fillet  16 . An effective impingement cooling of the leading edge and trailing edge region of the fillet  16  is enhanced by the second obstructions  28  which shield it from cooling gas  33  flowing from the platform  18  towards the airfoil. 
       FIGS. 3, 5, 6, and 7  show the cut A-A of the vane  10  indicated in  FIG. 2 a , 2 b   . They show different examples of the platform, fillet-airfoil connection with corresponding cooling schemes. Only a cut-out of the airfoil  11  region close to the platform is shown since a tip region is not subject of the invention. If the vane  10  comprises platforms on both ends of the airfoil  11  these can be designed in according to the same principles shown. 
     The vane of  FIG. 3  comprises a platform  18 , an airfoil  11  extending away from the platform  18  into a hot gas flow (during operation). The airfoil  11  is connected to the platform  18  by a fillet  16 . The fillet  16  is curved and asymptotic to the platform  18 , respectively to the airfoil  11  at the respective connection as can be seen here for the leading edge region. 
     A baffle structure  20  is positioned adjacent to the fillet  16  and follows the inside contour of the fillet  16 . A first cooling passage  23  is arranged between the fillet and the baffle structure  20 . In this example the baffle structure  20  is configured as an impingement plate for impingement cooling of the fillet  16  with pressurized cooling gas  33  supplied from a plenum  37  above the baffle structure  20 . 
     An impingement tube  22  is inserted into the airfoil  11  delimiting a cooling channel  26  between the impingement tube  22  and the side walls  14   a ,  15   a . The impingement tube  22  is arranged next to the leading edge of the airfoil  11  allowing an impingement cooling of the side walls  14   a ,  15   a  in the leading edge region. After impinging on the side walls  14   a ,  15   a  the cooling gas  33  can be used to further cool the airfoil by discharging it to the outer surface of the airfoil through film cooling holes (not shown) or by guiding it through a cooling channel  26  formed by the side walls  14   a ,  15   a  and the impingement tube  22  along the side walls  14   a ,  15   a  towards the trailing edge of the vane, and thereby convectively cooling the airfoil  11 . 
     Between the first cooling passage  23  and the cooling channel  26  a first obstruction  25  is arranged on the inside of the airfoil  11  at the connection of the fillet  16  to the side walls  14   a ,  15   a . The first obstruction  25  prevents cooling gas  33  from flowing out of the first cooling passage  23  directly into the cooling channel  26  and forces the cooling gas  33  to flow out of an opening of the first cooling passage  23  into the impingement tube  22 . Thus the cooling gas  33  can be used twice. A closing plate  38  above the upper end of the impingement tube prevents a direct flow of the cooling gas  33  from plenum  37  into the impingement tube  22 . 
     In this example the vane further comprises a second impingement structure  27  adjacent the platform  18 . This second impingement structure  27  is configured as an impingement plate arranged offset and parallel to the platform. A second cooling passage  24  is formed between the platform  18  and the second impingement structure  27 . Cooling gas  33  impinges on the platform  18  and then flows along the platform&#39;s  18  inner surface in the second cooling passage. 
     In this example the vane has a second obstruction  28  which is arranged on the inside of the platform  18  at the connection between the second cooling passage  24  and the first cooling passage  23 . The second obstruction at least partly separates first cooling passage  23  from the second cooling passage  24  and thereby prevents a cross flow of cooling gas  33  from the second cooling passage  24  in the impingement cooled first cooling passage  23 . 
     The cooling gas  33  leaves the second cooling passage  24  via an opening and can be guided directly to the impingement tube  22  (not shown) or can flow through the sections of the first cooling passage  23  which are not blocked by the second obstruction (not shown here but indicated in  FIG. 2 a , 2 b   ). 
     The airfoil region downstream of the impingement tube  22 , i.e. in flow direction of hot gases flowing around the vane during operation, can be convectively cooled with the cooling gas  33  leaving the impingement tube  22  or cooling gas directly fed into the space between the side walls  14   a ,  15   a  downstream of the impingement tube  22 . In this example a first and a second convective cooling section  30 ,  31  are arranged downstream of the impingement tube  22  in the airfoil  11  for convectively cooling the side walls  14   a ,  15   a . The first convective cooling section  30  is fed with cooling gas coming from the first cooling passage  23  after the cooling gas  33  has cooled the fillet  16 . The first convective cooling section  30  is separated from the second convective cooling section  31  by a wall  29  which extends basically parallel to the platform  18  and spans between the pressure side wall  14   a  and the suction side wall  15   a . The second convective cooling section  1  is feed from cooling gas  33  leaving the cooling channel  26  after impingement cooling. In this arrangement cooling gas  33  with a higher pressure level is feed to the first convective cooling section  30  near the platform to better cool this highly loaded region. In the examples shown here the first and second convective cooling sections  30 ,  31  are configured as pin fields. Instead of pin fields other heat transfer enhancements can be used or depending on the cooling requirements at least part of the side walls can have a smooth inner surface. 
       FIG. 4  shows a variation of the platform  18  cooling design of the detail IV indicated in  FIG. 3 . In this example the first cooling passage  23  and second cooling passage  24  are connected and no obstruction is interposed between them. Further, the baffle structure  20  and the second impingement structure  27  are incooperated into one impingement plate following the contour of the platform  18  and around the curvature of the fillet  16 . 
     In this example the cooling gas  33  feed to the first and second cooling passage is further used for film cooling the fillet  16  through film cooling holes  34  and for convectively cooling the upstream end of the platform  18  through convective cooling holes  35 . 
       FIG. 5  is based on the  FIG. 3 . However, the second cooling passage  24  is connected to the first cooling structure without any interposed obstruction. Further the baffle structure  20  is not configured as an impingement plate but as a guiding plate for guiding cooling gas  33  leaving second cooling passage  24  along the fillet  16  for convective cooling of the fillet  16 . In this arrangement the cooling gas first impingement cools the platform, then convectively cools the fillet  16  and is then fed into the impingement tube  22  to finally cool the airfoil  11 . 
       FIG. 6  is also based on the  FIG. 3 . The cooling design of the platform  18  is modified over the design of the example of  FIG. 3 . In this example the height of the second cooling passage  24  is changed. It is higher than the first cooling passage  23 . An increased cooling passage height can be advantageous to guide large volume flow of cooling gas  33  through the passage. This can be used for example to guide cooling gas  33  which was used to cool the platform  18  in the leading edge region around the second obstruction  28  to the pressure side  14 , respectively suction side  15  of the vane where it can be used for convectively cooling the fillet  16 : 
     In  FIG. 6  also a modification of the second convective cooling section  31  is shown. In this example a row of ribs  32  arranged at the trailing edge of the airfoil  11 . These ribs  32  can be used for further heat transfer enhancement. 
     Another modification based on  FIG. 3  is shown in  FIG. 7 . In this example the first and second convective cooling section  30 ,  31  are both supplied with cooling gas from the impingement tube  22  without a direct feed from the first cooling passage  23  into the first convective cooling section  30 . 
       FIG. 8  schematically shows the cross section VIII-VIII of  FIG. 7  as a schematic example for cross section of an airfoil  11 . The suction-side wall  15   a  and pressure-side wall  14   a  delimit a hollow cross section of airfoil  11 . Towards the leading edge of the airfoil  11  an impingement tube  22  is arranged inside this hollow cross section. Cooling gas  33  is feed into the impingement tube and impinges on the inside of the suction-side wall  15   a  and pressure-side wall  14   a  for cooling. Subsequently, a part of the cooling gas  33  is used for film cooling and discharged via airfoil film cooling holes  39 . Another part of the cooling gas  33  flows in the cooling channel  26  between the impingement tube  22  and the suction-side wall  15   a  respectively pressure-side wall  14   a  towards the second convective cooling section  31  and is discharge via the trailing edge of the airfoil  11 .