Abstract:
A sealing arrangement for sealing a gap ( 22 ) between two adjacent, thermally loaded components ( 14, 14 ′) of a thermal machine, especially a turbomachine or gas turbine, includes a seal ( 25 ) which is supported in a recess ( 23 ) which extends transversely to the gap ( 22 ) and traverses said gap ( 22 ). An efficient and simple improvement of the sealing arrangement is made by the seal ( 25 ) being formed at least partially of a shape memory alloy in such a way that, when a pre-specified temperature limit is exceeded, the seal changes its sealing behavior.

Description:
This application claims priority under 35 U.S.C. §119 to Swiss Application No. 00343/11, filed Feb. 28, 2011, the entirety of which is incorporated by reference herein. 
     BACKGROUND 
     1. Field of Endeavor 
     The present invention relates to the field of machine elements in conjunction with thermal machines, and more particularly to a sealing arrangement. 
     2. Brief Description of the Related Art 
     In thermal machines, such as gas turbines, or electromechanical machines, such as electric generators, the cooling of thermally loaded components is an essential parameter for the overall efficiency and the service life of the system. In most cases, the cooling medium is cooling air; however, steam from a steam generator can also be used for the same purpose. The subsequently disclosed subject matter is correspondingly applicable to all cooling media, regardless of the supply source, but is explained using the example of an air-cooled gas turbine. 
     In a gas turbine, air is compressed by a centrifugal or axial compressor from the ambient pressure to nominal pressure. After cooling of the hot parts of the combustion chamber, the main portion of the compressed air is used for combusting fuel in the combustion chamber. The rest of the compressed air is extracted at one or more points along, or at the end of, the compressor, and directed through cooling air passages to the hot parts of the gas turbine. In the turbine, the cooling medium is used for the internal or external cooling of the turbine components, such as stator blades and rotor blades. In addition, the cooling medium reduces the operating temperature of non-rotating and rotating components, such as the blade roots or rotor disks, which are exposed to large centrifugal forces. 
     Some of the air is also used for sealing purposes, particularly between rotating and stationary parts, by the air being purged through a gap into the hot gas passage of the turbine in order to prevent entry of hot gas and therefore local overheating. The seal plays an important role in the effective distribution and controlling of the cooling air. On account of mechanical and thermal stresses and the thermal expansion of the components during operation, however, the gaps which are to be sealed alter in their dimensions. In the stationary state of the machine, each rotating and non-rotating component is assembled with other parts, taking into consideration manufacturing and assembly tolerances and also the anticipated mechanical and thermal deformation of the components. The cold gaps which therefore result enable the unhindered thermal expansion and rotation-induced deformation of the components during operation by the gaps which result in the process, the so-called hot gaps, at no time adopting unacceptably small values. The unhindered expansions and deformations prevent mechanical defects in each component where they adjoin other components. 
     Under operating conditions, depending upon the thermal and mechanical load of the respective component, the resulting hot gaps can become smaller or larger than the cold gaps relative to the stationary state of the machine. An example of such changes in a gas turbine is shown in  FIGS. 1-3 .  FIG. 1  shows in a detail a typical arrangement of rotor blades and stator blades in a gas turbine, including the seals—in this case exemplarily of a strip-like construction—which are arranged between the components. The gas turbine  10  of  FIG. 1  includes a rotor  11  which is equipped with rotor blades  13  and is enclosed by a turbine casing or stator blade carrier  12 . The rotor blades  13  have an (inner) platform  14  ( 14 ′), beneath which the blade merges into a shank  15 , on the end of which is arranged a blade root  16  by which the rotor blade  13  is fastened in the rotor  11 . Stator blades  18 , which have a(n) (outer) platform  19 , are attached on the turbine casing or stator blade carrier  12 . The blade airfoils of the rotor blades and stator blades  13 ,  18  lie in the hot gas passage of the turbine. 
     For sealing in relation to the hot gas passage, provision is made for seals—in this case exemplarily constructed as sealing strips  20 ,  21  (FIG.  2 )—which extend in the axial direction (z-axis in  FIG. 2 ) and in the circumferential direction (u-axis in  FIG. 2 ), but can also extend in the radial direction and be arranged between adjacent platforms  14  ( 14 ′) of blades or between blades and adjacent heat shields  13 ,  17  or ring segments, as well as between adjacent heat shields (when provided). According to  FIG. 3   a , the sealing strips  21  lie transversely to the gap  22  between the platforms  14  and  14 ′ of adjacent components in a corresponding recess  23 . 
     As exemplarily indicated in  FIG. 3   b , the gap between the rotor blades of each turbine stage increases during operation. Under the influence of centrifugal forces, the rotor blades are stretched in the radial direction, which results in a larger gap c in the circumferential direction between the platforms of the blades. 
     This enlargement of the gap in the circumferential direction is partially compensated for by the thermal expansion of the blade platforms. Depending upon the blade mass and the operating temperature, the thermal deformations of the platforms are usually less than the blade deformations which are brought about as a result of rotation. Therefore, the gap c between the blade platforms of longer blades in the circumferential direction during operation can be either larger than the cold gap c 0  in the stationary state of the machine ( FIG. 3   a ), or it can be smaller, which applies typically to non-rotating components or light rotor blades, the deformations of which are principally determined by the thermal load. In general, during operation the gaps can vary considerably depending upon the mechanical and thermal load of the rotating and non-rotating components of the machine. Furthermore, the radius of the platforms  14 ,  14 ′, which in the stationary state can have the value R 0  ( FIG. 3   a ), can acquire another value R ( FIG. 3   b ) during operation. 
     In order to passively control the gaps during operation of the machine, according to  FIG. 2  provision is made for axially and circumferentially extending seals—in this case exemplarily constructed as strips  20  or  21 —which prevent an uncontrolled leakage of the cooling medium into the hot gas passage of the turbine. The seals  20 ,  21  generally are formed of an alloy which is suitable for the operating conditions of the machine. 
     The seals which are arranged in rotating components are pressed against the platforms of the rotor blades as a result of centrifugal forces. Consequently, a mechanical contact between the upper side of the seal  21  and the outer flanks of the seal groove  23  is created inside the platforms or heat shields  14 ,  14 ′, which is schematically shown in  FIG. 3   b . On account of the thermal expansion and rotation-induced deformations, the effective hot gap c ( FIG. 3   b ) becomes larger during operation, whereas the width b of the seal  21  remains practically unaltered on account of its small dimensions relative to the size of the blade, of the rotor and of the heat shields. The mechanical contact which is created can be limited to a narrow contact area on the seal which does not lead directly to the best possible sealing effect or can even lead to undesirable or increased leakage on account of local deformations of the platform. 
     In the prior art, considerations have already been made to purposefully control sealing conditions in thermal machines by the use of memory alloys. 
     An arrangement of a rotor and a stator of a turbine, in which rotor blades with a blade airfoil and (inner) platform and stator blades with a blade airfoil and (inner) platform alternate with each other, is known from printed publication US 2007/0243061. Between the platforms of the rotor blades and the platforms of the stator blades a seal is defined, wherein the platforms of the rotor blades and/or stator blades in the region of the seal partially are formed of a memory alloy in order to control the cooling air flow through the seal in dependence upon temperature. Such controlling through the platforms themselves is extremely costly in production and dimensioning, because the blades themselves have to be constructed from different material and correspondingly prefabricated. 
     U.S. Pat. No. 7,086,649 B2 discloses an annular seal for inserting between two parts which rotate relatively to each other. The seal includes a section which can bend for altering the sealing gap. Such a section can be formed of a bimetal or a memory alloy in order to alter the gap width in a temperature-controlled manner. 
     A device for controlling the clearance between the blade tip of a rotor blade and the opposite wall of an axial turbine, in which the radial position of the wall is controlled by a spiral spring, formed of a memory alloy, which shifts the wall, is known from printed publication JP58206807. 
     The seals which are described in the introduction are not the subject of known proposals. 
     SUMMARY 
     One of numerous aspects of the present invention includes a sealing arrangement of the aforementioned type for a thermal machine, which takes into account the altered conditions in an automatic and simple manner if the machine gaps between stationary state and operation alter. 
     Another aspect includes a sealing arrangement for sealing a gap between two adjacent, thermally and/or mechanically loaded components of a thermal machine, especially a turbomachine or gas turbine, comprising a seal which is supported in a recess which extends transversely to the gap and traverses the gap. The seal is formed at least partially of a memory alloy in such a way that when a pre-specified temperature limit is exceeded, it alters its sealing behavior. 
     One development of a sealing arrangement as described herein is characterized in that at least one of the components is a rotor blade of a rotor of a turbomachine. 
     According to another development, both components are rotor blades of a rotor of a turbomachine. 
     It is also conceivable, however, that the other component is a heat shield (or a ring segment) of a rotor of a turbomachine. 
     Another development is characterized in that the components have, in each case, a platform, and in that the seals are arranged in a manner extending between the platforms in the axial and/or circumferential direction and/or radial direction. 
     A further development is characterized in that the seal comprises in each case at least one first component (particularly a horizontal strip) which is supported in the recess which extends transversely to the gap, in that the seal furthermore has at least one second component (especially a vertical strip) which extends along the gap perpendicularly to the horizontal strip, and in that at least the at least one second component is formed of a memory alloy. 
     It is particularly advantageous if in this case the seal has two second components which extend along the gap perpendicularly to the first component if both second components are formed of a memory alloy, and if both second components are designed in such a way that when the pre-specified temperature limit is exceeded, they come to bear against opposite walls of the gap with sealing effect. As a result of this, it is possible to achieve additional sealing effects over and above the effect of the first component. 
     A further development is characterized in that the second components extend from the first component essentially towards one side beyond the recess into the gap and, when the pre-specified temperature limit is exceeded, form a second seal there. 
     Another development is characterized in that the second components extend from the first component towards opposite sides beyond the recess into the gap and, when the pre-specified temperature limit is exceeded, form a second and a third seal there. 
     The first component in this case can be formed of a metal alloy, for example steel. 
     It is also conceivable, however, that the first component is formed of a memory alloy. In this way, the first component can also contribute towards the controlling of the sealing behavior. 
     Another development is characterized in that provision is made for two first components formed of a memory alloy, which when the pre-specified temperature limit is exceeded, come to bear against opposite walls of the recess with sealing effect. As a result of this, the controlled sealing characteristics can be improved even further. 
     A further improvement can be achieved if the first and/or second components of a memory alloy are provided with thickened portions (especially side protrusions) for improving the sealing characteristics. 
     Finally, each seal can comprise at least one first component which is supported in the recess which extends transversely to the gap, the first component can have a transfer passage, and the transfer passage can be closed off by a valve strip formed of a memory alloy, in such a way that when a pre-specified temperature limit is exceeded, it frees the transfer passage. In this way, in the case of high thermal load the cooling can locally be altered in a directed manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention shall subsequently be explained in more detail based on exemplary embodiments in conjunction with the drawing. In the drawing 
         FIG. 1  shows, in a detail, a typical arrangement of rotor blades and stator blades in a gas turbine including the seals—in this case exemplarily of a strip-like design—which are arranged between the components; 
         FIG. 2  shows the section through the rotor blade from  FIG. 1  in the plane A-A; 
         FIG. 3  shows the behavior of a conventional sealing strip between adjacent rotor blades, or rotor blade and adjacent heat shield, in the stationary state of the turbine and at room temperature ( FIG. 3   a ) and also at nominal speed and operating temperature ( FIG. 3   b ); 
         FIG. 4  shows the behavior of a sealing arrangement with two-way sealing effect according to an exemplary embodiment of the invention between adjacent rotor blades, or rotor blade and adjacent heat shield, in the stationary state of the turbine and at room temperature ( FIG. 4   a ) and also at nominal speed and operating temperature ( FIG. 4   b ); 
         FIG. 5  shows the behavior of a sealing arrangement with three-way sealing effect according to another exemplary embodiment of the invention between adjacent rotor blades, or rotor blade and adjacent heat shield, in the stationary state of the turbine and at room temperature ( FIG. 5   a ) and also at nominal speed and operating temperature ( FIG. 5   b ); 
         FIG. 6  shows the behavior of a sealing arrangement with four-way sealing effect according to another exemplary embodiment of the invention between adjacent rotor blades, or rotor blade and adjacent heat shield, in the stationary state of the turbine and at room temperature ( FIG. 6   a ) and also at nominal speed and operating temperature ( FIG. 6   b ); and 
         FIG. 7  shows the behavior of a sealing arrangement with valve function according to another exemplary embodiment of the invention between rotor blade and adjacent heat shield in the stationary state of the turbine and at room temperature ( FIG. 7   a ) and also at nominal speed and operating temperature ( FIG. 7   b ). 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     It can be particularly advantageous to produce the seal wholly or partially from a memory alloy (shape-memory alloy, SMA). Above a pre-specified temperature limit, which can be lower than the nominal temperature, or in the course of a continuously rising operating temperature, the part of the seal formed of a memory alloy automatically activates and alters or improves the sealing characteristics. The shrinking-, stretching-, torsional-, and bending deformations of the memory alloy create a mechanism for improving the sealing characteristics in a simple sealing system, for example produced from steel, as is shown in  FIG. 3  in the form of the sealing strip  21 . 
       FIG. 4  shows an example of a sealing arrangement according to principles of the invention, which is based on a sealing strip  25  with a horizontal strip  26  (first component) formed of a conventional seal material. Connected to this horizontal strip  26  are two vertical strips  27  (second components), formed of a memory alloy, which extend downwards in parallel. The horizontal strip  26 , similar to the sealing strip  21  in  FIG. 3   a , is accommodated in a recess  23  which lies transversely to the gap  22  ( FIG. 4   a ). During an operation above the temperature limit or in the case of rising operating temperature, the vertical strips  27  bend outwards towards the respectively adjacent platform  14  or  14 ′ in order to lie in mechanical contact there with sealing effect ( FIG. 4   b ). In order to boost this deformation process, side protrusions  28 , which intensify the contact-producing deformation, can be arranged at the ends of the vertical strips  27 . The vertical strips  27  are of sufficient length in order to achieve the necessary elasticity of the coupled platform-seal system. This elasticity ensures that a permanent contact between the blades and the seals is provided even in the event of vibrations in the turbine stage. The vertical strips  27 , which make contact with sealing effect according to  FIG. 4   b , in addition to the horizontal strip  26 , create a second seal so that the sealing arrangement according to  FIG. 4 , in contrast to the single-stage sealing arrangement of  FIG. 3 , is a two-stage sealing arrangement. 
     In order to create a three-stage sealing arrangement, the horizontal strip  26  according to  FIG. 5  can be produced from a memory alloy. Above the temperature limit, or in the case of steadily rising operating temperature, the horizontal strip  26  of the sealing strip  30  stretches in opposite horizontal directions in order to better take up the increasing clearance c ( FIG. 5   b ). At the same time, the two vertical strips  27  of the memory alloy follow the expansion  29  of the horizontal strip  26  and separate in the direction of the adjacent platforms  14  and  14 ′. The centrifugal forces, which act upon the side protrusions at the upper ends of the two vertical strips  27 , bend these vertical strips outwards. This bending creates mechanical contacts with the upper sections of the adjacent platforms  14  and  14 ′, as is shown in  FIG. 5   b . The bending characteristics and expansion characteristics of the sealing system of memory alloy therefore enable a three-stage sealing arrangement with only one sealing strip  30 . 
     In the previous explanations, the joining technique for connecting the individual strips  26  and  27  to each other was not elaborated upon in more detail. However, all possible joining techniques for producing the proposed sealing arrangement can be used. Also, the configurations of the sealing strips which are explained in detail must be understood only as examples, wherein the seals may also be formed of non-strip-like elements or components. Other configurations of the sealing arrangement are possible within the scope of the invention and may contain other mechanisms for deformation of the memory alloy, and also differently formed constructions and combinations of different geometric shapes which are triggered as a result of thermal, mechanical or other load. Thus,  FIG. 6  illustrates for example a sealing arrangement with a sealing strip  31 , in which two horizontal strips  26   a  and  26   b  of a memory alloy first of all expand in the case of a thermal load above the temperature limit or in the case of a steadily rising operating temperature (expansion  29  in  FIG. 6   b ) and are then bent in opposite radial directions. In this way, an additional fourth sealing stage between the adjacent blades is achieved. 
     The memory alloy which is used may include different metallurgical compositions of different elements and be produced in a different way. The temperature and/or the mechanical change which is dependent upon the operating conditions of the machine initiate the process of the geometrical change of the strip of the memory alloy, which depends upon the production of that memory alloy. In the case of a decrease of the cold gap c 0 , the shrink behavior of the memory alloy is taken into consideration, instead of expansion in fact, as has been described here. 
     A sealing arrangement as described herein, which operates with a memory alloy, can also be used in all other machines where an active leakage control is necessary. Thus, such a sealing arrangement can be used for example in cryogenic apparatus such as helium and/or hydrogen liquefiers or, for example, refrigeration plants with operating temperatures below the freezing point. In this case, a reduction in the operating temperature is the principal mechanism which controls the active leakage control using the sealing arrangement. 
     A SMA sealing arrangement as described herein can also be equipped with additional emergency characteristics. Above a permissible temperature, the SMA sealing arrangement can be opened, for example in order to supply an overheated region of a selected component with more cooling medium. Such a system can be arranged in cooling passages of the machine or inside cooling passages of a hot component. Another emergency function of such a SMA sealing arrangement can be to open above a permissible temperature, which is exceeded on account of entry of hot gas, in order to avoid, for example, local overheating of the blade platform or of another component and also of the sealing arrangement itself. Such a function avoids damage to the sealing arrangement as a result of self-opening. 
     The sealing arrangement can vary in reaction to an excessively high metal temperature of the platform or to an excessively high gas temperature which is induced as a result of a hot gas flow ( 39  in  FIG. 7   b ) for example between the upper side of a heat shield ( 37  in  FIG. 7 ) and the tip of the stator blade ( 18  in  FIG. 7 ). As one of many possible configurations,  FIG. 7  exemplarily shows a sealing arrangement with a sealing strip  32  which features such an emergency characteristic. Above the temperature limit, a valve strip  36  shrinks, and in the stationary state of the machine covers a transfer passage  34  in a base strip or horizontal strip  33  (formed of steel) lying beneath the valve strip, and opens this (preferably slot-like) opening  34  in order to admit an unhindered cooling medium flow into the hot blade passage. The upper part of the vertical strip  35  of memory alloy bends in order to direct the cooling air flow  38  to the overheated regions of the platform  14  and of the rotor blade  13 . At the same time, the bent vertical strip  35  guides the hot gas flow  39  away from the overheated region. The last-named characteristic can also be used in conjunction with a horizontal strip, formed of steel, without transfer passage  34 . The overheated region is then protected by the bent vertical strip by the hot gas flow  39  being deflected towards the upper part of the blade. 
     Overall, with seals as described herein the following additional characteristics and advantages can ensue:
         The sealing arrangement can be used in different machines, the adjacent components of which expand variably during operation on account of diverse monotonic and/or cyclic loads between stationary state, partial load, nominal load and/or overload;   The sealing arrangement, depending upon requirement, can intensify sealing or can open;   The sealing arrangement can be used between adjacent components formed of the same or different material;   The sealing arrangement can improve the sealing behavior;   The sealing arrangement can act as an emergency valve which brings about additional cooling of an overheated region of a protected component if the system exceeds a limit in temperature, in pressure, in centrifugal load or in another parameter;   The service life of the components in question is consequently extended;   The higher cooling medium consumption of the machine which is brought about as a result can be used as a parameter for monitoring and possible shutting down of the machine.       

     LIST OF DESIGNATIONS 
     
         
         
           
               10  Gas turbine (turbomachine) 
               11  Rotor 
               12  Turbine casing 
               13  Rotor blade 
               14 ,  14 ′,  19  Platform 
               15  Shank 
               16  Blade root 
               17  Adjacent blade (or heat shield) 
               18  Stator blade 
               20 ,  21  Seal 
               22  Gap 
               23  Recess (groove) 
               24  Hot gas passage 
               25 ,  30 ,  31  Seal 
               26 ,  26   a,b  Component (for example horizontal strip) 
               27  Component (for example vertical strip) 
               28  Thickened portion (for example side protrusion) 
               29  Expansion 
               32  Seal 
               33  Component (for example base strip/horizontal strip) 
               34  transfer passage 
               35  Component (for example vertical strip) 
               36  Valve strip 
               37  Heat shield 
               38  Cooling air flow 
               39  Hot gas flow 
             b Width 
             c 0  Cold gap 
             c Hot gap 
             u Circumferential direction 
             r Radial direction 
             R, R 0 , R′ Radius 
             z Axial direction 
           
         
       
    
     While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.