Patent Publication Number: US-10323531-B2

Title: Airfoil device for a gas turbine and corresponding arrangement

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2014/074255 filed Nov. 11, 2014, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP13196280 filed Dec. 9, 2013. All of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The present invention relates to an airfoil device for a gas turbine and an airfoil arrangement for a gas turbine. 
     ART BACKGROUND 
     In gas turbines, airfoil devices are arranged in order to guide a working fluid through a gas turbine. The airfoil devices may comprise blades which are mounted to a rotating turbine shaft or vanes which are mounted for example to a housing of the gas turbine. The airfoil devices are mounted in a circumferential direction around the turbine shaft one after another. A gap may exist between adjoining airfoil devices such that leakage occurs. For this reason, a sealing arrangement is required between adjacent airfoil devices. By attaching a sealing arrangement between adjacent airfoil devices, an injection of hot working gas into inner cavities of the airfoil devices is prevented. Moreover, cooling air which flows through cavities inside the airfoil devices is prevented from disappearing out into the mainstream flow of the hot working gas before being put to use. Moreover, a sealing arrangement is beneficial because the working fluid is guided through the airfoil passage without losing energy through the gaps between adjacent sealing devices. 
       FIG. 5  illustrates a conventional airfoil device  400 . A conventional airfoil  401  is arranged onto a conventional platform  402 . The conventional platform  402  comprises a conventional root section  404 . Within the conventional route section, a groove for arranging a conventional seal strip  405  is formed. Below the platform  402 , a conventional cavity  403  is formed. The conventional seal strip  405  is decoupled from the conventional cavity  403 . 
     EP 2 054 588 B1 discloses an airfoil device, wherein a platform of the airfoil device comprises a slot into which is seal strip is arranged. 
     EP 2 201 271 B1 discloses an airfoil device, wherein a sealstrip is arranged with a first end section inside a first groove of a root section of the airfoil device and a second end section is arranged inside the second groove of the root section of the airfoil device. 
     EP 2 551 464 A1 discloses an airfoil device which comprises a platform, wherein under the platform a cavity is form. A seal strip is arranged inside the cavity without an underside support. 
     SUMMARY OF THE INVENTION 
     It may be an object of the present invention to provide an airfoil device comprising a seal strip which has a reduced weight and proper sealing properties. 
     This object is solved by an airfoil device for a gas turbine and a turbine arrangement for a gas turbine according to the independent claims. 
     According to a first aspect of the present invention, an airfoil device for a gas turbine is presented. The airfoil device comprises a root section which is mountable to an airfoil disc of the gas turbine and an airfoil element. 
     The root section comprises a platform at which the airfoil element is arranged. The root section comprises a cavity which is surrounded by an inner surface of the platform, a first edge side (e.g. a downstream edge side) of the root section and a second edge side (e.g. an upstream edge side) of the root section. The first edge side and the second edge side are spaced apart from each other along an axial direction of the gas turbine. 
     A seal strip is arranged at the inner surface. The seal strip has a first end section, a middle section and a second end section, wherein the first end section is spaced apart from the second end section along the axial direction and the middle section is arranged between the first end section and the second end section. 
     The first edge side comprises a recess (groove, slit) into which the first end section of the seal strip is inserted such that the recess (partially) surrounds the first end section and thereby fixes the first end section to the inner surface. 
     The root section comprises a supporting lever extending from the second edge side into the cavity such that a free end of the supporting lever forms a contact region with the middle section of the seal strip for fixing the middle section of the seal strip to the inner surface. The supporting lever is further formed such that a further cavity is formed between the inner surface, the second edge side and the supporting lever, wherein the second end section of the seal strip is arranged inside the further cavity. 
     The root section comprises the platform, the first (trailing) edge side and the second (leading) edge side. The platform has a first (outer) surface which faces to a mainstream flow channel of the gas turbine and a second (inner) surface which faces to an opposite region of the platform in comparison to the first surface. The airfoil element, such as a blade, is attachable to the first surface. 
     The platform extends generally along a circumferential direction and an axial direction of the gas turbine. The thickness of the platform i.e. its extension along the normal of the inner surface, e.g. along the radial direction, is generally smaller in comparison to the other extensions, e.g. to the extensions along the axial and circumferential direction. 
     The terms axial direction, circumferential direction and radial direction refer to directions with respect to a turbine shaft of the gas turbine. The circumferential direction describes a run around the gas turbine shaft, the radial direction describes a run through a point of the rotating axis of the turbine shaft and the axial direction describes a run parallel to the rotating axis of the turbine shaft. The axial direction and the radial direction are orientated in particular perpendicular with respect to each other. 
     The (second) leading edge side and the (first) trailing edge side are attached to the platform. The second edge side and the first edge side run from the inner surface of the platform along a substantially radial direction. The second edge side is located more upstream with respect to the first edge side or vice versa, wherein “upstream” and “downstream” describe a location of a part along a flow direction of the main stream of the working fluid of the gas turbine. Hence, the platform, the first edge side and the second edge side may form a U-shape inner cross-section inside the cavity and the further cavity is formed. The above-described structure of the airfoil device is valid for the described airfoil device and e.g. also for the further airfoil device described below. 
     The recess (slit or groove) of the first edge side may have an U-shaped cross section, wherein the first end section of the seal strip may be inserted and slipped into the recess through its open side. 
     The cavity and the further cavity may be flushed with cooling air, wherein the cooling air may be fed from or to a hollow airfoil or the blade root for cooling purposes. The cavity may also be surrounded additionally by a bottom side which is connected to the trailing edge side and the leading edge side and which bottom side is located on the opposite side of the cavity in comparison to the inner surface of the platform. 
     A plurality of airfoil devices are mounted adjacent to each other to an airfoil disc along the circumferential direction. In particular, the first platform and a further platform of an adjacent further airfoil device abut against each other, wherein, for example due to assembly tolerances and growth allowance (centrifugal and thermal) during operation, small gaps exist between both platforms. 
     The supporting lever extends from the second edge side into the cavity and hence protrudes into the cavity. The supporting lever is formed in such a way that a gap between a free end of the supporting lever and the inner surface is formed. The seal strip arranged onto the inner surface protrudes through the gap. In other words, the supporting lever forms with its free end contact region with the middle section of the seal strip and thereby presses and may fix or force the middle section of the seal strip to the inner surface. The seal strip is retained or secured in the cavity by the supporting lever preventing the second end section moving radially inwardly of the stopper section. Thus in normal operation the seal strip is held within the cavity. 
     Furthermore, the supporting lever divides the cavity such that the further cavity is formed between the inner surface, the second edge side and the supporting lever, wherein the second end section of the seal strip is arranged inside the further cavity. 
     The seal strip may be formed of a metal strip or a metal plate, accordingly. The seal strip is in contact with the inner surface of the airfoil device and a further inner surface of an adjacent further airfoil device and thus seals a gap between two adjacent platforms. 
     By the present invention, the support lever extends only part-way along the axial direction into the cavity and along the inner surface. In contrast to conventional approaches, where the complete underside of the seal trip is supported by a support surface, by the present invention the overall weight of the airfoil device is reduced and hence also stress at the airfoil device and turbine disc is reduced. 
     Furthermore, by the present invention only the first end section of the seal strip is inserted into the recess, wherein the opposed second end section of the seal strip is not surrounded by a recess or groove for support purposes. The seal strip is held against the inner surface by the free end of the support lever. Hence, an easy installation of the seal strip is provided. The seal strip is elastically deformable into a spring loaded condition during installation. In the spring loaded status, the seal strip is slid along the circumferential direction with its middle portion inside the gap between the free end of the supporting lever and the inner surface. The first end section and the second end section are movable within the cavity and the further cavity. During installation the first end section is inserted slideably into the recess at the first edge side of the root section. The spring loaded status of the seal strip is then released so that the second end section unfolds and contacts in a final position the inner surface and e.g. the below described stopper section. 
     According to a further exemplary embodiment of the present invention, a first part of the middle section of the seal strip between the contact region and the first end section is arranged inside the cavity. The second end section and a second part of the middle section of the seal strip between the contact region and the second end section is arranged inside the further cavity. Hence, the second end section is not supported in radial direction. 
     In a further exemplary embodiment, the supporting lever is formed such that a first axial length of the seal strip between the contact region and the first end section is larger than a second axial length of the seal strip between the contact region and the second end section. Hence, because of the shorter second axial length between the contact region and the second end section, the seal strip may be formed stiffener such that the shorter second axial length part of the seal strip does not deform due to the own weight of the shorter second axial length part. 
     According to a further exemplary embodiment, the second edge side comprises a seal strip inlet for inserting the seal strip into the further cavity. 
     The seal strip inlet is formed such that a fitment of the seal strip with blades already in-situ is enabled. In particular, the seal strip inlet may be formed at the upstream (second) edge side and connects an upstream environment of the airfoil device with the further cavity. 
     Hence, the seal strip may be inserted through the seal strip inlet along approximately the axial direction into the further cavity. Furthermore, the seal strip may be further moved along approximately the axial direction until the first end section  109  of the seal strip is arranged within the recess in the downstream (first) edge side. 
     According to a further exemplary embodiment, the seal strip inlet is formed such that air is streamable out of the further cavity. Hence, air may stream from the cavity via the contact region (i.e. the gap between the free end of the supporting lever and the inner surface of the platform) through further cavity and exits the seal strip inlet. This air flow is intentionally reduced by minimizing the gap between the free end of the supporting lever and the inner surface of the platform. 
     According to a further exemplary embodiment, the second edge side comprises a stopper section (step or protrusion) which is formed such that the second end section of the seal strip abuts against the stopper section. In particular, the stopper section comprises a surface which has a normal that is (at least with a component) parallel to the axial direction. The seal strip abuts against the stopper section, if the seal strip is moved out of the recess along the axial direction. Hence, the stopper section limits a movement of the seal strip along the axial direction, such that a slipping out of the recess is prevented. 
     According to a further exemplary embodiment, the first edge side is a trailing edge side of the root section, wherein the second edge side is a leading edge side of the root section. 
     According to a further exemplary embodiment, an airfoil arrangement is described, wherein the airfoil arrangement comprises an above described airfoil device and a further airfoil device. The airfoil device and the further airfoil device are arranged one after another along a circumferential direction of the gas turbine, wherein the seal strip is formed such that the seal strip extends between the airfoil device and the further airfoil device for sealing a gap between the airfoil device and the further airfoil device. 
     It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited. 
         FIG. 1  shows a schematic view of an airfoil device according to an exemplary embodiment of the present invention, 
         FIG. 2  shows an enlarged view of a stopper section of the airfoil device as shown in  FIG. 1 , 
         FIG. 3  shows a perspective view of the airfoil device as shown in  FIG. 1 , 
         FIG. 4  shows a gas turbine engine according to an exemplary embodiment of the present invention, and  FIG. 5  shows a conventional airfoil device. 
     
    
    
     DETAILED DESCRIPTION 
     The illustrations in the drawings are schematic. It is noted that in different figures similar or identical elements are provided with the same reference signs. 
       FIG. 1  shows an airfoil device  100  for a gas turbine according to an exemplary embodiment of the present invention. The airfoil device  100  comprises a root section  101  which is mountable to an airfoil disc of the gas turbine. The root section  101  may therefore comprise a mounting bottom section which comprises e.g. a mounting plug which may be formed in a fir tree shape (see  FIG. 3 ). 
     The airfoil device  100  further comprises an airfoil element  102 , wherein the root section  101  comprises a platform  103  at which the airfoil element  102  is arranged. The root section  101  comprises a cavity  104  which is surrounded by an inner surface  105  of the platform  103 , a first edge side  106  of the root section  101  and a second edge side  107  of the root section  101 . The first edge side  106  and the second edge side  107  are spaced apart from each other along an axial direction  121  of the gas turbine. 
     A seal strip  108  is arranged at the inner surface  105 . The seal strip  108  has a first end section  109 , a middle section  111  and a second end section  110 , wherein the first end section  109  is spaced apart from the second end section  110  along the axial direction  121  and the middle section  111  is arranged between the first end section  109  and the second end section  110 . The first edge side  106  comprises a recess  112  into which the first end section  109  of the seal strip  108  is arranged such that the recess  112  surrounds the first end section  109  and fixes the first end section  109  to the inner surface  105 . 
     The root section  101  comprises a supporting lever  113  extending from the second edge side  107  into the cavity  104  such that a free end of the supporting lever  113  forms a contact region  114  with the middle section  111  of the seal strip  108  for fixing the middle section  111  of the seal strip  108  to the inner surface  105 . The supporting lever  113  is further formed such that a further cavity  115  is formed between the inner surface  105 , the second edge side  107  and the supporting lever  113 . The second end section  110  of the seal strip  108  is arranged inside the further cavity  115 . 
     The root section  101  comprises the platform  103 , the first (trailing) edge side  106  and the second (leading) edge side  107 . The platform  103  has a first (outer) surface which faces to a mainstream flow channel of the gas turbine and a second (inner) surface  105  which faces to an opposite region of the platform  103  in comparison to the first surface. The airfoil element  102 , such as a blade, is attachable to the first surface. 
     The platform  103  extends generally along a circumferential direction  123  and an axial direction  121  of the gas turbine. The thickness of the platform  103  i.e. its extension along the normal of the inner surface  105 , e.g. along the radial direction  122 , is generally smaller in comparison to the other extensions, e.g. to the extensions along the axial direction  121  and circumferential direction  123 . 
     The terms axial direction  121 , circumferential direction  123  and radial direction  122  refer to directions with respect to a turbine shaft  20  (see  FIG. 4 ) of the gas turbine. The circumferential direction  123  describes a run around the gas turbine shaft  20  the radial direction  122  describes a run through a point of the rotating axis of the turbine shaft  20  and the axial direction  121  describes a run parallel to the rotating axis of the turbine shaft  20 . The axial direction  121  and the radial direction  122  are orientated in particular perpendicular with respect to each other. 
     The (second) leading edge side  107  and the (first) trailing edge side  106  are attached to the platform  103 . The second edge side  107  and the first edge side  106  run from the inner surface  105  of the platform  103  along a substantially radial direction  122 . The second leading edge side  107  is located more upstream with respect to the first edge side  106 . Hence, the platform  103 , the first edge side  106  and the second edge side  107  form a kind of a U-shape inner cross-section inside the cavity  104  and the further cavity  115  is formed. 
     The recess (slit or groove)  112  of the first edge side  106  has an U-shaped cross section, wherein the first end section  109  of the seal strip  108  is inserted and slipped into the recess  112  through its open side. 
     The cavity  104  and the further cavity  115  may be flushed with cooling air, wherein the cooling air may be fed from a hollow airfoil  102  or the root section  101  for cooling purposes. 
     The supporting lever  113  extends from the second edge side  107  into the cavity  104  and hence protrudes into the cavity  104 . The supporting lever  113  is formed in such a way that a gap between a free end of the supporting lever  113  and the inner surface  105  is formed. The seal strip  108  arranged onto the inner surface  105  protrudes through the gap. In other words, the supporting lever  113  forms with its free end contact region  114  with the middle section  111  of the seal strip  108  and thereby presses and fixes the middle section  111  of the seal strip  108  to the inner surface  105 . 
     Furthermore, the supporting lever  113  divides the cavity  104  such that a further cavity  115  is formed between the inner surface  105 , the second edge side  107  and the supporting lever  113 , wherein the second end section  110  of the seal strip  108  is arranged inside the further cavity  115 . 
     The seal strip  108  is in contact with the inner surface  105  of the airfoil device  100  and a further inner surface of an adjacent further airfoil device and thus seals a gap between two adjacent platforms  103 . 
     As can be taken from  FIG. 1 , the support lever  113  extends only part-way along the axial direction  121  into the cavity  104  and along the inner surface  105 . Only a first end section  109  of the seal strip  108  is inserted into the recess  112 , wherein the opposed second end section  110  of the seal strip  108  is not surrounded by a further recess or groove for support purposes. The seal strip  108  is held against the inner surface  105  by the free end of the support lever  113 . Hence, an easy installation of the seal strip  108  inside the inner cavity  104  is provided. 
     As shown in  FIG. 1 , a part of the middle section  111  of the seal strip  108  between the contact region  114  and the first end section  109  is arranged inside the cavity  104 . The second end section  110  and a second part of the middle section  111  of the seal strip  108  between the contact region  114  and the second end section  110  is arranged inside the further cavity  115 . 
     In particular, as can be taken from  FIG. 1 , the supporting lever  113  is formed such that a first axial length of the seal strip  108  between the contact region  114  and the first end section  109  is larger than a second axial length of the seal strip  108  between the contact region  114  and the second end section  110 . Hence, because of the shorter second axial length between the contact region  114  and the second end section  110 , the seal strip  108  may be formed stiffener such that the shorter second axial length part of the seal strip  108  does not deform due to the own weight of the shorter second axial length part. 
     The location of the contact region  114  and the length of the support lever  113  will depend on the length of the middle section  111  and the angle by which the support lever  113  is approaching the inner surface  105 . Or put differently, how much elastic deflection can be accomplished by the seal strip during installation with a controllable force. 
     The second edge side  107  comprises a seal strip inlet  116  for inserting the seal strip  108  into the further cavity  115 . 
     The seal strip inlet  116  is formed such that a fitment of the seal strip  108  with blades already in-situ is enabled. In particular, the seal strip inlet  116  may be formed at the upstream (second) edge side  107  and connects an upstream environment of the airfoil device  100  with the further cavity  115 . Hence, the seal strip  108  may be inserted through the seal strip inlet  116  along approximately the axial direction  121  into the further cavity  115 . Furthermore, the seal strip  108  may be further moved along approximately the axial direction  121  until the first end section  109  of the seal strip  108  is arranged within the recess  112  in the downstream (first) edge side  106 . 
     A further benefit of having the seal strip inlet  116  at the upstream side is that the pressure differences acting on and across the seal strip would push the seal strip further in and up in the groove rather than away and out. 
     The second edge side  107  comprises a stopper section  117  which is formed such that the second end section  110  of the seal strip  108  abuts against the stopper section  117 . 
       FIG. 2  shows an enlarged view of a stopper section  117  of the airfoil device  100  as shown in  FIG. 1 . The stopper section  117  comprises a step or protrusion which protrudes from second edge side  107  or the inner surface  105  into the further cavity  115 . The stopper section  117  has a surface which has a normal that is (at least with a component) parallel to the axial direction  121 . The seal strip  108  abuts against the stopper section  117 , if the seal strip  108  is moved out of the recess  112  upstream and along the axial direction  121 . Hence, the stopper section  117  limits a movement of the seal strip  108  along the axial direction  121 , such that a slipping out of the recess  112  is prevented. 
       FIG. 3  shows a perspective view of the airfoil device  100  as shown in  FIG. 1 . 
     When the seal strip  108  is assembled to the airfoil device  100  and/or between two circumferentially adjacent airfoil devices  100 , the seal the middle section  111  of the seal strip  108  is inserted via the seal strip inlet  116  and will contact both the inner surface  105  and supporting lever  113 . Continued insertion causes the supporting lever  113  to exert a force on the seal strip  108  which in turn forces against the inner surface  105 . The seal strip  108  elastically deforms and/or the supporting lever  113  elastically deforms to accommodate and permit continued insertion of the seal strip  108 . Once the first end section  109  is at least partly in the recess  112  and the second end  110  clears (i.e. is axially rearward) the stopper section  117 , the seal strip  108  springs into the position shown in  FIG. 1 . Although the seal strip  108  is shown as a straight member the seal strip  108  can be arcuate in the axial and/or circumferential direction to aid fitting and securing into its cavity  104 . To remove or disassemble the seal strip  108  from the cavity  104 , the second end section  110  is forced radially inwardly such that the seal strip  108  and/or the supporting lever  113  flexes or elastically deforms so that the second end section  110  is radially inwardly of the stopper section  117 . The seal strip  108  can then be moved axially forwardly and removed from the cavity  104 . 
     The seal strip  108 , as shown in  FIG. 1 , is retained or secured in the cavity by virtue of the supporting lever  113  preventing the second end section  110  moving radially inwardly of the stopper section  117 . It should be appreciated that during engine operation the seal strip  11  will be forced radially outwardly against the inner surface  105  by centrifugal effects. When the engine is not in operation, the seal strip  108  may rest against the supporting lever  113  and the recess  112  and not in contact with the inner surface  105 . Furthermore, the circumferential edges or parts of the circumferential edges of the strip seal  108  may be in contact with the inner surface  105 . 
     When the seal strip  108  is assembled to the airfoil device  100  and/or between two circumferentially adjacent airfoil devices  100  it seals the generally axial gap between each platform along their axial extents to prevent ingress of hot gases to the cavity  104 . It should be appreciated that circumferentially adjacent airfoil devices  100  each comprise a cavity  104  and one or both may have a supporting lever  113 . 
       FIG. 4  shows an example of a gas turbine engine  10  in a sectional view. The gas turbine engine  10  comprises, in flow series, an inlet  12 , a compressor section  14 , a combustor section  16  and a turbine section  18  which are generally arranged in flow series and generally in the direction of a longitudinal or rotational axis. The gas turbine engine  10  further comprises a shaft  20  which is rotatable about the rotational axis and which extends longitudinally through the gas turbine engine  10 . The shaft  20  drivingly connects the turbine section  18  to the compressor section  14 . 
     The terms upstream and downstream refer to the flow direction of the airflow and/or working gas flow through the engine unless otherwise stated. The terms forward and rearward refer to the general flow of gas through the engine. The terms axial, radial and circumferential are made with reference to a rotational axis of the engine. 
     In operation of the gas turbine engine  10 , air  24 , which is taken in through the air inlet  12  is compressed by the compressor section  14  and delivered to the combustion section or burner section  16 . The burner section  16  comprises a burner plenum  26 , one or more combustion chambers  28  defined by a double wall can  27  and at least one burner  30  fixed to each combustion chamber  28 . The combustion chambers  28  and the burners  30  are located inside the burner plenum  26 . The compressed air passing through the compressor section  14  enters a diffuser  32  and is discharged from the diffuser  32  into the burner plenum  26  from where a portion of the air enters the burner  30  and is mixed with a gaseous or liquid fuel. The air/fuel mixture is then burned and the combustion gas  34  or working gas from the combustion is channeled via a transition duct  35  to the turbine section  18 . 
     The turbine section  18  comprises a number of blade carrying discs  36  attached to the shaft  20 . In the present example, two discs  36  each carry an annular array of turbine blades  38 . The turbine blade devices  38  may be designed such as the above described airfoil devices  100 . However, the number of blade carrying discs could be different, i.e. only one disc or more than two discs. In addition, guiding vanes  40 , which are fixed to a stator  42  of the gas turbine engine  10 , are disposed between the turbine blades  38 . The guiding vanes  40  may be designed such as the above described airfoil devices  100 . Between the exit of the combustion chamber  28  and the leading turbine blades  38  inlet guiding vanes  44  are provided. 
     The combustion gas from the combustion chamber  28  enters the turbine section  18  and drives the turbine blades  38  which in turn rotates the shaft  20 . The guiding vanes  40 ,  44  serve to optimise the angle of the combustion or working gas on to the turbine blades  38 . The compressor section  14  comprises an axial series of guide vane stages  46  and rotor blade stages  48 . 
     It should be noted that the term “comprising” does not exclude other elements or steps and “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims. 
     LIST OF REFERENCE SIGNS 
     
         
           10  gas turbine engine 
           12  inlet 
           14  compressor section 
           18  turbine section 
           20  shaft 
           24  air 
           26  burner plenum 
           27  can 
           28  combustion chamber 
           30  burner 
           32  diffuser 
           35  transition duct 
           36  disc 
           38  turbine blade 
           40  guiding vanes 
           42  stator 
           44  guiding vanes 
           46  guide vane stage 
           48  rotor blade stage 
           100  airfoil device 
           101  root section 
           102  airfoil element 
           103  platform 
           104  cavity 
           105  inner surface 
           106  first edge side 
           107  second edge side 
           108  seal strip 
           109  first end section 
           110  second end section 
           111  middle section 
           112  recess 
           113  supporting lever 
           114  contact region 
           115  further cavity 
           116  seal strip inlet 
           117  stopper section 
           121  axial direction 
           122  radial direction 
           123  circumferential direction 
           400  conventional airfoil device 
           401  conventional airfoil 
           402  conventional platform 
           403  conventional cavity 
           404  conventional root section 
           405  conventional seal strip